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SALT SINK—A body of water too salty for most freshwater uses.

SALT WATER—Water which contains a relatively high percentage of sodium chloride.

SALT-WATER BARRIER—A physical facility or method of operation designed to prevent the intrusion of salt water into a body of fresh water.

SALT-WATER INTRUSION—The invasion of a body of fresh water by a body of salt water, due to its greater density. It can occur either in surface or ground-water bodies. The term is applied to the flooding of freshwater marshes by seawater, the migration of seawater up rivers and navigation channels, and the movement of seawater into freshwater aquifers along coastal regions.

SALT-WATER MARSH—Low, flat marshlands subject to inundation by salt waters; may be tidal or non-tidal; normally the only vegetation present is salt-tolerant bushes and grasses. Also see Marsh and Tidal Marsh.

SALVAGED WATER—The part of a particular stream or other water supply that is saved from loss, in respect to quantity or quality, and is retained and made available for use.

SAMPLE—(Statistics) The sample consists of a number of randomly-selected, representative observations about an economic phenomenon; a part of a population taken to estimate a parameter of the whole population. The underlying assumptions are that we do not have the means to measure the entire population of events and that the sample, if properly selected, will accurately represent the behavior and characteristics of the entire population (of events) within specified limits of probability. Types of samples include:

[1] Random—A sample drawn without bias from a population in which every item has an equal chance of being drawn;
[2] Representative—A sample drawn in such a way that it gives a true value for the population from which it was drawn.

Contrast with Population and Census.

SAMPLE SIZE—(Statistics) The number of individual observations. The sample size, i.e., number of observations about a particular event or phenomenon, has important implications on how representative the sample is of the Population.

SAND—Composed predominantly of coarse-grained mineral sediments with diameters larger than 0.074 mm (0.0029 inch) and smaller than 2 mm (0.079 inch) in diameter.

SANDBAR—A ridge of sand built up by currents, especially in a river or in coastal waters.

SAND FILTER—(Water Quality) A device used to remove particles from drinking water prior to distribution to customers. The water is allowed to Percolate through a chamber containing sand of various grain sizes, with the finest grain size located on the top. The particles in the water are removed at the surface of the sand and later discarded by reverse flushing.

SANDSTONE AQUIFER—The type of aquifer supplying groundwater to large parts of the United States upper Middle West, Appalachia, and Texas. The water-bearing formation is often contained by shale strata, and the water has high levels of iron and magnesium.

SAND TRAP—(Irrigation) A device, often a simple enlargement in a ditch or conduit, for arresting the heavier particles of sand and silt carried by the water.

SANITARY LANDFILL—(Water Quality) A disposal site employing methods of disposing of solid wastes in a manner that minimizes environmental hazards by spreading, compacting to the smallest practical volume and applying cover material over all exposed wastes at the end of each operating day.

SANITARY SEAL (WATER WELL)—The neat cement seal at the top of a water well intended to prevent well contamination from surface water or shallow ground water flows containing potential contaminants.

SANITARY SEWER—A pipe or network of pipes which transport only municipal, commercial, or industrial wastewater (sewage) and not rain or storm waters from streets.

SANITARY SURVEY—An on-site review of the water sources, facilities, equipment, operation and maintenance of a public water system to evaluate the adequacy of those elements for producing and distributing safe drinking water.

SANITARY WATER—Water discharged from sinks, showers, kitchens, or other nonindustrial operations, but not from commodes. Also referred to as Gray Water.

SAPROBIEN SYSTEM—(Water Quality) A European system of classifying organisms according to their response to organic pollution in slow-moving streams. Classifications include:

[1] Alpha-Mesaprobic Zone—Area of active decomposition, partly Aerobic, partly Anaerobic, in a stream heavily polluted with organic wastes;
[2] Beta-Mesaprobic Zone—That reach of a stream that is moderately polluted with organic wastes;
[3] Oligosaprobic Zone—That reach of a stream that is slightly polluted with organic wastes and contains the mineralized products of self-purification from organic pollution, but with none of the organic pollution remaining;
[4] Polysaprobic Zone—That area of a grossly polluted stream that contains the complex organic wastes that are decomposing primarily by anaerobic processes.

SAPROPEL—(1) A mud rich in organic matter formed at the bottom of a body of water. (2) A fluid slime found in swamps and bogs as a product of Putrefaction.

SAPROPHYTE—An organism, especially a fungus or bacterium, that grows on and derives its nourishment from dead or decaying organic matter that help natural decomposition of organic matter in water.

SAPWOOD—Newly formed outer wood that lies just inside the cambium of a tree trunk and is usually lighter in color and more active in water conduction than the heartwood.

SASTRUGA—(Russian) A long wavelike ridge of snow, formed by the wind and found on the polar plains.

SATELLITE (WASTEWATER TREATMENT) PLANT—Generally refers to a wastewater treatment facility in an outlying area, not connected to the main plant.

SATURATED—(1) Generally, filled to capacity; having absorbed all that can be taken up; soaked through with moisture. (2) (Hydrologic) A condition often used in reference to soils in which all voids or pore spaces between soil particles are filled with water. (3) (Chemistry) Describes a solution in its most concentrated state in which dissolved material can remain in solution under given conditions of temperature, pressure, etc.

SATURATED ADIABATIC LAPSE RATE—The lapse rate in the saturated lower layers of the air. It is equal to about 3.4F per 1,000 feet in elevation.

SATURATED AIR—Moist air in a state of equilibrium with a plane surface of pure water or ice at the same temperature and pressure; i.e., air whose vapor pressure is the saturation vapor pressure and its relative humidity is 100 percent.

SATURATED FLOW—The liquid flow of water in soils that occurs when the soil pores in the wettest part of the soil are completely filled with water and the direction of flow is from the wettest zone of higher potential to one of lower potential.

SATURATED THICKNESS (AQUIFER)—The thickness of the portion of the aquifer in which all pores, or voids, are filled with water. In a Confined Aquifer, this is generally the aquifer thickness. In an Unconfined Aquifer, this is the distance between the water table and the base of the aquifer.

SATURATED VAPOR PRESSURE—The pressure exerted by the vapor in a saturated space.

SATURATED ZONE—The zone in the earth's crust, extending from the water table downward, in which all open pore spaces in the soil or rock are filled with water at greater than atmospheric pressure. A termed used synonymously with the Zone of Saturation. Also referred to as Phreatic Zone.

SATURATION—The condition of a liquid when it has taken into solution the maximum possible quantity of a given substance at a given temperature and pressure.

SATURATION DEFICIT—The difference between the actual vapor pressure and the saturation vapor pressure as the existing temperature. Also referred to as the Vapor Pressure Deficit.

SATURATION MIXING RATIO—The maximum water vapor concentration in the atmosphere for a given air temperature. The higher the air temperature, the higher the saturation mixing ratio.

SATURATION POINT—That point at which a soil or an aquifer will no longer absorb any amount of water without losing an equal amount.

SATURATION VAPOR PRESSURE—The maximum possible partial pressure of water vapor in the atmosphere at a given temperature.

SATURATION, ZONE OF—The zone below the Water Table in which all pore spaces are filled with ground water. The water table is the top of the zone of saturation in an unconfined aquifer. Also referred to as the Phreatic Zone.

SAUNA—(1) A Finnish steam bath in which the steam is produced by pouring water over heated rocks; a bathhouse or room for taking such a steam bath. (2) A dry heat bath; a room or an enclosure for taking a dry heat bath.

SAUSAGE DAM—A dam composed of loose rock that has been wrapped with wire into cylindrical bundles and laid in a horizontal or vertical position.

SAVANNA, also Savannah—A flat grassland of tropical or subtropical regions dominated by grasses, sedges, and small shrubs with scattered trees, either as individuals or clumps. The savanna often represents a transitional region between true grassland and forest. Also see Biome and Grasslands.

SAXITOXIN—The primary toxin produced by dinoflagellate protozoans during blooms known as Red Tides in marine waters. The genus or protozoan involved in the generation of the red color in the water is Ganyaulax. Also see Paralytic Shellfish Poisoning.

SCABLAND—(Geography) An elevated area of barren, rocky land with little or no soil cover, often crossed by dry stream channels.

SCALD—(1) To burn with or as if with hot liquid or steam. (2) To subject to or treat with boiling water.

SCALE—(1) An accumulation of solid material on interior surfaces, such as pipelines, tanks, and boilers, as a result of the precipitation of mineral salts from water, most typically salts of calcium. Hard Water leaves a deposit (scale) in steam irons, coffee makers, and water heaters. (2) A hard incrustation usually rich in sulfate of calcium that is deposited on the inside of a vessel (as a boiler) in which water is heated.

SCARIFY—In land Restoration activities, to stir the surface of the ground with an implement in preparation for replanting.

SCAVENGING COEFFICIENT—The exponential constant (Þ) in an Exponential Decay model for the physical removal of particulate from the air by rainfall. Of the form:

X_t = X₀e^-Þt

where:

X_t is the particulate concentration at time t;
X₀ is the particulate concentration at time 0;
t is the number of time units (periods) since the rainfall began;
e is the base of the natural logarithm; and
Þ is the scavenging coefficient for the specific particulate.

SCENIC WATERWAY—Rivers or river segments chosen for scenic and recreation qualities to be preserved in their natural state.

SCHEDULE OF COMPLIANCE—Description of remedial actions to be accomplished by the permit holder (type of facility to be installed or alternative control measures to be established) and a sequence of actions leading to compliance with applicable standards.

SCHEDULED DELIVERY—Operation of a water delivery system to meet predetermined needs, generally based on user water orders. Also referred to as Arranged Delivery.

SCHISTOSOMIASIS—A debilitating tropical disease, common in underdeveloped regions of the world, particularly rural areas of Africa, Asia, and Latin America, caused by a small roundworm named Schistosoma. The disease is transmitted to humans through contact with water contaminated by fecal material. Infected humans discharge eggs in feces. These eggs hatch in freshwater, producing a small immature form of the parasite that infects snails common in streams, ponds, and lakes. The life cycle of the worm continues in the snail, and a second immature worm reenters the water where it later infects humans who come in contact with the infected water. Also referred to as Bilharziasis and Snail Fever.

SCIENCE ADVISORY BOARD (SAB)—An independent body established by the Administrator of the U.S. Environmental Protection Agency (EPA) in 1974 and by Congress in 1977. Its purpose is to review the scientific merits of EPA research and the scientific basis for the agency's proposed regulations and standards.

SCIENTIFIC METHOD—A systematic method of inquiry that includes the identification of a specific question or problem, the accumulation of the available data and information relating to that question, the proposal of a tentative answer or solution to the question or problem, the conduct of methodical observations or experiments to test the proposed answer, and the rational interpretation of the results of the observations or experiments.

SCOUR—(1) To clear, dig, or remove by or as if by a powerful current of water. (2) The erosive action of running water in streams, which excavates and carries away material from the bed and banks. Scour may occur in both earth and solid rock material.

SCOURING SLUICE—An opening in a dam controlled by a gate through which the accumulated silt, sand, and gravel may be ejected.

SCOUR POOLS—A pool formed by flow directed either laterally or obliquely against a partial channel obstruction or bank.

SCREENING—The use of screens to remove coarse floating and suspended solids from sewage.

SCRUBBER, SPRAY—An air pollution control device that removes particulates or gases from an airstream by spraying liquid into the air duct and then collecting the pollutant-containing droplets. Also see Spray Tower.

SCRUBBER, VENTURI—An air pollution control device that operates by the introduction of a liquid into a narrow throat section (venturi) of an air duct that is carrying a contaminant. The high velocity in the venturi, compared to the low initial liquid velocity, provides efficient contact between the injected scrubbing liquid and the contaminant to be removed.

SCUBA—A portable apparatus containing compressed air and used for breathing under water. [The name is derived from S(elf-)C(ontained)U(nderwater)B(reathing)A(pparatus).]

SCUD—Wind-driven clouds, mist, or rain; a gust of wind; ragged low clouds, moving rapidly beneath another cloud layer.

SCUM—(Biological) A filmy layer of extraneous or impure matter that forms on or rises to the surface of a liquid or body of water.

SCUPPER—(1) (Nautical) Openings along the side of a ship at deck level to allow water to run off. (2) An opening for draining off water, as from a floor or the roof of a building.

S-CURVE—The mass curve corresponding to a Unit Hydrograph or a distribution graph.

SCUTTLEBUTT—(1) A cask on shipboard to contain fresh water for a day's use. (2) A drinking fountain on a ship or at a naval or marine installation.

SEA—(1) One of the larger bodies of salt water, less than an ocean, more or less landlocked and generally forming part of, or connecting with, an ocean or a larger sea. (2) An inland body of water, especially if large or if salty or brackish.

SEA BREEZE—The sea-to-land surface wind that typically occurs in coastal areas during the day. It is caused by the thermal rising of the air above the land, which warms more readily than the water. Also see Land Breeze.

SEACOCK—(Nautical) A valve in the hull of a boat or ship that may be opened to let in water so as to flood a ballast tank, for example.

SEAL—A tight and perfect closure as against the passage of water.

SEA LEVEL—The level of the surface of the sea, especially measured at its mean position midway between mean high and low water. Also see Mean Sea Level (MSL).

SEA MILE—A unit of length used in sea and air navigation, based on the length of one minute of arc of a great circle, especially an international and U.S. unit equal to 1,852 meters (about 6,076 feet). More commonly referred to as Nautical Mile.

(WELL) SEAL—(Hydraulics) The watertight seal established in the annular space between the outermost water well casing and the drill hole to prevent the inflow and movement of surface water or shallow ground water, or to prevent the outflow or movement of water under artesian pressures. The term also includes a Sanitary Seal.

SEAS (World)—The following constitute a listing of the principal the seas of the world in alphabetical order: Adriatic Sea, Aegean Sea, Amandson Sea, Andaman Sea, Arabian Sea, Arafura Sea, Aral Sea, Baltic Sea, Banda Sea, Barents Sea, Bearing Sea, Beaufort Sea, Bellingshausan Sea, Bismark Sea, Black Sea, Caspian Sea, Caribbean Sea, Celebes Sea, Celtic Sea, Chukchi Sea, Coral Sea, Davis Sea, Dead Sea, D'Urville Sea, East China Sea, East Siberian Sea, Flores Sea, Greenland Sea, Halmahera Sea, Ionic (Ionian) Sea, Irish Sea, Java Sea, Kara Sea, Labrador Sea, Laptev Sea, Ligurian Sea, Mediterranean Sea, Molucca Sea, Norwegian Sea, North Sea, Philippine Sea, Red Sea, Ross Sea, Salton Sea, Savu Sea, Scotia Sea, Sea of Azov, Sea of Cortez, Sea of Crete, Sea of Galilee, Sea of Hebrides, Sea of Japan, Sea of Marmara, Sea of Okhotsk, Sibuyan Sea, South China Sea, Sulu Sea, Tasman Sea, Timor Sea, Tyrrhenian Sea, Weddell Sea, White Sea, Yellow Sea.

SEASHORE—(1) Land by the sea. (2) (Legal) Ground lying between high-water and low-water marks; the Foreshore.

SEASON—A period of time characterized by some distinguishable occurrence or feature, such as growing season, harvest season, winter season, dry season, etc. It is not to be used in reference to a 12-month period.

SEASONAL ADJUSTMENT—(Data Analysis) The adjustment of Time-Series Data, for example, commercial, residential, and irrigation water use, to eliminate the effects of intra-year normal variations (i.e., fluctuations within a 12-month period), such as those caused by weather patterns, agricultural cycles and irrigation patterns, school terms, holidays, and business patterns associated with such factors as product demand, tourism, etc. Also see Seasonal Factors.

SEASONAL ADJUSTMENT FACTORS—(Data Analysis) Mathematical indexes used to adjust for the intra-year normal seasonal fluctuations in time-series data. In its simplest form, such indexes may be derived from dividing the value of each observation by a simple moving average centered over each data observation with such an average encompassing the six-month period before and after the observation. More sophisticated seasonal adjustment processes may also account for the placement of weekends and holidays and other factors affecting data behavior. Also see Seasonal Factors.

SEASONAL APPLICATION EFFICIENCY (SAE)—The sum of the Evapotranspiration of Applied Water (ETAW), the Leaching Requirement (LR), and cultural practices (CP) (e.g., frost protection, heat protection, weed control), divided by the total applied water (AW), expressed as a percentage, or,

SAE = (ETAW + LR + CP)/AW

SEASONAL FACTORS—(Data Analysis) Factors such as weather conditions, agricultural production and irrigation requirements, business and tourism patterns, school schedules and holidays, etc., which cause normal annual fluctuations in business activity within individual and/or combinations of industries. These factors affect the demand for commercial, residential, and irrigation water use. Also see Seasonal Adjustment and Seasonal Adjustment Factors.

SEASONAL or INTERMITTENT STREAMS—Streams which flow only at certain times of the year when it receives water from springs, rainfall, or from surface sources such as melting snow. Also see Stream.

SEASONALITY—(Statistics) Periodic, repetitive, and generally predictable patterns in time series data. Typically, forecast models must explicitly incorporate seasonality in the estimation process either through Differencing techniques (transformations), through the introduction of seasonal Dummy Variables to explicitly account for these patterns, or by seasonal smoothing (Winters Model).

SEASONALLY ADJUSTED (S.A.)—(Data Analysis) Data which have been adjusted (modified) by Seasonal Adjustment Factors so as to remove the effects of Seasonality. Also sometimes referred to as a De-Seasonalization of the data. Also see Seasonal Adjustment.

SEASONALLY FLOODED (Estuarine)—Water regime in estuaries with seasonally-closed mouths and seasonally-flooded habitats.

SEASONAL WETLANDS—Wetland areas flooded or taking on the characteristics of a wetland only during specific periods of the year or seasons. Also see Wetlands and Prairie Potholes.

SEA STACK—A small, steep-sided rocky projection above sea level near a cliffed shore.

SEA WALL, also Seawall—An embankment to prevent erosion of a shoreline.

SEAWATER—The salt water in, or coming from the sea or ocean.

SECCHI DEPTH—A relatively crude measurement of the turbidity (cloudiness) of surface water. The depth at which a Secchi Disc (Disk), which is about 10-12 inches in diameter and on which is a black and white pattern, can no longer be seen.

SECCHI DISC (DISK)—A circular plate, generally about 10-12 inches (25.4-30.5 cm) in diameter, used to measure the transparency or clarity of water by noting the greatest depth at which it can be visually detected. Its primary use is in the study of lakes. Also see Secchi Depth.

SECONDARILY IMPROVED PASTURES—Livestock pasture areas that have not been intensively land leveled or serviced by modern conveyance structures. These areas receive primarily drainage water from improved irrigated areas.

SECONDARY (INDIRECT) ASSOCIATION—(Statistics) A situation in which there exists an apparent link between two Variables that is actually the result of a Confounding Variable. When the confounding variable is controlled, then the link is no longer evident.

SECONDARY BENEFITS—The values over and above the immediate products or services of a water resource development project.

SECONDARY DATA—(Data Analysis) Typically, data acquired from published sources as opposed to data acquired from direct observation or measurement such as a survey. Contrast with Primary Data.

SECONDARY DRINKING WATER REGULATIONS—Non-enforceable regulations applying to public water systems and specifying the maximum contamination levels that, in the judgement of the U.S. Environmental Protection Agency (EPA), are required to protect the public welfare. These regulations apply to any contaminants that may adversely affect the odor or appearance of such water and consequently may cause people served by the system to discontinue its use. Term may be used synonymously with Secondary Drinking Water Standards.

SECONDARY DRINKING WATER STANDARDS—Non-enforceable standards related to the aesthetic quality of drinking water such as those relating to taste and odor; generally set by the U.S. Environmental Protection Agency (EPA) or state water-quality enforcement agencies based on EPA guidance. Term may be used synonymously with Secondary Drinking Water Regulations. Also see Drinking Water Standards and Drinking Water Standards [Nevada]. [See Appendix D-5, Nevada Drinking Water Standards for a listing of Nevada's current drinking water primary and secondary quality standards.]

SECONDARY MAXIMUM CONTAMINANT LEVEL (SMCL)—The maximum concentration or level of certain water contaminants in public water supplies set by the U.S. Environmental Protection Agency (EPA) to protect the public welfare. The secondary levels are written to address aesthetic considerations such as taste, odor, and color or water, rather than health standards. Also see Primary Drinking Water Standards, Maximum Contaminant Level (MCL), and Maximum Contaminant Level Goal (MCLG).

SECONDARY POROSITY—The porosity that results from fractures and solution channels.

SECONDARY RECOVERY—The injection of water into an underground petroleum deposit to force the remaining oil into recovery wells. This technique is used to recover additional oil from old wells following the removal of the oil that can be easily pumped to the surface (Primary Recovery). Also referred to as Enhanced Oil Recovery. Also see Injection and Recharge.

SECONDARY SETTLING TANK—A tank used to hold wastewater that has been subjected to Secondary Wastewater Treatment. Floc or particles of organic matter formed during the secondary processes are allowed to settle from the suspension for subsequent removal.

SECONDARY STANDARDS—Allowable amounts of materials in air or water that are set to retain environmental qualities not related to the protection of human health. Secondary water standards are set for, among other things, taste, odor, and color, and some secondary air standards define concentrations that will not be harmful to plant life. Compare to Primary Standards.

SECONDARY SUCCESSION—The orderly and predictable changes that occur over time in the plant and animal communities of an area that has been subjected to the removal of naturally occurring plant cover. This type of succession occurs when agricultural fields are taken out of use or when forested areas are subjected to severe fires that destroy all vegetation. In both cases the top soil remains for the regrowth of natural plant communities. Compare to Primary Succession.

SECONDARY WASTEWATER TREATMENT—Treatment (following Primary Wastewater Treatment) involving the biological process of reducing suspended, colloidal, and dissolved organic matter in effluent from primary treatment systems and which generally removes 80 to 95 percent of the Biochemical Oxygen Demand (BOD) and suspended matter. Secondary wastewater treatment may be accomplished by biological or chemical-physical methods. Activated sludge and trickling filters are two of the most common means of secondary treatment. It is accomplished by bringing together waste, bacteria, and oxygen in trickling filters or in the activated sludge process. This treatment removes floating and settleable solids and about 90 percent of the oxygen-demanding substances and suspended solids. Disinfection is the final stage of secondary treatment. Also see Primary Treatment and Tertiary Wastewater Treatment.

SECTION—A unit of land area, generally equal to one square mile or 640 acres (259 hectares). Thirty-six sections also comprise a Township (6 miles long by 6 miles wide). The section is part of a description of the location of land using the survey system (Public Land Survey System—PLSS) of the United States Government and includes the 40-acre subdivision within a quarter, section, township and range. The public land survey system is based on the concept of a township as a square parcel of land six miles on each side. Its location is established as being so many six-mile units east of a north-south line (called the meridian) and so many six-mile units north or south of an east-west line (called the baseline). The township is described by township and range, e.g., T.4N, R.23E. Each township is further divided into 36 parts called sections one mile square (each section measuring 5,280 feet on each side). A typical section containing 640 acres may be further subdivided into quarters (measuring 2,640 feet on each side). The quarter of a section is equal to 160 acres and described as the NW1/4 or the NE1/4 or the SW1/4 or the SE1/4 of that particular section. The quarter section may be divided into four quarters, each being 1/16 of a section, or 40 acres (measuring 1,320 feet on each side). Each sixteenth is described as the NW1/4 or the NE1/4 or the SW1/4 or the SE1/4 of that particular quarter section. A person must determine which is north on a map (usually at the top) in this case the easterly direction would be to the right, south to the bottom, and west to the left. The written location of a specific 40-acre parcel of land would be NW1/4 of the NW1/4 of Section 13, T.4N., R.23E. Some sections may contain more or less acreage than 640.

SECTION 319 (CLEAN WATER ACT)—See Clean Water Act (CWA), Section 319.

SECTION 404 (CLEAN WATER ACT)—That section of the Clean Water Act (CWA) delineating restrictions on the dredging and filling (only) of Wetlands in the United States. While Section 404 Permits are issued by the U.S. Army Corps of Engineers (COE), Section 404 regulations are written by the U.S. Environmental Protection Agency (EPA).

SECTION 404 PERMIT—The Wetland dredge and fill permit issued under regulations written to conform to Section 404 of the Clean Water Act (CWA). The permit is actually granted by the U.S. Army Crops of Engineers (COE).

SECULAR TREND—(Data Analysis) A trend existing to some characteristic or phenomena over a relatively long period of time. Secular trends represent an important underpinning to any form of analysis as once such a trend is determined, deviations or perturbations from this trend may only be temporary and, if caused by man's activities, may not be reasonably sustainable in the long term.

SECURE LANDFILL—(Water Quality) A ground location for the deposit of hazardous wastes. The material, either in solid or liquid form, is placed above natural and synthetic liners than prevent or restrict the Leaching of dangerous substances (Leachates) into the groundwater and deep aquifers. A piping network called a Leachate Collection System is placed beneath the facility to allow the pumped removal of any liquid that penetrates the bottom or side liners. Access to the location is restricted and wells are used to monitor the leaching of any dangerous materials into the surrounding area. May constitute a type of treatment, storage, or disposal facility.

SECURE MAXIMUM CONTAMINANT LEVEL—The maximum permissible level of a contaminant in water delivered to the free flowing outlet of the ultimate user, or of contamination resulting from corrosion of piping and plumbing caused by water quality.

SEDIMENT—(1) In the singular the word is usually applied to material in suspension in water or recently deposited from suspension. In the plural the word is applied to all kinds of deposits from the waters of streams, lakes, or seas, and in a more general sense to deposits of wind and ice. Such deposits that have been consolidated are generally called sedimentary rocks. (2) Fragmental or clastic mineral particles derived from soil, alluvial, and rock materials by processes of erosion, and transported by water, wind, ice, and gravity. A special kind of sediment is generated by precipitation of solids from solution (i.e., calcium carbonate, iron oxides). Excluded from the definition are vegetation, wood, bacterial and algal slimes, extraneous light-weight artificially made substances such as trash, plastics, flue ash, dyes, and semisolids.

SEDIMENTARY CYCLE—The biogeochemical cycle in which materials primarily are moved from land to sea and back again.

SEDIMENTARY ROCK—(Geology) Rock formed of sediment, especially from mechanical, chemical, or organic processes, and specifically: (1) clastic rock, such as conglomerate, sandstone, and shale, formed of fragments of other rock transported from their sources and deposited in water; and (2) rocks formed by precipitation from solution, as rock salt and gypsum, or from secretions of organisms, such as most limestone. Many sedimentary rocks show distinct layering, which is the result of different types of sediment being deposited in succession. Also see Igneous Rock and Metamorphic Rock.

SEDIMENTATION—(1) Strictly, the act or process of depositing sediment from suspension in water. Broadly, all the processes whereby particles of rock material are accumulated to form sedimentary deposits. Sedimentation, as commonly used, involves not only aqueous but also glacial, aeolian, and organic agents. (2) (Water Quality) Letting solids settle out of wastewater by gravity during treatment.

SEDIMENTATION BASIN (SB)—A surface water runoff storage facility intended to trap suspended solids, suspended and buoyant debris, and adsorbed or absorbed potential pollutants that are carried by surface water runoff. The sedimentation basin may be part of an overall multipurpose detention and retention facility.

SEDIMENTATION TANKS—Wastewater tanks in which floating wastes are skimmed off and settled solids are removed for disposal.

SEDIMENT CONTROL—The control of movement of sediment on the land, in a stream or into a reservoir by means of manmade structures; such as debris dams, wing dams, or channelization; land management techniques, or natural processes.

SEDIMENT (CATCHMENT) DAM—A structure used specifically to trap sediment in water and prevent its transport further downstream.

SEDIMENT-DELIVERY RATIO—The ratio of sediment yield to gross erosion, expressed in percent.

SEDIMENT DISCHARGE, TOTAL—The rate at which dry weight of sediment passes a section of a stream or the quantity of sediment, as measured by dry weight or by volume, that is discharged in a given time.

SEDIMENT DISCHARGE, UNMEASURED—The difference between Total Sediment Discharge and measured Suspended-Sediment Discharge.

SEDIMENT LOAD—The total sediment, including bedload, being moved by flowing water in a stream at a specified cross section.

SEDIMENT OXYGEN DEMAND (SOD)—The amount of dissolved oxygen removed from the water covering the sediment in a lake or stream because of microbial activity.

SEDIMENT POOL—The reservoir space allotted to the accumulation of submerged sediment during the life of the structure.

SEDIMENTS—Soil, sand, and minerals washed from the land into water, usually after rain. They pile up in reservoirs, rivers, and harbors, destroying fish and wildlife habitat, and clouding the water so that sunlight cannot reach aquatic plants. Careless farming, mining, and building activities will expose sediment materials, allowing them to wash off the land after rainfall.

SEDIMENT STORAGE—The accumulation, in a reservoir, of sediment that would normally be carried downstream without the project.

SEDIMENT-TRANSPORT CURVE—Usually the relation between water discharge and Suspended-Sediment Discharge, but it can be between water discharge and Bed-Load Discharge, Unmeasured Sediment Discharge, or Total Sediment Discharge.

SEDIMENT YIELD—That amount of sediment transported by a stream system that may be measurable at a particular location. Usually expressed in volume or weight per unit of time.

SEDIMENT STORAGE (RESERVOIR)—That portion of total reservoir storage dedicated for sediment deposition and encroachment. Normally a part of Dead Storage.

SEEP—(1) To pass slowly through small openings or pores; ooze. (2) An area which slowly passes water out of the ground to the surface, or where water moves slowly from surface bodies to groundwater bodies, as from canals and ditches into the underlying groundwater table. (3) An area of minor groundwater outflow onto the land surface or into a stream channel or other waterbody. Flows are usually too small to be a spring.

SEEPAGE—(1) The slow movement of water through small cracks, pores, Interstices, etc., of a material into or out of a body of surface or subsurface water. (2) The Interstitial movement of water that may take place through a dam, its foundation, or its Abutments. (3) The loss of water by infiltration into the soil from a canal, ditches, laterals, watercourse, reservoir, storage facilities, or other body of water, or from a field. Seepage is generally expressed as flow volume per unit of time. During the process of priming (a field during initial irrigation), the loss is called Absorption Loss.

SEEPAGE BED—A trench or bed more than 36 inches (0.91 meter) wide containing at least 12 inches (30.5 centimeters) of clean, coarse aggregate and a system of distribution piping through which treated sewage may seep into the surrounding soil.

SEEPAGE LAKES—Lakes whose ecology is determined primarily by ground water rather than surface water.

SEEPAGE LOSSES—A measure of water losses in a conveyance system due to water being seeped into the surrounding soils. Seepage losses from canals and other conveyance systems depend on: (1) the permeability of the surrounding soil; (2) the wetted surface of the canal; and (3) the difference in level of the water in the canal and the adjacent groundwater table. Broadly, the seepage losses range from 15-45 percent of diverted flow for unlined canals and from 5-15 percent for lined canals.

SEEPAGE PIT—A covered pit with lining designed to permit treated sewage to seep into the surrounding soil.

SEEPS—Groundwater/surface water connections caused by river or stream erosion into a near-surface aquifer.

SEICHE—An oscillation of the water surface of a lake or other body of water due to variations of atmospheric pressure, wind, or minor earthquakes. The oscillation may be a foot or more in amplitude and may last several hours.

SELECTIVE ION PROBE—An electrode for which the measured voltage is proportional to the concentration of a specific ion in the solution being tested. The most common example is the pH probe, which responds to the hydrogen ions in solution.

SELF-PRODUCED WATER—A water supply (usually from wells) developed and used by an individual or entity. Also referred to as Self-Supplied Water.

SELF-PURIFICATION—The ability of a body of water to rid itself of pollutants. The removal of organic material, plant nutrients, or other pollutants from a lake or stream by the activity of the resident biological community. Biodegradable material added to a body of water will gradually be utilized by the microorganisms in the water, lowering the pollution levels. If excessive amounts of additional pollutants are not added downstream, the water will undergo self-cleansing. This process does not apply to pollution by non-biodegradable organic compounds or metals.

SELF-SUPPLIED WATER—Water withdrawn from a surface or ground-water source by a user rather than being obtained from a Public Water Supply System (PWSS). Also referred to as Self-Produced Water.

SELF-SUPPLIED WATER (INDUSTRIAL)—Water for industrial use, supplied from sources other than municipal distribution systems.

SELVA—A dense tropical rain forest usually having a cloud cover, especially one in the Amazon Basin.

SEMIAQUATIC—Adapted for living or growing in or near water; not entirely aquatic.

SEMIARID—A term applied to regions or climates where moisture is normally greater than under arid conditions but still definitely limits the growth of most crops. Dryland farming methods or irrigation generally are required for crop production. The upper limit of average annual precipitation in the cool semiarid regions is as low as 15 inches (38.1 cm). Whereas in tropical regions it is as high as 45 or 50 inches (114.3 or 127.0 cm).

SEMICONFINED (AQUIFER)—An aquifer that has a "leaky" confining unit and displays characteristics of both confined and unconfined aquifers, typically evidencing low permeability through which recharge and discharge can still occur. Also see Leaky Aquifer.

SEMIPERMEABLE—(1) Partially permeable. (2) Allowing passage of certain, especially small, molecules or ions but action as a barrier to others. Used of biological and synthetic membranes.

SEMIPERMEABLE MEMBRANE—A barrier, usually thin, that permits passage of particles up to a certain size or of a special nature. Also referred to as a Differentially Permeable Membrane.

SENESCENCE (SENESCENT)—Describing plants or specific ecosystems that are nearing the end of their normal life span. Sometimes used to describe lakes or other bodies of water in advanced stages of Eutrophication. For example, a lake that is filling with accumulated aquatic vegetation, dead plant material, and sediments can be described as senescent because it is nearing extinction as a productive lake environment. Also see Secular Trend.

SENILE—(Geology) Worn away nearly to the base level, as at the end of an erosion cycle.

SENSITIVE SPECIES—Those plant or animal species susceptible or vulnerable to activity impacts or habitat alterations. Species not yet officially listed but undergoing status review for listing on the U.S. Fish and Wildlife Service's (USFWS) official threatened and endangered list; species whose populations are small and widely dispersed or restricted to a few localities; and species whose numbers are declining so rapidly that official listing may be necessary. Also see Endangered Species Act (ESA), Endangered Species and Threatened Species.

SEPTAGE—Septic Tank sludge that is a combination of raw primary sludge and an anaerobically produced raw sludge.

SEPTIC SYSTEM—An on-site system designed to treat and dispose of domestic sewage. A typical septic system consists of a tank that receives waste from a residence or business and a system of tile lines or a pit for disposal of the liquid effluent (sludge) that remains after decomposition of the solids by bacteria in the tank. The remaining solids must be pumped out periodically.

SEPTIC TANK—A tank used to detain domestic wastes to allow the settling of solids prior to distribution to a leach field for soil absorption. Septic tanks and their associated Leaching Fields are used when a sewer line is not available to carry them to a treatment plant. A settling tank in which settled sludge is in immediate contact with sewage flowing through the tank, and wherein solids are decomposed by anaerobic bacterial action. Usually part of a rural on-site sewage treatment system. Typically, septic tanks would have a volume of 1-3 times the daily sewage flow. Also see Septic System.

SEPTIC TANK ABSORPTION FIELD—A soil absorption system for sewage disposal, consisting of a subsurface tile system laid in such a way that effluent from the septic tank is distributed with reasonable uniformity into the natural soil.

SEPTIC TANK SOIL ABSORPTION SYSTEM (ST-SAS)—A conventional wastewater treatment process used in isolated, rural, and other areas where normal sewer systems are not available whereby effluent is treated in a Septic Tank, then fed to soil-based disposal fields or trenches by gravity or under pressure. The quality of the ensuing filtration and absorption treatment is directly linked to soil and sit characteristics such as permeability, drainage, slope, and depth to limiting conditions such as groundwater or bedrock. Unfortunately, many soils are classified as unsuitable for conventional soil absorption systems. Under such unsuitable soil conditions, alternative pretreatment systems may be employed to include sand filters, mound systems, evapotranspiration systems, low pressure pipe systems and a more complex drip soil absorption system. Also see Drip (Trickle) Soil Absorption System.

SEQUENCING BATCH REACTORS (SBR)—(Water Quality) A wastewater treatment technique consisting of an activated sludge system which operates sequentially in time rather than in space, that is, all steps of the process take place, one after the other, in the same tank instead of moving to a second tank for the continuation of the treatment. The typical SBR operation involves filling a tank with raw wastewater or primary effluent, aerating the wastewater to convert the organics into a microbial mass, providing a period for settling, discharging the treated effluent, and a period identified as "idle" that represents the time after discharging the tank and before refilling. Typically, a multiple tank system is required, allowing incoming flow to be switched to one tank while the other is going through the aeration, clarification, discharge, and idle functions. A key element of the SBR process is that a tank is never completely emptied, but rather a portion of settled solids is left in the tank for the next cycle. The retention of sludge within the tank establishes a population of microorganisms uniquely suited to treating the waste.

SEQUESTERING AGENT—(Water Quality) A chemical compound such as EDTA or certain polymers that chemically tie up (sequester) other compounds or ions so they cannot be involved in chemical reactions.

SÉRAC—A large pointed mass of ice in a glacier isolated by intersecting crevasses.

SERIAL DISTRIBUTION—An arrangement of Absorption Trenches, Seepage Pits, or Seepage Beds so that each is forced to pond, utilizing the total effective absorption area, before liquid flows into the succeeding component.

SERVICE AREA—The geographical land area served by a distribution system of a water agency.

SERVICE CONNECTOR—The pipe that carries tap water from a public water main to a building.

SERVICE LINE SAMPLE—A one-liter sample of water collected according to federal regulations that has been standing for at least 6 hours in a service pipeline.

SERVICE PIPE—The pipeline extending from the water main to the building served or to the consumer's system.

SESTON—All material, both organic and inorganic, suspended in a waterway.

SETTLE—To cause to sink, become compact, or come to rest; to cause a liquid to become clear by forming a Sediment.

SETTLEABLE SOLIDS—Most generally, all solids in a liquid that can be removed by stilling the liquid. In the Imhoff cone test, the volume of matter in a one-liter sample that settles to the bottom of the cone in one hour. (Water Quality) Bits of debris, sediment, or other solids that are heavy enough to sink when a liquid waste is allowed to stand in a pond or tank. Also see Settling Chamber and Settling Pond.

SETTLING BASIN—An enlargement in the channel of a stream to permit the settling of materials carried in suspension.

SETTLING CHAMBER—(Water Quality) An enclosed container into which wastewater contaminated with solid materials is placed and allowed to stand. The solid pollutants suspended in the water sink to the bottom of the container for removal. Also referred to as a Settling Tank.

SETTLING POND—(Water Quality) An open Lagoon into which wastewater contaminated with solid pollutants is placed and allowed to stand. The solid pollutants suspended in the water sink to the bottom of the lagoon and the liquid is allowed to overflow out of the enclosure.

SETTLING TANK—A term used synonymously with Settling Chamber.

SETTLING VELOCITY—(Water Quality) The rate of downward movement of particles through water. This gravitational settling removes particles naturally and is used also in pollution control devices, for example the Settling Tanks and Ponds in a sewage treatment facility. The settling velocity of a particle is often the same as the Terminal Settling Velocity.

SEVEN SEAS—Figuratively, all the waters or oceans of the world. The phrase probably has its origins in Brahmanic mythology: the seven seas dividing and surrounding the seven land masses of the earth. In modern times it has been applied to the seven oceans: Arctic, Antarctic, North and South Atlantic, North and South Pacific, and Indian.

7Q10—The period of lowest stream flow during a seven-day interval that is expected to occur once every 10 years. During this time of low flow, the amount of Dissolved Oxygen in the water would be expected to be the lowest encountered under normal conditions. Since such conditions are considered to be the worst natural case, the dissolved oxygen levels during such episodes are used to establish Ambient Water Quality Standards for that stream.

SEWAGE—The liquid waste from domestic, commercial, and industrial establishments.

SEWAGE FUNGUS—A thick filamentous growth that develops in water contaminated with sewage. The filamentous material is composed predominately of the bacterium Sphaerotilus natans.

SEWAGE LAGOON—A shallow pond where natural processes are employed to treat sanitary waste from households or public rest rooms. Solid material settles to the bottom and is degraded by Anaerobic microbial communities. The enclosure is open to the atmosphere, which permits Aerobic mineralization of organic compounds in the upper layers of the water. The decomposition processes are analogous to those in effect in Primary and Secondary Wastewater Treatment processes. The effluent from these ponds is usually allowed to flow into nearby streams without further purification.

SEWAGE SLUDGE—Settled sewage solids combined with varying amounts of water and dissolved materials that are removed from sewage by screening, sedimentation, chemical precipitation, or bacterial digestion. The terms Biosolids, Sludge, and sewage sludge can be used interchangeably.

SEWAGE SYSTEM—Pipelines or conduits, pumping stations, force mains, and all other structures, devices, and facilities used for collecting or conducting wastes to a point for treatment or disposal.

SEWAGE TREATMENT—The processing of wastewater for the removal or reduction of contained solids or other undesirable constituents.

SEWAGE TREATMENT PLANT—A facility designed to receive the wastewater from domestic sources and to remove materials that damage water quality and threaten public health and safety when discharged into receiving streams or bodies of water. The substances removed are classified into four basic areas:

[1] greases and fats;
[2] solids from human waste and other sources;
[3] dissolved pollutants from human waste and decomposition products; and
[4] dangerous microorganisms.

Most facilities employ a combination of mechanical removal steps and bacterial decomposition to achieve the desired results. Chlorine is often added to discharges from the plants to reduce the danger of spreading disease by the release of pathogenic bacteria.

SEWAGE TREATMENT RETURN FLOW—Water returned to the hydrologic system by a Sewage Treatment Plant.

SEWER—The piping system or conduit used to carry runoff water or wastewater. Various types of sewer systems fulfill different functions, for example:

[1] Sanitary Sewer carries wastewater from a household or public facility;
[2] Storm Sewer carries runoff from rainfall from streets and parking lots; and
[3] Combined Sewer transports both rainfall runoff and sanitary waste.

SEWERAGE—The entire system of sewage collection, treatment, and disposal.

SHADOOF, also Shaduf—(1) A device consisting of a long suspended pole weighted at one end and having a bucket at the other end, used in the Near East and especially Egypt for raising water, as for the irrigation of land. (2) A counterbalanced sweep used since ancient times, especially in Egypt for raising water (as for irrigation).

SHAFT—A vertical or inclined opening of uniform and limited cross section made for finding or mining ore, raising water, or ventilating underground workings (as in a cave).

SHALLOW WELL—A well with a pumping head of 20 feet or less, permitting use of a suction pump.

SHARP-CRESTED WEIR—A device for measuring water, featuring a notch cut in a relatively thin plate and having a sharp edge on the upstream side of the crest.

SHEET—(Geology) A broad, relatively thin deposit or layer of Igneous or Sedimentary Rock.

SHEET EROSION—The removal of thin, fairly uniform layer of soil or materials from the land surface by the action of rainfall and runoff water.

SHEET FLOW, also Sheetflow—An overland flow or downslope movement of water taking the form of a thin, continuous film over relatively smooth soil or rock surfaces and not concentrated into channels larger than rills.

SHEETFLOW AREA—Designated Flood Zones AO and AH on a community's Flood Insurance Rate Map (FIRM) with a one percent or greater annual chance of flooding to an average depth of one to three feet where a clearly defined channel does not exist, where the path of flooding is unpredictable and where velocity flow may be evident. Such flooding is characterized by ponding or sheet flow. Also referred to as Area of Shallow Flooding.

SHEET PILING—Material, typically concrete or steel, placed vertically in the ground to contain erosion or the lateral movement of groundwater.

SHELDON NATIONAL WILDLIFE REFUGE (NWR) [Nevada]—One of the nine National Wildlife Refuges (NWR) located in the State of Nevada, the Sheldon NWR is located in the northwestern corner of Nevada and consists of over 575,000 acres (approximately 900 square miles) of high-desert habitat as a representative area for native plants and wildlife. The Sheldon NWR was formally established in 1978 and represented a consolidation of two refuge and range protection areas: (1) the Charles Sheldon Wildlife Range, created in 1931 by President Herbert Hoover from the purchase of the 30,000-acre Last Chance Ranch by the Boone and Crockett Club and the National Audubon Society; and (2) the Charles Sheldon Antelope Range, an area of over one-half million acres created in 1936 by President Franklin D. Roosevelt for the conservation and development of natural wildlife resources. Today, this area encompasses remote settings, scenic vistas, numerous natural geothermal hot springs, old ranches and homesteads plus sites of archaeological significance. Some 20 million years ago this area was covered with pine forests and lush grasslands, nurtured by a mild climate and more than 50 inches of annual rainfall. Today, this area receives only from 4-8 inches of precipitation, primarily in the form of snowfall, and the prominent geological characteristic of this refuge is the subsequent volcanic activity which spewed rhyolitic magma over much of its area with basalt flows up to 100 feet thick. These have formed the large, broad tables prevalent in this area today. Also see National Wildlife Refuge (NWR) System and National Wildlife Refuges (NWR) [Nevada].

SHELF ICE—An extension of glacial ice into coastal waters that is in contact with the bottom near the shore but not toward the outer edge of the shelf.

SHIFTING CONTROL—A stream-gaging control which is affected by scour, fill, or backwater.

SHIGELLA DYSENTERIAE—A waterborne microorganism which is the causative agent of bacillary dysentery, a disease characterized by severe diarrhea with bold and pus in the feces. The disease is transmitted through the consumption of water, food, or beverages contaminated with fecal material.

SHOAL—(1) A shallow place in a body of water. (2) A sandy elevation of the bottom of a body of water, constituting a hazard to navigation. (3) A sandbank or sandbar that makes the water shallow; specifically an elevation which is not rocky and on which there is a depth of water of six fathoms (11 meters or 36 feet) or less. Also referred to as a Sandbank or Sandbar.

SHOCK LOAD—(Water Quality) The arrival at a water treatment plant of raw water containing unusual amounts of algae, colloidal matter, color, suspended solids, turbidity, or other pollutants.

SHOOT—(1) A rush of water down a steep or rapid. (2) A place where a stream runs or descends swiftly. (3) To cause to move suddenly or swiftly forward as motion down rapids.

SHOREBIRD—One of several families of often long-legged wading birds that inhabit the seacoast and shallow fresh-water bodies including sandpipers, stilts, killdeer, avocets, plovers and others.

SHOREFRONT—Land situated on the edge of a body of water.

SHORE LIFE-The essentially marine organisms that inhabit the region bounded on one side by the height of the extreme high tide and on the other by the height of the extreme low tide. Within these boundaries, organisms face a severe environment imposed by the rise and fall of tides. For up to half of a 24-hour period, the environment is marine; the rest of the time it is exposed, with terrestrial extremes in temperature and the drying effects of wind and sun. Life on rocky shores, best developed on northern coasts, is separated into distinct zones that reflect the length of time each zone is exposed. At the highest position on the rocks is the so-called black zone, marked by encrusted bluegreen algae. This transition area between land and the marine environment is flooded only during the high spring or fortnightly tides. The algae, enclosed in gelatinous sheaths to resist drying, are grazed on by periwinkles, which are protected under tightly sealed, conical shells. Below the black zone lies a so-called white zone, where barnacles are tightly glued to rocks. Living among the barnacles are rock-clinging mollusks called limpets. At low tide, barnacles keep their four movable plates closed to avoid drying; at high tide they open the plates and extend six pairs of wand-like tentacles to sweep the water for microscopic life. Preying on the barnacles are hole-drilling snails called dog whelks. Below the white zone and in some places overlying the barnacles are rockweeds, which have no roots but are attached to rocks by holdfasts. Brown algae (Phaeophyta) are rockweeds that grow more than 8 feet (2.5 meters) long; the most common are the bladder wracks, with branching thalli up to 6 inches (15 centimeters) wide. Looking like fronds, the thalli have conspicuous, bubble-like swellings that buoy the plants at high tide. Blue mussels also inhabit this tide zone, especially where sediment accumulates. They attach themselves to the substrate by threadlike byssuses secreted by a gland in the foot. In the lowest zone, uncovered only during the spring tides, is the large brown alga Laminaria, one of the kelps. Beneath its frond-like thalli live starfish, sea cucumbers, limpets, mussels, and crabs. Also see Marine Life.SHORE LIFE-The essentially marine organisms that inhabit the region bounded on one side by the height of the extreme high tide and on the other by the height of the extreme low tide. Within these boundaries, organisms face a severe environment imposed by the rise and fall of tides. For up to half of a 24-hour period, the environment is marine; the rest of the time it is exposed, with terrestrial extremes in temperature and the drying effects of wind and sun. Life on rocky shores, best developed on northern coasts, is separated into distinct zones that reflect the length of time each zone is exposed. At the highest position on the rocks is the so-called black zone, marked by encrusted bluegreen algae. This transition area between land and the marine environment is flooded only during the high spring or fortnightly tides. The algae, enclosed in gelatinous sheaths to resist drying, are grazed on by periwinkles, which are protected under tightly sealed, conical shells. Below the black zone lies a so-called white zone, where barnacles are tightly glued to rocks. Living among the barnacles are rock-clinging mollusks called limpets. At low tide, barnacles keep their four movable plates closed to avoid drying; at high tide they open the plates and extend six pairs of wand-like tentacles to sweep the water for microscopic life. Preying on the barnacles are hole-drilling snails called dog whelks. Below the white zone and in some places overlying the barnacles are rockweeds, which have no roots but are attached to rocks by holdfasts. Brown algae (Phaeophyta) are rockweeds that grow more than 8 feet (2.5 meters) long; the most common are the bladder wracks, with branching thalli up to 6 inches (15 centimeters) wide. Looking like fronds, the thalli have conspicuous, bubble-like swellings that buoy the plants at high tide. Blue mussels also inhabit this tide zone, especially where sediment accumulates. They attach themselves to the substrate by threadlike byssuses secreted by a gland in the foot. In the lowest zone, uncovered only during the spring tides, is the large brown alga Laminaria, one of the kelps. Beneath its frond-like thalli live starfish, sea cucumbers, limpets, mussels, and crabs. Also see Marine Life.

SHORT-CIRCUITING—(Water Quality) A condition that exists when some of the water in water treatment tanks or basins flows faster than the rest; may result in shorter contact, reaction, or settling times than calculated or presumed.

SHOWER—A brief fall of precipitation, such as rain, hail, sleet, or snow.

SHOWERHEAD—A fixture for directing the spray of water in a bathroom shower.

SHRINK-SWELL POTENTIAL—The susceptibility of soil to volume change due to loss or gain in moisture content.

SHRUB—A woody plant which at maturity is usually less than 6 m (20 feet) tall and generally exhibits several erect, spreading, or prostrate stems and has a bushy appearance; e.g., speckled alder (Alnus rugosa) or buttonbush (Cephalanthus occidentalis).

S-HYDROGRAPH—The direct surface discharge hydrograph resulting from a continuous succession of Unit Storms.

SIC CODE—See Standard Industrial Classification (SIC) Code.

(THE) SIERRA CLUB—A national environmental organization founded in 1892 based in San Francisco, California. The Sierra Club is dedicated to the exploration, enjoyment, and protection of the wild places of the earth; to practice and promote the responsible use of the earth's ecosystems and resources; to educate and enlist humanity to protect and restore the quality of the natural and human environment; and to use all lawful means to accomplish these objectives. The Sierra Club is active in public education, lobbying of legislative and administrative bodies, and, through its affiliated Sierra Club Legal Defense Fund, in the courts.

SIERRA CLUB LEGAL DEFENSE FUND—See Sierra Club.

SIERRA VALLEY DECREE [California-Nevada]—Adjudication (1958) allowing the Sierra Valley Water Company to divert a portion of the Little Truckee River in California into Webber Creek for irrigation purposes in the Sierra Valley in the Feather River Basin. The maximum allowable diversion is 60 cubic feet per second (cfs), averaging approximately 5,700 acre-feet (AF) per year (although as a supplemental water source, diversions typically vary between 1,500 AF and 10,000 AF per year). Waters may be diverted only between March 15th and September 30th of each year. The Priority Date of this water right was set at 1870.

SIERRA WAVE [Sierra Nevada Mountains]—The condition along the eastern slope of the Sierra Nevada Mountain range in California and Nevada which manifests itself in the creation of Lenticular Clouds. This condition is facilitated by the north-south orientation of the mountains and the prevailing westerly winds, providing ideal conditions for the formation of such clouds. The condition is most prevalent in the winter months when the jet stream reaches further south. The resultant lenticular clouds typically form at between 20,000 and 25,000 feet and appear as flat or elongated clouds which appear "trapped" beneath the rising and falling air stream over the mountains. In fact, the stationary effect is an optical illusion; the lenticular cloud is actually forming on its front edge and vanishing on its trailing edge.

SIGMOID GROWTH—(Data Analysis) A growth rate trend characterized by an elongated S-shaped, or sigmoid curve. Typical of population growth rate trends which begin rapidly at an exponential rate but slow as limiting factors are encountered until a limit is approached asymptotically. Also see Arithmetic Growth, and Exponential Growth.

SIGNIFICANT (Statistical)—A term applied to differences, correlations, cause-and-effect relationships, etc., to indicate that they are probably not due to chance alone. Significant ordinarily indicates a probability of not less than 95 percent, while highly significant indicates a probability of not less than 99 percent.

SIGNIFICANT HYDROLOGIC RESOURCES (SHR)—Generally refers to either federally significant resources, e.g., wetlands, which meet federal definitions and guidelines, or regionally designated significant resources which do not meet such federal definitions or guidelines, e.g., stream and riparian environments, playas, spring fed stands of riparian vegetation, and other wetland areas.

SIGNIFICANT MUNICIPAL FACILITIES—Those publicly owned sewage treatment plants that discharge a million gallons per day or more and are therefore considered by states to have the potential to substantially effect the quality of receiving waters.

SILICA—(Geology) Silicon dioxide (SiO₂). It occurs in crystalline (quartz), amorphous (opal), or impure (silica sand) forms.

SILL—(1) A submerged ridge at relatively shallow depth separating the basins of two bodies of water. (2) A horizontal beam forming the bottom of the entrance to a lock. (3) Also, a low, submerged dam-like structure built to control riverbed scour and current speeds.

SILT—Sedimentary particles smaller than sand particles, but larger than clay particles.

SILTATION—The deposition of finely divided soil and rock particles upon the bottom of stream and river beds and in reservoirs.

SILVER IODIDE—A compound of silver and iodine, the crystalline structure of which closely approximates that of ice crystals; used as ice nuclei in weather modification.

SILVER IODIDE GENERATOR—Any of several devices used to generate a smoke of Silver Iodide crystals for Cloud Seeding.

SILVER IODIDE SEEDING—A method of Cloud Seeding in which Silver Iodide crystals are introduced into the supercooled portions of clouds to induce the Nucleation of ice crystals and, thus, precipitation.

SILVICS—(1) The science treating of the life of trees in the forest. (2) Habit or behavior of a forest tree.

SILVICULTURE—The art of producing and caring for a forest.

SIMPLE REGRESSION (MODEL)—(Statistics) A model structure characterized by only one explanatory (Exogenous) variable, of the form:

Y_t = a + ß X_t + e_t

where:

t represents the time periods of observation (where t=1, 2, ..., n);
Y_t represents the dependent (Endogenous) variable in time period t;
a (alpha) represents the model equation's constant term (without a time reference);
ß (beta, also a constant term without a time reference) represents the coefficient of the independent variable;
X_t represents the independent variable in time period t; and
e_t (epsilon), the error term, represents the value of the unexplained disturbance term. t represents the time periods of observation (where t=1, 2, ..., n);
Y_t represents the dependent (Endogenous) variable in time period t;
a (alpha) represents the model equation's constant term (without a time reference);
ß (beta, also a constant term without a time reference) represents the coefficient of the independent variable;
X_t represents the independent variable in time period t; and
e_t (epsilon), the error term, represents the value of the unexplained disturbance term.

Also see Multiple Regression (Model).

SIMULATION ANALYSIS—(Statistics) A procedure or process by which an Econometric Model (Regression Analysis) is designed to test the effects on outputs of various changes (simulations) of inputs. Simulations of a model might be performed for a variety of reasons, including model testing and evaluation, historical policy analysis, and forecasting. Simulation may be ex post, or simulation beyond the estimation period of the model but for periods of time for which actual data is available for validation purposes. Simulation may also be ex ante which includes forecasting beyond the last actual data point. Therefore, forecasting represents a form of simulation of an econometric model forward in time beyond the model's estimation period.

SINK—(1) Generally, a dry or intermittently dry lakebed in the lowest spot of a closed valley; a depression in the land surface, especially one having a central playa or saline lake with no outlet. Salt contents are generally quite high. The term sink is interchangeable with the term Playa. Also see Natural Sink. (2) (Environmental) A place in the environment where a compound or material collects.

SINKHOLE—A depression in the earth's surface caused by dissolving of underlying limestone, salt, or gypsum. Drainage is provided through underground channels which may be enlarged by the collapse of a cavern roof. Also see Karst.

SINKING—(Environmental) Controlling oil spills by using an agent to trap the oil and sink it to the bottom of the body of water where the Sinking Agent and the oil are biodegraded.

SINKING AGENT—A chemical additive that, when applied to a floating oil discharge, will cause oil to sink below the surface of the water. The application of sinking agents is generally not permitted by regulations of the U.S. Environmental Protection Agency (EPA) governing the treatment of oil discharges.

SINTER—A chemical sedimentary rock deposited as a hard incrustation on rocks or on the ground by precipitation from hot or cold mineral waters of springs, lakes, or streams.

SINUOUS STREAM—Characterized by many curves or turns; winding.

SIPHON, also Syphon—(1) A pipe or tube fashioned or deployed in an inverted U shape and filled until atmospheric pressure is sufficient to force a liquid from a reservoir in one end of the tube over a barrier higher than the reservoir and out the other end. (2) (Zoology) A tubular organ, especially of aquatic invertebrates such as squids or clams, by which water is taken in or expelled.

SIPHON TUBES—(Irrigation) Small curved pipes, typically 0.5-4.0 inches (1.3-10.2 centimeters) in diameter, that deliver water over the side of a head ditch or lateral to furrows, corrugations, or borders.

SITE—(Environmental) An area or place within the jurisdiction of the U.S. Environmental Protection Agency (EPA) and/or a state.

(LOCAL TEST-WELL) SITE DESIGNATION [Nevada]—The local test-well site designation used in Nevada is based on the identification of a site by hydrographic area and by the official rectangular subdivision of the public lands referenced to the Mount Diablo (located east of Walnut Creek, California) base line and meridian and is based on the Public Land Survey System (PLSS). Each site designation consists of four units: The first unit is the hydrographic area number. The second unit is the township, preceded by an N or S to indicate location north or south of the base line. The third unit is the range, preceded by an E to indicate location east of meridian. The fourth unit consists of the section number and letters designating the quarter section, quarter-quarter section, and so on (A, B, C, and D indicate the northeast, northwest, southwest, and southeast quarters, respectively), followed by a number indicating the sequence in which the site was recorded. For example, site 210 S12 E63 29DABC2 is in Coyote Spring Valley (Nevada Hydrographic Area 210). It is the second site recorded in the southwest quarter (C) of the northwest quarter (B) of the northeast quarter (A) of the southeast quarter (D) of Section 29, Township 12 South, Range 63 East, Mount Diablo base line and meridian.

(TEST-WELL) SITE IDENTIFICATION (U.S. GEOLOGICAL SURVEY)—The standard U.S. Geological Survey (USGS) site identification is based on the grid system of latitude and longitude. The number consists of 15 digits. The first six digits denote the degrees, minutes, and seconds of latitude; the next seven digits denote degrees, minutes, and seconds of longitude; and the last two digits (assigned sequentially) identify the sites within a 1-second grid. For example, site 365227114554401 is at 36°52'27" latitude and 114°55'44" longitude, and it is the first site recorded in that 1-second grid. The assigned number is retained as a permanent identifier even if a more precise latitude and longitude are later determined. Also see Local Site Designation [Nevada].

SI UNITS—The International System of Units (Le Système International d'Unités) defined by an international gathering convened to establish agreements on the most frequently used units of measurement. The Conference on Weights and Measures in 1960 adopted standard measures based on the meter/kilogram/second units and radiation quantities based on the becquerel, gray, and sievert. The units adopted by the conference represent the currently preferred measures of length, mass, time, radiation, and so on.

SKERRY—(Scandinavian) A rocky isle; a reef.

SKIM—(1) To remove floating matter from the surface of a liquid. (2) To throw in a gliding path; especially to throw so as to ricochet along the surface of water.

SKIMMING—(1) The diversion of water from a stream or conduit by a shallow overflow in order to avoid diversion of sand, silt, or other debris carried as bottom load. (2) Using a machine to remove oil or scum from the surface of the water.

SKIN DIVING—The sport of swimming under water with a face mask and flippers and especially without a portable breathing device.

SKIN SAMPLE—Sampled water that is not representative of the cross-sectional flow in a pipe or conduit. A skin sample can result if the tap opens on the inside wall of the pipe and can be avoided by using sampling taps that extend toward the center of the pipe, away from the wall.

SLACK TIDE—See Slack Water.

SLACK WATER—(1) The period at high or low tide when there is no visible flow of water. (2) An area in a sea or river unaffected by currents; still water. Also referred to as Slack Tide.

SLACKWATER CHANNEL—A navigation channel in a canal or river where the water surface elevation is controlled by a dam or dams with locks.

SLAKE—(1) To satisfy (a craving) as to quench a thirst. (2) To cool or refresh by wetting or moistening. (3) To become mixed with water so that a true chemical combination takes place, as in the slaking of lime.

SLAKED LIME—Calcium hydroxide which is formed by the addition of water to quicklime; a process termed Slaking.

SLAKER—A mechanical device in which dry lime (calcium oxide) or magnesium oxide is powdered and mixed with water to produce alkali consisting of calcium or magnesium hydroxide. Calcium hydroxide is used as a water softener.

SLASH AND BURN—(Environmental) An agricultural practice involving the rapid destruction of natural forest for limited farming activity. The natural forest is cleared, the residue from the clearing process is burned, and crops are planted for a few years. When the fertility of the soil is depleted, the process is repeated in a new area, and the initial land is abandoned.

SLEET—(1) A form of precipitation consisting of frozen raindrops cooled to the ice stage while falling through air at subfreezing temperatures. (2) A mixture of rain and snow or hail. (3) A thin icy coating that forms when rain or sleet freezes, as on trees or streets.

SLICKENSIDES—(Geology) A smooth striated polished surface produced on rock by movement along a fault.

SLICK SPOTS—Barren areas having puddled or crusted, very smooth, nearly impervious surfaces, usually because of high salinity or alkalinity.

SLIMES—(Water Quality) Substances of viscous organic nature, which are usually formed from micro-biological growth and which attach themselves to other objects forming a coating.

SLIP—(1) (Nautical) A docking place for a ship or boat between two piers; a Slipway. (2) The downslope movement of a soil mass under wet or saturated conditions.

SLIP-OFF SLOPE BANK—The bank of a meandering stream which is not eroded by stream action, and which may be built up gradually.

SLIPWAY—(Nautical) A sloping surface leading down to the water, on which ships are built or repaired and then more readily moved back into the water.

SLOPE—The side of a hill or mountain, the inclined face of a cutting, canal or embankment or an inclination from the horizontal. In the United States, it is measured as the ratio of the number of units of horizontal distance to the number of corresponding units of vertical distance. The term is expressed as a percent when the slope is gentle, in which cast the term Gradient is also used.

SLOPE-DISCHARGE (DIAGRAM)—A curve which shows the discharge at a given Gaging Station, taking into account the slope of the water surface, as well as the Gage Height. This curve may be plotted either by using the difference between elevations of water surface at two fixed gages on the stream, instead of the slope or by using the three variables of discharge, elevation of water surface at the lower gage, and elevation of the water surface at the upper gage.

SLOPE PROTECTION—The protection of a Slope against wave action or erosion.

SLOPE WASH—Soil and rock material that is being or has been moved down a slope predominantly by the action of gravity assisted by running water that is not concentrated into channels. The term applies to the process as well as the materials.

SLOPING GAGE—A staff gage used to register the elevation of the water surface in a stream channel, conduit, reservoir or tank, with a scale graduated to represent vertical elevation. A sloping gage is usually installed on a flat sloping bank where it is desirable to increase the accuracy of reading the gage.

SLOSH—(1) To spill or splash (a liquid) copiously or clumsily. (2) To agitated in a liquid.

SLOUGH—(1) A place of deep mud or mire; a wet or marshy place as a swamp or marshland creek. Also a side channel or inlet as from a river; ordinarily found on or at the edge of the flood plain or a river; a Bayou. (2) (Localized) In the Mississippi Valley and in California, a tide flat or bottom-land creek. (3) (Sewage Disposal) Of a filter, to cast off a thin film of scum or a mass of bacterial growth or fungus. (4) Also Slue. A stagnant swamp, marsh, bog, or pond, especially as part of a bayou, and inlet, or a backwater.

SLOW SAND FILTER—Essentially, a concrete basin covered with graded gravel and about 3 feet of sand used to pass raw water through at low velocity, resulting in substantial removal of chemical and biological contaminants. Cleaning is accomplished by removing the surface layers of sand at periodic intervals. These are then washed and stored for reuse.

SLUDGE—(1) (Water Quality) Semisolid material such as the type precipitated by a Sewage Treatment Plant. The terms Biosolids, sludge, and Sewage Sludge can be used interchangeably. (2) Mud, mire, or ooze covering the ground or forming a deposit, as on a riverbed. (3) Finely broken or half-formed ice on a body of water, especially the sea.

SLUDGE AGE—A measure of the time biological solids are retained in a basin calculated by dividing the mass of volatile solids in the basin by the total mass of volatile solids wasted during a given time period. Expressed in units of time.

SLUDGE BULKING—A common problem encountered in the operation of activated sludge plants in which a portion of the sludge does not settle properly and is carried out with the effluent.

SLUDGE DEPOSITS—Accumulations of settled, usually rapidly decomposing organic material in the aquatic system.

SLUDGE DIGESTER—A tank in which complex organic substances like sewage sludges are biologically dredged. During these reactions, energy is released and much of the sewage is converted to methane, carbon dioxide, and water.

SLUDGE DIGESTION—The biological decomposition of solids collected during the operation of a facility designed to remove organic wastes from domestic or industrial sources. The total volume of solids is reduced by the Mineralization activity of bacteria, and the sludge remaining is rendered less reactive because the easily degraded compounds have been removed.

SLUDGE DISPOSAL—The removal and discarding of thick watery suspensions of particulate waste matter. Final disposal may involve the removal of excess water and the subsequent burning of the solids, or placing the dewatered material in a landfill.

SLUDGE VOLUME INDEX (SVI)—A laboratory test result used to indicate the rate at which Sludge is to be returned from the discharge end of an Aeration Tank to the inflow (upstream) end. It is calculated as:

SVI = (SV/MLSS) X 1,000

with the result in milligrams/gram and where SV is the sludge volume (solids settled in a 1-liter graduated cylinder after 30 minutes, in milliliters/liter) and MLSS is the level of Mixed Liquor Suspended Solids (MLSS), in milligrams/liter. The SVI has units of milliliters/gram. Used in conjunction with the Activated Sludge Process. Also known as the Mohlman Index.

SLUE—See Slough.

SLUICE—(1a) An artificial channel for conducting water, with a valve or gate to regulate the flow; (1b) A valve or gate used in such a channel; a Floodgate or Sluice Gate. (2) A body of water impounded behind a floodgate. (3) A Sluiceway. (4) A long inclined trough, as for carrying logs or separating gold ore.

SLUICE GATE—A gate which can be raised or lowered by sliding in vertical guides.

SLUICEWAY—(1) An artificial channel, especially one for carrying off excess water. (2) An opening at a low level from a reservoir generally used for emptying or for scouring sediment and sometimes for irrigation releases. In such instances it is also referred to as a Bottom Outlet or Low Level Outlet.

SLUMP—The sliding or gravitational movement of an overlying layer of soil, typically from becoming saturated, and lying on a rock layer or other relatively impermeable layer.

SLURRY—A thin, watery muck, or any substance resembling it, such as a lime slurry. The mixture is pourable and can be transported by pipe. The form in which some raw material is added to an industrial process. Compare with Liquor.

SLURRY WALL—(1) Material placed vertically in the ground to prevent the lateral movement of groundwater. (2) Barriers used to contain the flow of contaminated water or a subsurface liquid. Slurry walls are constructed by digging a trench around a contaminated area and filling it with an impermeable material that prevents water from passing through it. The ground water or contaminated liquids can then be exacted or treated within the area confined by the slurry wall.

SLUSH—(1) Partially melted snow or ice. (2) Soft mud; slop; mire.

SMALL STREAM FLOODING—Nuisance flooding of very small creeks and streams due to excessive rainfall over small drainage areas. Should not be confused with flash flooding. Usually 0.5-1.0 inch (1.3-2.5 cm) of rainfall in an hour is sufficient to cause some small stream flooding.

SNOW—Precipitation in the form of branched, hexagonal crystals, often mixed with simple ice crystals, which fall more or less continuously from a solid cloud sheet. These crystals may fall either separately or in cohesive clusters forming Snowflakes.

SNOW ABLATION—The removal of snow by the force of the wind.

SNOWCAP—Snow covering a mountain peak, especially such snow existing year-round.

SNOW, COLD CONTENT OF—The amount of heat required to raise the temperature of the snowpack to 0°C (32°F). Usually expressed as the number of inches of liquid water (produced at the surface by either rain or melt) which, on refreezing within the pack, will warm the snowpack to 0°C (32°F).

SNOW COURSE—A line laid out and permanently marked on a drainage area along which the snow is sampled at definite distances or stations. Sampling occurs at appropriate times to determine snow depth, water equivalent, and density.

SNOW CRYSTAL—A configuration of ice crystals, usually in the shape of a hexagonal plane or delicate column, formed around a nucleus by sublimation, condensation and freezing, coalescence, or a combination thereof.

SNOW DENSITY—The ratio of the volume of meltwater derived from a sample of snow and the initial volume of the sample. This is numerically equal to the specific gravity of the snow.

SNOW DEPTH—The vertical distance between the surface of a snow layer and the ground beneath.

SNOWDRIFT—A mass or bank of snow piled up by the wind.

SNOWFALL—The amount of snow, hail, sleet, or other precipitation in solid form which reaches the earth's surface. It may be expressed in depth in inches as it falls, or in terms of depth in inches of the equivalent amount of water.

SNOW FENCE—A fence of slat and wire or other material used in winter to intercept drifting snow, thus protecting roads, railways, and other areas from snowdrifts. Also, fences used to impound snow where melting in place will add to the soil moisture content.

SNOW FIELD—An area, usually at high elevation or in polar latitudes, where snow accumulates and remains on the ground throughout the entire year.

SNOWHEDGE—A planting of shrubs or other plants to intercept drifting snow. Also referred to as Snowbreak and Snow Catch.

SNOWFLAKE—Precipitation which is made up of a number of snow crystals fused together.

SNOW LINE—(1) The general altitude to which the continuous snow cover of high mountains retreats in summer, such as the Snowcap of a mountain, chiefly controlled by the depth of the winter snowfall and by the summer temperature. (2) A line, sometimes drawn on a map during the winter, which shows the lower elevation of the snow cover at a particular time. (3) The fluctuating latitudinal boundaries around the polar regions marking the extent of snow cover.

SNOW LINE, TEMPORARY—A line sometimes drawn on a weather map during the winter showing the southern limit of the snow cover.

SNOW, LIQUID WATER DEFICIENCY OF—The difference between the liquid water-holding capacity and the water content of a snowpack.

SNOW, LIQUID WATER-HOLDING CAPACITY OF—The maximum amount of liquid water that a snowpack can hold against gravity at a given stage of metamorphism and density. When the liquid water-holding capacity is reached, the snowpack is said to be "ripe".

SNOW MAKING—The production of artificial snow in the form of granular ice particles for use on ski slopes.

SNOW MANAGEMENT—The management of snow in such a way as to increase moisture for crop production. This is generally accomplished through the use of wind barriers, including grass and grass stubble barriers, but may also include trees and shrubs.

SNOWMELT—(1) The runoff from melting snow. (2) The net decrease in water equivalent of the snowpack after allowing for increases due to precipitation. It does not include water which refreezes or is retained as liquid water within the Snowpack. (3) A period or season when such runoff occurs.

SNOWMELT RATE—The rate of conversion of ice into water within a snowpack.

SNOW METAMORPHISM—The transformation of snowflakes within a snowpack into different forms or structures.

SNOWMOLD—(1) A disease of grasses appearing as grayish-white or pinkish patches after heavy snow has melted and caused by fungi that thrive at low temperatures. (2) A fungus that causes this disease.

SNOWPACK—A field of naturally packed snow that ordinarily melts slowly during the early summer months.

SNOW PELLET—A small white ice particle that falls as precipitation and breaks apart easily when it lands on a surface. Often used in the plural. Also referred to as Graupel or Soft Hail.

SNOW QUALITY—The ratio of the weight of the ice within a snowpack to the total weight of the snowpack, usually expressed as a percentage.

SNOW SAMPLE—A core taken from the snow mantle on a snow course from which the depth and density of snow may be determined.

SNOW SAMPLER—An instrument used in obtaining snow samples, which consists of a set of light, jointed metal tubes for taking samples and a spring scale graduated to read directly the corresponding depth of water contained in the sample.

SNOW STAKE—A graduated fixed pole or staff used to measure snow depth.

SNOW SURVEY—The process or operation of determining the depth, water content, and density of snow at various selected points on a drainage basin. This is done in order to determine the amount of water stored there in the form of snow for the purpose of forecasting subsequent runoff.

SNOW, THERMAL QUALITY OF—The ratio of the amount of heat required to produce a specific quantity of water from a snowpack to the amount of heat required to produce the same quantity of water from pure ice at 0C (32F).

SNOW, WATER EQUIVALENT OF—The amount of water that would be obtained if the snowpack were melted, usually expressed in inches of water.

SOAK—(1) To make thoroughly wet or saturated by or as if by placing in liquid; to immerse in liquid for a period of time. (2) To absorb a liquid through or as if through pores or interstices.

SOAKAGE—(1) The process of soaking; the condition of being soaked. (2) The amount of liquid that soaks into, through, or out of an object.

SOAKER HOSE—A low-flow watering device with small holes throughout the surface of the hose used to soak plant beds and gardens.

SOCIOECONOMICS—The study of the economic, demographic, and social interactions of humans.

SOCs—See Synthetic Organic Chemicals (SOCs).

SODA ASH—(Water Quality) Also known as Sodium Carbonate, typically of chemical symbol Na₂CO₃, a salt of strong alkaline taste used in making glass, soap, paper, chemical reagents and to remove non-carbonate hardness from water.

SODIC (Soils)—Soils having an excess concentration of sodium ions. Excess sodium results in poor aeration, slow infiltration rates, and causes serious nutritional disturbances in plants by affecting the availability of calcium, magnesium, and other ions required by plants.

SODICITY (of Soils)—A measure of the excess sodium in a soil which imparts a poor physical condition to the soil. Sodic Soils are generally impermeable to water, which makes it difficult to germinate crops.

SODIUM ADSORPTION RATIO (SAR)—An expression of relative activity of sodium ions in exchange reactions with soil, indicating the sodium or alkali hazard to soil. It is calculated from the expression:

SAR = (Na)/[(Ca + Mg)/2]

where all quantities are expressed in milliequivalents per liter (meg/l). It is a particularly important measure in waters used for irrigation purposes.

SODIUM BICARBONATE—(Wastewater Treatment) A white crystalline salt, NaHCO₂, less soluble than Sodium Carbonate and having only a slight alkaline taste. Used as a Coagulant Aid in the neutralization process of wastewater treatment plants, it promotes more rapid settling, increases the efficiency of the coagulation process and extends the pH range to a level at which Alum (aluminum sulfate), a common inorganic coagulant, is effective.

SODIUM CARBONATE—(Water Quality) Any carbonate of sodium, typically Na₂CO₃, a salt of strong alkaline taste, found in nature, as in soda lakes, but more often made artificially and used extensively in making glass, soap, paper, chemical reagents and in the softening of water. Also referred to as Soda Ash.

SODIUM HYPOCHLORITE—(Water Quality) A water solution of sodium hydroxide and chlorine, NaOCl, used as a primary disinfectant in water treatment.

SOFT DETERGENTS—Cleaning agents that break down in nature.

SOFT HAIL—See Snow Pellet.

SOFT WATER—Water that contains low concentrations of metal ions such as calcium and magnesium. This type of water does not precipitate soaps and detergents. Compare to Hard Water.

SOFTENING—The removal of metal ions such as calcium and magnesium from water supplies. The converting of Hard Water to Soft Water.

SOIL—The meaning of this term varies depending on the field of consideration: (1) Pedology—the earth materials which have been so modified and acted upon by physical, chemical, and biological agents that it will support rooted plants; (2) Engineering Geology—the layer of incoherent rock material that nearly everywhere forms the surface of the land and rests on Bedrock, also called Regolith; (3) Ecology—A dynamic natural body on the surface of the earth in which plants grow, composed of mineral and organic materials and living forms.

SOIL ABSORPTION FIELD—A sub-surface area containing a trench or bed with clean stones and a system of piping through which treated sewage may seep into the surrounding soil for further treatment and disposal.

SOIL ABSORPTION SYSTEM, DRIP OR TRICKLE—A shallow slow rate pressure-dosed system used for land application of treated wastewater, particularly under soil conditions unsuitable for normal septic tanks and gravity-fed soil absorption systems. In agriculture, drip soil irrigation systems irrigate crops by means of a network of shallow underground pipes fed by a pump. Such a system conserves water used in crop irrigation by applying it at a controlled rate in the root zone, minimizing evaporation and percolation losses. In the drip soil absorption system, the filtered effluent is delivered via supply lines to a subsurface drip field consisting of parallel rows of polyethylene tubing, known as dripper lines. Emitters are installed along these tubes to uniformly distribute and control the flow of effluent. The key to the effective operation of drip soil absorption systems is the slow and controlled rate at which it applies effluent over a large surface area, allowing relatively shallow placement of the dripper lines and long-term use without risk of saturating soils. This allows such systems to be effectively used for subsurface irrigation of trees, shrubs, and gardens in arid regions. Also see Soil Absorption System, Septic Tank.

SOIL ABSORPTION SYSTEM, SEPTIC TANK—A conventional wastewater treatment process used in isolated, rural, and other areas where normal sewer systems are not available whereby effluent is treated in a Septic Tank, then fed to soil-based disposal fields or trenches by gravity or under pressure. The quality of the ensuing filtration and absorption treatment is directly linked to soil and sit characteristics such as permeability, drainage, slope, and depth to limiting conditions such as groundwater or bedrock. Unfortunately, many soils are classified as unsuitable for conventional soil absorption systems. Under such unsuitable soil conditions, alternative pretreatment systems may be employed to include sand filters, mound systems, evapotranspiration systems, low pressure pipe systems and a more complex drip soil absorption system. Also see Soil Absorption System, Drip or Trickle.

SOIL AIR—Below-ground air in the pore spaces between soil particles. Decomposition of organic matter in the soil can cause high concentrations of carbon dioxide in the soil air. The carbon dioxide combines with water to form carbonic acid, thereby increasing the acidity of the groundwater.

SOIL AMENDMENT—Any material added to soil that enhances plant growth.

SOIL AND SEDIMENT ADSORPTION ISOTHERM TEST—A test method that measures the Adsorption of a chemical substance to soil or sediment and thus indicates the likely distribution pathways in the environment. If a substance is readily adsorbed, it will increase in concentration in a soil or sediment layer; if it is not adsorbed, it will be free to move through the soil into groundwater or run off into surface water.

SOIL AND WATER CONSERVATION PRACTICES—Control measures consisting of managerial, vegetative, and structural practices to reduce the loss of soil and water.

SOIL AND WATER ASSESSMENT TOOL(SWAT)— a river basin, or watershed scale model developed by Dr. Jeff Arnold for the USDA Agricultural Research Service (ARS). SWAT was developed to predict the impact of land management practices on water, sediment and agricultural chemical yields in large complex watersheds with varying soils, land use and management conditions over long periods of time.

SOIL CLASSIFICATION——The systematic arrangement of soils into groups or categories on the basis of their characteristics. Broad groupings are made on the basis of general characteristics and subdivisions on the basis of more detailed differences in specific properties. Soil Taxonomy is the study of soil classification systems. For a description of soil classifications, see Land Capability Classes.

SOIL CONDITIONER—An organic material like humus or compost that helps soil absorb water, build a bacterial community, and take up mineral nutrients.

SOIL CONSERVATION—The use of land, within the limits of economic practicability, according to its capabilities and its needs to keep it permanently productive.

(UNITED STATES) SOIL CONSERVATION SERVICE (SCS)—Former name of the U.S. Natural Resources Conservation Service (NRCS).

SOIL CORE—A sample of soil taken by forcing a cylindrical device into the ground perpendicular to the horizontal. The resulting sample contains a circular section of each layer of sediment.

SOIL CREEP—The slow mass movement of soil materials down slopes primarily under the influence of gravity, but facilitated by saturation with water and/or by alternating freezing and thawing.

SOIL ERODIBILITY—An indicator of a soil's susceptibility to raindrop impact, runoff, and other erosive processes.

SOIL EROSION—The detachment and movement of soil from the land surface by wind or water.

SOIL FLUSHING—A treatment technique for cleaning soil contaminated with inorganic or organic hazardous waste. The process involves the flooding of the soil with a flushing solution, which may be acidic, basic, or contain Surfactants, and the subsequent removal of the Leachate via shallow wells or subsurface drains. The recovered leachate is then purified.

SOIL MANAGEMENT—The basis of all scientific agriculture, which involves six essential practices: (1) proper tillage; (2) maintenance of a proper supply of organic matter in the soil; (3) maintenance of a proper nutrient supply, including water; (4) control of soil pollution; (5) maintenance of the correct soil acidity; and (6) control of erosion.

SOIL MOISTURE (SOIL WATER)—Water diffused in the upper part of the Unsaturated Zone (Zone of Aeration) of the soil, from which water is discharged by the Transpiration of plants, by Evaporation, or Interflow.

SOIL-MOISTURE DEPLETION—The process where soil water is removed by crops through Evapotranspiration.

SOIL-MOISTURE DEFICIENCY (or DEFICIT)—The difference between the water-holding capacity of the soil and the instantaneous soil moisture.

SOIL MOISTURE REGIME—The changes in the moisture content of soil during a year.

SOIL-MOISTURE RETENTION—That part of the soil moisture retained by surface tension and molecular forces against the influence of gravity.

SOIL-MOISTURE SENSOR—A device attached to an automatic irrigation or sprinkler system that monitors the level of drops or water administered to the soil based on the level of Soil Moisture.

SOIL MONOLITH—A vertical section of a soil profile removed and mounted for display or study.

SOIL MORPHOLOGY—The physical constitution, particularly the structural properties, of a soil profile as exhibited by the kinds, thickness, and arrangement of the horizons in the profile and by the texture, structure, consistency, and porosity of each horizon.

SOIL PROFILE—A vertical section of the soil from the surface through all its horizons. The three basic horizontal layers that may be observed in a soil profile are the A, B, and C horizons. The A horizon, or topsoil, is the main source of plant nutrients. Soluble materials leach from the A horizon down to the subsoil, or B horizon. This is the zone of clay accumulation. The deepest layer, the C horizon, is composed of partially weathered bedrock. Also referred to as Soil Horizons. Also see Soil Core.

SOIL SORPTION COEFFICIENT (K_d)—A parameter relating the partitioning of a chemical between soil and water in a soil-water mixture. The coefficient is computed by:

K_d = C_s/C_w K_d = C_s/C_w

where C_s is the amount of a specific chemical bonded to the soil (micrograms chemical per gram of soil) and C_w is the concentration of the same substance dissolved in the water (micrograms of the chemical per gram of water).

SOIL STRUCTURE—The physical properties of different soils. Sand has little structure because sand particles do not tend to stick together, whereas clay has a firm structure because clay particles tend to pack tightly together to form a solid material.

SOIL TAXONOMY——The systematic study and arrangement of soils into classifications based on their characteristics. For a description of soil classifications, see Land Capability Classes.

SOIL TEXTURE—A classification of soils based on the size distribution of mineral grains comprising the soil. The relative proportions of silt, sand, clay, and gravel are normally given.

SOIL THIN-LAYER CHROMATOGRAPHY—A method used to estimate the potential for leaching a chemical from soil by measuring the mobility of the chemical in soil under controlled conditions.

SOIL TOXICITY—A measure of the toxic effects that a soil and its constituents may have on plants and/or animals. A number of elements have varied effects on plant growth as well as aquatic habitat and downstream drinking water supplies. Some of the more common toxic substances include excess levels of boron, arsenic, molybdenum, selenium, nitrate, and mercury. Other elements of growing concern include Nickel, Uranium, Copper, Manganese, Lead, Calcium, Zinc, Silver, Chromium, Beryllium, Lithium, and Cobalt.

SOIL VAPOR SURVEY—A non-invasive method for the detection of volatile or semivolatile organic contaminants in shallow subsurface soil. The technology is especially useful in the analysis of soils with a high sand content and of shallow groundwater. A probe is driven into the ground, and samples of vapors in the soil are drawn to the surface for analysis.

SOIL WATER—Water present in the soil pores. Also referred to as Soil Moisture which includes Water Vapor.

SOIL WATER TENSION—The work that must be done per unit quantity of pure water to transport it from free water at the same elevation to soil water. Also referred to as Matric or Capillary Potential.

SOIL ZONE—The Root Zone.

SOLAR AQUATIC SYSTEMS (SAS)—A technology which, under controlled conditions, duplicates the natural water purification processes of streams and wetlands. The solar greenhouse wastewater treatment system includes solar tanks which optimize photosynthetic reactions and biological activity, and artificially-controlled ponds and wetlands which replicate a natural environment. Within such a system, wastewater is circulated through ecologically-engineered aquatic environments where the contaminants and nutrients are metabolized or "bound up." Typically, tertiary treatment of wastewater requires up to four days.

SOLAR POND—A pool of salt water heated by the sun and used either as a direct source of heat or to provide power for an electric generator.

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