A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Saline Water
Water that contains significant amounts of dissolved solids.
Fresh water   Less than 1,000 parts per million (ppm)
Slightly saline water  From 1,000 ppm to 3,000 ppm
Moderately saline water  From 3,000 ppm to 10,000 ppm
Highly saline water From 10,000 ppm to 35,000 ppm.
See also brine, seawater, brackish.

Sand & Gravel Aquifers
A term often used to describe alluvial or stratified drift aquifers comprised of unconsolidated sediments.

Saturated Zone
The zone within sediment and rock formations where all voids are filled with water. The level below the water table in an unconfined aquifer. The saturated zone may be considered to include water held above the water table by capillary rise. Soils and the vadose zone in rock formations are not fully saturated. Saturated thickness describes the vertical extent of an aquifer below the water table.

Screen (well screen)
A cylinder of steel or plastic material used to allow water to enter a well while preventing sediment or rock particles from entering the well. A screen operates something like a sieve. Well screens may be wire wrapped, louver or perforated, and can be made from different materials and at different opening sizes. The selection of well screen design and opening size may depend on characteristics of the geologic formation, required yield and the thickness of the aquifer.
See also Grain size.

Seawater intrusion (salt water intrusion)
The inland movement of saline water in coastal aquifers. Saline intrusion usually results from ground water withdrawals and is a problem in areas such as southern California and Florida.

Secondary Standard
A water quality standard of the maximum recommended concentration of substances in drinking water based on aesthetic, not health-risk criteria.

Sediment
A term usually applied to material in suspension in water or recently deposited from suspension. The word sediments is used to describe different kinds of geologic deposits:
Sediment Type   Size in inches   size in millimeters
Boulder   greater than 10.8    greater than 256
Cobble    2.52 - 10.8    64 - 256
Very Coarse Gravel   1.26 - 2.52    32 - 64
Coarse Gravel   0.63 - 1.26    16 - 32
Medium Gravel    0.31 - 0.63    8 - 16
Fine Gravel    0.16 - 0.31    4 - 8
Very Fine Gravel   0.08 - 0,16    2 - 4
Very Coarse Sand   0.04 - 0.08    1 - 2
Coarse Sand    0.02 - 0.04    0.5 - 1
Medium Sand    0.01 - 0.02    0.25 - 0.5
Fine Sand   0.005 - 0.01    0.125 - 0.25
Very Fine Sand    0.002 - 0.005    0.063 - 0.125
Silt    0.0002 - 0.002    0.004 - 0.063
Clay    Less than 0.0002    Less than 0.004


Sedimentary Rock
Rock formed of sediment, (1) fragments of other rock transported from their sources, (2) rocks formed by or from organisms, such as most limestone, (3) rocks formed by precipitation of chemicals. Many sedimentary rocks show distinct layering, which is the result of different types of sediment being deposited in succession.

Sedimentation Tanks
Wastewater tanks in which floating wastes are skimmed off and settled solids are removed for disposal.

Seepage
(1.) The definition is similar to that for springs, however, the movement of ground water to the surface is often slower and generally not as concentrated as in springs. By some definitions, seepage is the process and springs are the result.
(2.) The slow movement of ground water through small cracks, pores, interstices, etc., into surface water or the loss of water by infiltration into the soil from fields, canal, ditches or from any natural stream or water body.
See also springs.

Self-Supplied Water
Water withdrawn from a surface or ground water source by a user rather than being obtained from a public supply. An example would be homeowners getting their water from their own well. Fifteen million US homes have their own water well.

Septic Tank
A tank (usually made of concrete) used to detain domestic wastewater to allow the settling of solids prior to distribution to a leach field for soil absorption. Most solids that are retained by septic tanks are decomposed by anaerobic bacterial action.

Sewage Treatment Plant
A facility designed to receive wastewater (principally from domestic sources) and by processes such a aeration and settling, restore water quality before returning it yo rivers or the ocean.

Sewer
A system of underground pipes that collect and deliver wastewater to treatment facilities or streams.

Sinkhole
A depression in the Earth's surface caused by collapse of overlying soils or rock into pre-existing cave systems formed by dissolving of underlying limestone, salt, or gypsum. Drainage is provided through underground channels that may be enlarged by the collapse of a cavern roof. Solution of limestone is a slow process. The creation of a sink-hole however may occur rapidly.

Soil Moisture
Water occurring in the pore spaces between the soil particles in the unsaturated zone. This water is available for uptake by plants.

Solute
A substance that is dissolved in another substance, thus forming a solution.

Solvent
A substance that dissolves other substances, thus forming a solution. Water dissolves more substances than any other and is known as the "universal solvent".

Specific capacity
The rate of discharge of a well per unit depth of drawdown. Expressed as gallons per minute per foot, (liters per minute per meter). It is used as a measure of well efficiency. The ideal for a well is high discharge and low drawdown.

Specific Conductance
A measure of the ability of water to conduct an electrical current and expressed in units of electrical conductance, i.e., Siemens per centimeter at 25 degrees Celsius. Specific conductance can be used for approximating the total dissolved solids content of water as an indication of the presence of ions of chemical substances.

Specific Yield
The ratio (%) volume of water yielded by a rock to the volume of rock. In practice some water always "sticks" to the rock and so not all the water stored in a unit volume of rock is available to flow to a well.

Spray Irrigation
A common irrigation method where water is sprayed from high-pressure nozzles onto crops. In spray irrigation there is greater likelihood of water being lost to evaporation.

Spreading Basins
Areas used for the purpose of recharging aquifers. Spreading basins may have to be periodically scraped to remove fine sediments that restrict rates of recharge.

Springs
Areas where there is a concentrated discharge of ground water that appears as a flow of water at the surface. The distinction between springs and seepages is arbitrary. Vast amounts of groundwater discharges continuously to rivers and lakes, the majority of which occurs unseen in streambeds or as bank seepage. A "spring-fed" river may not have a visible "spring." There are many different geologic and hydrologic circumstances that can result in springs. Wetlands, springs, and seepages may occur where the water table intersects the land surface.

Storm Sewer
A sewer that carries only surface runoff, street wash, and snow melt from the land. In a separate sewer system, storm sewers should be completely separate from those that carry domestic and commercial wastewater (sanitary sewers).

Stratified Drift
Sedimentary deposits comprised of sands and gravels deposited by glacial meltwater streams. Layers of silts and clays may be interbedded among the sand and gravel layers.

Stream
A general term for a body of flowing water in a natural watercourse containing water at least part of the year. In hydrology, it is generally applied to the water flowing in a natural channel as distinct from a canal.

Streamflow
The water discharge that occurs in a natural channel. A more general term than runoff, streamflow may be applied to river discharge whether or not it is affected by diversion or regulation.

Sublimation
Evaporation occurring directly from ice or snow without passing through the liquid state.

Subsidence
A dropping of the land surface as a result of ground water being pumped. Cracks and fissures can appear in the land. Subsidence resulting from over pumping is usually an irreversible process. Parts of eastern Texas have subsidence districts that are managed to reduce pumping and curtail subsidence. Venice and Mexico City have subsidence problems resulting from ground water withdrawals.

Subsurface Water
All water occurring beneath the earth's surface. It includes soil moisture and ground water.

Surface Water
Water that is on the Earth's surface, such as in a stream, river, lake, or reservoir.

Surfactant
A substance used to reduce surface tension in a liquid. Foam sufactants are used in some drilling processes. Sufractants are also used to increase the efficiency of some ground water remediation techniques.

Suspended Sediment
Very fine soil particles that remain in suspension in water for a considerable period of time without contact with the bottom. Such material remains in suspension due to the upward components of turbulence and currents and/or by suspension.

Suspended Sediment Concentration
The ratio of the mass of dry sediment in a water/sediment mixture to the mass of the water/sediment mixture. Typically expressed in milligrams of dry sediment per liter of water sediment mixture. Reducing suspended sediment is crucial for water used in recharge wells and spreading basins.

Swale
A natural depression or engineered wide shallow ditch designed to temporarily store, route or filter runoff.