Safe Drinking Water Act: a regulatory program passed by the U.S. Congress in 1974 to help ensure safe drinking water in the United States; sets maximum contaminant levels for a variety of chemicals, metals, and bacteria in public water supplies.
saline intrusion: the saltwater infiltration of freshwater aquifers in coastal areas, when groundwater is withdrawn faster than it is being recharged.
salinity: an indication of the amount of salt dissolved in water.
salt marsh: an area where salt water from an ocean, bay, or gulf meets fresh water from a river.
salt water: water associated with the seas distinguished by high salinity.
sanitary landfill: rehabilitated land in which garbage and trash have been buried.
saturated air: air that contains as much moisture as it is possible to hold under existing conditions.
saturated zone: underground layer in which every available space is filled with water.
saturation: the state of being infused with so much of a substance (Example: water) that no more can be absorbed, dissolved, or retained.
secondary treatment: the wastewater process where bacteria are used to digest organic matter in the wastewater.
sediment: insoluble material suspended in water that consists mainly of particles derived from rocks, soil, and organic materials; a major nonpoint source pollutant to which other pollutants may attach.
sediment pollution: the introduction of sediment into a water body.
sediment pond: a natural or artificial pond for recovering the solids from effluent or runoff.
septic system: a domestic wastewater treatment system (consisting of a septic tank and a soil absorption system) into which wastes are piped directly from the home; bacteria decompose the waste, sludge settles to the bottom of the tank, and the treated effluent flows out into the ground through drainage pipes.
settling: the process of a substance, such as dregs or sediment, sinking or being deposited.
settling tank: a vessel in which solids settle out of water by gravity during drinking and wastewater treatment processes.
sewage contamination: the introduction of untreated sewage into a water body.
sewage outfall: the point of sewage discharge, often from a pipe into a body of water, in turn called the outfall area.
sewer system: an underground system of pipes used to carry off sewage and surface water runoff.
silage: livestock food prepared by storing and fermenting green forage plants in a silo.
silt: particles of small size left as sediment from water.
sinkhole: a natural depression in a land surface connected to a subterranean passage, generally occurring in limestone regions and formed by solution or by collapse of a cavern roof.
siphon: a bent pipe or tube through which liquid can be drawn by air pressure up and over the edge of a container; to draw off by a siphon.
slope: to take a slanting direction, such as a bank sloping down to a river; a piece of slanting ground, such as a hillside; the upward or downward slant, such as that of a roof.
slough: a stagnant swamp, marsh, bog, or pond, especially as a part of a bayou, inlet, or backwater.
sludge: solid matter that settles to the bottom of septic tanks or wastewater treatment plant sedimentation; must be disposed of by bacterial digestion or other methods or pumped out for land disposal or incineration.
solar radiation: radiation emitted by the sun.
solution: the result of solving a problem; a liquid in which something has been dissolved.
solvent: a liquid capable of dissolving another substance (Examples: paint thinner, mineral spirits, and water).
stormwater runoff: surface water runoff that flows into storm sewers or surface waters.
stream: a body of water flowing in a channel, as a brook, rivulet, or river.
stream use classification: a system for classifying streams according to the intended use of the water (Examples: recreation, industrial cooling, irrigation).
strip mine: an open mineral mine (Examples: coal, copper, zinc, etc.) where the topsoil and overburden is removed to expose and extract the mineral.
subsidence: the compacting and sinking of an area.
substance: a material of a particular kind or constitution.
substrate: the substance acted upon by an enzyme or a fermenter, such as yeast, mold, or bacteria.
suffocate: to die due to the lack of oxygen.
sulfuric acid: the acid formed when sulfur oxides combine with atmospheric moisture; a major component of acid rain.
supersaturation: the state of being infused with more of a substance (Example: water) than is normally possible under given conditions of temperature and pressure.
surface tension: the elastic-like force in a body, especially a liquid, tending to minimize, or constrict, the area of the surface.
surface water: precipitation that does not soak into the ground or return to the atmosphere by evaporation or transpiration. It is stored in streams, lakes, rivers, ponds, wetlands, oceans, and reservoirs.
swamp: land having soils saturated with water for at least part of the year and supporting natural vegetation of mostly trees and shrubs.