Acidic -- In water a condition in which the pH is less than 7.
Average ground-water velocity -- The speed and direction of travel of ground water through an aquifer. The average ground-water velocity is calculated from the specific discharge and the aquifer porosity.
Basic -- In water a condition in which the pH is greater than 7.
Batch tests -- Laboratory tests performed on samples of contaminated ground water, aquifer materials, and chemicals in a reactor to evaluate how the chemicals used by a remediation technology might be useful for remediation.
Capture zone -- The area contributing to flow to a well. The shape of the capture zone depends on the average linear ground-water velocity, the rate at which the well is being pumped, and the hydraulic conductivity of the aquifer. The upgradient extent of the capture zone depends on how long the well is being pumped.
Concentration -- The amount of contaminant (or other constituent) in a given volume of water, often as milligrams per liter.
Cone of depression -- The depression, roughly conical in shape, produced in a water table by the extraction of water from a well at a given rate. The size of the cone of depression depends on the duration of pumping, the pumping rate, and the hydraulic conductivity, specific yield, and thickness of the aquifer.
Cone of impression -- A rise, roughly conical in shape, produced in the water table from the addition of water from an injection well at a rate that is faster than the ability of the aquifer to transmit the water away from the well. The size of the cone of impression depends on the duration of pumping, the pumping rate, and the hydraulic conductivity, specific yield, and thickness of the aquifer.
Darcy's law -- A mathematical equation stating that the flow rate of water through porous materials is proportional to the hydraulic gradient. The constant of proportionality is the hydraulic conductivity.
Drinking-water standard or guideline -- A threshold concentration for a constituent or compound in a public drinking-water supply, designed to protect human health. As defined here, standards are U.S. Environmental Protection Agency regulations that specify the maximum contamination levels for public water systems required to protect the public welfare; guidelines have no regulatory status and are issued in an advisory capacity.
Ground water -- (1) water that flows or seeps downward and saturates soil or rock, supplying springs and wells. The upper surface of the saturate zone is called the water table. (2) Water stored underground in the pores of geologic materials that make up the Earth's crust.
Heterogeneous -- When used in connection to aquifers, the term means nonuniform becuase the aquifer consists of more than one material type (gravel, sand, silt or clay) distributed as continuous or discontinuous layers.
Hydraulic head -- The elevation of the water level in wells penetrating an aquifer and in this case, it is equivalent to the elevation of the water table. Also, the elevation of the water surface in surface water bodies, such as lakes and streams.
Hydrograph -- A graph of time versus the water-level elevation above sea level or depth to water below surface in a well. The graph is typically used to assess the magnitude of water-level fluctuations or change in an aquifer.
ion -- an atom or a group of atoms that have lost their electrically neutral status by acquiring or losing one or more electrons The number in he superscript above the chemical symbol indicates the number of electrons gained or lost by the ion. Cations are positively charged ions (such as the hydrogen ion [H+1] or the ferric ion [ Fe+3 ]). Anions are negatively charged ions (such as the chromate ion [ CrO4-2 ].
Maximum Contaminant Level (MCL) -- (1) The greatest amount of a contaminant that can be present in drinking water without causing a risk to human health. (2) Maximum permissible level of a contaminant in water that is delivered to any user of a public water system. MCLs are enforceable standards established by the U.S. Environmental Protection Agency. EPA sets MCLs at levels that are economically and technologically feasible. Some states set MCLs which are more strict than EPA's.
Monitoring -- (1) Repeated observation, measurement, or sampling at a site, on a scheduled or event basis, for a particular purpose. (2) Testing that water systems must perform to detect and measure contaminants. A water system that does not follow EPA's monitoring methodology or schedule is in violation, and may be subject to legal action.
National Pollutant Discharge Elimination System (NPDES) -- This is a permit system that allows industries and cities to legally discharge treated water to surface water bodies if the fluids being discharged meet US Environmental Protection Agency water quality or other standards.
Organics -- Shortened term used to refer to manmade organic chemicals made up primarily of carbon, hydrogen, and oxygen. Common examples include pesticides, solvents such as methanol and acetone, and Trichloroethylene (TCE). TCE is a chemical compound that is often used to remove grease and clean metal parts and products.
Oxidizing conditions -- A chemical environment in which oxidation is favorable. Eh values are relative measure of the oxidation potential. Positive values indicate a likelihood of oxidation.
Particle size -- The diameter, in millimeters, of suspended sediment or bed material. Particle-size classifications are:
 Clay-0.00024-0.004 millimeters (mm);
 Silt-0.004-0.062 mm;
 Sand-0.062-2.0 mm; and
 Gravel-2.0-64.0 mm.
Permeability -- The ability of a material to allow the passage of a liquid, such as water through rocks. Permeable materials, such as gravel and sand, allow water to move quickly through them, whereas unpermeable material, such as clay, don't allow water to flow freely.
pH -- A measure of the hydrogen ion concentration in water. The range of pH values is from 0 to 14. pH values below 7 are considered to be acidic, whereas pH values above 7 are considered basic. A pH of 7 is neutral.
Plume -- See contaminant plume.
Pollutant -- Any substance that, when present in a hydrologic system at sufficient concentration, degrades water quality in ways that are or could become harmful to human and/or ecological health or that impair the use of water for recreation, agriculture, industry, commerce, or domestic purposes.
Pollution -- An alteration in the character or quality of the environment, or any of its components, that renders it less suited for certain uses. The alteration of the physical, chemical, or biological properties of water by the introduction of any substance that renders the water harmful to use.
Porosity -- The ratio of the pore or void space to the total volume occupied by a material. With respect to water movement, it is not just the total magnitude of porosity that is important, but the size of the voids and the extent to which they are interconnected, as the pores in a formation may be open, or interconnected, or closed and isolated. For example, clay may have a very high porosity with respect to potential water content, but it constitutes a poor medium as an aquifer because the pores are usually so small.
Public supply -- Water withdrawn by public governments and agencies, such as a county water department, and by private companies that is then delivered to users. Public suppliers provide water for domestic, commercial, thermoelectric power, industrial, and public water users. Most household water is delivered by a public water supplier. The systems have at least 15 service connections (such as households, businesses, or schools) or regularly serve at least 25 individuals daily for at least 60 days out of the year. There are more than 170,000 public supplies providing water from wells, rivers and other sources to about 250 million Americans. The others drink water from private wells. There are differing standards for public supplies of different sizes and types.
Pump-and-treat method -- This is the most common method of aquifer remediation. Contaminants are removed from the aquifer by means of a production well. They produced water is treated to remove the contaminants and the treated water is returned to either a surface water body or the aquifer.
Redox reactions -- A shorthand term for all chemical reactions in which the atoms have their oxidation number (oxidation state) changed. Oxidation occurs when an atom loses electrons and reduction occurs when an atom gains electrons.
Reducing conditions -- A chemical environment in which reduction is favorable. Eh values are relative measure of the reduction potential. Negative values indicate a likelihood of reduction.
Specific conductance -- A measure of the ability of a water to transmit an electric current. The higher the value, the greater ability. Specific conductance is also a measure of the total dissolved solids content of the water. The total dissolved solids in a water is the concentration of all the dissolved ions in water.
Tailing and rebound -- This occurs near the end of a remediation cycle when most of the contaminant has been removed from the more permeable zones of the aquifer while significant contamination remains in the less permeable parts. The contaminants recovered decrease over time during the cycle to the point where the apparent concentration is below the maximum environmental standard. If remediation ceases, the contaminants in the less permeable part of the aquifer move into the more permeable part, thereby causing the apparent concentration in the ground water to rise above he standard.
Test drilling -- Drilling for exploratory purposes that may or may not include the installation of wells for water sampling and water-level and water-quality determination. Test drilling is also done to provide information of the types and distribution of earth materials below the ground surface and the nature of the layering.
Time-of-travel -- The time required for ground water to move from one point beneath the surface to another.
Treatment zone -- Used in reference to the aquifer volume created by application of the in situ redox manipulation or pump and treat in which the remediation effort has chemically altered the environment or by developed a hydraulic control within a capture zone.
Unconfined aquifers -- An aquifer in which the upper boundary is the water table.
Water quality standard -- Recommended or enforceable maximum contaminant levels of chemicals or materials (such as chlorobenzene, nitrate, iron, arsenic) in water. These levels are established for water used by municipalities, industries, agriculture, and recreationists.
Water table -- (1) The level below the earth's surface at which the ground becomes saturated with water. (2) The top of an unconfined aquifer; indicates the level below which soil and rock are saturated with water.
Well screen -- A perforated interval usually in the lower part of the well casing where ground-water flows into the well when the pump is turned on and a filtering devise used to keep sediment out the well.
Wellfield design -- The layout of production or production and injection wells based on the effects of pumping or injection on the aquifer and the contaminant plume. Pumping/injection rates are usually considered as part of the wellfield design. Placement of the wells is usually governed by the desired effect, such as controlling the movement of a contaminant plume.