GLOSSARY

Groundwater & Water Risk Glossary

Plain-language definitions of groundwater, aquifer, drawdown, contamination and data center water terms — from acre-foot and cone of depression to PFAS, MCL and WUE.

A

Acre-foot: The volume of water that covers one acre to a depth of one foot — about 325,851 gallons or 1,233 cubic meters. The standard unit for large water volumes in the U.S. West.
Advection: Transport of dissolved contaminants with the bulk flow of groundwater. It sets the average speed at which a plume moves down-gradient.
Aquifer: A saturated body of rock or sediment that stores groundwater and transmits it to wells and springs in usable quantities.
Aquitard: A low-permeability layer, such as clay or shale, that slows groundwater movement between aquifers. Compaction of aquitards during heavy pumping causes most land subsidence.

B

BTEX: Benzene, toluene, ethylbenzene and xylenes — the volatile aromatic compounds in gasoline that drive most petroleum groundwater cleanups. The federal drinking water limit for benzene is 0.005 mg/L.

C

Cone of depression: The cone-shaped lowering of the water table or pressure surface around a pumping well, deepest at the well and flattening with distance. See cone of depression and well interference.
Confined aquifer: An aquifer bounded above and below by low-permeability layers, so its water is under pressure. Pumping effects spread much farther in confined aquifers than in unconfined ones.
Consumptive use: Water withdrawn and not returned to the local source — evaporated, transpired or built into a product. Data center evaporative cooling is mostly consumptive.

D

Delineation: Defining the horizontal and vertical extent of contamination, usually with monitoring wells placed until concentrations fall below the applicable limit at the plume edge.
Desired future conditions (DFCs): Measurable aquifer goals — such as an allowable average drawdown over 50 years — adopted jointly by Texas groundwater conservation districts in a groundwater management area. See Texas groundwater.
Dispersion: Spreading and mixing of a dissolved contaminant as groundwater moves through variable pore paths, which widens a plume and dilutes its edges.
DNAPL: Dense non-aqueous phase liquid — a separate-phase liquid denser than water, such as chlorinated solvents, that sinks through an aquifer and forms long-lived source zones.
Drawdown: The decline in water level at a point caused by pumping, measured from the static level before pumping began.

E

Evaporative cooling: Rejecting heat by evaporating water, as in cooling towers. It is the most water-intensive way data centers cool equipment. See AI data center water use.

G

Groundwater: Water that fills pores and fractures below the water table. About 140 million people in the U.S. rely on it for drinking water (USGS).
Groundwater conservation district (GCD): A local political subdivision in Texas that regulates groundwater production through permits, well spacing and production limits. Texas has 98 confirmed districts (TWDB).
Groundwater depletion: A long-term decline in the volume of water stored in an aquifer, caused by pumping that exceeds recharge over many years. See groundwater depletion.

H

Hydraulic conductivity: A measure of how easily water moves through a porous material, usually expressed in feet per day or meters per day.
Hydrograph: A chart of water level (or flow) over time at a well or stream gauge. Long hydrographs are the primary evidence of aquifer trends.

L

Land subsidence: Sinking of the land surface, most often from compaction of clay layers when groundwater is pumped. USGS attributes more than 80% of identified U.S. subsidence to groundwater withdrawal. See Houston subsidence.
LNAPL: Light non-aqueous phase liquid — a separate-phase liquid lighter than water, such as gasoline or diesel, that floats on the water table. Also called free product or phase-separated hydrocarbon (PSH).

M

Managed aquifer recharge (MAR): Intentionally adding water to an aquifer — through infiltration basins, injection wells or aquifer storage and recovery — to store water or offset depletion.
Maximum contaminant level (MCL): The highest level of a contaminant legally allowed in public drinking water under the Safe Drinking Water Act, for example 0.005 mg/L for benzene and 0.000004 mg/L for PFOA and PFOS.
Modeled available groundwater (MAG): The amount of groundwater that can be produced annually while still achieving an aquifer’s desired future conditions, as estimated by the Texas Water Development Board.
Monitored natural attenuation (MNA): A remedy that relies on natural processes — mainly biodegradation, dispersion and sorption — to reduce contaminant concentrations, verified by ongoing monitoring.

N

Nitrate: A highly soluble form of nitrogen from fertilizer, manure and septic systems, and one of the most widespread groundwater contaminants; the federal drinking water limit is 10 mg/L as nitrogen. See nitrate in groundwater.

O

Ogallala aquifer: The principal formation of the High Plains aquifer, which underlies about 174,000 square miles in eight states and supplies about 30% of the groundwater used for U.S. irrigation (USGS). See Ogallala aquifer depletion.

P

PFAS: Per- and polyfluoroalkyl substances — persistent synthetic “forever chemicals” that form long groundwater plumes. See PFAS in groundwater.
Plume: The zone of contaminated groundwater that spreads down-gradient from a source such as a leaking tank or spill.
Potentiometric surface: The level to which water would rise in wells tapping a confined aquifer — the pressure-head equivalent of the water table.
Protective concentration level (PCL): Under the Texas Risk Reduction Program (TRRP), the concentration of a chemical that is protective of human health and the environment for a given exposure pathway; used to set cleanup and closure targets.
Pumping test: A controlled test that pumps a well at a known rate while measuring drawdown in it and nearby wells, used to estimate transmissivity, storativity and well yield.

R

Radius of influence: The distance from a pumping well at which drawdown becomes negligible — the outer edge of the cone of depression.
Recharge: Water that enters an aquifer, mainly from infiltrating rain and snowmelt or seepage from streams and lakes.
Retardation: Slowing of a contaminant relative to groundwater flow because it sorbs to aquifer solids, especially organic carbon.
Rule of capture: The doctrine, followed in Texas since 1904, that a landowner may pump groundwater beneath their land without liability to neighbors — today subject to groundwater conservation district rules.

S

Saltwater intrusion: Movement of seawater or deep saline water into a fresh aquifer, usually because pumping lowered fresh water levels. See saltwater intrusion.
Saturated thickness: The vertical thickness of an aquifer that is filled with water. Declining saturated thickness reduces well yields and is a key measure of remaining supply.
Specific capacity: A well’s yield per unit of drawdown, typically in gallons per minute per foot. Falling specific capacity can signal well fouling or declining aquifer conditions.
Specific yield: The volume of water an unconfined aquifer releases from storage per unit area per unit decline of the water table, expressed as a fraction.
Storativity: The volume of water a confined aquifer releases from storage per unit area per unit decline in head. It is very small, which is why confined-aquifer drawdown spreads far.
Sustainable yield: The maximum pumping rate a well or wellfield can maintain for a target period without drawing water levels below a defined threshold, such as minimum saturated thickness or pump submergence.

T

Theis equation: The 1935 analytical solution by C.V. Theis for transient drawdown around a well pumping a confined aquifer at a constant rate.
TPH: Total petroleum hydrocarbons — a bulk measure of the mixture of hydrocarbons from crude oil and fuels in soil or water.
Transmissivity: The rate at which water moves through the full saturated thickness of an aquifer under a unit hydraulic gradient — hydraulic conductivity multiplied by saturated thickness.

U

Unconfined aquifer: An aquifer whose upper surface is the water table, open to the atmosphere through the unsaturated zone. It releases water by draining pore space.

V

Vadose zone: The unsaturated zone between the land surface and the water table. Contaminants such as PFAS can be held there and leach to groundwater for years.

W

Water stress: The degree to which demand consumes available renewable water supply. WRI Aqueduct defines “high” stress as withdrawals of 40–80% of available supply. See water scarcity in the U.S..
Water table: The top of the saturated zone in an unconfined aquifer, where water pressure equals atmospheric pressure.
Water usage effectiveness (WUE): A data center metric from The Green Grid: liters of site water used per kilowatt-hour of IT equipment energy (L/kWh).
Well interference: Additional drawdown at a well caused by pumping at nearby wells whose cones of depression overlap it.