A layer of low-permeability rock or sediment that hampers groundwater movement. Often applied to the confining bed in a confined aquifer


A water-bearing body of rock or sediment that will yield water to a well or spring in usable quantities

Artesian-pressure surface

An imaginary surface defined by the level to which water will rise in an artesian well

Cone of depression

The cone shape formed by the water table around a pumping well

Confined aquifer

A water bearing formation bounded above and below by impermeable beds or beds of distinctly lower permeability

Darcy's law

A derived formula for the flow of fluids in a porous medium


The lowering of a water table immediately adjacent to a pumping well


That portion of precipitation that is returned to the air through evaporation and transpiration, the latter being the escape of water from the leaves of plants

Gaining stream

a stream that receives water from the zone of saturation


That part of subsurface water that is in the zone of saturation

Hydraulic conductivity

a groundwater unit expressed as the volume of water that will move in a unit of time through a unit of area measured perpendicular to the flow direction. commonly called permeability measured in cubic meters

Hydraulic gradient

The slope or vertical change in water-pressure head with horizontal distance in an aquifer


a person who studies the geology and management of underground water and related aspects of surface water and groundwater

Hydrologic cycle

the constant circulation of water from the sea to the atmosphere, to the land, and eventually back to the sea. the cycle is driven by solar energy

Karst terrain/ Karst topography

Area underlain by soluble limestone or dolomite and riddled with caves, caverns, sinkholes, lakes, and disappearing streams

Losing stream

a stream or reach of a stream that contributes water to the zone of saturation

Maximum Contaminent Level (MCL)

the highest level of a contaminent that is allowed in drinking water. MCLs are set as close to the MCL goals as feasible using the best available treatment technology

Maximum Contaminent Level Goal (MCLG)

The level of a contaminant in drinking water below which there is no known or expected risk to health. MCLG's allow for a margin of safety


process by which stagnant bottom waters in a lake come to the surface and are refreshed with oxygen

Perched Water Table

the upper surface of a body of groundwater held up by a discontinuous impermeable layer above the static water table


The degree of ease with which fluids flow through a porous medium


a materials ability to contain fluid. the ratio of the volume of pore spaces in a rock or sediment to its total volume, usually expressed as a percentage

Potable water

water that is suitable for drinking

Potentiometric surface

the hydrostatic head of groundwater and the level to which the water will rise in a well. the water table is the potentiometric surface in an unconfined aquifer

Residence time

The average length of time a substance (atom, ion, molecule) remains in a given reservoir

Saltwater intrusion

the movement of denser saltwater into the pore space of an aquifer as less dense freshwater is removed by groundwater pumping and not replaced

Specific yield

the ratio of the volume of water drained from an aquifer by gravity to the total volume considered

Sustained yield (hydrology)

the amount of water an aquifer can yield on a daily basis over a long period of time

Unconfined aquifer

Underground body of water that has a free (static) water table-that is, water that is not confined under pressure beneath an aquiclude

Water table

the contact between the zone of aeration and the zone of saturation