Groundwater Information Center
Following is a list of common terms that are used in conjunction with groundwater. For a more detailed list of terms, please refer to
Groundwater Glossary
Bulletin 118 Update 2003
California Water Plan
Aquifer
A body of rock or sediment that is sufficiently porous and permeable to store, transmit, and yield significant or economic
quantities of groundwater to wells and springs.
Aquitard
A confining bed and/or formation composed of rock or sediment that retards but does not prevent the flow of water to or
from an adjacent aquifer. It does not readily yield water to wells or springs, but stores ground water.
Artesian Aquifer
A body of rock or sediment containing groundwater that is under greater than hydrostatic pressure; that is, a confined aquifer.
When an artesian aquifer is penetrated by a well, the water level will rise above the top of the aquifer.
Artificial Recharge
The addition of water to a groundwater reservoir by human activity, such as putting surface water into dug or constructed
spreading basins or injecting water through wells.
Basin/Subbasin Number
The basin numbering format is x-xxx.xx. The first number in the sequence assigns the basin to one of the nine Regional Water Quality
Control Board boundaries. The second number is the groundwater basin number. Any number following the decimal identifies that the
groundwater basin has been further divided into subbasins.
Basin or Subbasin Name
Basin names are based on published and unpublished reports, topographic maps, and local terminology. Names of more recently
delineated basins or subbasins are based on the principal geographic feature, which in most cases corresponds to the name
of a valley. In the case of a subbasin, its formal name should include the name of the basin (for example, Sacramento Valley
Groundwater Basin, North American Subbasin). However, both locally and informally, the term subbasin is used interchangeably
with basin (for example, North American Basin).
Confined Aquifer
An aquifer that is bounded above and below by formations of distinctly lower
permeability than that of the aquifer itself. An aquifer containing confined ground water. See artesian aquifer.
Contaminant
Any substance or property preventing the use or reducing the usability of the water for ordinary purposes such as drinking,
preparing food, bathing washing, recreation, and cooling. Any solute or cause of change in physical properties that renders
water unfit for a given use. (Generally considered synonymous with pollutant).
Critical Conditions of Overdraft
A groundwater basin in which continuation of present practices would probably result in significant adverse overdraft-related
environmental, social, or economic impacts. The definition was created after an extensive public input process during
the development of the Bulletin 118-80 report.
Groundwater Basin
An alluvial aquifer or a stacked series of alluvial aquifers with reasonably well-defined boundaries in a lateral direction and having a definable bottom.
Groundwater Table
The upper surface of the zone of saturation in an unconfined aquifer.
Hydraulic Conductivity
A measure of the capacity for a rock or soil to transmit water; generally has the units of feet/day or cm/sec.
Overdraft
The condition of a groundwater basin in which the amount of water withdrawn by pumping exceeds the amount of water that recharges
the basin over a period of years during which water supply conditions approximate average conditions.
Permeability
The capability of soil or other geologic formations to transmit water. See hydraulic conductivity.
Porosity
The ratio of the voids or open spaces in alluvium and rocks to the total volume of the alluvium or rock mass.
Potentiometric Surface
A hypothetical surface representing the level to which groundwater would rise if not trapped in a confined aquifer (an aquifer in which
the water is under pressure because of an impermeable layer above it that keeps it from seeking its level). The potentiometric surface
is equivalent to the water table in an unconfined aquifer.
Recharge
Water added to an aquifer or the process of adding water to an aquifer. Ground water recharge occurs either naturally as the net gain from precipitation, or artificially as the result of human influence. See artificial recharge.
Safe Yield
The maximum quantity of water that can be continuously withdrawn from a groundwater basin without adverse effect.
Specific Yield
The ratio of the volume of water a rock or soil will yield by gravity drainage to the total volume of the rock or soil.
Transmissivity
The product of hydraulic conductivity and aquifer thickness; a measure of a volume of water to move through an aquifer. Transmissivity generally has the units of ft2/day or gallons per day/foot. Transmissivity is a measure of the subsurface's
ability to transmit groundwater horizontally through its entire saturated thickness and affects the potential yield of wells.
Unconfined Aquifer
An aquifer which is not bounded on top by an aquitard. The upper surface of an
unconfined aquifer is the water table.