Ground Water Glossary
An aquifer is a water-bearing unconsolidated sediment or rock layer that yields water in usable quantities to a well or spring.
- Bedrock consists of consolidated rock that underlies unconsolidated surficial material.
- The depression of heads around a pumping well is known as a cone of depression and is caused by the withdrawal of ground water.
- Confining units or layers of clay rich material separate aquifers and slow the movement of ground water.
- Drawdown is a measure of the reduction in head at a location and is caused by the withdrawal of ground water.
- Water in porespace of subsurface sediments or rock is called ground water.
- The capacity of subsurface materials to transmit ground water is measured by hydraulic conductivity. It is the volume of water that will move in unit time under unit hydraulic gradient through a unit cross-sectional area.
- Hydraulic gradient is the change in head per unit distance measured in the direction of the steepest change.
- Porosity is a measure of the voids in unconsolidated sediments or bedrock. It is the ratio of volume of openings to the total volume of the material.
- The potentiometric surface represents the total head of the ground water in an aquifer. It is the height of a datum plane at which the water level stands in tightly cased wells that tap that aquifer.
- The level in the subsurface where all openings are filled with water is known as the saturated zone.
- Specific capacity is the yield of a well per unit of drawdown.
- Specific yield is the ratio of the volume of water that will drain under the influence of gravity to the volume of saturated subsurface material.
- The volume of ground water released from storage in a unit prism of an aquifer when the head is lowered a unit distance is unitless number referred to as the storage coefficient.
- Total head is the height of a datum plane (e.g. mean sea level) of a column of ground water. It is the sum of elevation head and pressure head.
- Transmissivity is the rate at which ground water is transmitted through a unit width of an aquifer under a unit hydraulic gradient. It also equals the hydraulic conductivity multiplied by the aquifer thickness.
- The water table is the surface at which the hydraulic pressure of ground water equals atmospheric pressure.