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Groundwater Definitions

Note: The following definitions are not for regulatory purposes.

Aquifer: An underground geological formation able to store and yield water (see artesian, confined, and unconfined aquifers).

Artesian (or confined) aquifer: exist where the groundwater is bounded between layers of impermeable substances like clay or dense rock. When tapped by a well, water in confined aquifers is forced up, sometimes above the land surface.

Artesian well: A well tapping a confined (or artesian) aquifer. Water in the well rises above the top of the aquifer under artesian pressure, but does not necessarily reach the land surface. When a well in which the water level is above the land surface (see "Pontiometric surface"), a natural flow of water out of the well occurs. When water flows above the land surface the well is defined as a flowing artesian well.

Cone of depression: The zone around a well in an unconfined aquifer that is normally saturated, but becomes unsaturated as a well is pumped, leaving an area where the water table dips down to form a cone shape. The shape of the cone is influenced by porosity and the water yield or pumping rate of the well. The land surface overlying the cone of depression is referred to as the area of influence.

Confining layer: Geologic material with little or no permeability or hydraulic conductivity. Water does not pass through this layer or the rate of movement is extremely slow.

Decommissioning a well: An environmentally sound process of sealing a well that is no longer being used in order to prevent groundwater contamination and harm to people and animals. "Well abandonment" can be a synonymous term unless it is used to describe a situation in which the use of a well is discontinued and the well is ignored or improperly sealed.

Drawdown: A lowering of the groundwater level caused by pumping.

Image depicting groundwater drawdown terms

Groundwater: Water found in the spaces between soil particles and cracks in rocks underground located in the saturation zone. Cracks in rocks can be due to joints, faults, etc. Groundwater is a natural resource that is used for drinking, recreation, industry, and growing crops.

Hydraulic conductivity: a measure of a material's capacity to transmit water. It is independent of the thickness of that material. See "Transmissivity" for related definition. Another term for hydraulic conductivity is "coefficient of permeability".

Impermeable layer: A layer of material (such as clay) in an aquifer through which water does not pass or passes extremely slowly.

Induced recharge: The recharge to an aquifer that occurs when a pumping well creates a cone of depression that lowers an adjacent water table below the level of a stream or lake, causing the stream or lake to lose water to the adjacent groundwater aquifer.

Infiltration: Flow of water from the land surface into the subsurface.

Infiltration rate: The quantity of water that enters the soil surface in a specified time interval. Often expressed in volume of water per unit of soil surface area per unit of time.

Karst: A geologic formation of irregular limestone deposits that dissolve to form sink holes, underground streams, and caverns.

Groundwater Mining: Withdrawal (removal) of groundwater over a period of time that exceeds the recharge rate of the supply aquifer. Also referred to as overdraft or mining the aquifer.

Monitoring well: A non-pumping well, generally of small diameter, that is used to measure the elevation of a water table or water quality. A piezometer, which is open only at the top and bottom of its casing, is one type of monitoring well.

Perched aquifer: Localized zone of saturation above the main water table created by a laterally limited layer of underlying impermeable material.

Permeable/Permeability: Capable of transmitting water (porous rock, sediment, or soil); the rate at which water moves through rocks or soil.

Permeable layer: A layer of porous material (rock or unconsolidated sediment); in an aquifer, the layer through which water freely passes as it moves through the subsurface.

A cross-section diagram showing the precipitation seepage into the groundPore space: Openings between geologic material found underground. Also referred to as void space or interstices.

Porosity: The ratio of the volume of void or air spaces in a rock or sediment to the total volume of the rock or sediment. The capacity of rock or soil to hold water varies with the material. For example, saturated small grain sand contains less water than coarse gravel.

Potentiometric surface: The potential level to which water will rise above the water level in an aquifer in a well that penetrates a confined aquifer; if the potential level is higher than the land surface, the well will overflow. See "Artesian well" and "Confined aquifer".

Pumping test: Evaluation of an aquifer by "stimulation" through controlled pumping and observing the aquifer's "response" (drawdown) in the production and observation wells. See NYSDEC protocol for pumping tests. Also "aquifer test". As opposed to "pump test" which measures the performance of a pump.

Recharge: Water added to an aquifer. For example, when rainwater seeps into the ground. Recharge may occur artificially through injection wells or by spreading water over groundwater reservoirs.

Recharge rate: The quantity of water per unit of time that replenishes or refills an aquifer.

Saturated thickness: Total water-bearing thickness of an aquifer.

Saturated zone: Located immediately below the unsaturated zone (see definition below) where the pores are totally saturated with water. Same as "groundwater".

Spring: The emergence of groundwater at the land surface, usually at a clearly defined point; it may flow strongly or just ooze or seep out.

Transmissivity: A measure of the capability of the entire thickness of an aquifer to transmit water. In other words, it is product of hydraulic conductivity and aquifer thickness. Also "coefficient of transmissivity". Technically defined as the rate of flow at which water is transmitted through a unit width of the saturated thickness an aquifer under a unit hydraulic gradient. In the English Engineering system it is flow in gallons per minute through the vertical section of an aquifer one foot wide and extending the full saturated height of an aquifer under a hydraulic gradient of 1.

Unconfined aquifers: An aquifer in which the water table is at or near atmosphere pressure and is the upper boundary of the aquifer. Because the aquifer is not under pressure the water level in a well is the same as the water table outside the well.

Unsaturated zone: Located immediately below the land surface where the pores contain both water and air, but are not totally saturated with water. Plant roots can capture the moisture passing through this zone, but it cannot provide water for wells. Also known as the unsaturated zone or vadose zone.

Watershed: The land area from which surface runoff drains into a stream, channel, lake, reservoir, or other body of water. Also called a drainage basin, watersheds may encompass an area of millions of square miles or as little as a few acres. They are often considered to be more of a local definition.

A drawing shows the wellhead protection arear by distances
Wellhead Protection Area

Water table: The top of an unconfined aquifer; indicates the level below which soil and rock are saturated with water. See "potentiometric surface" for definition of water level of confined aquifer.

Well: A bored, drilled or driven shaft, or a dug hole whose depth is greater than the largest surface dimension and whose purpose is to reach underground water supplies to inject, extract or monitor water.

Wellhead protection area: A protected surface and subsurface zone surrounding a well or well field supplying a public water system to keep contaminants from reaching the well water.

Wetlands: Lands where water saturation is the dominant factor in determining the nature of soil development and the types of plant and animal communities. Other common names for wetlands are sloughs, ponds, and marshes.

All definitions after The Groundwater Foundation.

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