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Groundwater is any water that is pooled or moving beneath the surface of the earth. In some areas groundwater can be found in continuous flowing formations, rather like an underground river that is continually replenished by rainfall or snow melt. In other areas the groundwater, or aquifer, is more static, like a lake. These static areas are also replenished by water from the surface working its way down into the earth, but the process may be quite slow. As a consequence, some of the water in aquifers may have been there for a long time. For example, some of the water in the Madison Aquifer in the western US has been there for 20,000 years.
Groundwater mining is the process of locating aquifers and pumping the water out of them faster than the aquifer can replenish itself. Groundwater in these slow-refilling aquifers is essentially a nonrenewable resource.
Groundwater mining is the removal, or withdraw, of water in the natural ground over a period of time that exceeds the recharge rate of the supply aquifer. It is also called "overdraft" or "mining the aquifer."
Ground water is contained in specific rock units called aquifers. Water, ultimately from rain or snow, percolates downward directly from rain, or from a river bed or lake bed, through soil, sediment, and rock, following the route of least pressure, to reach a level where it is saturated. It is then ground water, occupying the microscopic spaces between the rock particles in the aquifer. In natural circumstances an aquifer is close to equilibrium in its water content, with recharge balancing outflow.
The water level in a natural aquifer is called the water table. Although it may rise and fall from season to season and year to year, the water table usually varies round some average depth. If the water table reaches the ground surface, water will tend to ooze out, as a natural seep or spring. In the end, every drop of ground water eventually leaves the aquifer by outflow as a natural spring, or as seepage into a lake, river, or the sea, or pumped out of a well; but by that time it has been replaced by other water. Water flow above or below ground follows physical laws that are well understood. In general, ground water flows very slowly compared with the unconfined flows that are familiar in rivers and streams: rates are more of the order of feet per day rather than feet per second.
Even rain water is not pure, and ground water, reacting over long periods of time with the solid and liquids it contacts underground, is likely to be quite impure. Water can dissolve chemicals that it then carries in solution, and it can deposit minerals. In particular, water may have oxygen in solution or not, with very different chemical results. Finally, waters from different sources may mix freely, mingling their load of chemicals as they do so.
Hydrogeology is the study of aquifers and the water contained in them. It is crucial in assessing the impact of human activities on ground water, and in planning for the wise use of water in the future.
The process, deliberate or inadvertent, of extracting groundwater from a source at a rate in excess of the replenishment rate such that the groundwater level declines persistently, threatening exhaustion of the supply or at least a decline of pumping levels to uneconomic depths.pits, ditches, furrows, streambed modifications, or injection wells.
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