Water table

In geology, the water table is a subterranean layer of soil, sand, or other permeable rock[?] which is saturated with water. Most land areas on Earth have some form of water table underlying them, though often at significant depth. Wells drilled down to the local water table are a common source of fresh water, and where the water table reaches the surface natural springs often form.

The water content of a water table, known as an aquifer, usually originates as rain water that sinks into the ground. Small amounts of "new" water are also introduced into aquifers from the interior of the planet, upwelling from the mantle. In some regions the water table contains brine, either due to infiltration of sea water or from salts leached out of local minerals.

Water table conditions are frequently of importance to agricultural irrigation, waste disposal[?] (including nuclear waste), and other ecological issues.

A major water table related problem currently exists in West Bengal, India, and Bangladesh, where the groundwater relied upon by over 1 million people has become contaminated with dangerous levels of arsenic. It is thought that irrigation for rice production since late 1970s resulted in the withdrawal of large quantities of underground water, which caused the local water table to drop, allowing oxygen to enter the ground and touching off a reaction that leaches[?] out arsenic from pyrite in the soil. The actual mechanism, however, is yet to be identified with certainty.