Water is cycled around the planet – it is evaporated from the sea and land water masses, falls to the ground as rain, sleet, hail or snow, where it collects in rivers, streams and lakes, or permeates the soil to form underground reservoirs of water. Water travels downwards through air spaces in the soil until it is trapped by a layer of impermeable rock or clay, where it forms an aquifer – a large underground body of water that can provide a steady flow of water to the surface via a spring or well point. The point where the soil becomes saturated, or the surface of the aquifer, is referred to as the water table.
Groundwater can be accessed from springs or wells, and used for irrigating agriculture, for industrial purposes, or as a source of drinking water. According to the Safe Drinking Water Foundation, 97% of all liquid freshwater reserves are held as groundwater in underground aquifers, providing a source of fresh drinking water to 1.5 billion people around the world. Groundwater supplies water to 50% of people in the US, with almost all rural households receiving water from wells that pump out groundwater. In addition, groundwater performs a number of ecological functions by naturally replenishing, rivers, lakes, and wetlands, and preventing them from running dry.
While groundwater is protected from many contaminants commonly found in surface water, and soil particles and organic matter act as a natural filter medium, some contamination can still leach through soils to contaminate groundwater held in underground aquifers. Some common contaminants of groundwater include pollutants from septic tanks, landfill sites, gas leaks from underground tanks, and leaching of pesticides and fertilizers used on crops. Another common Pollutant is arsenic, which is a mineral that occurs naturally in rocks and soils that can be leached from the subsurface layers and enter the groundwater within aquifers and wells. Cases of arsenic poisoning from drinking water pumped from wells that utilize water held in underground aquifers has been reported in many countries around the world, including the USA.
The tendency to overexploit underground water reserves means that very often, more water is extracted than what is replenished by precipitation, causing the water table to drop, and the availability of groundwater to diminish. This can severely affect crop production, and consequently food supply, resulting in food shortages. Excessive removal of groundwater due to pumping for irrigation or other purposes can also cause land subsidence. Particularly in rocks composed of fine grain sediments, the groundwater assists in holding up the land above it. When the groundwater is pumped out, there is nothing to support the soil above it, resulting in it subsiding to fill the void that was previously filled with water. The effect of rocks and soil caving in and compacting underground actually reduces the amount of air spaces between the soil particles, which effectively reduces the ability of the soil to hold water in the long term, and the future viability of the aquifer. It is therefore extremely important that our underground water is utilized in a responsible and sustainable manner.