An aquifer is an underground body of water that is associated with a formation of permeable rock, sand or gravel that allows water to pass through. Some aquifers are huge, going on for miles, and others are might be tiny collections of water in otherwise dry, desert environments.
If we ignore the freshwater that is stored in the polar ice caps, aquifers are said to provide over 97 percent of all of the freshwater on the planet.
There are confined aquifers that have an impermeable cover of rock or clay soil, and there are unconfined aquifers that may be at the surface in the form of artesian wells. They are generally associated with riparian zones, where rivers, oceans or other waterways are nearby enough for their waters to work their way through the permeable layers.
The permeable rock and soil covers of confined and unconfined aquifers usually would filter and clean out most surface pollutants or pollutants in incoming water, but some pollutants cannot be filtered out.
The major aquifer contaminants that are not naturally occcuring pollutants include nitrate, saline, water soluble organic compounds and some pathogens, especially fecal pathogens. Fertilizers, large animal husbandry operations and chemical processing are the major culprits.
Surface pollutants come from dumping, leaking, leaching, depositing or incorporating into soil where the surface pollutant goes in a reverse process to add to the water, rather than filter out as the porous rock or sand does. In other words, some pollutants are not filtered out, or attenuated as others are when they work their way through any protective soil and covering rock.
Arsenic is a naturally occurring substance in rocks and soils that is a major pollutant of ground water in the US. A test of 31,500 wells in the United States and Puerto Rico was conducted to find out that in any area of a 50 mile radius, about 25 percent of wells had lower levels and about 75 percent of wells had higher levels of arsenic. The highest level allowed by the government is 10μg/L.
Saline intrusion is another factor of pollution that will occur when too much of an aquifer that is near a saline body of water is extracted, leaving an imbalance that allows the salt water to move into the spaces of the permeable rock before filtering can occur.
Aquifers will also have different levels of other substances, making them either brackish (a mix of saline and freshwater), iron rich, sulfur rich and so on. With deeper wells, there can be the extraction of more substances that would have settled out or been left at lower levels of the aquifer.
What mankind can do is to take more care with what we introduce into the soil, since we depend on the freshwater that is under that soil.