Groundwater initially fell to the earth as rain or snow and then percolated through the soil. It became trapped in underground layers called aquifers, which consist of sand, gravel and rock. Some of the aquifers are located near the surface, and others are far underground. They are irregular in shape, and wells drilled a few hundred feet apart may reach different aquifers. There may be several aquifers under a particular site, and wells may be drilled into any of these.
Extensive regulations control the construction of wells.
In addition, many local health departments require well-water testing before a new well is put into service. However, no regulations govern water quality in private wells. There are no enforceable limits for particular contaminants and no requirements for tests to be made on any regular basis. Often, health agencies will make recommendations as to the suitability of well water based on standards established for public drinking water supplies. These, however, are only recommendations.