Water Use: Supply - Ground Water
Groundwater quality
Until the 1970s, groundwater was seen to be protected from contamination by the
layers of soil, rock, and sand that acted as filters to trap contaminants. About 10% of all
groundwater public water supply systems are in violation of drinking water standards
because of biological contamination (Citizen's guide to groundwater protection).
Additionally, 74 pesticides, some of them carcinogenic, have been found in the
groundwater of 38 states. Although there have been estimates of the extent of
groundwater contamination they have been difficult to verify because of the nature of the
resource and difficulties in monitoring.
The extent of groundwater contamination depends on many factors, including
water elevation and amount of time it takes for the contaminant to reach groundwater
levels. Because groundwater moves slowly and lacks turbulence, once a contaminant
reaches it, it forms a concentrated plume that flows at the same rate as surrounding
groundwater. As a result, groundwater contamination incidences can go undetected for
years.
Groundwater contamination can be either natural or man-made, however most
sources of contamination are anthropogenic byproducts: hydrocarbons and synthetic
organic chemicals (e.g. solvents, pesticides, petroleum), leachates (from Superfund or
landfill sites, for example), heavy metals or organic decomposition products. Sources of
groundwater contamination are myriad, the principal being: septic tanks, surface
impoundments, agricultural activities, landfills, underground storage tanks, abandoned
wells, accidents or illegal dumping.
Improperly installed or monitored septic tanks threaten groundwater sources
because of their potential to leak bacteria, viruses, household chemicals, detergents,
nitrates and chlorides into the groundwater. “Approximately one fourth of all homes in
the United States rely on septic systems to dispose of their human wastes” (“Cititzens’
Guide to Groundwater Protection”).
Surface impoundments in the form of ponds or lagoons used by businesses and
industry to store, treat and dispose of liquid waste and wastewater also threaten
groundwater sources as their clay or plastic liners can develop leaks.
Agriculture, with its application of fertilizers and pesticides as well as storage
areas for animal waste, threatens groundwater sources with contamination.
Landfills, of which there are approximately 500 hazardous waste land disposal
facilities, and more than 16,000 municipal and other landfills nationwide, threaten
groundwater sources. Although facilities are required to use clay or synthetic liners and
leachate collection systems, older landfills lack adequate containment systems. Under
the Superfund program, sites are being cleaned up by owners, operators, state and federal
government.
Underground storage tanks, of which there are between five and six million in the
U.S., are primarily used to store gasoline and fuel oil. Although the average life span of
each unit is 18 years, units degrade due to environmental exposures. Hundreds of
thousands of groundwater tanks are leaking nationwide.
Rural communities rely disproportionately upon well water and in the past
community water supply systems were more prevalent. As more rural areas attach to
municipal water systems, wells become abandoned and in some cases are direct sources
of groundwater contamination.