Volume 30 No. 2 of the Renewable Resources Journal features “An Assessment of Aquifer Water Quality,” an article that identifies the various contaminants in groundwater that have human-health and ecological consequences. This article is adapted from a 2015 report by the U.S. Geological Survey (USGS), Circular 1360, Water Quality in Principal Aquifers of the United States, 1991-2010.
Anatomy of an Aquifer
About 80 billion gallons of water is pumped from U.S. aquifers daily. Groundwater provides one-third of the water that is pumped by public supply systems to provide the water used in homes, schools, and businesses in cities and towns. Overall, about 130 million people currently get their drinking water from groundwater.
The aquifers assessed in the USGS report provide about 90% of the groundwater pumped for public supply, irrigation, and other uses nationally. However, about 43 million people, or 15% of the U.S. population, rely on private wells for their drinking water. Many of these people live in rural areas where there is no other source of drinking water. Groundwater quality is of particular concern for domestic well users because there are no regulations that require routine testing or treatment for contaminants in domestic wells in most states.
Nationally, nearly 80% of the 3,700 wells sampled in drinking water aquifers had concentrations of measured chemical constituents less than human-health benchmarks. Groundwater from 22% of sampled wells contained at least one chemical constituent at a concentration greater than a U.S. Environmental Protection Agency (EPA) Maximum Contaminant Level (MCL), or other human-health benchmark for concentrations in drinking water. Most of these contaminants were from geologic sources. Nitrate is the only constituent from manmade sources that exceeded its human-health benchmark in more than 1% of wells. Some of the potential health effects associated with elevated concentrations of these contaminants include increased risk of cancer; various neurological, developmental, and reproductive effects; liver problems; and blue-baby syndrome. Sixteen contaminants—11 from geologic sources and 5 from human sources—accounted for nearly all (98%) of the instances in which concentrations were greater than human-health benchmarks.
As groundwater flows, it reacts with the minerals, rocks, and sediments that make up the aquifer and soil. Chemical constituents from these geologic sources are released into the groundwater. These contaminants include the following:
- Arsenic occurs naturally as a trace component in many rocks and sediments, and it can also be released as a result of human activities. Arsenic was detected in nearly half of the wells sampled in parts of aquifers used for drinking water; concentrations were greater than the MCL in 6.7%. Arsenic is toxic at low levels and is a known carcinogen.
- Radionuclides in groundwater are primarily from geologic sources and include isotopes of uranium, radon, radium, polonium, and lead. Several radionuclides are toxic or carcinogenic.
- Manganese stains plumbing and laundry, and it can cause neurological effects at elevated concentrations. Concentrations were greater than the human-health benchmark in about 7% of sampled wells.
- Dissolved solids determine if water is freshwater or salt water. Dissolved solids can lead to unpleasant taste, higher water treatment costs, accumulation of minerals in plumbing, staining, corrosion, reduced equipment lifespan, and restricted use for irrigation.
Contaminants from Human Activities
Activities associated with agriculture, industry, and urbanization can all contribute various contaminants to groundwater. Additionally, the development of water resources for human purposes alters the flow of aquifers, potentially making those aquifers more vulnerable to human sources of contamination.
- Concentrations of nitrate have increased from synthetic fertilizer use, water disposal, and fossil-fuel combustion. Nitrate can cause blue-baby syndrome, and it stimulates algal and plant growth in lakes, rivers, and oceans. Concentrations were greater than the MCL in about 4% of sampled wells.
- Pesticides and Volatile Organic Compounds (VOCs) are pervasive in modern life, and many of these chemicals are toxic and can pose human-health or ecological concerns. Pesticides were detected in 32% and VOCs in 40% of sampled wells. Concentrations of individual pesticides and VOCs were mostly low, however, and human-health benchmarks were rarely exceeded.
- Atrazine, simazine, prometon, and metolachlor were the most frequently detected pesticides. Agricultural herbicides account for about 80% of the total amount of conventional pesticides in the U.S.
- Chloroform, perchloroethene (PCE), trichloroethene (TCE), and 1,1,1-trichloroethane (TCA) are among the most frequently detected VOCs. Chloroform is used in industry and is formed when water is treated with chlorine. PCE, TCE, and TCA have many commercial and industrial uses, including degreasing and dry cleaning.
- Methyl Tert-Butyl Ether (MTBE) is an oxygenate that was added to reformulated gasoline during the 1980s and 1990s to reduce air pollution. The widespread occurrence of MTBE in groundwater, despite its relatively short history of intense use, illustrates how vulnerable shallow groundwater is to contamination by newly introduced chemicals.
- Fumigants were detected infrequently in groundwater. However, in areas where they were used, some fumigants are still detected in groundwater at concentrations greater than human-health concentrations decades after their use was banned because of health concerns.
Identifying trends in groundwater quality and investigating their causes is essential to helping water managers prepare for the future. Once contaminants in groundwater reach levels that impair its use, restoration is difficult, costly, and can take decades, if such restoration is even possible.