USGS: Elevated geogenic contaminants common in drinking water aquifers across the US

Groundwater present in aquifers is a source drinking water for almost 150 million people in the United States. However, the use of groundwater for drinking is often limited by the presence of geogenic contaminants— elements that occur in groundwater because of natural water–rock interactions.

Some geogenic contaminants, such as manganese, molybdenum, and sulfate, are essential for human health in small quantities. But at high concentrations, these contaminants can damage organs, cause cancer, or impair reproductive function.

A multi-decadal analysis of geogenic contaminants across the lower 48 states arms water-resource managers and public health professionals with new information for protecting the health and safety of communities who rely on groundwater.

Elevated, increasing, and all mixed up

Between 1988 and 2022, elevated concentrations (exceeding half of an established human-health benchmark) of geogenic contaminants were widespread in groundwater wells across the country. Some of the most commonly elevated contaminants included arsenic, lithium, radium, sulfate, and uranium. Elevated concentrations of these contaminants are particularly concerning for the 37 million people who use domestic wells for their drinking water, as domestic wells are typically not regulated and require individual users to monitor and treat their own water. This study represents groundwater quality for about 5 million people. According to the USGS analysis, more than 2.3 million Americans with domestic wells may be exposed to elevated concentrations of one or more geogenic contaminants. Although this study provides unprecedented national-scale information, its results represent water quality for only a small portion of domestic well users nationwide (5 of 37 million).

Overall, increasing concentration trends were more common than decreasing trends for most geogenic contaminants. However, concentrations and trend patterns differed by aquifer type, constituent geochemistry, groundwater geochemistry, well depth, and land use. The drivers of chloride concentrations, for instance, varied regionally, with road-salt application likely driving increased chloride in the northeastern U.S. compared to evaporative processes driving concentration increases in the western U.S.

Overall, increasing concentration trends were more common than decreasing trends for most geogenic contaminants

In domestic well networks, many contaminants had elevated concentrations that affected 90 thousand people or more. Some contaminants also saw increasing trends while others saw more stable dynamics. For example, concentrations of lithium, manganese, radium, and uranium tended to increase through time, whereas concentrations of fluoride and molybdenum remained stable.

Mixtures of elevated concentrations of two or more contaminants were common, and the complexity of mixtures increased over the study time span. The regulated contaminants arsenic and uranium were present in many of the most common mixtures, indicating an emerging threat to human health. Human-health benchmarks do not account for the effects of contamination mixtures, and it is unknown whether exposure to multiple contaminants has adverse, additive human-health effects.

Signs of stability

While there were overall more increasing concentration trends, there were some contaminants that showed a more promising story. For example, only one well network showed significant increases in fluoride, with the rest having either decreasing trends or stable values over time. On top of that, only about 1% of sites had elevated or high concentrations of fluoride. Fluoride helps prevent tooth decay in small amounts but is harmful for bone health at high concentrations.

People’s role in groundwater quality

Human activities such as groundwater extraction and land-use change can affect the geochemistry of groundwater, ultimately altering concentrations of geogenic contaminants. Concentrations of trace elements like arsenic, manganese, strontium, and lithium are more likely to be a concern in groundwater than in surface water. That’s because when groundwater moves through the rocks and sediments that make up an aquifer, some of the minerals in or adhered to those rocks and sediments are released into the water. Humans affect the release of geogenic contaminants when they modify groundwater flow patterns or geochemistry through activities such as groundwater extraction or accelerated aquifer recharge.

For example, summer irrigation of crops and urban landscapes has caused shallow, young groundwater that contains high concentrations of uranium to migrate deeper to drinking water aquifers. Such is the case in eastern San Joaquin Valley in California, where uranium concentrations in groundwater commonly exceed federal and state drinking water standards. Studies such as these are essential to understanding the potential long-term effects of groundwater development and irrigation-supported agriculture on water quality in arid and semiarid regions around the world.