Not All Arsenic Tests are Created Equal

In 2014-2016, the USGS and partners sampled study wells in northeast, northwest and central Minnesota—areas that commonly have elevated arsenic concentrations in well water—and examined the effects of various water-sampling methods for each of the wells. The researchers found that arsenic levels were most reliable when they were filtered, collected from household plumbing instead of from the drill rig pump or collected several months after well construction and installation.

“Improving the reliability of arsenic tests can help protect the health of people who drink well water in Minnesota by ensuring that residents receive the best possible information about the quality of their water,” said Melinda Erickson, a USGS hydrologist and the lead author of the study.

Chronic exposure to high levels of naturally occurring arsenic through drinking water is a human health hazard that can cause certain cancers, skin abnormalities and other adverse health effects. Minnesota state code requires that all new potable drinking wells be tested for arsenic. However, the code does not specify how to best collect samples for testing, and test results can vary depending on which sampling methods are used.

Particles and fine sediments within well water samples can result in inconsistent arsenic concentration measurements. The new study found that reducing the amount of sediments in water samples used for testing can improve the precision and consistency of arsenic measurements for private wells.

“Establishing guidance for drillers that includes specific sampling protocols for the filtration of water samples and/or collection of samples from household plumbing would improve the reliability of information provided to well owners because those samples have less undesirable sediment,” Erickson said.

Public water supplies are regulated by the U.S. EPA, but maintenance, testing and treatment of private water supplies are the sole responsibility of the homeowner. The maximum arsenic level allowed for public water supplies is 10 micrograms of arsenic per liter. In Minnesota, arsenic concentrations exceed 10 micrograms of arsenic per liter in about 11 percent of newly constructed private wells, and in certain counties, more than 35 percent of tested wells exceed the benchmark. 

The USGS partnered with the Minnesota Department of Health on the new study, which is published in the journal Groundwater. The research was funded by the State of Minnesota Clean Water Fund through the Minnesota Department of Health and the USGS Cooperative Matching Fund. The work was also supported by the National Science Foundation Graduate Research Fellowship Program and an internship provided through the Graduate Research Internship Program.