Of the sample analyses results available, radon was the contaminant more likely to have elevated concentrations. About 55% of the radon analyses results were above 2,000 picocuries per liter (pCi/L), while about 7% of the uranium results were above 30 micrograms per liter (µg/L), two commonly used standards or health advisory levels.

Although a metal commonly found in bedrock around the world, uranium in water can increase the risk of cancer and kidney toxicity when ingested if it exceeds the maximum contaminant level set by the EPA. Radon is a radioactive gas that is a byproduct of uranium radioactive decay. It often seeps into homes from the ground through foundation cracks, but it also dissolves into groundwater. If present in the well water, radon can dissipate from household water-use into the air. According to the World Health Organization, long-term exposure to high concentrations of radon in indoor air increases the risk of lung cancer.

“The Environmental Public Health Tracking Program at NH DHHS is committed to sharing data that drives public health action,” said Dr. Katie Bush, EPHT Program Administrator. “These probability estimates help us identify high-risk areas across the state and focus our outreach and education efforts in places where it will have the most impact.”

The presence of both radon and uranium is partly correlated with the geologic make-up of an area’s bedrock, which is the solid rock that lies beneath loose materials such as soil, clay, sand, till, or gravel.  New Hampshire’s bedrock mostly comprises plutonic igneous rocks, such as granite, and metamorphic rock, which has a high potential for containing uranium and its byproducts, including radon.

Published in the Journal of the American Water Resources Association, this USGS study provides uranium estimates and expands upon previous findings that link the concentration of radon in groundwater with mapped bedrock units.  The study also produced more precise approximations of radon occurrence in groundwater by including additional predictors and more comprehensive statistical analyses. In addition to geology, the researchers considered the long-recognized idea that groundwater quality is impacted by its location within a watershed.

“With groundwater, it’s not just the local watershed that’s defined by the immediate landscape,” said Moore. “Groundwater can travel much more regionally, so as you go deeper into the ground, or closer to a large river, you are more apt to be capturing water from farther away, from a larger groundwater watershed.”

The scientists looked at variables related to the depth of the groundwater flow paths, the distance to a receiving water body, and where the groundwater is located within local and regional watersheds, by using a dataset called the Multi-Order Hydrologic Position. Variables associated with both large and small watersheds showed up as noteworthy predictors for radon presence in the study.

“To me, the way in which the local landscape showed up as significant for radon indicates that the closer the groundwater well is to the local watershed divide, the more likely it is that there will be a higher concentration of radon - other factors being equal,” Moore said. “This is because the groundwater originating from local precipitation picked up by some wells has not been transported underground for very long and has not yet lost its radon from radioactive decay.  Radon has a very short half-life of only 3.8 days - unlike uranium, which has a half-life of 4.5 billion years!”

A lot of the high probabilities for radon and uranium are found in the White Mountains of northern New Hampshire, where the state population density is lower. However, radon and uranium can be at a concentration that is of health concern in any groundwater source in the state, and the only way to be positive of its concentration is through well-water testing.

The State of New Hampshire and the EPA recommend all private well owners test for common groundwater contaminants every three to five years. New Hampshire residents interested in testing their private wells for radon and uranium can find more information on the New Hampshire Department of Environmental Services website. The NH Radon Program within NH DHHS offers free radon in air testing through an EPA funded program, request your free test kit here.

Well water should be treated if uranium concentrations are greater than the EPA standard of 30 micrograms per liter (µg/L) or if radon concentrations are greater than the NH standard of 2,000 picocuries per liter (pCi/L).

The geospatial data used in this USGS study are available for download here.

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