Drought may lead to elevated levels of naturally occurring arsenic in private domestic wells

An estimated 4.1 million people in the lower 48 states are potentially exposed to arsenic levels that exceed EPA’s drinking water standards.

A new U.S. Geological Survey study highlights the importance of homeowners testing their well water to ensure it is safe for consumption, particularly in drought-prone areas. The first-of-its-kind national-scale study of private well water, conducted in collaboration with the Centers for Disease Control and Prevention, showed that drought may lead to elevated levels of naturally occurring arsenic and that the longer a drought lasts, the higher the probability of arsenic concentrations exceeding U.S. Environmental Protection Agency’s standard for drinking water.

Jacks Pond in Hancock, New Hampshire. Groundwater from this area supplies nearby private wells. (Credit: Melissa Lombard, USGS. Public domain.)

Researchers estimate that during drought conditions, 4.1 million people in the lower 48 states who use private domestic wells are potentially exposed to unsafe levels of arsenic. This is an increase of 54% from the estimated 2.7 million people exposed to unhealthy arsenic levels in private wells during normal, non-drought conditions. 

Arsenic is a metal that can occur naturally in bedrock and sediments around the world and is commonly reported in drinking-water supply wells. However, chronic exposure to arsenic from drinking water is associated with an increased risk of several types of cancers, including bladder, lung, prostate and skin cancers. Other adverse effects include developmental impairments, cardiovascular disease, adverse birth outcomes and impacts on the immune and endocrine systems.

The study’s findings can help public health officials and emergency managers notify well owners in areas potentially affected and further refine their strategies for addressing the issue. The EPA regulates public water supplies, but maintenance, testing and treatment of private water supplies are the responsibility of the homeowner. Private well owners can work with their local and state officials to determine the best way to test and, if necessary, treat their water supply.

“The population potentially exposed to arsenic levels exceeding the EPA standard during simulated drought conditions amounts to roughly one-tenth of the estimated 37.2 to 43.2 million people in the conterminous U.S. who use domestic wells for household water supply,” said Melissa Lombard, a USGS hydrologist and lead author of this study.

This is the first national-scale study to assess the potential impact of drought on arsenic levels in private domestic wells. It is also the first to estimate the population of private well users who are potentially exposed during droughts to arsenic levels above EPA’s limits, which are intended to protect human health.

The study also estimated that 2.7 million people are exposed to elevated arsenic levels above EPA standards under normal conditions. This is an increase from a 2017 study by the USGS and CDC that estimated 2.1 million people were exposed to elevated arsenic levels. The increase reflects new estimates of well locations and the population reliant on private wells.

The new study, which did not examine private domestic wells in Alaska or Hawaii, includes maps showing where simulated drought conditions are likely to increase the probability of high arsenic levels and the number of people potentially exposed. 

The states with the largest populations facing elevated arsenic levels in private domestic well water during the simulated drought conditions are Ohio (approximately 374,000 people), Michigan (320,000 people), Indiana (267,000 people), Texas (200,000 people) and California (196,000 people).

Even without drought conditions, relatively large numbers of people are estimated to be exposed to elevated arsenic levels in private domestic well water. Under normal conditions, the largest populations potentially exposed to high levels of arsenic are in Ohio (approximately 241,000 people), Michigan (226,000 people), Indiana (162,000 people), California (157,000 people) and Maine (121,000 people).

This study is the first to explore the potential large-scale impact of drought on naturally occurring arsenic in private drinking water wells,” said Lombard. “While the results suggest that drought will have a negative impact, the study cannot predict what might happen at an individual well, further highlighting the importance of testing.”

The occurrence of arsenic in groundwater is due to a variety of complex interactions, added Lombard. The reasons for the increase in arsenic during drought and as drought persists could vary depending on changes to groundwater flow, alterations in water chemistry and other factors.

Further exacerbating these challenges, climate models predict increasing temperatures and decreasing precipitation in portions of North America during the 21st century. USGS findings suggest that as the duration of drought increases, the probability of arsenic concentrations greater than EPA’s drinking water standard will also increase.

This study used an existing USGS statistical model that predicts the probability for elevated arsenic concentrations in domestic well water. In the new research, scientists used the model to simulate drought conditions by changing precipitation and groundwater levels. The researchers also used data from the drought of 2012, one of the worst on record in the U.S., to investigate how drought duration can impact arsenic levels.