COMMUNICATION OF RISK - URANIUM OCCURRENCE IN GROUNDWATER IN NORTHEASTERN WASHINGTON STATE

A recent study by Kahle and others (2018) made a preliminary assessment of naturally occurring uranium in groundwater relying on historical data and limited sampling. Of the 1,742 historically sampled wells, uranium was detected in 60 percent of the samples and 87 samples (5 percent) exceeded the health standard for uranium. Of the 13 recently (2017) sampled wells, uranium was detected in all samples and 6 samples (46 percent) exceeded the health standard; uranium concentrations in two samples were nearly 40 times the health standard. U.S. Environmental Protection Agency (EPA; 2000) regulates uranium in Group A community water systems at a Maximum Contaminant Level (MCL) of 30 micrograms per liter (µg/L). This health standard was developed to protect from toxic kidney effects of uranium, and to reduce the risk of cancer in those served by Group A systems.

The study area encompasses about 13,000 square miles in the northeastern part of Washington with a 2010 population of about 563,000. Most of the study area is rural with small towns interspersed throughout the region. The geology of northeastern Washington is composed of various types of bedrock overlain in places by unconsolidated sediment. Naturally occurring uranium is associated with granitic and metasedimentary rocks, as well as younger sedimentary deposits. Uranium ore was discovered in Washington in the 1950s and was mined at several locations within the study area.

Most rural residents rely on private wells for their source of drinking water. Although public-water supply operators are required to test their water periodically for uranium and other constituents, no monitoring requirements exist for private wells. Therefore, individual well owners are responsible for the quality and safety of their well water. Unfortunately, many well owners are unaware of the presence of uranium in area groundwater and most private wells go untested unless motivated by personal health concerns. To improve the understanding of uranium in groundwater and reduce exposure and associated health risks, USGS is developing informational products in partnership with health agency personnel to inform well owners about the risk that uranium in area groundwater poses, and to provide guidance on testing for uranium in their wells and water treatment options.