USGS Releases Results of Gem County Groundwater Quality Study

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BOISE, Idaho — The entire population of southwestern Idaho’s Gem County depends on aquifers for its drinking water. The water in those aquifers is of generally good quality according to a U.S. Geological Survey report released today.

The study was prompted by concerns that rapidly growing population and changes in land use, including potential natural gas exploration and development, might affect the quality of the county’s drinking-water supplies. As a non-regulatory agency with nationwide experience in this type of study, the USGS was uniquely positioned to help.

USGS scientists analyzed water samples from 47 domestic and municipal wells, one spring and two surface-water sites on the Payette River collected during the fall of 2015. Results included:

• Arsenic: Samples from six wells contained arsenic at concentrations greater than the U.S. Environmental Protection Agency’s maximum contaminant level, or MCL, of 10 micrograms per liter, the maximum arsenic concentration detected was 42 micrograms per liter.
• Coliform bacteria: Four groundwater samples contained coliform bacteria, both total coliform and were detected in the two Payette River samples.
• Fluoride: Samples from two wells had fluoride concentrations greater than the EPA MCL of 4 milligrams per liter.
• Methane: Naturally occurring methane was detected in 36 wells. There is no drinking-water standard for methane.
• Nitrate: Only one sample slightly exceeded the IDEQ nitrate threshold of 5 milligrams per liter.
• Pesticides: Five triazine-class herbicides were detected in samples from five wells, none of the concentrations were greater than applicable EPA MCLs.
• Volatile organic compounds: Three VOCs were detected in samples from five wells at concentrations well below EPA MCLs; toluene, a common industrial chemical and solvent, tetrahydrofuran, a common solvent and carbon disulfide, which can have a natural source or may be manufactured for use as a solvent.

These results provide a baseline for future monitoring of the county’s water resources. The USGS used cooperative matching funds to conduct the study with Gem County and the Idaho Department of Environmental Quality. All data from the study are publicly available from the USGS National Water Information System database and are provided in the report.