Overall, the groundwater in Idaho has been found safe for human consumption and other beneficial uses at most of the Statewide Program sites. However, some sites had concentrations of one or more constituents that exceeded the safe levels (known as Maximum Contaminant Levels [MCLs]) that have been established by the U.S. Environmental Protection Agency (EPA) for public water systems. Southern Idaho had a higher percentage of these sites than central and northern Idaho. Nitrate, bacteria, arsenic, fluoride, gross alpha, radon, and uranium are the main constituents found to exceed MCLs.
Nitrate is the most commonly-occurring constituent with elevated concentrations and MCL exceedences. Nitrate in Idaho's groundwater is considered to be caused by activities on the land surface (agriculture, wastewater applications, and septic systems). Five percent of the Statewide sites have had nitrate concentrations over the MCL of 10 milligrams per Liter (mg/L), and 33% of the sites have had impacted levels of nitrate (2 to 10 mg/L). Nitrate trends have been studied using three approaches:
Hydrogeologic Subareas (as defined by the IDWR)
Nitrate Priority Areas (as defined by the Idaho Department of Environmental Quality)
Nitrate has increased in a number of areas—mostly in southern area. The most significant increasing trends have been documented in four Nitrate Priority Areas: Twin Falls, Ada/Canyon, NE Star, and Marsing.
Arsenic has also been studied for patterns and trends. The MCL for arsenic in groundwater is 10 parts per billion. Arsenic has exceeded the MCL at 15% of the Statewide Program sites. Arsenic concentrations in Idaho's groundwater are considered to be naturally-occurring. In Idaho, arsenic occurs commonly in groundwater in certain areas in the southern part of the state, such as the Weiser area (southern Washington County), the Treasure Valley (Ada and Canyon Counties), and Twin Falls County.
Fluoride and Gross Alpha
Other constituents that have results that exceeded the MCLs are fluoride, and gross alpha.
Pesticides have also been detected at some Statewide sites, with Atrazine being the most commonly detected compound. Fortunately, the concentrations of the detected pesticides have been well below any MCLs or Health Advisories.