For a swath of acreage stretching across portions of Idaho and Lewis counties, a state report ranks this area 15th in the state for nitrate concentrations. This region, the Clearwater Plateau, is one of 35 monitored by the Idaho Department of Environmental Quality (DEQ), with results listed in the 2020 Nitrate Priority Area (NPA) Delineation and Ranking Process report.
In a department statement provided by Kathryn D. Elliott, DEQ ground water coordinator, the data used in in the 2020 evaluation, “indicate nitrate concentrations in the Clearwater Plateau NPA are stable.” The report outlines the techniques used to evaluate and rank Idaho’s nitrate priority areas in order to help federal, state, and local agencies prioritize ground water protection and remediation activities.
The report is open for public comment through April 15.
According to DEQ, nitrate is the most widespread ground water contaminant in Idaho and is commonly found in public water supply systems. Nitrate priority areas were developed to identify aquifers with ground water quality degradation due to nitrate so resources could be directed to those areas to help protect public health.
Sources of nitrate contaminant in water can result from runoff from fertilizer use; leaking from septic tanks, sewage; and erosion of natural deposits. According to DEQ, the state and federal drinking water standard, as well as the Idaho Ground Water Quality standard, for nitrate is 10 mg/L.
DEQ states sensitive populations (babies, persons in poor health and the elderly) can be susceptible to problems from short-term nitrate exposure. For infants younger than 6 months in age, those who drink water containing nitrate in excess of the maximum contaminant level (MCL) could become seriously ill and, if untreated, may die. Symptoms include shortness of breath and blue-baby syndrome.
In 2020, DEQ and the Idaho Ground Water Monitoring Technical Committee revised the list of nitrate priority areas using data collected from 1990 through 2016. The re-evaluation process is designed to use more recent data to assess ground water quality changes in the existing nitrate priority areas and identify new areas with nitrate-degraded ground water. The 419-square-mile Clearwater Plateau area encompasses the communities of Cottonwood, Greencreek, Ferdinand, Craigmont and Nezperce, extending to just north of Fenn and the northern outskirts of Grangeville, encompassing a population of 3,760.
The plateau is one of 17 areas — out of the 35 total monitored — in the state listed as a “moderate-high” nitrate priority. According to the report, 138 sites were sampled (a combination of private wells, public water system wells and dairy sites). Of these, 98 sites (71 percent) found nitrate concentrations equal to or greater than 2 mg/L (milligrams per liter); 61 sites (44 percent) were equal to or greater than 5 mg/L; and 31 sites (22 percent) were equal to or greater than 10 mg/L.
The report lists the 2 mg/L category as the concentration threshhold indicating human-caused impacts. The upper limit for naturally occuring nitrate concentrations is about 2 mg/L. Conentrations in excess of 10 mg/L presents health risks to certain individuals. The 5 mg/L category indicates significant degradation, and public water systems are required to increase monitoring frequency when this level is reached. In its annual Water Quality Report, published in the Idaho County Free Press March 24, the City of Grangeville listed a nitrate concentration of 2.6 mg/L.
While the Clearwater Plateau showed an increasing trend for nitrate concentrations in 2002, the report subsequently showed no trend — up or down — in subseqent monitoring years: 2008, 2014 and 2020.
According to Elliott, the Clearwater Plateau NPA was initiated in Augus 2005 as part of a regional ground water monitoring network. The objective of this longterm groundwater quality monitoring is to determine the effectiveness of implemented projects. “DEQ has a long history working with various partners within the Clearwater Plateau to identify and address nitrate concerns in ground water,” Elliott said. “Most of that work involves developing a ground water management plan (2008), conducting extensive monitoring (initiated in 2005), and identifying and securing federal funding that can be passed through to the local conservation districts.”
According to Elliott, in the Clearwater Plateau area, the Lewis Soil Conservation District manages the implementation of best management practices designed to reduce nitrogen loads that can potentially leach into the ground water.
Past projects in the Clearwater Plateau area have utilized various funding sources such as Clean Water Act 319, State NonPoint Source dedicated funds, Snake River Basin Adjudication, Pacific Coast Salmon Recovery, and Water Quality Program for Agriculture funds. These grants also helped fund cooperative projects with local volunteer landowners to implement a variety of activities that focused both on surface and ground water concerns. Information on these projects is available from the local soil and water conservation districts or the NRCS.