Tools for monitoring and evaluating potential sources of nitrates to ground water, Eastern Idaho
Idaho Department of Environmental Quality
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Ground water is the primary source of drinking water for more that 95 percent of Idaho residents, including public water systems and private wells. Agricultural and domestic fertilizer applications, animal feeding operations and dairies, septic and onsite waste water treatment systems, and industrial wastewater sources are potential sources of nitrate in ground water. Idaho Department of Environmental Quality (IDEQ) has a leading role in monitoring to identify areas of degraded ground water, understand potential nitrate sources, and evaluating best management practices to address those sources. Understandings gained by IDEQ studies at the regional and local level is aimed at protecting public and private drinking water sources, aiding groups such as local soil and water conservation districts to protect ground water quality through improving agricultural land use practices, and helping to restore degraded ground water. The following suggested tools are based on IDEQ-led investigations of ground water in Eastern Idaho and are meant to be used as a resource for other IDEQ regional offices or other decision makers to understand potential regional sources of nitrates to ground water.In this study, monitoring wells are identified within randomly selected square-mile sections for regional-scale (100’s of square miles) characterization. Smaller scale monitoring networks are also identified for subareas sharing similar hydrogeology and land use. Well selection criteria favors newer wells producing from the shallow-most aquifer. The number of sites selected for the subareas is based on the mean and variance of existing nitrate data, which estimates the bounds of confidence and prediction intervals for future sampling. Data collected from each site include: field parameters (pH, temperature, conductivity, dissolved oxygen); bacteria (total coliform, E. coli); nutrients (nitrite + nitrate, ammonia); major anions and cations (Ca, Mg, Na, K, total alkalinity, Cl, SO4, F); nitrogen isotopes (d15NNO3, d18ONO3); characteristic tracers (Br, B); isotopes of water (d18OH2O, d2HH2O); and tritium.Data analysis includes the following: 1) Review field parameters to establish the general chemistry and to identify if conditions are oxidizing or reducing, and to confirm that the water sampled is representative of the aquifer; 2) Review bacteria concentrations, specifically E. coli, as a direct indication of human or animal wastes; 3) Plot characteristic ratios of major ions and mixing plots of major ions, tracers, and nitrate or nitrogen isotopes to confirm recharge and nitrate source mixing and to identify signatures of potential nitrate sources; 4) Prepare piper trilinear diagrams in order to show mixing between major recharge and contamination sources; 5) Plot characteristic ratios of major ions, nitrates, tracers, and nitrogen isotopes to provide signatures of potential recharge nitrate sources; 6) Employ mixing plots to identify significant indicators and mixing between both recharge and contaminant sources; 7) Dual isotope plots of nitrogen and oxygen of nitrates are used to identify original nitrate sources and distinguish nitrification and denitrification; 8) Isotopes of oxygen and hydrogen in water are used to confirm timing and source of recharge; and 8) Tritium age dating provides the age of recharge.