Estimating the probability of elevated nitrate (NO2+NO3-N) concentrations in ground water in the Columbia Basin Ground Water Management Area, Washington

Logistic regression was used to relate anthropogenic and natural factors to the occurrence of elevated nitrate concentrations in the Columbia Basin GWMA and develop models that predict the probability that nitrate concentrations in ground water exceed either 3 mg/L or 10 mg/L. Explanatory variables that were considered for inclusion in the models included surficial geology, soils, ground-water recharge rates, land use, presence of canals near the well, presence of irrigation near the well, well depth, well diameter, depth to the bottom of well casing, and the amount of fertilizer applied annually near the well. When each variable was individually related to elevated nitrate concentrations, most were significantly related to nitrate concentrations above 3 mg/L and about half were significantly related to nitrate concentrations above 10 mg/L. The variables that best predict the occurrence of elevated nitrate in a multivariable model were casing depth and the amount of fertilizer applied within 2 km of a well for the 3-mg/L model, while casing depth, soil hydrologic group, and the amount of fertilizer applied within 3 km of a well make up the variables in the 10-mg/L model. Maps showing the predicted probability for the occurrence of elevated nitrate concentrations indicate that the irrigated agricultural regions are most at risk. The predicted depths to which wells need to be cased in order to have an 80-percent chance of obtaining low nitrate ground water exceed 600 feet in the irrigated agricultural regions, while wells in dryland agricultural areas generally need a casing in excess of 400 feet. The predicted depth to which wells need to be cased to have at least an 80-percent chance to draw water with a nitrate concentration less than 10 mg/L generally did not exceed 800 feet, with a 200-foot casing depth being typical of the majority of the study area.