Extent and Sources of Nitrate Contamination of Drinking Water in an Agricultural Watershed in Southwest Ohio

Elevated nitrate concentrations have been reported from municipal drinking water wells in the Twin Creek watershed in southwest Ohio, where residents depend on ground water as the sole drinking-water source.  While the nitrate problem has been persistent in this rural agricultural area, the sources and spatial extent of nitrate contamination were unknown.  The extent and spatial and temporal distribution of contamination were investigated and compared with well characteristics, site characteristics (including distance from potential sources) and hydrogeology.  Samples were collected throughout the watershed from domestic wells (25 and 36 samples in April and July 2008, respectively) and streams (8 and 16 samples in April and July).  Samples were analyzed for temperature, pH, conductivity, dissolved oxygen, total coliform and E. coli bacteria and major cations and anions.  Isotopic analyses of δ ¹⁸O and δ¹⁵N in nitrate were used to determine nitrate sources, suspected to be septic systems or agricultural applications.  δ¹⁸O and δD in water were also measured at select sites to investigate ground-water and surface-water interactions that could influence nitrate concentrations in ground water.  In April nitrate concentrations ranged from 0 - 8.09 mg/L with a mean of 0.89 mg/L).  In July, concentrations ranged from 0 - 6.52 mg/L with a mean of 1.34 mg/L.  Nitrate concentrations were higher in July than in April in surface water, after fertilizer application.  The highest ground-water concentrations were clustered near the municipality.  In July, nitrate concentrations were inversely correlated to the total well depth and to the depth to the water table.  These relations did not apply in April.  Nitrate sources inferred from isotope data will be compared to the spatial distribution of contamination to provide insights in the best approaches for reducing nitrate contamination in the future.