Determining the source of nitrate in groundwater by nitrogen isotope studies
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Nitrogen isotope ratios of ammonium and nitrate ions from soil and water samples can be analyzed reproducibly with an experimental error of approximately ±1⁰/₀₀. Two isotopic ranges of soil nitrate are found in the soils of southern Runnels County, Texas. Nitrate from the decomposition of animal waste nitrogen has a δN¹⁵ of +10⁰/₀₀ to +22⁰/₀₀. The isotopic ratio is controlled by the volatilization of isotopically light ammonia gas during the decomposition of urea in urine. Nitrate derived from the mineralization of organic nitrogen in cultivated soils has a δN¹⁵ of +2⁰/₀₀ to +8⁰/₀₀. In southern Runnels County the major source of nitrate in groundwater is natural soil nitrate. The isotopic composition of groundwater nitrate beneath cultivated 15 fields corresponds with δN¹⁵ of natural soil nitrate. Groundwaters beneath farmhouse-barnyard complexes have a higher average δN¹⁵, indicating the addition of animal waste nitrate. Eleven samples of groundwater from Macon County, Missouri, have δN¹⁵ of +10⁰/₀₀ to +19⁰/₀₀ indicating that the waters are contaminated with nitrate from animal wastes. Nitrates in groundwaters from the Upper Glacial aquifer in Queens County, New York, appear to be from an animal waste source, whereas nitrates in groundwaters from the Magothy aquifer in Nassau County, New York, appear to be from either natural soil nitrogen or artificial fertilizer.