Nitrogen isotope ratios of ammonium and nitrate ions from soil and water samples can be analyzed reproducibly with an experimental error of approximately +/-1 parts per thousand (ppt). 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 dN15 of +10 ppt to +22 ppt. 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 dN15 of +2 ppt to +8 ppt. In southern Runnels County, the major source of nitrate in ground water is natural soil nitrate.
The isotopic composition of ground-water nitrate beneath cultivated fields corresponds with dN15 of natural soil nitrate. Ground waters beneath farmhouse-barnyard complexes have a higher average dN15, indicating the addition of animal waste nitrate.
Eleven samples of ground water from Macon County, Missouri, have dN15 of +10 ppt to +19 ppt, indicating that the waters are contaminated with nitrate from animal wastes. Nitrates in ground waters from the Upper Glacial aquifer in Queens County, New York, appear to be from an animal waste source, whereas nitrates in ground waters from the Magothy aquifer in Nassau County, New York, appear to be from either natural soil nitrogen or artificial fertilizer.