Generally, the ranges in nitrogen isotope values for synthetic fertilizer, soil nitrogen, and human and animal waste differ from each other. These differences in isotope values make it possible to use nitrogen isotopes to identify nitrate sources. However, identifying nitrate sources based on nitrogen isotope measurements does have limitations. Some natural processes, such as microbial denitrification in anaerobic water, can alter the initial isotopic composition of nitrate. Assuming that no other processes significantly altered the isotopic composition of the nitrate, then the isotope measurements and knowledge about land use surrounding the wells can be used to identify possible nitrate sources.
Land use around that well consisted of irrigated fields fertilized with animal waste.
Taking into consideration that isotope value, and other chemical measurements made on water from that well, it is likely that the nitrate was derived from animal waste, further indicating that agriculture has affected water quality in the Ogallala Formation. The tritium, pesticide, nitrate, and isotope data demonstrate the benefit of developing multiple lines of evidence to characterize the effects of land use on water quality. The tritium and pesticide data indicate that recently recharged water was present in the Ogallala Formation. The pesticide, nitrate, and isotope data indicate that the recently recharged water was affected by agricultural practices. U.S. Geological Survey Circular 1136, p. 24.