Use of multiple isotope tracers to evaluate denitrification in ground water: Study of nitrate from a large-flux septic system plume

This study explores the use of multiple isotopic tracers to evaluate the processes involved in nitrate attenuation in ground water. delta(15)N and delta(18)O are used to provide information about the role of denitrification on nitrate attenuation, and delta(34)S, delta(18)O, and delta(13)C are used to evaluate the role of reduced sulfur and carbon as electron donors for nitrate reduction. The focus of this study is a zone of significant NO3-1 attenuation occurring in a sand aquifer impacted by septic system contamination. The NO3-1 pattern, the ground water flow system, and changes in other chemical parameters suggest that the NO3-1 depletion is caused by denitrification. This is supported by the nitrate delta(15)N and delta(18)O data which both show significant isotopic enrichment as NO3-1 depletion proceeds along the flowpath, The increase of sulfate and dissolved inorganic carbon observed in the zone of nitrate attenuation suggests that reduced sulfur in addition to carbon is also involved in denitrification. This is supported by a trend toward depleted sulfate delta(34)S and delta(18)O values in the zone of sulfate increase, which reflects the input of sulfate formed by the oxidation of biogenic pyrite present in the aquifer sediments. The trend toward depleted delta(13)C values in the zone of increasing dissolved inorganic carbon reflects the input of organic carbon into this carbon pool. Chemical mass balance indicates that carbon is the dominant electron donor; however, this study demonstrates the effectiveness of using multiple isotopic tracers for providing insight into the processes affecting nitrate attenuation in ground water.