AQUIFER DENITRIFICATION: CORRELATION OF 15N ISOTOPIC ENRICHMENT AND FIRST-ORDER RATE CONSTANTS

Denitrification is the process by which potentially dangerous nitrate is biogeochemically reduced into harmless nitrogen gas. This process is mediated by bacteria in the presence of suitable electron donors, such as organic carbon, sulfide, and ferrous iron. Due to its potential utility in the remediation of contaminated environments, the process of denitrification has received considerable attention. However, a relationship between the rate constant of the reaction and isotopic enrichment of ¹⁵N has received little consideration. A strong correlation between these two factors could lead to a technique to estimate denitrification rates based on ¹⁵N isotopic enrichment. Mariotti, et al. (1982) were the first researchers to investigate this relationship. After observing lab experiments, they proposed the hypothesis that a negative correlation exists between ¹⁵N enrichment and reaction rates. That is, higher rates of denitrification result in lower (less negative) values of isotopic enrichment. This hypothesis was tested using bromide tracer tests carried out in seven different sites across eastern North Dakota and western Minnesota. A decent correlation exists; however, the correlation is a positive one, where the highest denitrification rates resulted in the highest (most negative) enrichment values. Furthermore, significant variability, particularly that of the Larimore site, remains unexplained. This variability could be due to a regional effect or some other cause.  Further research, including investigation of bacteria types, variations due to temperature, and more correlation studies are suggested.