Multiple Geochemical and Isotopic Approaches for Assessing Ground Water NO3 Elimination in a Riparian Zone


Corresponding author, present address: EAWAG, Limnological Research Center, 6047 Kastanienbaum, Switzerland.


During the last 40 years, nitrate (NO3) has become one of the most common ground water contaminants. Stream riparian zones are considered important ecological ecotones that decrease the NO3 load of ground water discharging into streams. This study uses NO3/Cl ratios, natural abundances of15N and18O in NO3, and an in situ15NO3 tracer experiment to evaluate NO3 reducing processes occurring in ground water within a narrow grassed buffer strip bordering a stream in an agricultural watershed. The NO3/Cl ratios (1.9-0.0 [M/M]) indicate that both NO3 consuming processes and mixing of two ground water flow regimes with different NO3 loads contribute to the drop observed in ground water NO3 concentrations within the riparian zone. δ15N and δ18O of the ground water NO3 within the riparian zone (δ15N = 5.1 to 48.8 %c; δ18O = 1.1 to 17.8 %c) were enriched compared to the ground water below the adjacent cultivated field (δ15N = 3.8 to 10.1 %0; δ18O = 0.5 to 6.2 %c). A significant linear relationship (r2=0.97) between ground water δ15N and δ18O in NO3 was found, which is consistent with NO3 consumption by microbial denitrification. The estimated enrichment factors for15N are a factor of 1.5 higher than for18O. The in situ15NO3 tracer experiment conclusively confirmed that denitrification is occurring within the ground water of the riparian zone and demonstrates that denitrification rates can be directly measured in situ.