Multiple Geochemical and Isotopic Approaches for Assessing Ground Water NO ₃ ⁻ Elimination in a Riparian Zone

Abstract

Abstract During the last 40 years, nitrate (NO 3 − ) has become one of the most common ground water contaminants. Stream riparian zones are considered important ecological ecotones that decrease the NO 3 − load of ground water discharging into streams. This study uses NO 3 − /Cl − ratios, natural abundances of 15 N and 18 O in NO 3 − , and an in situ 15 NO 3 − tracer experiment to evaluate NO 3 − reducing processes occurring in ground water within a narrow grassed buffer strip bordering a stream in an agricultural watershed. The NO 3 − /Cl − ratios (1.9‐0.0 [M/M]) indicate that both NO 3 − consuming processes and mixing of two ground water flow regimes with different NO 3 − loads contribute to the drop observed in ground water NO 3 − concentrations within the riparian zone. δ 15 N and δ 18 O of the ground water NO 3 − within the riparian zone (δ 15 N = 5.1 to 48.8 %c; δ 18 O = 1.1 to 17.8 %c) were enriched compared to the ground water below the adjacent cultivated field (δ 15 N = 3.8 to 10.1 %0; δ 18 O = 0.5 to 6.2 %c). A significant linear relationship (r2=0.97) between ground water δ 15 N and δ 18 O in NO 3 − was found, which is consistent with NO 3 − consumption by microbial denitrification. The estimated enrichment factors for 15 N are a factor of 1.5 higher than for 18 O. The in situ 15 NO 3 − 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.