Numerical simulation of organic carbon, nitrate, and nitrogen isotope behavior during denitrification in a riparian zone

Riparian zones have been investigated in order to determine the processes that control the removal of agriculturally derived nitrate from groundwater; however, many gaps exist in our understanding of the functioning of riparian zones as regulators of nitrate fluxes in shallow groundwater. In this study, a reactive-transport model is applied to simulate organic carbon, nitrate, and nitrogen isotope behavior during denitrification in a river riparian zone. The model includes the reactive-transport processes presented by MacQuarrie and Sudicky [J. Contam. Hydrol. 47 (2001) 53], but has been extended to compute nitrogen isotope fractionation during biogeochemical reactions. Numerical simulation results have been compared to field data from a well-characterized river riparian aquifer and the comparison shows that the model captures the essential reactive-transport processes for major reactive species, including δ15N. A surficial peat zone is the main source of dissolved organic carbon (DOC), and the supply of this DOC to underlying sands and gravels by hydrodynamic processes controls the denitrification of upland nitrate flowing into the riparian zone. The model results support the previously proposed conceptual models for denitrification in riparian aquifers in that enriched δ15N, declining DOC, and declining groundwater NO3−–N concentrations all occur within a relatively thin (1.5 m) zone. The simulated isotope enrichment factors for selected locations within the model domain range from −2% to −8‰, and it is noted that these values are significantly lower than the value specified in the model input. This suggests that it would be very difficult to derive accurate isotopic enrichment factors from a limited number of piezometers in such riparian aquifers. Numerical sensitivity analyses show that denitrification can occur at depths as great as 4 m below the ground surface, but the overall nitrate mass removal capacity of the riparian zone will decrease as the groundwater flow system becomes thicker.