NUMERICAL SIMULATION OF ORGANIC CARBON, NITRATE, AND NITROGEN ISOTOPE BEHAVIOR DURING TRANSPORT IN A RIPARIAN ZONE AQUIFER

Riparian zones have been the focus of previous field investigations in which the objective has been 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. Consistent with an approach often expounded by Ed Sudicky that mathematical models can be used to interpret field data and increase our understanding of fundamental processes, in this study we apply a reactive transport model to simulate organic carbon, nitrate, and nitrogen isotope behavior during transport in a river riparian zone. In addition to multicomponent reactive transport, the model computes nitrogen isotope fractionation during biogeochemical reactions.  Simulation results have been compared to field data from a well-characterized river riparian aquifer in southern Ontario and the comparison shows that there is good agreement for major reactive species, including delta¹⁵N. 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 delta¹⁵N, declining DOC, and declining groundwater NO₃^--N concentrations all occur within a relatively discrete (1.5 m thick) zone. The simulated isotope enrichment factors for selected locations within the model domain range from -2 ‰ to -8 ‰, and 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 thickness of the saturated zone increases.