SIMULATION OF NITRATE TRANSPORT THROUGH THE GROUNDWATER/SURFACE WATER INTERFACE INFLUENCED BY TRANSIENT ADVECTIVE FLOW

The ground water-surface water interface (GSI) is an active zone for nitrate removal through denitrification in some low-relief coastal streams. Transient hydrological events may affect nitrate removal by movement of the redox front and by altering the residence time in the microbiologically active zone. Continuous-flow experiments using intact sediment columns were conducted to investigate hydrological controls on nitrate transport across the GSI. The concentration of nitrate in the influent was held constant, and the flow velocity and direction were adjusted to simulate transient hydrological episodes. We observed that the redox sequence along the flowpath moved in response to flow velocity, the active nitrate-removal zone expanded or shrank, and nitrate reduction rates changed. The transient redox processes were quantified by applying a macroscopic numerical model that combined biological and physical processes. The microbial processes were described by nonlinear multiple Monod kinetics. The resulting partial differential equations were solved by the finite-element method. The consistency between the experimental and modeling results indicates that the processes defined and the controlling parameters chosen describe the coupled interactions between transport and biogeochemical reactions for the studied system.