MASS TRANSPORT BETWEEN SURFACE WATER AND GROUNDWATER: THE INFLUENCE OF DISPERSIVE MIXING

In humid regions a strong coupling between surface water bodies and groundwater systems exists. In these environments the exchange of water flux is dependent on the hydraulic gradient between the reservoirs but mass transport between the reservoirs is additionally reliant on the concentration gradients. We hypothesize that daily changes in river stage associated with anthropogenic water releases (such as those from a hydroelectric dam) cause anomalous mixing in the region of adjacent aquifer material by creating large hydraulic head gradients in the aquifer region adjacent to the stream. Field observations of hydraulic head reversals in a shallow aquifer are presented. Hydraulic conditions observed in the field are explicitly reproduced in a physically-based 2-dimensional numerical model of groundwater flow coupled through a simplistic surface water boundary condition. The important physical processes observed in the field are reproduced in the numerical model. Mass transport simulations of a conservative solute introduced into the surface water are performed and examined relative to a stream conditions without stage fluctuations. Simulations of 20 days for both fluctuating river stage and steady high river stage show more mass is introduced into the aquifer from the stream in the oscillating case by a factor of 2. This is the case even though no net water flux is driven into the aquifer for the oscillating case. Velocity and concentration gradients control this mixing and is dispersive in nature. This excess mixing is likely to be important for understanding dissolved mass transport in near-stream aquifer environments and likely influence hyporheic zone processes.