Numerical Modeling of an Abiotic Hyporheic Mixing-Dependent Reaction: Chemical Evolution of Mixing and Reactant Production Zones

Abstract:

The hyporheic zone is sometimes defined as where upwelling groundwater mixes with surface water. Such mixing may allow reactions dependent on chemicals in both source waters (mixing-dependent reactions) to occur. Mixing-dependent reactions can attenuate contaminants, especially in upwelling flowpaths thus reducing contaminants reaching surface water. Here we used MODFLOW and SEAM3D to numerically simulate a lab experiment involving a mixing-dependent reaction between sodium sulfite in upwelling groundwater and dissolved oxygen (DO) in a hyporheic flow cell induced by a sediment partition. We evaluated changes in the DO mixing zone location and thickness and sulfate production zone location and thickness in response to variations in ratio of flows between hyporheic flow cell and upwelling groundwater (i.e. upwelling groundwater to downwelling surface water), DO and sulfate source concentrations at model boundaries, and kinetic rate. We also analyzed model scenarios of conservative physical transport with the same source location and concentration as the DO to understand how the abiotic reaction influenced the mixing zone that developed. Our results show that DO mixing zones were thin (~1 cm), consistent with previous porescale studies. DO mixing zone location moved upstream as inflow ratio (i.e. upwelling groundwater to downwelling surface water) increased given greater upwelling. The sulfate production zone was consistently downstream of the DO mixing zone location, i.e. near the location of prior reactant consumption. Mixing zone thickness for the conservative model was thicker than the reactive model simulations calibrated to experimental data, indicating that as DO is consumed the DO mixing zone narrows. The DO mixing zone thickness was most sensitive to variations in the kinetic rate. Inflowing DO and sulfate concentrations had minimal or no effect on the mixing zone location and thickness. Sulfate production zone thickness and DO mixing zone thickness decrease as inflow ratio increases. This study provides insight into the chemical compositions found in and around the hyporheic zones. Furthermore, it shows the importance in knowing the location of chemicals to quantify attenuation of contaminants especially those that use DO as an electron acceptor such as degradation of hydrocarbons.