Water Quality Responses to Seasonal, Hydrologically-driven Redox Cycling in a Floodplain Aquifer

Abstract:

Quality of surface and ground water is dependent on the biogeochemical processes that control contaminant mobility. Biogeochemical cycling and hydrologic variability are especially strong in aquifer systems that experience extreme seasonal shifts in precipitation. Insight into biogeochemical-hydrologic process coupling in the capillary fringe, where gradients are sharp and transient, is critical to understanding feedbacks between hydrologic drivers and groundwater quality. Seasonal saturation of the capillary fringe forms transiently reduced zones (TRZs) accompanied by dissolution/precipitation of iron minerals, carbonates and sequestration or release of uranium (U). Characterized by 1 to 2 wt % organic carbon and low hydraulic conductivity, TRZ sediments have a high capacity for anaerobic microbial metabolisms due to slow diffusion and rapid consumption of oxygen. Microbially-driven iron (Fe) and sulfur (S) cycling likely play key roles in controlling the mobility of redox-sensitive contaminants by mediating the reducing potential of the system as well as producing and consuming complexing agents like Fe-colloids and organic matter. To support quantitative mechanistic models, our investigation links temporal and spatial redox-driven mineral transformations to water quality indicators.

Depth-resolved porewater compositions of a floodplain aquifer system over the course of a full runoff-to-drainage cycle coupled with hydrologic data, reveal coincident shifts in water level, redox potential, and contaminant concentrations. Runoff events that saturate the aquifer induce upward propagation of a reducing front through the soil profile within weeks of the flood event. High U concentrations correlate with low Fe concentrations, and are highest in the upper half of the profile where evapotranspiration concentrates U in evaporite minerals. Notably, U concentrations in porewater remain above 500 μg/L for all times and depths sampled, even when Fe and S reducing conditions exist. This observation belies the paradigm that U will be immobilized in reducing environments. Further, sharp redox gradients over spatial scales of inches demonstrate that biogeochemical processes in the TRZ profile are linked to, but not governed by the chemical potential of the water supplied from the aquifer below.