Modeling of Biogeochemical Responses to Hydrologic Transitions in Floodplain Aquifers

In the intermountain West, large shifts in hydrologic conditions associated with seasonal snowmelt trigger important fluctuations in the biogeochemical functions of alluvial aquifers. Such alluvial aquifers often host contamination derived from human activities or geogenic sources. The presence of fine-grained, organic-enriched sediments (i.e. naturally reduced zones, NRZs) further mediates water quality, including the mobility of redox-sensitive contaminants like U or As, by acting as sinks or as secondary sources. In this work, we develop a reactive transport network describing cm-scale NRZs embedded in a more permeable sand, based on a set of column experiments. We show that diffusive transport limitations combined with fast aerobic respiration rates are required to maintain reducing conditions inside the NRZs. In the case where large concentrations of organic carbon are available, reducing conditions tend to spread away from the NRZs into the surrounding sand, creating an inventory of easily accessible reduced species with the potential to be remobilized during future oxygenation events. This inventory is however small relatively to the main NRZ. Overall, our results suggest that, in water saturated conditions, NRZs are resilient to a change of geochemical conditions inside the aquifer, which would have important implications for predictions of water quality. Large functional shifts, as observed on the field, are likely associated with physical mechanisms like partial drying of the sediments.