HYDRAULIC AND GEOCHEMICAL EVIDENCE OF RECHARGE OF AN ALLUVIAL AQUIFER THROUGH ZONES OF HIGHER VERTICAL CONDUCTIVITY IN AN OXBOW LAKE-WETLAND

The Mississippi River Valley Alluvial Aquifer is one of the most over-drafted aquifers in the US. Withdrawals for agricultural use have resulted in declines in the water table, raising concerns about the long-term sustainability of crop production in the region. To manage this resource, a greater understanding is needed of the sources of recharge. One potential source is from oxbow lakes common to the floodplains of large meandering river systems, though the degree to which they communicate with the aquifer is poorly understood. Fine-grained bottom sediments create a low permeability barrier to vertical flow, but the heterogeneous nature of wetland sediments that surround many oxbows may provide conduits for flow. At Sky Lake, MS, an ancient meander of the Mississippi River, a series of piezometers around the lake monitored groundwater levels in conjunction with changes in lake level, precipitation, and nearby river stage. A prolonged dry spell interrupted by a 4-meter rise in lake level provided a unique opportunity to observe the impact on groundwater. Prior to the rise, the groundwater gradient was uniformly westward. After the lake rise, a clear groundwater mound formed beneath the lake, rising by 2 m at the center and temporarily reversing the groundwater gradient at its eastern edge. Flow returned to a generally westward gradient after lake levels fell below 1 m. In a companion study, redox probes were placed within the Sky Lake wetland 0.3 and 0.6 m below the ground surface at several locations. When water levels were low in the wetland, all probes recorded low redox potentials, consistent with expectations for saturated sediments. When water levels rose above 1 m, isolated locations recorded significant increases in redox potential, suggesting the presence of localized zones of higher vertical hydraulic conductivity. The apparent threshold of 1 m is likely a manifestation of the hydraulic gradient needed to transport oxygenated surface water downward at a rate faster than the rate at which oxygen is consumed by biological demand. Both studies point to the important role oxbows play in recharging the aquifer. These results suggest that maintaining high water levels in oxbows in the winter months may serve to enhance recharge to mitigate against annual groundwater declines.