Groundwater recharge from an oxbow lake‐wetland system in the Mississippi Alluvial Plain

The Mississippi River Valley Alluvial Aquifer ranks among the most overdrafted aquifers in the United States due to intensive irrigation. Concern over declining water levels has increased focus on understanding the sources of recharge. Numerous oxbow lakes overlie the aquifer that are often considered hydraulically disconnected from the groundwater system due to fine‐grained bottom sediments. In the current study, groundwater levels in and around a 445‐ha oxbow lake‐wetland in Mississippi were monitored for a 2‐year period that included an unusually long low‐water condition in the lake (>17 months), followed by a high‐water event lasting over 4 months before returning to earlier low‐water levels. The high‐water pulse (>4 m rise) provided a unique opportunity to track the impact in the underlying alluvial aquifer. During low‐water conditions, groundwater flowed westward beneath the lake. Following the lake rise, groundwater beneath and near the perimeter responded as quickly as the same day, with more delayed responses moving away from the lake. Within 2 months, a groundwater mound formed near the centre of the oxbow (>3 m increase), with a reversal in the local hydraulic gradient towards the east. Flow returned to a westward gradient when the lake level dropped back below 0.3 m. Analysis of precipitation and nearby river stage could not account for the observed behavior. Recharge to the aquifer is attributed to rising water levels spreading over point bar deposits and into the surrounding forested wetlands where preferential flow pathways are likely to exist due to buried and decomposing tree remains. An earlier study in the wetland demonstrated an increasing redox potential in isolated zones, consistent with the existence of preferential flow pathways through the bottom sediments (Lahiri & Davidson, 2020). Retaining high‐water levels in oxbow lakes could be a relatively low‐cost water management practice for enhancing aquifer recharge. Conventional understanding often considers oxbow lakes and their riparian wetlands to be hydraulically disconnected from the underlying alluvial aquifer due to the fine‐grained bottom sediments. Groundwater levels in and around a large oxbow lake‐wetland in Mississippi indicate that recharge to the aquifer occurs during periods of high‐surface water levels via two processes: diffuse infiltration through point bar deposits and discrete flow in the surrounding forested wetlands where preferential pathways are likely to exist