Oxbow Lake-Wetland Systems as a Source of Recharge to the Mississippi River Valley Alluvial Aquifer

Author(s): Gratzer, M.; Davidson, G.; O'Reilly, A.; Rigby, J.

This project investigates whether the Sky Lake oxbow lake-wetland system contributes significant recharge to the Mississippi River Valley Alluvial Aquifer through preferential flow pathways created by tree limbs and roots embedded in the wetland bottom sediment. The problem is being investigated by monitoring water levels in 11 wells in and around the Sky Lake oxbow lake-wetland system. These data are being used to determine the shape of the potentiometric surface and how the aquifer responds to precipitation and surface water level changes (changes in surface-water/groundwater head difference) at different locations. Temperatures at various depths in these wells are also being monitored to see how subsurface temperatures respond to air and surface-water temperature changes at different locations, thereby potentially allowing inference of different recharge sources. Soil temperatures are being measured at two locations in the wetland at 30 and 60 cm belowground and analyzed for evidence of spatial heterogeneity. Generally, the potentiometric surface is relatively flat upgradient (eastward) of the oxbow and steeper downgradient (westward) of the oxbow. It is also curved, forming a possible groundwater ridge. Hydrographs of four wells are consistent with vertical recharge beneath the lake raising the groundwater level beneath the lake. Consequently, groundwater backs up, causing a decrease in the hydraulic gradient inside and upgradient of the meander loop and an increase in the hydraulic gradient outside and downgradient of the meander loop. The hypothesis that preferential flow paths convey wetland surface water into the subsurface is supported by the temperatures recorded at 30 and 60 cm belowground, because the trends shown by these soil thermistors vary spatially.

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