Quantifying Recharge to the Mississippi River Valley Alluvial Aquifer from Oxbow-Lake-Wetland Systems

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

Irrigation-related groundwater withdrawals have caused declining water levels in the Mississippi River Valley Alluvial Aquifer (MRVAA) since the late 1920s. To manage this resource, recharge sources must be quantified. This study examines recharge through oxbow lakes, which are numerous in the Mississippi Delta. Previous investigations at Sky Lake, an ancient Mississippi River oxbow with an associated wetland, near Belzoni, Mississippi, suggest that oxbow wetlands may contribute significant recharge to the MRVAA. Multiple methods using geologic, hydrologic, and temperature data are being employed to identify and quantify recharge from the Sky Lake oxbow lake-wetland system. Two wetland soil cores were collected to depths of approximately 7 m, encountering 6 m of clay and silt before penetrating into sands and gravels. Monitoring of MRVAA water levels in two piezometers in the wetland and nine monitoring wells in Sky Lake’s vicinity began during the 2016 drought and will continue through the rainy season to track groundwater responses. The potentiometric surface will be mapped to identify possible groundwater mounding beneath the lake, which would indicate vertical recharge. Preliminary results from mid-December, 2016, indicate a general groundwater flow direction to the west beneath the lake. Wells are outfitted with temperature-recording data loggers at specific depth intervals. The groundwater temperature profiles have the potential to differentiate localized wetland-recharge from regionally distributed infiltration, or from recharge from the nearby Yazoo River. Soil temperatures 30 and 60 cm below ground at ten points in the wetland are also being monitored over time to characterize small-scale variations in downward flux. Preliminary results are consistent with earlier work indicating preferential flow pathways through the fine-grained bottom sediments due to an abundance of buried trees and limbs in various stages of decomposition.

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