Variable pathways and geochemical history of seepage under the Mississippi River levee: Observations from the 2011, 2015, and 2016 floods

Author(s): Davidson, G.; Voll, K.; Corcoran, M.; Kelley, J.

During flood stage on the lower Mississippi River, water levels on the river side of a levee can be several meters higher than the surrounding land surface, creating steep hydraulic gradients that drive seepage of water beneath the levees. Sand boils form when sediment is eroded and transported to the surface on the opposite side of a levee, leaving open conduits that can compromise the structural integrity of the levee. The flow path of seepage beneath the levee may be deep or shallow, depending on the surficial geology, with deeper flow pathways found where a levee sits on top of low-permeability channel-fill deposits. For levees over the Mississippi River Valley alluvial aquifer, deeper flow pathways may encounter anoxic water with distinct geochemistry, raising the possibility that flow pathways for individual seeps or sand boils can be elucidated based on their geochemical signatures. Exploratory sampling north of Vicksburg, MS, from the river, from relief wells, and from sand boils during the 2011, 2015, and 2016 flood events shows considerable promise. Relief wells and a small number of sand boils had high iron and arsenic concentrations, consistent with deeper water being driven up to the surface. Most of the sand boils analyzed had iron and arsenic concentrations more similar to river water, consistent with shallow pathways through sandbar deposits. Many sand-boil samples also showed evidence of redox reactions during transit, not just simple mixtures of river water and groundwater. In select relief wells and sand boils, sampling was repeated after three weeks of continuous flow (2015), and again during a subsequent flood (2016), to identify short term geochemical and isotopic changes that may occur as flood waters move increasingly into the subsurface. Preliminary results show significant changes in tritium concentration over time.

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