ARS | Publication request: Elemental chemistry of sand-boil discharge used to trace variable pathways of seepage beneath levees during the 2011 Mississippi River flood

Submitted to: Applied Geochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 23, 2012
Publication Date: January 1, 2013
Citation: Davidson, G., Rigby Jr, J.R., Pennington, D., Cizdziel, J. 2013. Elemental chemistry of sand-boil discharge used to trace variable pathways of seepage beneath levees during the 2011 Mississippi River flood. Applied Geochemistry. 28:62-68.

Interpretive Summary: The integrity of the levee system along the Mississippi River is of the utmost concern for the prevention of human loss of life and serious economic damages to communities and cropland. During floods water may bypass levees through subsurface pathways and re-surface through “sand boils” beyond the levee. The pathway taken by the water, whether seeping through the levee or upwelling from deep groundwater, is a major factor in determining whether a sand boil behind the levee is indicative of a potential levee failure. Scientists at the USDA-ARS National Sedimentation laboratory and the University of Mississippi in cooperation with the Yazoo Water Management District collected and analyzed water samples from a number of sites along the levee system north of Vicksburg, MS during the flood of May 2011. Sample sites included sand boils, relief wells, an irrigation well and the river itself. Results of a chemical analysis of the water samples show that a chemical signature can be used to differentiate deep and shallow water flow pathways. This analysis may be developed into a near real-time method to diagnose potential weak points in levees during flood conditions.

Technical Abstract: Water samples were collected from the Mississippi River, from sand boils near the toe of the levee on the Mississippi side of the river, and from actively flowing relief wells shortly after peak stage of the 2011 Mississippi River flood. Two distinct pathways for seepage under the levee were identified based on the elemental composition of water samples. Fe, Mn, and As proved particularly useful to differentiate shallow and deep seepage pathways. The elemental composition of sand boil discharge was similar to water from relief wells only at a location where the levee sits on channel fill deposits. Seepage at this site is forced under the channel fill, forcing deeper aquifer water to the surface. Where the levee sits on point bar deposits, shallow lateral flow beneath the levee is unimpeded. The chemical composition of discharge from sand boils at these sites was clearly not just river water, nor a simple mixture of river and groundwater, but reflected the unique weathering and redox interactions occurring within the upper portion of the alluvial aquifer. Distinguishing shallow and deep seepage pathways may prove useful for evaluating site specific risk of levee failure.