Date of Award
M.S. in Engineering Science
Geology and Geological Engineering
Gregg R. Davidson
Seepage beneath levees during flood stage becomes a concern when piping occurs, creating channels beneath the levee and forming sand boils where transported sediments discharge. Along the lower Mississippi River, pathways beneath the levee vary with surface geology, following deeper paths where the levee overlies channel fill deposits, and shallower paths where it overlies sand bar deposits. A preliminary investigation north of Vicksburg, MS, during the 2011 flood, demonstrated the potential for using aqueous geochemistry to differentiate sand boils forming at the end of deep and shallow flow pathways, though the study was limited to cation and trace metal analyses. Sampling during the 2015 and 2016 events for temperature, conductivity, redox potential, bulk chemistry, trace metals, tritium, and stable isotopes of oxygen, hydrogen, and strontium, enhanced understanding of the nature of flow and the geochemical evolution of the local groundwater. Geochemical signatures identify distinct deep and shallow flow pathways. Deeper flow pathways have lower redox potential, high fe and as, and higher barium to calcium ratios. Tds values were of the same magnitude for shallow and deep zones of the aquifer. Oxygen and hydrogen isotopes suggest that river water experiences significant evaporation before recharging to the Mississippi River valley alluvial aquifer, likely due to temporary surface storage in wetlands between the river and levee system. Strontium isotope results were equivocal, with a distinct difference between river and groundwater, but no clear distinction between deep and shallow pathways. Tritium generally decreased for deeper flow paths, though temporal variability did not allow calculation of residence times. Unique geochemical signatures of groundwater between the two flood events may reflect variation over time in the relative contributions of water in the river from different regions of the Mississippi River watershed.
Voll, Kaitlin Marie, "Characterizing Variable Flow Pathways And Geochemical History Of Seepage Under The Mississippi River Levee: 2011, 2015, And 2016 Floods" (2018). Electronic Theses and Dissertations. 1014.
Emphasis: Geological Engineering