Selected sediment properties of quaternary deposits, Shelby County, Tennessee: Implications for contaminant hydrogeology and quaternary stratigraphy

Contaminant hydrology and stratigraphic characteristics of Quaternary sediments in Shelby County, Tennessee, were investigated using field observations and laboratory analysis of samples from five boreholes. Thirty-nine samples of Quaternary sediments and four samples of underlying Mid-Tertiary sediments were analyzed for grain size, organic carbon (OC) content, cation exchange capacity (CEC), and clay mineralogy. Three different Quaternary stratigraphic units are present: Pliocene(?)-Pleistocene fluvial-terrace deposits, Pleistocene loess, and Pleistocene and Holocene alluvium. Many of the measured sedimentary characteristics of the loess and fluvial-terrace deposits are significantly different from each other, especially if those of a transition zone in the upper part of the fluvial-terrace deposits are discounted. The sedimentary characteristics of the alluvium are generally intermediate between those of the loess and fluvial-terrace deposits, although the presence of some features, such as abundant vermiculite and large organic fragments, are unique to the alluvium. The variations in the sedimentary characteristics of a given Quaternary stratigraphic unit between the borehole sites are minor and generally are related to topographic position and age of deposits. The clay mineralogy of the Quaternary stratigraphic units is related to specific sources of sediment and weathering processes in the Mississippi Embayment. The CEC and OC values of the Quaternary deposits, in concert with hydrologic inference from grain-size characteristics, suggest that contaminant migration may be retarded during matrix infiltration through the loess and upper alluvium. The potential for contaminant migration in the fluvial-terrace deposits and lower alluvium is much greater owing to lower CEC and OC values and higher hydraulic conductivity in these units