per Geology and Soils Manual, Missouri State Highway Commission, 1962, p.77-90.
These neighborhoods occupy two areas comprised of different overburden (soil above bedrock). The floodplain area, at lower elevations near the Mississippi River, consists of alluvial (river deposited) soils, often silts and sands. This soil is fairly permeable, easily transporting groundwater and any potential contaminants. At normal river levels, groundwater lies approximately 20 feet below surface. The water table here rises with rising river levels.
The natural overburden away from the Mississippi River, at higher elevations, is best described in the above-referenced manual. This silty/clay soil predominating the City is described as being any one or a combination of the following: Memphis, Clinton, and Marion silt loams (wind-deposited soils which became clayey due to downward percolating water and compaction). These loams are characterized by yellowish-brown to brown to light grayish-brown silty clay.
Because of their silty character, these loams are susceptible to capillary water movement (pulling water up from the natural water table surface), thus mainitaining a higher than usual water table. This is especially significant in areas that are not topographically high (at a low elevation). In such areas, groundwater would be encountered closer to the surface. These neighborhoods have some of these areas, but most areas are topographically high, well above this influence. Basements dug to normal depths would not be expected to encounter water. With respect to construction projects, these loams are considered somewhat elastic: left exposed, will slump when wet and dust (erode) when dry.
Migration of percolating water (as well as contaminants) through these loams are relatively slow and move downward to the capillary fringe (upper water table). Thus, property and basements in this area, unless immediately near hazardous sites, are well above and away from potential contaminants. As in all neighborhoods developed for over 100 years, the natural overburden has been criss-crossed with utility lines, trenches, pipes, and impervious fill sands and materials, all of which serve as conduits, speeding the migration of any contaminants more quickly and further away from their source(s) than allowable through the natural overburden.
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