Influence of illuviated clays and soil cements on hydraulic conductivity in sedimentary saprolite

A stepped pit was excavated to a depth of 3.4 meters In soil and saprolite formed from the Maryville Limestone (Middle Cambrian) at a site near Oak Ridge, TN. Field-scale measurements of hydraulic conductivity, K_sat and K(Ψ), and thin-section microscopy were used to test the hypothesis that illuviated clays and Fe/Mn cements are common occurrences in sedimentary saprolite and are responsible for the formation of a low K zone (depth <0.5-1.5 m) typically found in these materials. Results show that the onset of deposition of pedogenic clay and Fe/Mn oxides at about 60 to 100 cm depth corresponds to a two-orders-of-magnitude reduction in saturated hydraulic conductivity. Observations also indicate that accumulations of pedogenic clays and Fe/Mn oxides can occur to depths of at least 3 m, and that the relative amount of infilling in the deep saprolite does not correlate well to variations in hydraulic conductivity. The influence of lithology becomes increasingly important as one proceeds deeper into the section, because the frequency and nature (fractures vs. root holes) of the macropores varies according to lithology. Limestone-derived saprolite tends to have more frequent root holes and a higher abundance of pedogenic clay and Fe/Mn oxide infilling than shale/siltstone saprolite. However, variations in hydraulic conductivity tend to cross lithologic boundaries, indicating that variations in K depend on the interaction of a variety of factors, and suggesting that very complex flow systems are likely to develop in the saprolite. These findings further our understanding of groundwater flow through the unsaturated zone in sedimentary saprolite, and have implications for agriculture, soil drainage, aquifer recharge, and contaminant migration in these materials.