Hydrogeology and pedology of saprolite formed from sedimentary rock, eastern Tennessee, USA

Groundwater flow in sedimentary rock saprolite, and in soils derived from this material, is strongly influenced by sedimentary layering and fractures inherited from the parent bedrock. The main objectives of this study were to determine whether parent bedrock lithology and infilling of fractures and other macropores with pedogenically derived clays and Fe/Mn oxides also play major roles in controlling hydraulic conductivity and groundwater flow. The study was carried out by measuring profiles of saturated and unsaturated hydraulic conductivity, Ksat and K(ψ), and comparing them to soil and saprolite pedology and lithology in a 3.4 m deep pit excavated in interbedded limestone and shale saprolite. In the depth interval of 50 to 100 cm, there was an abrupt decline in Ksat and K(0) by a factor of up to 250. This corresponds to the occurrence of a zone where almost all of the fractures and other macropores are occluded with pedogenic clays and Fe/Mn oxides. Below a depth of 100 cm, both degree of pore infilling and hydraulic conductivity tend to vary, with a spatial regularity that is similar to the thickness of sedimentary layering in the saprolite. However, variations in hydraulic conductivity do not always correspond to changes in lithology, suggesting that hydraulic conductivity is a result of the complex interaction of several factors, including parent bedrock lithology and the degree of infilling of the macropores. Similarities between hydrogeologic conditions at this site and at research sites in weathered sedimentary shale/siltstone and carbonate rock settings at the Oak Ridge Reservation in Tennessee and in weathered crystalline rock in North Carolina indicate that macropore infilling plays an important role in controlling hydraulic conductivity and groundwater flow for a variety of different types of parent bedrock.