CONTROLS ON HYDRAULIC CONDUCTIVITY IN SOIL AND SAPROLITE DERIVED FROM SEDIMENTARY ROCK

Field investigations in soil and saprolite derived from weathering of interbedded shale and limestone at a site near Oak Ridge, TN, indicate that parent bedrock lithology and the extent of pore and fracture infilling with pedogenic clays and Fe/Mn oxides play a major role in controlling hydraulic conductivity in these materials. Field measurements of saturated hydraulic conductivity, made with a constant head permeameter, show the presence of a zone of high values of saturated hydraulic conductivity (Ksat of about 10-5 to 10-4 m/s) in the A- and B-horizons, overlying a zone of low values (about 2 x 10-7 m/s) in the upper C-horizon from 50 –150 cm depth. Below 150 cm depth, Ksat values show greater variability, which didn’t correlate well with parent bedrock lithology. Vertical profiles of K(psi)/K(0), measured with a tension disc infiltrometer, show large variations with depth, indicating changes in the relative influence of macropore and micropore flow that generally correspond with the variations in Ksat. Microscopic examination of thin sections of the soil and saprolite indicate that below 50 cm depth, a large proportion (15-50%) of each section consisted of pedogenic clays and Fe/Mn oxides which infilled macropores (fractures and rootholes) and matrix pores. The infillings were most common in the limestone saprolite, but again there wasn’t a direct correlation between degree of infilling and Ksat or K(psi). It is likely that K-values are influenced by a combination of factors, including parent bedrock lithology, frequency and size of macropores, and the degree of macropore infilling. The sharp decrease in Ksat below 50 cm depth is expected to provide a significant barrier to infiltration and likely leads to rapid surface runoff of the type previously observed in saprolite in similar materials at Oak Ridge National Laboratory (ORNL). This study and the previous studies at ORNL suggest that abrupt changes in hydraulic conductivity, at least partly controlled by macropore infilling, are common features in sedimentary rock saprolite.