A new Conceptual Model for Controls on Hydraulic Conductivity in Soil and Saprolite Derived from Sedimentary Rock
Abstract
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 with a constant head permeameter showed the presence of a zone of low saturated hydraulic conductivity (Ksat of about 2 x 10-7 m/s), just below the B-horizon, from about 50 -150 cm depth. Vertical profiles of K(0)/K(psi) for psi values of 5 and 14 cm showed large increases in this zone, indicating the influence of matrix flow, rather than macropore flow. Microscopic examination of thin sections of the soil and saprolite indicated that the cause of the reduction in Ksat and the absence of macropore flow from 50-150 cm depth was the high degree of infilling of macropores with pedogenic clays and Fe/Mn oxides. Below this low-K zone, hydraulic conductivity was highly variable and appeared to be related to both the lithology of the parent bedrock and the degree of pore and fracture infilling. Biopores (mainly root holes) were common in the limestone to depths of 3.4 m (top of intact bedrock), but many of these were infilled. Fractures were the dominant macropore type in the shale, but typically had much less infilling than macropores in the limestone. As a result, high K-values can occur in either material, although they appear to be more prevalent in the limestone. The field studies corroborate earlier research in similar deposits at Oak Ridge National Laboratory (ORNL), where frequent perched water table conditions and rapid shallow subsurface runoff were observed in the upper 1 m of the soil and saprolite. This study identifies the principal causes of the "storm-flow zone" at ORNL, and suggests that unsaturated and saturated flow in the saprolite underlying the zone of maximum pore infilling is likely very complex, because of the high degree of variability of macropore size, frequency and infilling, as well as variability in hydraulic conductivity.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFM.H31C0253M
- Keywords:
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- 1829 Groundwater hydrology;
- 1875 Unsaturated zone;
- 5100 PHYSICAL PROPERTIES OF ROCKS;
- 5114 Permeability and porosity