Lithologic and Pedogenic Influences on Porosity Distribution and Groundwater Flow in Fractured Sedimentary Saprolite: A New Application of Environmental Sedimentology

Abstract

ABSTRACT Soil and fractured saprolite formed from weathered Middle Cambrian sedimentary rocks were analyzed from two shallow (< 2.5 m deep) pits located at the Oak Ridge Reservation in eastern Tennessee. The effects of both parent heterogeneity and pedogenic processes on fracture and matrix porosity were examined using thin-section petrography and geochemistry. The A and Bw horizons of the soil profiles comprise thin (20-60 cm), weakly developed, loamy inceptisols characterized by high (40-50%) matrix porosity. Subjacent saprolite with pore-occluding pedogenic clay (Crt horizon) occurs beneath a saprolite without pedogenic clay (Cr horizon) and varies lithologically depending on parent material (limestone, siltstone and shale, or sandstone). The soil and saprolite commonly contain root pores; the saprolite additionally contains bedding-parallel and bedding-perpendicular (orthogonal) fracture sets. The fractures and root holes were initially expected to control bulk or mesoscale hydraulic conductivity of the materials. This study indicates, however, that extensive pedogenic clay and Fe/Mn-oxide accumulations in the pores and fractures in the saprolite, at depths of 100-250 cm, cause porosity reduction that is so extensive it serves to create a low-hydraulic-conductivity barrier within the upper vadose zone. This barrier is known from prior work to cause formation of perched water-table conditions and rapid downslope flow during rainfall events, but this is the first study to investigate the origin of the low-conductivity barrier. Geochemical mass balance, using Ti as an immobile index, also demonstrates that three distinct weathering environments defined by element translocation patterns occur in each pit, which correspond to the soil zone (A and Bw horizons), shallow saprolite (Cr horizon), and deep saprolite (Crt horizon). The geochemical zonation is also consistent with the pedogenic and hydraulic interpretation of the system. The potential environmental significance of the zone of pore occlusion is also discussed from the perspective of runoff, recharge, and contaminant migration.