Lithologic and pedogenic influences on porosity distribution and groundwater flow in fractured sedimentary saprolite: A new application of environmental sedimentology

Soil and fractured saprolite formed from weathered Middle Cambrian sediment ary 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 het erogeneity and pedogenic processes on fracture and matrix porosity were exa mined using thin-section petrography and geochemistry. The A and Bw horizon s of the soil profiles comprise thin (20-60 cm), weakly developed, loamy in ceptisols characterized by high (40-50%) matrix porosity. Subjacent saproli te with pore-occluding pedogenic clay (Crt horizon) occurs beneath a saprol ite without pedogenic clay (Cr horizon) and varies lithologically depending on parent material (limestone, siltstone and shale, or sandstone). The soi l and saprolite commonly contain root pores; the saprolite additionally con tains 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, h owever, that extensive pedogenic clay and Fe/Mn-oxide accumulations in the pores and fractures in the saprolite, at depths of 100-250 cm, cause porosi ty reduction that is so extensive it serves to create a low-hydraulic-condu ctivity barrier within the upper vadose zone. This barrier is known from pr ior work to cause formation of perched water-table conditions and rapid dow nslope flow during rainfall events, but this is the first study to investig ate 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 translocatio n patterns occur in each pit, which correspond to the soil zone (A and Bw h orizons), shallow saprolite (Cr horizon), and deep saprolite (Crt horizon). The geochemical zonation is also consistent with the pedogenic and hydraul ic interpretation of the system. The potential environmental significance o f the zone of pore occlusion is also discussed from the perspective of runo ff, recharge, and contaminant migration.