Hydraulic Conductivity of Saprolite as Determined by Channels and Porous Groundmass

Abstract

AbstractSaprolite (weathered bedrock) is being evaluated in some regions for use in on‐site disposal of household wastewater. As part of this evaluation, contributions of channels and planes to the saturated and unsaturated hydraulic conductivities were estimated for a mica‐schist saprolite. Undisturbed cores (76 mm in diam. by 76 mm in height) were collected from a 5‐ha forested site. Cores were oriented both vertically and parallel to saprolite foliation, and some contained continuous shear planes. Saturated hydraulic conductivity (Ksat) was measured on all cores, and then a dye tracer was passed through the cores. Unsaturated hydraulic conductivities were estimated for nine additional core samples. Volume percentages of dyed (water‐conducting) and undyed components, channels, groundmass, and other characteristics were determined from thin sections made from all dyed core samples. The Ksat values ranged from 0.01 to 1.71 cm h−1 and had a geometric mean of 0.27 cm h−1. Channels (primarily 0.1–0.5 mm in diam.) comprised 1.9% of the sample volume and accounted for 93% of the Ksat. At a soil water potential of −10 cm, 50% of the K value was due to flow through channels. The channels conducted no water when the soil water potential was ≤‐30 cm and the hydraulic conductivity was ≤0.02 cm h−1. Foliation planes and shear planes were plugged with Fe‐Mn oxides or clay, and their effect on hydraulic conductivity was negligible.