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Hydraulic Conductivity of Saprolite as Determined by Channels and Porous Groundmass

Dep. of Soil Science, Box 7619, North Carolina State Univ., Raleigh, NC 27695-7619

A. G. Jongmans and J. Bouma

Dep. of Soil Science and Geology, Agricultural Univ., P.O. Box 37, 6700 AA, Wageningen, the Netherlands

*Corresponding author.

ABSTRACT

Saprolite (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 (K_sat) 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 K_sat 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 K_sat. 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.

Contribution from the Dep. of Soil Science, North Carolina State Univ.

Received for publication July 10, 1990.

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