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Hydraulic Conductivity of Saprolite as a Function of Sample Dimensions and Measurement Technique

Department of Soil Science, North Carolina State Univ., Raleigh, NC 27695

*Corresponding author (michael_vespraskas{at}ncsu.edu).

ABSTRACT

Accurate hydraulic conductivity (K) data are needed to evaluate soils for on-site waste disposal, but such data are difficult to obtain where K values vary with sample dimensions. This study evaluated the K of a quartz-diorite saprolite (Cl horizon) by comparing saturated hydraulic conductivities (K_sat) for sample volumes of 347, 6280, and 675000 cm³. Unsaturated hydraulic conductivities [K(h)] for sample volumes of 347 and 6280 cm³ were compared using a one-step outflow procedure and the crust test. A dye was also applied to the largest samples to determine the depth distribution of water-conducting macropores. The mean K_sat value found for samples with volumes of 347 cm³ (0.04 cm h^–1) was significantly lower than the mean K_sat values found for larger samples (0.16–0.20 cm h^–1); means for the latter samples were not significantly different (P = 0.10). Means for K(h) also differed between the two sample volumes for soil water pressure heads of –5 and –10 cm. Channels were the major kind of water-conducting macropore observed. The results from both the pore studies and K measurements showed that the minimum sample size needed in situ for K measurements in this horizon was 5000 cm³ for soil water pressure heads between 0 and –10 cm. Detached cores with smaller cross-sectional areas could be used for measuring K(h) at soil water pressure heads of -20 cm or less.

The research was funded by Grant no. 70121 from the Water Resources Research Inst. of the Univ. of North Carolina. The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service of the product named, nor criticism of similar ones not mentioned.

Received for publication March 11, 1994.

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