Hydraulic Conductivity of Saprolite as a Function of Sample Dimensions and Measurement Technique
- M. J. Vepraskas and
- J. P. Williams
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 (Ksat) 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 Ksat value found for samples with volumes of 347 cm3 (0.04 cm h−1) was significantly lower than the mean Ksat 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.
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