HYDRAULIC CONDUCTIVITY OF SAPROLITE AS A FUNCTION OF SAMPLE DIMENSIONS AND MEASUREMENT TECHNIQUE

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 (C1 horizon) by comparing saturated hydraulic conductivities (K-sat) for sample volumes of 347, 6280, and 675000 cm(3). Unsaturated hydraulic conductivities [K(h)] for sample volumes of 347 and 6280 cm(3) 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 cm(3) (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 approximate to 5000 cm(3) 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.