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Sampling Strategies for Assessing Hydraulic Conductivity and Water-Conducting Voids in Saprolite

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

J. Bouma

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

* Corresponding author.

ABSTRACT

A saprolite's saturated hydraulic conductivity (K_sat) and types of water-conducting voids must be known to assess its suitability for liquid waste disposal. This study determined optimum numbers of pits and numbers of samples per pit needed to estimate a saprolite's K_sat and volume percentages of water-conducting macrovoids and groundmass (microvoids). Three pits were dug across a 5-ha area to expose B horizons and saprolite (2-m depth), and undisturbed cores were collected from each pit. The K_sat was measured for each core, and water-conducting voids were dyed. Volume percentages of water-conducting (dyed) macrovoids (e.g., root channels with diam. 0.1 mm) and water-conducting groundmass were determined from thin sections. Mean values for saprolite K_sat and volume percentages of water-conducting macrovoids and groundmass were 0.25 cm h^–1 and 1.1 and 47.9%, respectively. In order to minimize both the variance of each mean value and the time required for analysis, it was most efficient to collect only one core per pit. The most efficient number of pits depended on the desired size of the confidence interval and the property of interest. A 95% confidence interval for saprolite K_sat that was within 10% of the true mean would require over 700 pits per 5 ha, while an estimate of the percentage of water-conducting groundmass would require only 10 pits per 5 ha. Increasing the width of the 95% confidence intervals to be within 50% of the means reduced the required number of pits for these properties to 28 and 1, respectively.

This research was funded by the North Carolina Agric. Res. Service.

Received for publication January 22, 1990.

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