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Porosity Factors that Control the Hydraulic Conductivity of Soil-Saprolite Transitional Zones

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

W. R. Guertal

U.S. Geological Survey, Water Resources Div., Yucca Mountain Project Branch, P.O. Box 327, Mail Stop 721, Mercury, NV

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

ABSTRACT

Slowly permeable transitional horizons separate soil and saprolite, but these horizons cannot be identified easily in the field. The objectives of this study were to determine why the soil-saprolite transitional zone (BC and CB horizons) is slowly permeable, and to evaluate ways for identifying it in the field. Two saprolite deposits were studied in the North Carolina Piedmont. At each site, saturated and unsaturated hydraulic conductivities (K_sat and K_unsat) were measured for major horizons. Volume fractions of water-conducting pores were also compared with the changes in hydraulic conductivity with depth. Horizon mean K_sat values at both sites ranged from virtually 0 to approximately 3 cm h^-1. The lowest K_sat values (<0.3 cm h^-1) occurred in or near the transitional horizons that were directly below the Bt horizons. Changes in the volume of pores within or between mineral grains (termed inter/intraparticle pores) with depth corresponded to changes in both K_sat and K_unsat. In the transitional horizons, the inter/intraparticle pores were plugged with clay and this caused the horizons to have low K values. In situ measurements of K_sat with depth were the most accurate technique to use for identifying transitional zones in the field. Examination of both the soil and rock structures in pits was also an acceptable technique. Texture and consistence were not considered reliable for pin-pointing transitional horizons.

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 April 17, 1995.

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