Physical Property Variation of a Soil and Saprolite Continuum at Three Geomorphic Positions

Effective land use planning for waste disposal purposes requires knowledge of water movement in the vadose zone in and below the soil solum. This study was conducted to assess the changes in the saturated hydraulic conductivity ( K sat ), water retention, particle‐size distribution, and bulk density of a Typic Kanhapludult soil and its underlying gneiss‐schist saprolite across a typical landscape in the Piedmont region of North Carolina. Intact cores (6.6‐cm diam., 8.0‐cm length) were collected from Bt, B/C, and C (saprolite) horizons at the center of 15 by 30 m areas located at three adjacent geomorphic positions (ridge top, shoulder, and ridge nose). Large L‐shaped observation pits were then excavated over the sample locations for profile description and other data collection. In situ K sat of the three horizons was also determined by the constant‐head well permeameter technique around each of the observation pits. Regression analysis was employed to assess the relationship between K sat and depth at each geomorphic position, and an analysis of variance was used to compare K sat values between horizons and geomorphic positions. At all three geomorphic positions, the laboratory‐determined K sat was consistently the highest in the clayey Bt horizon (arithmetic mean values 1.56 × 10 −6 to 2.19 × 10 −6 m s −1 ), diminished with depth until reaching a minimum value in the transitional, less clayey B/C horizon (arithmetic mean values 7 × 10 −8 to 2.3 × 10 −7 m s −1 ), and then increased with depth into the upper part of the massive, low clay content saprolite (arithmetic mean values 8.1 × 10 −7 to 1.57 × 10 −6 m s −1 ). The patterns of K sat with depth were consistently associated with major horizons and not strongly influenced by texture or bulk density.