Longitudinal and transverse dispersion coefficients in unsaturated plainfield sand

Abstract

The relationship between the longitudinal and transverse dispersion coefficients (DL and Dτ) and the pore water velocity (υ) and the effective diffusion coefficient (De) was determined for 0.01 < υ < 0.28 cm/min in a vertical unsaturated column (L = 23 cm) of C horizon Plainfield sand. The inflow and outflow control systems on the soil column consisted of rows of porous ceramic tubes with individual adjustable pressure controls. Uniform inflow and outflow were achieved across the column, and υ was maintained constant during each experiment. An analysis of the transverse spread produced indicated that the effect of apparatus‐induced dispersion was <10%. DL (cm2/min) was linearly related to υ (cm/min) (DL = 0.216υ + 0.0032; r2 = 0.90). Slightly larger DL was found in experiments with H2O replacing Cl than in experiments with Cl replacing H2O. DL at the lowest υ was 2 orders of magnitude greater than De. DT(cm2/min) was essentially independent of υ(DT = 0.0035υ + 0.0031; r2 = 0.15) and was about 2 orders of magnitude greater than De at the lowest υ studied. The spread of tracer at the outflow was inversely proportional to υ, and transverse dispersion had reduced the concentration peak of the inflow by 84% at L = 23 cm and υ = 0.01 cm/min. The ratio DL/DT was proportional to υ and ranged from ∼ 1 (υ = 0.01 cm/min) to ∼20 (υ = 0.28 cm/min).