Tracer vs. Pressure Wave Velocities through Unsaturated Saprolite

Abstract

Saprolite is a form of weathered bedrock that is commonly used as the host material at waste disposal sites in the Southeastern Piedmont. However, estimating the unsaturated hydraulic and transport properties of saprolite is difficult due to saprolite's low permeability. We demonstrate the use of short‐duration fluid irrigation pulses for maintaining unsaturated conditions in intact saprolite columns. Concomitant Cl− tracer experiments demonstrate that irrigated waters moved through an effective volumetric porosity (0.038–0.108 cm3 cm−3) substantially less than the ambient water‐filled porosity (0.44 cm3 cm−3). We observed the unexpected result that irrigation‐induced pressure wave velocities (1983–3670 cm d−1) were ≈1000 times faster than tracer velocities (2.04–6.00 cm d−1). The relationship between pressure wave velocities and fluid velocities is described using kinematic wave theory, presented for four parametric representations (Brooks–Corey, van Genuchten–Mualem, Broadbridge–White, and the Galileo Number), that predicts fluid pressure velocities to be from approximately two to fifteen times faster than saprolite tracer velocities. None of the kinematic models was able to reproduce observed rapid pressure wave velocities. A hydraulic form of the advection–diffusion equation based on Richards' equation is presented that favorably predicts the shape of pressure response curves only when the kinematic velocity is ignored and the hydraulic diffusivity of the unsaturated saprolite is considered. Based on the advection–diffusion equation, diffusion‐dominated soil water pressure wave velocities should decrease with depth, eventually conforming with kinematic wave theory. Pressure pulse velocity monitoring may be an additional tool for estimating unsaturated hydraulic properties in low permeability media.