Tracer vs. pressure wave velocities through unsaturated saprolite

Saprolite is a form of weathered bedrock that is commonly used as the host material at waste disposal sites in the Southeastern Piedmont. However, est imating the unsaturated hydraulic and transport properties of saprolite is difficult due to saprolite's low permeability. We demonstrate the use of sh ort-duration fluid irrigation pulses for maintaining unsaturated conditions in intact saprolite columns. Concomitant Cl- tracer experiments demonstrat e that irrigated waters moved through an effective volumetric porosity (0.0 38-0.108 cm(3) cm(-3)) substantially less than the ambient water-filled por osity (0.44 cm3 cm-3). We observed the unexpected result that irrigation-in duced pressure wave velocities (1983-3670 cm d(-1)) were approximate to 100 0 times faster than tracer velocities (2.04-6.00 cm d(-1)). The relationshi p between pressure wave velocities and fluid velocities is described using kinematic wave theory, presented for four parametric representations (Brook s-Corey, van Genuchten-Mualem, Broadbridge-White, and the Galileo Number), that predicts fluid pressure velocities to be from approximately two to fif teen times faster than saprolite tracer velocities. None of the kinematic m odels was able to reproduce observed rapid pressure wave velocities. A hydr aulic 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 o f the unsaturated saprolite is considered. Based on the advection-diffusion equation, diffusion-dominated soil water pressure wave velocities should d ecrease with depth, eventually conforming with kinematic wave theory. Press ure pulse velocity monitoring may be an additional tool for estimating unsa turated hydraulic properties in low permeability media.