Tracer vs. pressure wave velocities through unsaturated saprolite

Journal Article · Tue Feb 01 04:00:00 EST 2000 · Soil Science Society of America Journal

DOI:https://doi.org/10.2136/sssaj2000.64175x· OSTI ID:20075798

Rasmussen, T C; Baldwin, Jr, R H; Dowd, J F; Williams, A G

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. The authors demonstrate the use of short-duration fluid irrigation pulses for maintaining unsaturated conditions in intact saprolite columns. Concomitant Cl{sup {minus}} tracer experiments demonstrate that irrigated waters moved through an effective volumetric porosity substantially less than the ambient water-filled porosity. The authors observed the unexpected result that irrigation-induced pressure wave velocities were {approx}1,000 times faster than tracer velocities. The relationship between pressure wave velocities and fluid velocities is described using kinematic wave theory, presented for four parametric representations (Brooks-Corey, van Genuchten-Mualen, 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.

• Research Organization: Univ. of Georgia, Athens, GA (US)
• OSTI ID: 20075798
• Journal Information: Soil Science Society of America Journal, Journal Name: Soil Science Society of America Journal Journal Issue: 1 Vol. 64; ISSN 0361-5995; ISSN SSSJD4
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
ENVIRONMENTAL TRANSPORT
HYDRAULIC CONDUCTIVITY
MATHEMATICAL MODELS
ROCKS
TRACER TECHNIQUES
UNDERGROUND DISPOSAL
VELOCITY
WASTE DISPOSAL