The UFA method for rapid, direct measurements of unsaturated transport properties in soil, sediment, and rock

Abstract

These studies illustrate the efficacy of a newly developed unsaturated/saturated flow apparatus (UFA) to decrease significantly the time required to obtain direct measurements of transport parameters. The UFA method can be applied to cores of soil, sediment, rock, concrete, or any other porous media, especially highly unsaturated and relatively impermeable materials. The UFA method is effective because it allows the operator to set variables in Darcy"s Law and impose any desired steady-state hydraulic conductivity. The UFA achieves hydraulic steady-state in hours by using an adjustable body force (provided by a centrifugal acceleration in the rotating frame of reference) to drive the fluid, coupled with precision fluid flow through a rotating seal assembly. The normal operating range of unsaturated hydraulic conductivity that can be imposed by the UFA is from the saturated conductivity for that specimen [up to about 10-4 cm/s (10-1 darcy; 10-9 cm2)] down to about 10-11 cm/s (10-8 darcy; 10-16 cm2). For strictly saturated conditions, the UFA can operate down to 10-14 cm/s. Temperatures can be adjusted from -20° to 150°C, and constant flow can be imposed during rotation down to 0·001 mL/h. Several comparisons are made among K(θ) results, determined on splits of the same samples using the UFA and other methods, to the van Genuchten/Mualem estimations derived from water retention data using RETC. A case study is also discussed involving subsurface hydrostratigraphic mapping and recharge distribution at a mixed waste site. This study demonstrates the speed, economy, and direct measurement capability of the UFA that make it optimal for determining transport behaviour under unsaturated conditions.