GRAVITY-DRIVEN UNSATURATED FLOW IN SMALL-DISPLACEMENT DEFORMATION BAND FAULTS FOUND IN POORLY LITHIFIED PARENT SANDS

Many small-displacement (< 1 m) faults cross-cut the poorly lithified basin-fill sands found in New Mexico’s Rio Grande rift. These faults have lower porosity and saturated hydraulic conductivity than their parent sands. The same deformational processes that lower these saturated properties, creating barriers to saturated flow, enhance unsaturated properties, creating preferential flow paths through vadose zone sediments. If sufficiently continuous, numerous, or conductive, such faults may hasten the subvertical movement of water and solutes through the vadose zone. We report the first measurements of unsaturated hydraulic properties for these materials, together with preliminary estimates of gravity-driven, liquid-phase water fluxes under unsaturated conditions.

Using the UFA centrifuge, hydraulic relations were measured on undisturbed samples from a small-displacement (0.3m) normal fault and adjacent sands in the Bosque del Apache Wildlife Refuge, central New Mexico, USA. Experimental data were fit to standard models, which were used to estimate 1D gravity-driven flux densities over matric potentials representative of arid vadose zones and to calculate the number of faults needed to significantly increase downward flux per unit area.

Saturated conductivity is three orders of magnitude less in these faults than undeformed sand. As matric potential decreases from 0 to -200 cm of tension, unsaturated hydraulic conductivity decreases only one order of magnitude in the fault, but six orders of magnitude in undeformed sand. Fault conductivity is greater by 2-4 orders of magnitude at matric potentials between -200 and -1000 cm. Gravity-driven downward flux through faulted domains is 10% to >100-fold larger than unfaulted domains for –200 > matric potential > -1000 cm and 1:1000 m < fault density < 1:10 m. Field site fault density is much less than 1:10 m and in situ moisture content data show matric potential lies between –300 and –600 cm. These results suggest small-displacement faults can move significant amounts of recharge through sandy parts of arid vadose zones given sufficient fault densities.