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Description
Double-ring infiltrometer tests were done at eight sites in alluvial deposits near Joshua Tree, California to measure surface infiltration rates and aid in evaluating the study area for construction of infiltration ponds for groundwater recharge. Surface infiltration rates ranged from 3.61x10__ ft_/s within the wash north of Pinto Mountain Fault, to 6.68x10__ ft_/s south of the fault outside of an active wash. Hydraulic conductivity of alluvium was estimated from infiltration data using the computer program VS2D. Hydraulic conductivities estimated using VS2D ranged from 1.74x10__ ft/s to 4.63x10__ ft/s and were within the range of fine sand to silty sand. Hydraulic conductivities also were measured by laboratory analysis of surface cores from each site and ranged from 7.59x10__ ft/s to 4.33x10__ ft/s. Hydraulic conductivities computed from particle size data using the pedotransfer function (PTF) based computer program Rosetta ranged from 6.71x10__ ft/s to 1.73x10__ ft/s. Laboratory hydraulic conductivities matched the field data better than hydraulic conductivities calculated using Rosetta, but neither method correctly predicted infiltration rates measured using the double-ring infiltrometer. Evidence for a surficial low-permeability layer was seen in the data for six of the eight test sites, suggesting the formation of runoff depositional crusts and sieving crusts during intermittent wash flow. Surficial crusts can be removed before pond infiltration, but will distort infiltration measurements and should be accounted for. Simulation results using VS2D were sensitive to low-permeability layers at shallow depths, 0 to 4 feet below land surface, within the alluvium. Results were not sensitive to the thickness of the layer. The length of the infiltrometer test should be long enough to ensure detection of any subsurface layering. VS2D was used to simulate infiltration from one-acre spreading ponds on either side of the Pinto Mountain Fault to provide an estimated range of recharge times for the study area. The time required before the wetting front reaches the water table 400 feet below land surface was 14 - 60 days, implying that the area may be suitable for recharge through surface infiltration. Model simulations do not account for low-permeability layers within the alluvium or decreases in hydraulic conductivity with depth caused by compaction, consolidation, or changes in subsurface geology. As a consequence actual infiltration rates will probably be less than simulated rates. A more extensive survey of the subsurface geology and hydraulic properties of the study area including test drilling would help to asses subsurface conditions that would inhibit infiltration and recharge.
