HR: 1330h
AN: H32A-0524    [PDF]
TI: Measured versus predicted recharge and bromide transport through a sandy soil in the San Joaquin Valley, California
AU: * Green, C T
EM: ctgreen@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Stonestrom, D A
EM: dastones@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Bekins, B A
EM: babekins@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Akstin, K C
EM: kakstin@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AB: The unsaturated zone is a complex system of interacting processes that are often poorly characterized, making transport prediction inherently difficult. This study investigated the ability of Darcy-based models that included hysteresis, sedimentary heterogeneity, evapotranspiration (ET), and preferential flow to achieve accurate predictions of recharge and solute transport in a real-world setting. A natural-gradient bromide-tracer experiment was performed on a late-Pleistocene terrace deposit along the Tuolumne River in California. Bromide moved into the soil at the onset of the rainy season and then formed two peaks: one that continued moving downward into the profile, and one that reformed at the land surface. Sediment hydraulic properties were measured in the field and in the laboratory. Numerical models of water and solute movement allowed evaluation of various conceptualizations of the unsaturated flow system. Model outputs were compared to field measurements of matric pressure, moisture content, and bromide concentrations for the first eight months following application of tracer at the land surface. Prediction errors were large compared to measurement errors for all scenarios. The most consistent failures of the models were: (1) underestimation of the travel velocity of the downward-moving bromide peak, and (2) underestimation of matric pressures following passage of the wetting front. Realistic matches between model predictions and measured data could not be attained regardless of choice of retention-curve model or treatment of ET, hysteresis, or sedimentary heterogeneity. A composite-porosity preferential-flow model also failed to provide a match. Inclusion of hysteresis improved prediction of the surface bromide peak. Calibration of the hydraulic conductivity resulted in more marked improvements elsewhere, although the best match required conductivities roughly two orders of magnitude above measured values. None of the conceptual models captured all of the prominent features of the moisture, pressure, and concentration histories, illustrating the need for alternative models of unsaturated flow and transport.
DE: 1818 Evapotranspiration
DE: 1832 Groundwater transport
DE: 1866 Soil moisture
DE: 1875 Unsaturated zone
SC: Hydrology [H]
MN: 2003 Fall Meeting