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ORIGINAL RESEARCH

Deterministic Analysis and Upscaling of Bromide Transport in a Heterogeneous Vadose Zone

^a Dep. of Environmental Sciences, Univ. of California, Riverside, CA 92507
^b U.S. Salinity Lab., Riverside, CA 92507
^* Corresponding author (Scott.Bradford{at}ars.usda.gov).

Received 6 December 2008.

Conservative solute transport experiments were conducted at a field plot and on an undisturbed soil core from the same site. The hydraulic and solute transport properties were extensively characterized so that the data could be analyzed from a deterministic perspective. To investigate the influence of scale and conceptual model on solute transport in the relatively homogeneous upper soil profile of the field site, breakthrough curves that were collected at different depths in the undisturbed column and in the field experiments were compared and analyzed. The mobile-immobile model provided a physically realistic description of the column data that was largely independent of the transport distance, and upscaling from the undisturbed core to the field plot was relatively successful. Mean and variance of the Br^– travel times were controlled by plot-scale variability in soil water content and heterogeneity, which was overcome by averaging the concentration values from four locations at the same depth and time, and analyzing the Br^– transport behavior as a one-dimensional process. Differences between the measured and simulated mass balances, average concentration, and the variations in concentration were predicted reasonably well across depth and time. Although prediction of the exact concentration information at a given point was not achieved, this study demonstrates that area-average Br^– transport in a heterogeneous vadose zone can be deterministically quantified.

Abbreviations: BTC, breakthrough curve • CDE, convection–dispersion equation • MIM, mobile–immobile model • PSD, particle size distribution • TDR, time domain reflectometry