EXPERIMENTAL AND SIMULATED SOLUTE TRANSPORT AND SHALLOW SUBSURFACE FLOW AT AN AGRICULTURAL SITE IN NORTHWESTERN MISSISSIPPI

Solute transport and subsurface flow through a Sharkey Clay soil typical of a soybean field in the alluvial plain of northwestern Mississippi were simulated using the two-dimensional, variably-saturated flow model of solute transport (VS2DTI) developed by the U.S. Geological Survey. The model was developed and validated using data collected from a 2-m ring infiltration test, which include: calcium bromide (CaBr) concentrations at depth, water flux, and soil moisture content. Local and State agencies are attempting to develop a plan for sustainable use of the Mississippi River Valley alluvial aquifer, which is heavily pumped for irrigation and has documented water-level declines of tens of feet in some areas over time. A critical component to determining the sustainable yield of the aquifer is recharge, both the amount and source. The most recent groundwater model simulation by the U.S. Geological Survey in 2001 estimated that about 5 percent of precipitation recharges the alluvial aquifer annually; more localized studies found that number is as high as 17 percent. Due to the complexity of recharge processes, a tool for local estimation of recharge is necessary. In this study, simulated results, using VS2DTI, were compared to observed infiltration rates along with flow direction and extent of the CaBr tracer. Observed tracer concentrations and flow were found to be more spatially variable than simulated solute transport and subsurface flow. This suggests flow in the vadose zone is not only dependent on the medium of soil and its physical properties, but also on anisotropic anomalies, such as capillary or layer barriers, or mudcracks and large organic particles, which can produce preferential flow pathways.