|Van Genuchten, Martinus|
Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/20/2009
Publication Date: 8/20/2009
Citation: Yakirevich, A., Gish, T.J., Simunek, J., Van Genuchten, M.T., Pachepsky, L., Nicholson, T.J. 2009. Potential impact of seepage face on solute transport to a pumped well. Vadose Zone Journal. 9:686-696. Interpretive Summary: Simulations of water flow dynamics and solute migration are important for understanding the mechanisms governing chemical transport to groundwater as well as subsurface lateral transport of chemicals to surface water bodies. Accurate monitoring protocols with mathematical modeling are needed to provide an approach for assessing field-scale water and chemical transport through the unsaturated and saturated zones. Recently, USDA developed a chemical transport procedure that for the first time captures over 95% of the solutes moving through soil (regardless of the transport processes involved). In this study, HYDRUS-2D was used to simulate data from this chemical transport procedure. The results of the simulation show that processes near the water table have a dramatic influence on chemical transit times and the shape of the chemical pulse arriving at the water table. This study will help state and federal agencies to monitor and evaluate the critical flow processes influencing chemical transit times through soil to underlying groundwater.
Technical Abstract: In order to develop predictive chemical transport models the mechanisms governing transport must be understood and quantified. In this study, the variably saturated flow and transport model HYDRUS-2D (which has a seepage boundary routine) was applied to describe tracer mass flux breakthrough curves of three surface applied tracers (bromide, chloride and pentaflorobenzoic acid) through soil to a single well from which the tracers were pumped (Gish and Kung, 2007). Axi-symmetrical chemical transport simulations were performed for several scenarios with different hydrodynamic dispersivity values. Simulations indicate that an active seepage face exists at the well-soil interface near the water table. Calculated cumulative water fluxes to the well through the active seepage face boundary were 1.18 times larger than the variable flux (specified by Neumann boundary condition along the well screen where the well is saturated). In addition, calculated mass fluxes of the tracers through this seepage interface were around 8.2, 3.8 and 10.6 times larger than those through the variable flux boundary for Br, Cl and PFBA, respectively. The model suggests that a seepage mixing zone could help explain the early arrival time and overall shape of the solute BTC’s. As a result, the potential role of a seepage face boundary in flow and transport modeling in unsaturated soil should not be underestimated and may need to be accounted for in experiments in shallow aquifers.