Preferential flow modelling in the vadose zone using MACRO 5.0: cape flats sands and Mpumalanga clays case studies (South Africa)

Abstract

Understanding fluid flow and solute transport within the vadose (unsaturated) zone is an essential prerequisite for protection of groundwater from contaminant sources occurring overland. Preferential flow paths in the vadose zone pose a significant problem because they are potential avenues for rapid transport of chemicals from contamination sources to the water table. The objectives of this study were i) to test the adequacy of a model, namely MACRO 5.0, in simulating bimodal water flow and generic contaminant transport in the vadose zone, and ii) to run a sensitivity analysis with MACRO 5.0 in order to determine which input parameters are the most relevant in describing the effects of preferential water flow, solute transport and exchange between macropores and matrix. Two case studies were investigated, the first at a landfill site overlying sandy unconfined aquifer (Coastal Park, Cape Town), and the second at an industrial site overlying cracking clayey soil and fractured rocks (Secunda, Mpumalanga Province). For the Coastal Park site, simulations of soil water content and leaching of a mobile tracer were compared to monitored soil water contents and chloride concentrations in groundwater. For the Secunda site, simulations of soil water content and concentrations of boron and fluoride originating from effluent irrigation were compared to soil profile measurements. In both cases, the MACRO 5.0 model predictions agreed well with measurements, appropriate input calibration data were used. The sensitivity analysis indicated that soil water properties related to preferential flow (hydraulic conductivity at the boundary between macropores and matrix, soil water content and tension, and diffusion pathlength) have influence on simulation results. Similarly, the solute balance is mostly influenced by degradation rate coefficients (both in solid and liquid phases), sorption distribution coefficients and solute concentrations.