Simulation of chromium transport in the unsaturated zone for predicting contaminant entries into the groundwater

Heavy metal contaminations in the top-soil layers can impose serious threats to groundwater quality. A seepage water prognosis of probable future contaminant entries at the groundwater surface has to take into account the emission characteristics of the source zone and the reactive transport of the contaminated leachate through the unsaturated zone. Here, a possible approach is presented, exemplarily for a site contaminated with chromium, encompassing batch elution experiments, unsaturated soil column leaching experiments with flow interruption, soil monolith lysimeter experiments, and numerical modeling of non-equilibrium solute transport. The prognosis provides a long-term prediction of Cr concentrations at the groundwater surface. It must be emphasized, that the modeling results are uncertain, because several of the parameters in the simulations can be determined only with significant errors. Additionally, the approach is not applicable routinely for every hazardous waste site. Our study reveals, however, that reactive contaminants can possibly reach the groundwater at hazardous concentrations within very short time.