This paper illustrates how sensitivity analysis and worst case scenario analysis can be useful tools in risk assessment of groundwater pollution. The approach is applied to a study area in Hungary with several known groundwater pollution sources and nearby drinking water production wells. The main concern is whether the contamination sources threaten the drinking water wells of the area. A groundwater flow and transport model is set up to answer this question. Due to limited data availability, the results of this model are associated with large uncertainty. Sensitivity analysis and worst case scenario analysis are applied to estimate this uncertainty and to build confidence in the model results. words in abstractKeywords: sensitivity analysis, worst case scenario analysis, groundwater pollution, groundwater and transport modeling 2

A complex hydrogeological study was carried out in the framework of the CIVAQUA program organized by the Trans-Tisza Region Environmental and Water Authority to provide the multifunctional artificial recharge of Great-Forest in Debrecen, East-Hungary. Due to several decades of overproduction of groundwater resources, the groundwater heads below the famous park area in the heart of the city has decreased to 12-14m, causing significant destruction of the valuable and protected surface ecosystem. The groundwater model of the investigated area was utilized to investigate several technical solutions in order to solve the severe water quantity deficit of the park. The source of the infiltration recharge is provided from near-site surface water bodies, which raises inevitable issues concerning the groundwater quality. In addition to the urgent need for finding appropriate recharge alternatives, the environmental authorities are aware of the slow mobilization of a complex hydrocarbon groundwater contamination from an adjacent industrial complex of the city. The contaminant plume was proved to evolve in the direction of the waterworks wells. The hydrodynamic and transport modeling aided planning has to give answers to several questions of a complex groundwater quality and quantity problem of this major city in Hungary. The modeling aided concept wanted to find a carefully designed artificial recharge strategy that serves the reconciliation of ecological and drinking water demand, but at the same time forming a hydrodynamic barrier in the way of the contamination plume.

Recharging from open-surface water resources is investigated as a method for remediation of over-produced and contaminated aquifers. The hydraulic and contaminant modeling using the Modflow-2000 and MT3DMS simulation software are resorted for evaluation of the available options. Methodology is developed and illustrated for reviewing the alternative remediation approaches for optimal selection of the best remediation approach to aquifer recharging from surface waters. The effectiveness of the present systematic approach is demonstrated by identifying the best choice among the multifunctional artificial recharge options available for the Great Forest Park in Debrecen, Hungary.

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