Optimal sampling design for well catchment investigation towards transient analysis of wellhead protection areas

For many drinking water protection programs, Wellhead Protection Areas (WHPAs) represent the first protective strategy in order to safeguard groundwater abstraction against sources of pollution. However, if we consider that a complete characterization of the aquifer at the field scale and of transient flow conditions is financially unfeasible, unresolved heterogeneity and transient conditions introduce unavoidable uncertainty during WHPA analysis. In order to reduce this combined uncertainty, optimal sampling strategies become an indispensable tool. An optimal sampling strategy could guarantee, during an exploration campaign, to provide the maximum information about hydro(geo)logical conditions while limited to a given budget for collecting hydro(geo)logical data. It identifies those measurement locations and experiment types that together provide the highest information content towards specific goals such as reduced safety margins in WHPA delineation. Recently, it has been recognized that time variant groundwater flow leads to important variations in WHPA delineation when compared to equivalent steady-state WHPA solutions. Thus, when addressing optimal catchment investigation for WHPA analysis it becomes necessary to include transient groundwater conditions. The goal of this study is to implement and demonstrate optimal catchment investigation strategies for transient WHPA analysis while considering the influence of a dynamic environment triggered by transient groundwater flow considerations. Our objective is to provide an optimal sampling strategy that reduces efficiently the combined geological and transient uncertainty during WHPA delineation. We show preliminary numerical results of a synthetic three-dimensional transient groundwater scenario influenced by four transient drivers that can be found on seasonal scale: (I) regional groundwater flow direction, (II) strength of the regional hydraulic gradient, (III) natural recharge to the groundwater and (IV) pumping rate.