Protection of drinkable groundwater resources from contamination has increased importance in the last decades. European countries, and recently also Italy, have taken the time of travel (TOT) criterion as a rule for the delineation of wellhead protection areas (WHPAs) near drinkable wells. This method defines WHPAs as the surface which contaminants move through and reach the well over a specified period of time if advection mechanism is considered. This is a simplistic assumption because it doesn't account for another important transport mechanism like hydrodynamic dispersion. The aim of this paper is to investigate the effects arising from the inclusion of hydrodynamic dispersion mechanism in delineation of WHPAs. This operation was performed implementing, under the framework Visual Modflow (i.e. a widespread flow and contaminant transport interface to MODFLOW and MT3D), a backward probabilistic method based on an ad joint of contaminant transport ADE (Neupauer & Wilson, 1999). Visual Modflow is not ready to solve ad joint of transport equation, so a Matlab script was developed to reverse the flow field and allow running of MT3D.
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