A numerical investigation of the effects of model parameterization on the delineation of source protection zones under uncertainty

Source protection zone delineation has evolved over the past decades from fixed radius or analytical and numerical methods which do not consider uncertainty, to more complex stochastic numerical approaches. In this paper we explore options for delineating a source protection zone, while considering the inherent uncertainty involved in characterizing hydraulic conductivity. We consider a representative pumping well in an unconfined alluvial aquifer under steady-state flow conditions, with the hydraulic conductivity distribution inferred from borehole lithology data in the West Melton area near Christchurch, New Zealand. Lithologies are categorized according to their inferred hydraulic flow and transport properties, using two to four hydrofacies groupings. Probabilistic source protection zones are determined for alternative lithology categorization scheme and hydrofacies conductivity parameterization methods. Results show that the choice of calibration method significantly impacts the delineated source protection zone. In heterogeneous aquifers, the degree of protection offered by deeper pumping wells may be overstated, and forward particle tracking proved more comprehensive than backward tracking due to the complexity of flow paths near the well screen. Simple models, such as homogeneous models, require upscaled parameters to effectively represent aquifer heterogeneity, providing insights into how simplified source protection zone delineation could be made more robust in highly heterogeneous contexts.