The stability of groundwater flow systems in unconfined sandy aquifers in the Netherlands

Groundwater flow patterns are temporally variable and uncertain, due to climatologically or anthropogenically induced variation in boundary conditions, and uncertainties in hydraulic model parameters. The quantification and mapping of uncertainty in flow patterns is especially essential in relatively flat areas where flow direction is sensitive to centimetre-scale head variations. In this study we aim to quantify and map the sensitivity of shallow groundwater flow patterns to uncertainties in anisotropy, drainage resistance, variations in drainage level and groundwater recharge for a sandy unconfined aquifer in the Salland region, the Netherlands. For this purpose, the most probable configuration of current groundwater flow systems was mapped using particle tracking and Monte Carlo analysis. Sensitivity was represented by the membership of each model cell to the defined groundwater flow systems given the uncertainties and variations in the hydraulic parameters and boundary conditions. In addition, the current configuration of groundwater flow systems was compared to the historical situation without artificial drainage. The average groundwater flow system size was found to be in the order of a few square kilometres, with a relatively stable configuration. In contrast to the intrinsic and temporally invariant hydraulic parameters, which were shown to have a minor influence on the spatial configuration of groundwater flow systems, natural variation in recharge and variations in drainage level management exert a large influence.