Reasons and implications of fossil hydraulic gradients in large-scale aquifer systems

Due to limited availability of surface water, many arid and semi-arid countries have to rely on their groundwater resources. Despite the quasi-absence of present-day replenishment, some of these aquifers contain large amounts of water that was recharged during wetter periods in the past. Although these recharge events often occurred several thousand years ago, some of these so-called fossil aquifer systems exhibit considerable hydraulic gradients and flow towards their discharge areas, even without pumping. As a result, these systems have more discharge than recharge and are therefore not in steady state, which makes their modelling, in particular the calibration, very challenging. Probably the most relevant explanation for this phenomenon is the long groundwater residence time and the associated long-term head decay of the paleo-recharge mounds. In this study, we propose a possible approach to deal with this phenomenon during model calibration. Moreover, we analyse the effect of considering and ignoring fossil gradients on aquifer parameterization for the Upper Mega Aquifer system on the Arabian Peninsula. Further details can be found in a related publication (Schulz et al., 2017). References Schulz S, Walther M, Michelsen N, Rausch R, Dirks H, Al-Saud M, Merz R, Kolditz O, Schüth C. 2017. Improving large-scale groundwater models by considering fossil gradients. Advances in Water Resources 103: 32–43 DOI: 10.1016/j.advwatres.2017.02.010