Assessment of the impact of sea-level rise on seawater intrusion in sloping confined coastal aquifers

Abstract

Previous studies of the impact of sea-level rise on seawater intrusion (SLR-SWI) are mostly based on horizontal coastal aquifers, despite that most coastal aquifers are sloping. A recent study has shown that the effect of aquifer slope can not be neglected when assessing SLR-SWI in unconfined coastal aquifers. However, the effect of slope in confined aquifers has not been evaluated. In this study, we apply existing analytical solutions to quantify the effect of the slope of both upper and lower confining layers on SLR-SWI. The results show that for flux-controlled sloping confined coastal aquifer systems, SLR does not change the steady-state interface location, consistent with previous findings for horizontal aquifers. Therefore, our analysis focuses on head-controlled sloping confined coastal aquifer systems, for which we find the assumption of horizontal confining units causes significant errors. We demonstrate this by assessing the sensitivities of the seawater toe location and seawater volume to SLR for various combinations of sloping confining layers. The results indicate that the influence of the slope on SLR-SWI is small in thin confined aquifers. However, the slope is an important factor in thick confined aquifers. The influence of the slope in the lower confining layer on SLR-SWI is much greater than that of the upper confining layer. Aquifers that thin towards the sea are most susceptible to SLR-SWI, whereas those that thin towards inland are least susceptible. The assessment of SLR-SWI in sloping confined coastal aquifers in the current study demonstrates application of a rapid, first-order assessment tool that can be applied to a wide range of other sloping coastal confined-aquifer problems.