Seawater intrusion in fractured coastal aquifers influenced by tides: Laboratory and numerical investigations

Abstract

Fractured aquifers are particularly vulnerable to seawater intrusion (SWI), but the mechanism of SWI in such aquifers remains largely unknown. This study aims to investigate the influence of tides on SWI in fractured aquifers through laboratory experiments and numerical models. Six fractured aquifers and one homogeneous aquifer were employed to conduct laboratory and numerical experiments to preliminary analyze the comprehensive effects of tides and fractures on salinity distributions, flow fields and submarine groundwater discharge (SGD). Sensitivity analyses were performed to further explore the impact of each fracture characteristic (i.e., horizontal position, vertical position, length and orientation) on SWI properties (i.e., saltwater wedge, mixing, saline volume) and concentration distribution under non-tidal and tidal conditions. The results indicate that tides can expand the variation range of most SWI properties in fractured aquifers and alter the influence patterns of fracture characteristics. Fractures tend to converge around freshwater, and their effect on SWI properties depends on their relative position to the saltwater. Toe length (TL) reached its maximum when the fractures had just separated from the saltwater wedge and were fully submerged in freshwater. Longer fractures promoted SWI and relieved the tides’ restriction on TL. Moreover, horizontal fractures can significantly amplify high-concentration (50%-90%) saltwater distribution, SGD and upper saline plume (USP) with the help of tides, while vertical fractures only had an impact on the mixing zone. These results have important implications for improving our understanding of comprehensive impact of tides and fractures on SWI in coastal subsurface environments.