Decreasing groundwater temperature relieves seawater intrusion in coastal aquifers

Fresh submarine groundwater discharge (FSGD), as an important natural and renewable water resource, occurs along much of the global coast but finally gets lost into the sea. Exploiting FSGD, however, could induce seawater intrusion in coastal aquifers. Here, we show that decreasing groundwater temperature (by extracting fresh groundwater from and recharging cooled and more viscous water back into coastal aquifers) can relieve seawater intrusion and lead to safe exploitation of FSGD, based on laboratory experiments and numerical simulations. The performance of groundwater cooling can be approximately quantified by an analytical solution which shows its dependence on the FSGD extraction ratio and temperature contrast. The seawater retreat ratio (ratio of the seawater retreat distance to the length of seawater intrusion prior to water extraction) can reach 47.4% with groundwater cooling and without fresh groundwater exploitation. Furthermore, without incurring further seawater intrusion, groundwater cooling can lead to 35.7% FSGD exploitation.