Seawater intrusion in coastal aquifers: Combined effects of salinity and temperature

Abstract

Seawater intrusion occurs commonly in coastal aquifers around the world, threatening the availability and usability of fresh groundwater resources for vegetation and human uses. The rapid growth of the world population and urbanization requires sound strategies for protection and management of the freshwater resources, especially coastal groundwater. This goal can only be achieved through proper understanding of the processes that underlie seawater intrusion in coastal aquifers. Major insights have been gained over the past several decades, in particular, roles of the density-driven flow in driving and maintaining the invasive flow of saltwater. However, most studies have linked the seawater intrusion process merely to the level of salt content through the free convection induced by the salinity contrast between groundwater and seawater but ignored their temperature difference. In reality, the thermal contrast between coastal groundwater and marine seawater may range up to 15°C in absolute value with either warmer or colder seawater. Such thermal contrast can alter seawater circulation through the coastal aquifer, which in turn affects the biogeochemical reactions of land-sourced pollutants in the aquifer prior to discharge to the marine ecosystem.This research aimed to investigate the combined effects of salinity and temperature contrasts on the interactions between freshwater and seawater in unconfined coastal aquifers. Using laboratory experiments and numerical simulations, this research explored the coastal groundwater dynamics under various boundary settings with regards to thermal variations, tidal forcing and seasonal changes of seawater temperature. Findings from this research provided insights into the importance of temperature variations on various key processes in coastal aquifers under the condition of either static sea level or tidal oscillation.The effects of temperature contrast were first investigated for the seaward boundary of static level using physical experiments and numerical models in combination with tracer tracking. With the static sea level, the thermal contrast induced long-term impacts on the aquifer and altered background flow patterns and transport activities. The position of the saltwater wedge toe was modified significantly by the presence of temperature gradient either landward or seaward. Colder seawater enhanced the advancement of saltwater while warmer seawater hindered it. More importantly, the seawater circulation pattern changed dramatically in the latter case. A second circulation cell was discovered for the first time near the seaward boundary. The regular landward circulation cell was pushed to the vicinity of the interface where considerably larger velocity was observed. In-depth sensitivity analysis revealed the important role of spatial correlation between temperature-induced and salinity-induced density gradients, especially at the base of the aquifer, in driving the formation of the new cell.Both laboratory experiments and numerical simulation were carried out to investigate the thermal effects under the condition of tidal oscillation. The responses of the saltwater wedge were found to be similar to those under the static sea level, in particular, the retreat and advance of the wedge with warmer and colder seawater, respectively. The mixing zone widened as a result of the tidal fluctuation. Meanwhile, the upper saline plume and the freshwater discharge zone expanded in the warmer seawater case and contracted with colder seawater. The increased seawater temperature also intensified water exchange across aquifer-ocean interface, seawater circulation and the submarine groundwater discharge. Furthermore, tidally induced seawater circulation intensified with increased contribution to the submarine groundwater discharge compared with density-driven seawater circulation. All these characteristics were persistent over a range of tidal amplitudes. These results shed light on the importance of the thermal effects and have important implications for the assessment of the biogeochemical processes in coastal aquifers.The seasonal variations of the temperature contrast were then examined based on numerical simulations. The results showed clearly seasonality of the aquifer – ocean exchange and seawater circulation induced by the seasonal variation of seawater temperature in both cases with the static sea level and tidal conditions. Compared with the cases of the isothermal condition, all fluxes increased during colder months and decreased during warmer months. The periodic oscillation of the thermally induced density gradient resulted in a continuously changing mode of saltwater flow in the saltwater wedge. The flow path and transit time of circulating seawater shortened considerably in comparison with that in the isothermal case. This finding is particularly important for the evaluation of transport of land-sourced contaminants to the marine environment.The insights into the thermal effects on coastal unconfined aquifers gained from laboratory experiments and numerical simulations were applied to calculate a thermal impact factor and a thermal sensitivity index for aquifers along global coastlines based on local conditions of freshwater temperature and temperature contrast. The results suggested that the temperature effect is significant and would either amplify or reduce the impact of sea level rise on the vulnerability of coastal aquifers over a large proportion of the global coastlines.