Thermal Effects on Flow and Salinity Distributions in Coastal Confined Aquifers

Abstract

Coastal aquifers provide an important hydrologic connection between terrestrial freshwater and oceanic seawater. Understanding the water flow and salinity distributions in those aquifers is essential to manage seawater intrusion and regional groundwater resources. For the last 50 yr, these processes have been extensively examined; however, previous studies typically assumed isothermal conditions and overlooked groundwater-seawater temperature contrasts, as commonly found along the global coastline. Here, we validated a solute and heat transport model against laboratory experiments and then revisited the classical Henry problem for coastal confined aquifers. We found that as the seawater temperature increases or freshwater temperature decreases, the freshwater-seawater interface retreats seaward and thus the freshwater storage increases. This occurs as a result of temperature-induced changes to the fluid density and aquifer hydraulic conductivity. Furthermore, double diffusion of salt and heat can induce two circulation cells in opposing directions in the saltwater wedge once the seawater temperature exceeds that of groundwater by a certain extent. Consequently, the circulating seawater flux and associated travel time change dramatically compared with isotheral conditions. Overall, the effect of thermal forcing can be significant, as demonstrated by a sensitivity analysis considering a large range of groundwaterseawater temperature contrasts. These results highlight the impact of changing thermal regimes on the flow and salinity distributions in coastal confined aquifers and provide guidance for better assessment of water resources in coastal zones. Plain Language Summary Coastal zones are often densely populated with associated significant economic activity. Groundwater in coastal aquifers provides an essential freshwater resource for domestic and industrial use as well as food production. Intensive human activities and climate change cause water regime shifts and thus threaten water security in coastal areas. Coastal water resource assessments often overlook groundwater-seawater temperature contrasts, even though such contrasts are globally ubiquitous. Here, we show those assessments could be improved with thermal effects considered as temperature contrasts can markedly influence salinity distributions and water circulations in coastal aquifers. The results provide guidance for better assessment of water resources in coastal zones. Water temperature affects oceanland exchange and various biogeochemical processes, which should be considered in future studies including climate change evaluations.

Influence of Dynamically Stable‐Unstable Flow on Seawater Intrusion and Submarine Groundwater Discharge Over Tidal and Seasonal Cycles

“…It has been established that external forces acting on the landward and seaward boundaries, for example, the tidal stage (Henderson et al., 2010), tidal amplitude (Abarca et al., 2013), seawater temperature (Befus et al., 2013; Nguyen et al., 2020; Pu et al., 2020), rainfall (Yu et al., 2017), and inland hydraulic gradient (Kuan et al., 2012; Yu, Xin, & Lu, 2019), influence the USP configuration. These forces operate at various timescales, such as seasonal or daily seawater temperatures (Anderson, 2005), spring‐neap tides with different amplitudes (Robinson, Gibbes, et al., 2007), and seasonality in inland hydraulic gradients (Liu et al., 2016), thus, creating a dynamic intertidal environment.…”

Thermal effects on flow and salinity distributions in heterogeneous coastal aquifers with fixed-flux inland boundaries

“…FW-SW temperature contrasts have been widely reported around global coastlines (Wilson, 2005;Befus et al, 2013;Paldor et al, 2020;Pu et al, 2020). Temperature could alter the fluid density and viscosity in coastal aquifers, regulating density-driven flow and FW-SW mixing processes (van Lopik et al, 2015).…”

“…Temperature could alter the fluid density and viscosity in coastal aquifers, regulating density-driven flow and FW-SW mixing processes (van Lopik et al, 2015). Salinity distributions and groundwater discharge could thus be significantly regulated as the thermal regime changes in coastal aquifers (Nguyen et al, 2020;Pu et al, 2020). Wilson (2005) pointed out that geothermal convection could be developed in regional-scale coastal flow systems, contributing to the saline submarine groundwater discharge.…”

“…They found that higher seawater temperatures significantly increase tide-induced seawater circulations and thus affect fresh groundwater discharge zones and saltwater wedges. Recently, Pu et al (2020) revisited the classical Henry problem for coastal confined aquifers and considered FW-SW temperature contrasts. Both laboratory experiments and numerical simulations demonstrated that warmer freshwater or colder seawater can lead to landward FSI in homogeneous coastal aquifers.…”

Climate and Seasonal Temperature Controls on Biogeochemical Transformations in Unconfined Coastal Aquifers

“…Temperature is an important control on groundwater flow and biogeochemical activity in shallow unconfined aquifers due its effect on fluid viscosity and microbial reaction kinetics. Pu et al (2020) conduced laboratory tank experiments and used numerical variable-density flow and heat transport models of an unconfined coastal aquifer without tidal influence and showed that the saltwater-freshwater interface shifted seaward with an increase in seawater temperature or a decrease in fresh groundwater temperature. Temperature controls on flow and salt transport have also been investigated in beach aquifers.…”