Flow and Transport in Coastal Aquifer‐Aquitard Systems: Experimental and Numerical Analysis

• Published in: Water resources research Vol. 60; no. 4
• Main Authors: Zhang, Jiaxu, Lu, Chunhui, Shen, Chengji, Liu, Yuxuan, Werner, Adrian D., Zhang, Chenming
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.04.2024
• Subjects: Aquifer permeability; Aquifer systems; Aquifers; Aquitards; Chemical analysis; Coastal aquifers; Coastal plains; Coastal structures; Confined aquifers; Confined groundwater; Efflux; Freshwater; Geochemistry; Groundwater; Groundwater discharge; Groundwater flow; Hydrologic cycle; Hydrologic processes; Hydrological cycle; Inland water environment; Laboratories; Laboratory experimentation; Laboratory experiments; Leakage; Leaky aquifers; Mass balance; Mathematical models; Numerical analysis; Numerical models; Numerical simulations; Oceans; Offshore; Permeability; Saline water; Saline water intrusion; Saline-freshwater interfaces; Salinity; Salt advection; Salt water intrusion; sand tank experiments; Seawater; Seawater intrusion; submarine groundwater discharge; Tidal effects; tidal fluctuation; Tides; Transport processes; Unconfined aquifers; Water analysis; watertable overheight
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1029/2023WR035200

Coastal aquifers are commonly layered, and thus, a clear understanding of groundwater flow and salt transport in layered coastal aquifers is important for managing fresh groundwater. However, the influence of leakage between adjacent aquifers on flow and transport processes remains largely unknown where the influence of tides is considered. This study used laboratory experiments and numerical simulation to examine the processes of flow and transport within a tidal aquifer‐aquitard system (i.e., an unconfined aquifer underlain by a semi‐confined aquifer, with an intervening thin aquitard). The laboratory‐scale observations of the current study are the first observations of offshore fresh groundwater within a semi‐confined coastal aquifer. The numerical and laboratory results are in close agreement, revealing that upward leakage from the semi‐confined aquifer into the saltwater wedge of the overlying unconfined aquifer caused buoyant instabilities to form. The development of freshwater fingers created complex saltwater‐freshwater mixing, leading to mixed saltwater influx‐efflux patterns across the sloping aquifer‐ocean interface. Compared with non‐tidal conditions, tidal forces reduced the net upward leakage from the semi‐confined aquifer to the overlying unconfined aquifer. This increased the horizontal flow toward the sea, which in turn reduced the extent of the saltwater wedge in the semi‐confined aquifer. The higher rates of both fresh and saline submarine groundwater discharge (SGD), caused by tides, led to lower groundwater ages in the semi‐confined aquifer. These findings have important implications for unveiling the complex characteristics of seawater intrusion, SGD and geochemical hotspots within layered coastal aquifers. Plain Language Summary Coastal aquifers contain complex and dynamic hydrological and geochemical processes, which profoundly influence the global water cycle and chemical mass balance. Many coastal aquifers exhibit layerd structures in the form of high‐permeability aquifers alternating with thin aquitards. Inter‐aquifer leakage is a common issue in layered coastal aquifers, but few studies explore its impact on the mixing zone of saltwater wedges. The effects of tides on groundwater dynamics and the seawater extent in layered aquifers also remain poorly known. This study used laboratory experiments and numerical modeling to explore the effects of inter‐aquifer leakage and tides on flow and salinity dynamics within layered aquifer systems. We found that the upward leakage extended the mixing zones from the edges of the saltwater wedge to its interior within unconfined aquifers, leading to mixed saltwater influx‐efflux patterns across the aquifer‐ocean interface. The introduction of tides restricted the seawater extent in semi‐confined aquifers. This is a primary consequence of the tide‐induced increase in the horizontal freshwater flow toward the sea through the semi‐confined aquifer, in addition to the increases in the density‐driven seawater recirculation caused by tides. These findings highlight the important role of inter‐aquifer leakage and tides in layered coastal aquifers. Key Points Leakage from semi‐confined aquifers caused mixed‐convective flow within the saltwater wedge of overlying unconfined aquifers Tidal fluctuations reduced the net upward freshwater leakage, and consequently the extent of seawater, in semi‐confined coastal aquifers Tides increased both fresh and saline submarine groundwater discharge in semi‐confined aquifers, reducing groundwater ages