Water table salinization due to seawater intrusion

• Published in: Water resources research 2015-10, Vol.51 (10), p.8397-8408
• Main Authors: Badaruddin, Sugiarto, Werner, Adrian D., Morgan, Leanne K.
• Format: Article
• Language: English
• Subjects: Aquifers; Chemical analysis; Coastal aquifers; Computer simulation; density-dependent modelling; Discharge; Dispersion; Experiments; Fresh water; Freshwater; Freshwater resources; Groundwater; Groundwater table; Hydraulic conductivity; Hydraulic gradient; Hydraulics; Inland water environment; Laboratory experiments; Marine; Mathematical models; Modelling; Numerical simulations; Saline water intrusion; Salinization; Salt water intrusion; sand-tank experiments; Seawater; Seawater intrusion; Slope; Slope gradients; Slopes; Soil; Unconfined aquifers; Water analysis; Water table; watertable salinization
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1002/2015WR017098

Seawater intrusion (SWI) is a significant threat to freshwater resources in coastal aquifers around the world. Previous studies have focused on SWI impacts involving salinization of the lower domain of coastal aquifers. However, under certain conditions, SWI may cause salinization of the entire saturated zone of the aquifer, leading to water table salinization (WTS) in unconfined aquifers by replacing freshwater within the upper region of the saturated zone with seawater, thereby posing a salinity threat to the overlying soil zone. There is presently limited guidance on the extent to which WTS may occur as a secondary impact of SWI. In this study, physical experiments and numerical modeling were used to explore WTS associated with SWI in various nontidal, unconfined coastal aquifer settings. Laboratory experiments and corresponding numerical simulations show that significant WTS can occur under active SWI (i.e., the freshwater hydraulic gradient slopes toward the land) because the cessation of freshwater discharge to the sea and the subsequent landward flow across the entire sea boundary eventually lead to water table salinities approaching seawater concentration. WTS during active SWI is larger under conditions of high hydraulic conductivity, rapid SWI, high dispersivity and for deeper aquifers. Numerical modeling of four published field cases demonstrates that rates of WTS of up to 60 m/yr are plausible. Under passive SWI (i.e., the hydraulic gradient slopes toward the sea), minor WTS may arise as a result of dispersive processes under certain conditions (i.e., high dispersivity and hydraulic conductivity, and low freshwater discharge). Our results show that WTS is probably widespread in coastal aquifers experiencing considerable groundwater decline sustained over several years, although further evidence is needed to identify WTS under field settings. Key Points: Physical experiments of seawater intrusion and WTS reproduced in numerical model Field‐scale models show temporarily faster WTS than interface toe movements Largest WTS produced by aggressive seawater intrusion in deep, high conductivity aquifers