Water table salinization due to seawater intrusion

Abstract

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.

Plausibility of freshwater lenses adjacent to gaining rivers: Validation by laboratory experimentation

“…Salinities adopted within GHB representations of the freshwater reservoir in the various experiments were taken from the experimental measurements listed in Table , thereby accounting for the mixture of freshwater and saltwater contained therein. Dispersive simulations adopted longitudinal and transverse dispersivities of 0.1 and 0.01 m, respectively, based on values used in other experiments of similar scales [e.g., Jakovovic et al ., ; Badaruddin et al ., ]. Parameters were otherwise as listed above.…”

Assessment of Seawater Intrusion into Shallow Coastal Aquifer in Kanyakumari District, Tamilnadu, India

“…Excessive extraction of groundwater, particularly from coastal areas, resulting in seawater intrusion, is a global problem [11,12]. Measurement has been performed by means of a number of methods and techniques, including geoelectrical resistivity [13][14][15], analytical design methods [16][17][18][19] as well as the commonly used quantitative model of simulation [20][21][22][23][24]. The Chloride Bicarbonate Ratio method was used in this study to determine the occurrence of seawater intrusion, using an ion ratio method.…”

Evaluating plant community response to sea level rise and anthropogenic drying: Can life stage and competitive ability be used as indicators in guiding conservation actions?

“…During the dry season, the vadose zone can dry up, drawing more saline groundwater from below and increasing soil salinity. Conversely, during the wet season, when precipitation is high, rainwater seeps into the vadose zone, flushing salt from the soil column, recharging the freshwater lens, and freshening the groundwater table Badaruddin et al, 2015). The seasonal desalinization process, along with slightly higher elevation, allows glycophytic (freshwater) plant communities to exist in salty coastal areas (Sternberg et al, 2007;Teh et al, 2008;.…”

“…Increases in groundwater salinity and inundation periods and depth drive shifts in plant community composition (4,9); however, organisms themselves can act as ecosystem engineers, modifying habitats to increase their survival (16). The effect of salt on plants, communities, and plant competition is well established (17)(18)(19)(20), but there are conflicting results on whether and how plants influence soil salinity (21)(22)(23)(24)(25). As SLR salinizes coastal groundwater and soil, it becomes important to understand how halophytes (salt-tolerant plants) invading glycophytic (non-salt-tolerant) communities affect soil salinity (24), to determine whether a biological component should be added to the causes of community change predicted with SLR.…”

“…As part of that project, they created a map from 1978 aerial photos of the coastal communities stretching across the Flamingo portion of ENP and sampled vegetation on a 700m transect along an elevational gradient ranging from less than 0.5m to 1.5m above mean sea level 23 . The transect included 5 of 6 coastal south Florida plant communities (black mangrove forest, coastal tropical hardwood hammock (tropical hardwood hammock), buttonwood forest, halophyte prairie, and white mangrove forest) and one transition zone (buttonwood/manchineel hammock).…”