Aquifer Response to Estuarine Stream Dynamics

Abstract

While seawater intrusions are widely discussed, the salinization of coastal aquifers via narrow rivers is hardly documented. This study investigates groundwater dynamics in an aquifer next to an estuarine stream on the eastern Mediterranean coast. Groundwater levels and salinization patterns were examined as a response to dynamic changes in estuary water, both in low-and high-permeability aquifer units. In the high-permeability unit, the extent of salinization was relatively constant, reaching a distance of at least 80 m from the river, with no long-term changes in fresh-saline interface depth, indicating that the system is in a quasi-steady state. Groundwater salinity in the low-permeability unit showed frequent and large fluctuations (up to 36 and 22 at 5 and 20 m from the river, respectively). We suggest that the river may have a more immediate impact on a low-permeability than on a high-permeability aquifer. This is dependent on the history of seawater encroachments to the river, which are better preserved in the low-permeability unit, and on the hydrogeology of this unit, where sand lenses can serve as high-permeability conduits. However, this unit can efficiently prevent a large extent of salinization of the regional coastal aquifer by the estuary water.2 of 20 settings (e.g., References [10,25] and the references therein) [26][27][28][29], the salinization of coastal aquifers via estuarine streams was hardly studied in the field [14,16,30,31].The rate at which water exchanges between surface water and the aquifer is controlled by hydraulic gradient and sediment permeability. In rivers and estuaries, this exchange is also known as a hyporheic exchange [31][32][33][34]. Groundwater-surface water interaction in the estuarine environment is influenced by density contrasts between the typically fresh groundwater and saline-to-brackish estuarine water, which leads to convective circulation in the aquifer [7,19,[35][36][37]. Circulation occurs even if the hydraulic gradient in the aquifer dips toward the estuary [31,38]. The size of the resultant salt wedge is dependent on the hydraulic gradient, aquifer conductivity, and estuary salinity [31,36,39].In this paper, we investigate the dynamics and mechanisms of aquifer salinization next to a bar-built estuarine river by integrating high-resolution level and salinity measurements in the river and the adjacent aquifer. The Study AreaThe Alexander River is a perennial river on the Mediterranean coast of Israel (Figure 1a). The drainage area of the Alexander River is relatively small (555 km 2 , Figure 1a) and, thus, it experiences relatively small peak discharges (up to~180 m 3 /s in recent years; data from the Hydrological Service of Israel). Although it is a perennial river, the river's natural flow to the sea is insignificant during the dry season (the base flow entering the estuarine part is typically ≤0.3 m 3 /s), which is mostly composed of effluents from sewage treatment plants and aquaculture drainage [40].

The Issue of Groundwater Salinization in Coastal Areas of the Mediterranean Region: A Review

“…Another point that can be stressed from the reported literature, is that there is a net prevalence of the spatial analyses [211][212][213][214][215][216], with respect to the temporal trend analyses [217][218][219][220][221][222][223]. This is due to the fact that it is much more challenging to obtain consistent hydrochemical information for long time periods, because the monitoring network must have been sampled and analyzed for the major and minor water constituents without important gaps.…”

Assessment of sea-level rise impacts on salt-wedge intrusion in idealized and Neretva River Estuary

“…Note that value V * represents the total seawater to fresh-water ratio along the salt-wedge. This information may sometimes be more important than the intrusion length as it directly relates to salinity intrusion into aquifer, especially in highly permeable mediums (Shalem et al, 2019).…”