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Modeling of Groundwater Pollution by Former Seawater in Submarine Groundwater Discharge Driven by Saltwater Intrusion

Al-Taliby Wissam, Dekhn Hadeel

Journal of Ecological Engineering

2022

Vol. 23, nr 2

166--175

Coastal aquifers are usually vulnerable to contamination by saltwater intrusion. The degree of contamination depends on the amount of former seawater intruding the aquifer as submarine groundwater discharge (SGD). A three-dimensional numerical SEAWAT model was developed to provide insights to the responses of saltwater intrusion and amount of seawater portion of SGD, to groundwater withdrawal from four wells at different spatial locations. The results showed that saltwater wedge encroachment varies with a withdrawal rate and well location. Increasing withdrawal rates from 0.165 m3/s to 0.53166 m3/s resulted in a noticeable increase in wedge encroachment into the aquifer from 589 m to 1319 m, respectively, regardless of the distance from the coastline. However, higher withdrawals from the wells closer to the coastline caused the hydraulic head near the seaside to drop below the terrestrial head, causing the saltwater wedge to be pushed back toward the sea. Simulations also showed that a coastal well might act as a hydraulic barrier that prevents the brackish zone from moving further inland, which is critically important in terms of groundwater management. Seawater contributed from 5% to above 33% in SGD. Although the withdrawal rates from Well 3 were 10 to 20 times larger than the Well 2 rates, the seawater contribution associated with Well 3 was about the same. This is attributed to the horizontal vicinity of Well 2 to the seacoast boundary compared to Well 3 even though Well 3 is deeper than Well 2, demonstrating that it is the horizontal location that governs the amount of abstracted former seawater.

Effect of Variable-Density and Constant-Density Representations of Flow on Simulating Terrestrial Groundwater Discharge into a Coastal Lagoon

Al-Taliby Wissam, Dekhn Hadeel

Journal of Ecological Engineering

2021

Vol. 22, nr 11

188--197

Terrestrial groundwater discharge (TGWD) can be an important pathway for pollutants into coastal water bodies. Thus, a reliable way to quantify it is essential for efficient coastal management practices. This study evaluated the feasibility of using constant-density models for estimating TGWD amounts into the Indian River Lagoon, which is a variable-density estuarine environment. Constant-density models were developed using MODFLOW, while variable-density models were developed using SEAWAT. The numerical models were calibrated to match the field measured head data under the lagoon. The amounts of TGWD into the IRL and hydraulic head distributions calculated by the two codes were compared over eight pairs of numerical experiments. Two of those numerical experiments used the calibrated model and field measured conditions, while the rest of them used modified versions of the calibrated models, including variable anisotropy ratio k, variable lagoon salinity LS, and increased water table elevation by 5%. The results showed that the constant-density model is fairly accurate in estimating TGWD and head distributions at the calibrated k in the range of 1000–20,000 with an error not exceeding 9.4% under the actual measured field conditions. Even when LS was assumed to increase to ocean salinity value of 1.0, a case that rarely occurs in IRL, the calibrated constant-density model’s accuracy was not affected substantially. However, the constant-density model failed to represent the physics of the variable-density environment at k values lower than 1000, where the error exceeded 129%. Generally, the accuracy of the constant-density model was found to increase substantially at lower LS and higher water table elevations.