This paper compares the results produced by MODFLOW, a constant-density model, to results produced by SEAWAT, a variable-density model, to investigate the feasibility of using MODFLOW in a saline environment below an estuary known as the Indian River Lagoon. The comparison was conducted over sixteen numerical simulation cases at different conditions of estuarine salinity C L , hydraulic conductivity anisotropy ratio K r , and water table elevations on the freshwater boundaries in a two-dimensional vertical domain. The use of MODFLOW at the study site under the calibrated K r distribution ranging from 1000-20,000 was found to accurately match the field-measured and SEAWATsimulated results with a remarkable increase in accuracy at higher groundwater elevations. The study determined a critical value of K r of 1000 above which, MODFLOW simulations of the variable-density problem produced results that agreed well with those produced by SEAWAT. However, MODFLOW starts to produce significant errors with K r below the critical value and hence, it should not be used for simulating variable-density environments when K r <1000. The amount of submarine groundwater discharge (SGD) predicted by either model, and also MODFLOW accuracy in predicting the SGD are directly proportional to the head difference between the groundwater divide elevation and the lagoon water surface, but to a lower extent, are inversely proportional to C L .
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 m 3 /s to 0.53166 m 3 /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.
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