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Numerical Modeling of Seawater Intrusion into Coastal Karst Aquifers

Sousanabadi Farahani, Reza | 2023

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 56628 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Ataie Ashtiani, Behzad
  7. Abstract:
  8. Coastal groundwater aquifers are among the prime resources for providing fresh water for coastal cities that are in danger of intensified seawater intrusion due to climate change and excessive pumping. These coastal aquifers extensively exist in different cities around the world, for example, in the south and north of Iran, and due to their complex inherent, they have yet to receive enough attention among scholars. The prime objective of this research is to simulate seawater intrusion by considering preferential path flows using COMSOL Multiphysics. Based on numerical approaches merits, this study aims to simulate seawater intrusion in karst aquifers. It explores the effects of conduits' characteristics on SWI parameters such as Toe Length (TL) and the width of the mixing zone (WMZ). The developed numerical models are verified against the experimental models in the literature. Simplifying assumptions are applied to simulate SWI due to complexity and heterogeneity like Darcy equations describing flow and contaminant transport within the fractures and preferential path flows. However, numerous studies demonstrating a high Reynolds number in this environment show nonlinear flow. Thus, this research uses the Darcy-Brinkman equation (DB) to simulate SWI in Karst aquifers and compares the results with the Darcy model. Results demonstrate that in a steady state, the seawater progress rate in the Darcy model is 2.63 which is higher than the DB model which is 2.22. However, seawater progress in the DB model is faster during the pumping and resting periods. In addition, Comparing seawater intrusion parameters in the two models do not show significant differences. Additionally, this study investigates the conduits' features (conduit's position in the length and depth of the aquifer, permeability, orientation, diameter, and length) effect on SWI under pumping and resting periods. Among the considered features, TL has the highest sensitivity to the conduit's length, and among the developed models, the TL varies between 0.96 and 1.98 meters by changing the conduit’s length. To present extraction policies in such unique environments, remediation techniques such as intermittent pumping, injection well position, and Groundwater circulation wells (GCWs) have been studied. Injection well's position effect on SWI has been studied in homogeneous aquifers in the literature, and this study explores this effect for karst aquifers for the first time. The results reveal that relocating injection wells away from the sea boundary and toward the end of the conduit increases the efficiency of this system. For instance, an aquifer’s salinity percentage (concentration equal or more than 350 mol per cubic meter) of 31.65 can be alleviated to 27.99 by inserting an injection well close to the sea boundary (SB) and moving the well away from the SB decreases this rate to 20.91. Studying intermittent pumping demonstrates that shortening the pumping and resting duration decreases the salt concentration at the pumping well; however, it keeps the aquifer more saline, which is undesirable. The results show that the maximum relative salinity can be decreased from 0.56 to 0.396, but the aquifer remains 17 percent more saline. Finally, GWCs have been explored as a hydraulic barrier in controlling SWI, and the results prove that the efficiency of this system depends on the wells' distance, location in depth, and location in the length of the aquifer. Increasing the wells' distance leads to a greater circular velocity field, making the system more efficient. It is shown that the system's efficiency is highly sensitive to the depth of the wells, and installing them in a shallow depth not only prevents SWI but also intrigues its extent
  9. Keywords:
  10. Karst Coastal Aquifers ; Darcy-Brinkman Model ; Seawater Intrusion ; Numerical Modeling ; Coastal Aquifer ; Fractured Coastal Aquifers

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