Development of seawater-freshwater interface in heterogeneous coastal aquifers

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Rathore, Saubhagya Singh Singh
Luo, Jian
Georgakakos, Aristidis P.
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The effects of the hydraulic conductivity heterogeneity on seawater intrusion (SWI) and submarine groundwater discharge (SGD) was studied. We, for the first time, identified a single parameter—Transmissivity Centroid Elevation (TCE)—encompassing the effects of the spatial distribution of hydraulic conductivities. Higher values of TCE, i.e., higher conductivity zones lying in the upper part of the aquifer, represent a greater proportion of discharge in the upper aquifer and result in greater SWI extent and SGD. Using the proposed novel TCE framework, we derived compact analytical solutions for SWI and SGD in stratified aquifers. To homogenize stratified aquifers, we then derived effective hydraulic conductivity as a function of TCE which represents layer placement. For uncertain conductivity fields modeled as random stratification, we derived explicit analytical solutions for the moments of toe-position and discharge to quantify uncertainties. We found that the elevation of the preferential flow layer has a significantly more dominant effect than hydraulic conductivity contrast. To delineate the seawater-freshwater interface profile separating zone of distinct salinity, we extended the TCE concept to the local transmissivity parameters premised on the insight that the extent of SWI only depends on the transmissivity field above the interface represented by local TCE and local transmissivity. Leveraging the effectiveness of local-transmissivity parameters in estimating SWI, we developed a semi-analytical technique to compute the seawater-freshwater interface in aquifers with hydraulic conductivity varying along two dimensions. The semi-analytical technique is able to compute the interface with great accuracy when compared with numerical solutions of coupled variable-density flow which is time-consuming and computationally expensive. This rapid computation of the interface allowed us to perform a comprehensive stochastic and sensitivity analysis of SWI in a 2D heterogeneous case. The uncertainty in the SWI is heavily influenced by the degree of heterogeneity and weakly influenced by the scale of heterogeneity. We found that geometric mean as an effective parameter to homogenize the aquifer underestimate SWI on an average. We also found that the near-coast near-top region of the aquifer controls the SWI extent, and aquifer characterization efforts should be concentrated mainly in this region. The findings have significant implications like fast delineation of the seawater-freshwater interface, improved SWI control, uncertainty estimation, impacts of preferential flow paths, aquifer homogenization and upscaling, optimization of field characterization and transient analysis of SWI.
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