Freshwater-seawater interface is one of the most important regions in coastal aquifer systems, delineating the subsurface into zones with distinct fluid density and biogeochemical properties. The growth and decay of the interface control the subsurface flow field and water and chemical exchange processes between groundwater and ocean environments such as seawater intrusion (SWI) and submarine groundwater discharge (SGD). Although there are extensive studies regarding seawater intrusion and submarine groundwater discharge, there remain huge knowledge gaps in understanding the impacts of formation heterogeneity making it challenging to accurately estimate the seawater intrusion extent and submarine groundwater discharge rates, upscale effective aquifer parameters and quantify uncertainties. In this study, we aim to develop a series of analytical and numerical solutions to quantify seawater intrusion and groundwater discharge and identify key governing parameters encompassing the effects of heterogeneity. Using the obtained new insights, we aim to achieve fast delineation of the freshwater-seawater interface, control of SWI, understanding the impact of the preferential flow path, aquifer homogenization and upscaling, fast ensemble computations for uncertainty analysis, optimization of aquifer characterization.
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 when the higher conductivity zones lie in the upper part of the aquifer represents a greater proportion of discharge in the upper aquifer and results in greater SWI extent and SGD. Based on the TCE concept, we derived compact analytical solutions for the SWI and SGD for stratified aquifers. To homogenize stratified aquifers, we then derived effective hydraulic conductivity as a function of TCE which represents layer placements. For uncertain conductivity fields modeled as random stratification, we derived explicit analytical solutions for the moments of toe-positon 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 salinities, 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 the aquifer 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 takes hours to converge. This rapid computation of the interface allowed us to perform a comprehensive stochastic and sensitivity analysis of SWI in a 2D heterogeneous case. We found that the near-coast-near-top region of the aquifer controls the SWI extent, and aquifer characterization efforts should mainly be focused mainly on this region. Recommendations on future work are made to analyze the effect of heterogeneity on the transience of SWI.
Dr. Jian Luo
Dr. Aris Georgakakos
Dr. Jinfeng Wang
Dr. Yi Deng (EAS)
Dr. Marc Stieglitz (NSF)