Yu Ye

The spatial variability in coastal aquifer hydraulic properties significantly affects the extent of seawater and its movement. Yet, previous analytical solutions have almost exclusively evaluated only homogeneous and stratified aquifers. This study derived analytical solutions for the steady‐state, sharp‐interface seawater extent in two‐dimensional coastal aquifers (in the horizontal plane) comprising two alongshore regions of differing hydraulic conductivities ( K ), based on Laplace's and Poisson's equations. Numerical simulations were performed to validate the analytical solutions. The derived analytical solutions demonstrate that an alongshore change in K has a negligible effect on the steady‐state seawater extent under a constant and uniform head inland boundary, because the head is constrained uniformly alongshore, causing no groundwater to flow across the two regions. Any spatial change in head (e.g., the groundwater mound caused by distributed recharge) would reintroduce sensitivity. Otherwise, a constant‐flux inland boundary induces an alongshore variation of the seawater extent when an abrupt change in K is considered. Fresh groundwater flows from the lower‐ K to the higher‐ K zone, producing an over‐representation of freshwater discharge to the sea in the higher‐ K zone. The alongshore distance from the K transition boundary where the seawater extent is affected is approximately two to two‐and‐a‐half times the distance from the shoreline to the inland boundary, based on a defined threshold for negligible influence. The analytical solutions provide a convenient method for rapidly assessing the steady‐state seawater extent in aquifers with an alongshore change in K , with potential applications in seawater intrusion prevention and coastal groundwater management.

Accelerating sea-level rise (SLR) exacerbates coastal seawater intrusion (SWI), yet how fracture heterogeneity regulates SLR-driven SWI in fractured coastal aquifers remains underexplored. This study investigates the coupled impacts of fracture characteristics and SLR on SWI through laboratory experiments and numerical simulations based on the discrete fracture matrix (DFM) model. Seven fractured aquifers and one homogeneous aquifer were employed to analyze the effects of fracture position and inclination on the saltwater wedge (SW) and mixing zone (MZ) under SLR. The validated laboratory-scale model was then scaled up to perform sensitivity analyses, further exploring how individual fracture attributes (i.e., horizontal position, vertical position, and inclination) and varying SLR magnitudes affect SWI properties. Results reveal that SLR modulates the regulatory effects of fractures on SWI by altering the relative spatial relationship between fractures and the SW. For horizontal fractures, longer freshwater-exposed length and greater separation from the SW shift their role from restraining to facilitating saltwater expansion. However, SW submergence or re-entry into the fracture’s impact zone can negate or activate its effects. Inclined fractures deflect underlying saltwater, directing MZ expansion and concentration while limiting SW extent. Fractures approaching parallel to the fresh-saltwater interface can more significantly affect SWI following SLR. Furthermore, increasing SLR magnitude generally induces linear increases in SW variables, but can cause highly non-linear MZ variable responses. These findings provide mechanistic insights into SWI mitigation and fresh groundwater management in fractured coastal aquifers under ongoing global SLR.

The accurate prediction of groundwater contamination is challenging due to uncertainties arising from the inherent heterogeneity of aquifers, inadequate site characterization, and limitations in conceptual mathematical models. These factors can result in an underestimation of contaminant concentrations. For effective contaminant prevention and control, it is important to estimate the probability of exceeding the allowed threshold for contaminant concentrations, known as the failure probability of groundwater contamination. Computing small failure probabilities using classical Monte Carlo simulation (MCS) requires computing a large number of samplers to converge to a stationary target value, which is time‐consuming. To address this, in this paper, we develop a novel approach for calculating small failure probabilities, known as subset simulation (SS) coupled with preconditioned Crank‐Nicolson Markov chain Monte Carlo (pCN‐SS), which combines subset simulation with preconditioned Crank‐Nicolson Markov chain Monte Carlo (pCN‐MCMC) to promote computational efficiency. We have tested the performance of the proposed algorithm in both a mathematical example and a numerical case study of groundwater contamination. The results demonstrate that pCN‐SS provides improved accuracy and efficiency for evaluating small failure probabilities for high‐dimensional groundwater contamination, specifically for hydraulic conductivity as a source of uncertainty. Compared to classical MCS and traditional SS, pCN‐SS requires fewer model evaluations but produces stable and accurate results.