A Semianalytical Method to Fast Delineate Seawater‐Freshwater Interface in Two‐Dimensional Heterogeneous Coastal Aquifers

Abstract

The 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. Heterogeneity in hydraulic conductivity is inherent to geological formations, resulting in distinct interface profiles. Currently available analytical solutions are limited to homogeneous and stratified aquifers, and numerical simulations of variable-density flow and transport are the only option to characterize the interface in a two-or three-dimensional heterogeneous aquifer. This study presents a novel semianalytical method to fast delineate the steady seawater-freshwater interface in a two-dimensional, confined, heterogeneous aquifer with a constant flux inland boundary and indistinct seepage face at the vertical coastal boundary. The heterogeneous domain is conceptualized as a series of thin columns of stratified aquifers with the horizontal flow in each of them. The proposed approach is based on the concept of local transmissivity parameters and shows high accuracy in the interface delineation. Leveraging the ability to delineate the interface rapidly along with the Monte Carlo approach, we numerically investigate the stochastic behavior of the interface profile. The uncertainty in seawater intrusion is heavily influenced by the degree of heterogeneity and weakly influenced by the scale of heterogeneity. To homogenize the aquifer, we found that geometric mean generally underestimates the seawater intrusion and cannot be used as an effective parameter. We also showed that the near-coast, near-top region of the aquifer is the most influential region and should be characterized at a higher resolution to reduce uncertainties in estimating seawater intrusion.

Impacts of the Scale of Representation of Heterogeneity on Simulated Salinity and Saltwater Circulation in Coastal Aquifers

“…In the upscaling procedure, we used a simple mean and did not consider hydraulic conductivity in the diagonal direction. This problem has been pointed out by Rathore et al (2020), who found that the geometric mean underestimates the seawater intrusion extent on average. Using a full hydraulic conductivity tensor may preserve more detailed heterogeneity (Li et al, 2011;Zhang et al, 2006).…”

“…We know that all aquifers are heterogeneous, and incorporating any heterogeneity, either as layers or more complex systems, strongly influences both the salinity distribution and position of the freshwater‐saltwater mixing zones and therefore large‐scale density‐driven SGD. Previous studies have evaluated the impacts of heterogeneity on SGD by numerical simulations of groundwater flow through continuous random fields (e.g., multi‐Gaussian distributions) of K (De Vriendt et al., 2020; Geng & Michael, 2020; Held et al., 2005; Kerrou & Renard, 2010; Rathore et al., 2020). The results showed stronger effects on saline SGD than fresh SGD.…”

Analytical Solutions for Fresh Groundwater Lenses in Small Strip Islands With Spatially Variable Recharge

“…This assumption is reasonable for the rainfall intensity in a small area, but it is not realistic for the recharge of aquifers, because the process of infiltration is affected by geological characteristics such as topography, soil type and evapotranspiration, land use type, and human activities (Park et al., 2014; Werner et al., 2017). Field investigations have showed that the groundwater lens shape and thickness can be affected by spatially variable recharge rates induced by ridge topography (Ruppel et al., 2000), terrain and vegetation patterns (Schneider & Kruse, 2003, 2006), distribution of different soil types (Park et al., 2014) and heterogeneous hydrogeological properties including hydraulic conductivity, impermeable layer, and boundary conditions (Cellone et al., 2018; Holt et al., 2019; Rathore et al., 2018; Rathore, Lu & Luo, 2020; Sheng et al., 2020). In addition, the knowledge and insights on how the volume and shape of a groundwater lens change in response to spatially variable recharge rates can provide very useful information for determining the location and recharge rate of artificial recharge to control seawater intrusion (Bouwer, 2002; Rajasekhar et al., 2019; Senanayake et al., 2016).…”