Background/Objectives

Changes in sea level, precipitation patterns, and water usage can impact saltwater intrusion and groundwater levels along the coast. Reduced-order models that avoid overly complex, data intensive, and computationally expensive techniques but maintain robust relationships to relevant physical processes are favored for rapid, large-scale simulations. The objective of this project is to produce reduced-order models of subsurface saltwater intrusion and water table levels as they change due to various climate change scenarios. The results of this work will be used to inform screening-level assessments of saltwater intrusion impacts to freshwater resources and subsurface infrastructure at coastal installations.

Approach/Activities

A new sharp-interface modeling package is currently being developed to quickly assess future impacts of saltwater intrusion due to sea-level rise at DoD installations worldwide. Our approach uses a rigorous, physics-based reduced-order numerical model to simulate the interface between fresh and saltwater, and is designed to be practical, flexible, and robust. The new modeling package will incorporate sea-level change scenarios, recharge, surface-water impacts, and confined and unconfined groundwater flow. It is capable of predicting current and future water levels and the elevation of the interface between freshwater and saltwater in the aquifers with much lower computational effort than traditional variable-density modeling methods and is therefore suitable for rapid and large-scale simulations. This new modeling package will be available for MODFLOW-6 and will also be implemented into Aquaveo’s Groundwater Modeling System (GMS) graphical user interface.

Results/Lessons Learned

The team has reviewed and tested various numerical methods to solve two partial differential equations (for freshwater and saltwater flow) with two unknowns (the freshwater and saltwater heads). The sharp interface elevation can be then obtained from the Ghyben-Herzberg relationship. A fully-implicit finite difference formulation was developed, that uses upstream weighting of freshwater and saltwater thickness, and solves for freshwater head and saltwater head simultaneously using a modified Jacobian matrix with the Newton-Raphson method. This numerical formulation is stable and compatible with the MODFLOW-6 approach of solving multiple models. We have simulated steady-state and transient interface movement for advancing and receding interfaces for a number of example situations.