The overarching objective of this project is to develop reduced-order frameworks to assess groundwater-related risks to coastal infrastructure from sea level change. Future changes in sea level and recharge are expected to increase saltwater intrusion into coastal aquifers (horizontal migration) and change water table elevations (vertical migration). The magnitudes of these changes depend on regionally variable drivers and boundary conditions, as well as spatially differentiated landscape physical properties. Thus, tools for supporting management need to appropriately consider these factors. Pursuant to the overall objective, the project team seeks to address the following specific questions:

Q1: How do climate and landscape geophysical factors interact to determine groundwater responses to future sea level change?

Q2: What local factors will determine the magnitudes and relative importance of specific components of the groundwater responses?

Q3: Which Department of Defense (DoD) sites are most vulnerable to sea level changes, and what is the overall hierarchy of site vulnerability?

For each of the research questions, presented are the corresponding primary hypotheses:

H1: Climatic drivers and geophysical conditions can each be described in reduced-order model frameworks by dimensionless parameters that respectively describe sea level rise versus recharge change, and river baseflow drainage versus coastal subsurface groundwater discharge.

H2: Magnitudes of the essentially horizontal saltwater movement between ocean and aquifer cause vertical movement of the saltwater interface and vertical movement of the water table, which can be determined based on the dimensionless parameters that capture the interaction of climate drivers and geophysical conditions. These vertical movements are important components of coastal aquifer freshwater storage changes, in addition to the more commonly understood horizontal migration of the interface.

H3: Site vulnerability may be categorized into a two-dimensional typology matrix based on the climatic and geophysical dimensionless parameters from H1. The project team will test these hypotheses with a reduced-order model and data from coastal DoD sites. Fully developing the model framework is Task 1. Extending this framework beyond site-average values to account for intra-site spatial variability is Task 2. H3 is tested by combining the model with the entire database of coastal DoD sites, and then with higher-resolution information at selected sites in Task 3. Additional mechanisms and secondary hypotheses are tested in Task 4. Finally, Task 5 focuses on information transfer by developing robust user-friendly applications.

The work will culminate in a tool for prediction and management of the conjoined effects of sea level rise, altered recharge regimes, and landscape physical properties on subsurface coastal infrastructure. The reduced-order approach developed in this work is suitable for transferable application to coastal DoD facilities worldwide, in a broad cross-section of settings. All coastal DoD sites will be categorized in typologies of groundwater-related sea level rise risk in terms of expected changes in groundwater table and saltwater interface elevations. The fundamental advances of this work are applicable to coastal infrastructure in general, and will thus also be of broad interest to scientists and practitioners beyond DoD applications. The final tool will be a stand-alone executable program. (Anticipated Project Completion- 2026)