SELECTED U.S. GEOLOGICAL SURVEY INVESTIGATIONS OF COASTAL AND ISLAND HYDROGEOLOGY

The discipline of hydrogeology has a history of addressing coastal saltwater intrusion. Historically, most applied research addressed impacts on coastal water supplies, and this presentation will highlight examples from U.S. Geological Survey (USGS) density-dependent groundwater flow studies along the U.S. eastern seaboard that focused on urban water supplies. But with an increased understanding of sea level rise, studies began to include investigations of infrastructure impacts from the rising groundwater levels that are expected from sea level rise, and an example from New Haven, Connecticut will be presented. In low-lying urban areas, investigations began to jointly address rising surface water and groundwater, and an example from Miami-Dade County, Florida will be shared, as well as a new version of the USGS code MODFLOW6 that has been developed to update capabilities previously simulated using the SEAWAT code.

For many island nations with a large number of atolls, groundwater is the only reliable source of freshwater for use by human and natural biological communities. As we combine our collective understanding of sea level rise and the interconnectedness of natural systems, we have begun to incorporate multiple disciplines to address coastal and island issues. In the Pacific, for example, long-term research at the low-lying Palmyra Atoll National Wildlife Refuge has shown that one of the few surviving native tree stands (Pisonia grandis) in the central Pacific is at risk from groundwater salinization and wave overwash, but there is little basic groundwater information available to guide decision making.

Groundwater resources in these low-lying islands are vulnerable, especially where coastal vegetation and/or nearshore coral reefs have been degraded, but all aspects of these interconnected natural systems should be considered to more accurately address future conditions and associated risks in an age of rising seas. This will be illustrated through a recent USGS-led investigation demonstrating that wave-driven overwash accelerated the forecasted timeline of sea level rise impacts on freshwater availability in island nations. Investigations should consider this hazard and the interconnectedness of these systems to more accurately evaluate socio-economic impacts to populations and infrastructure.