ABSTRACT:

Subsurface barriers have been proposed to protect coastal aquifers from sea-level rise induced seawater intrusion, but the
potential for groundwater emergence near subsurface barriers remains unknown. Here, we investigated how emergence
changes groundwater flow conditions and influences the protective performance of subsurface barriers with sea-level rise. We
tested the subterranean consequences of sea-level rise for cutoff walls and subsurface dams with cross-shore groundwater
flow and salt transport models, investigating how barrier design, aquifer properties, and hydrological conditions control the
potential for emergence, groundwater partitioning at the barrier, and seawater intrusion with sea-level rise. We find that most
subsurface infrastructure cannot prevent seawater intrusion and emergence simultaneously. Subsurface dams spanning more
than half of the aquifer thickness created emergence hazards and subsequent groundwater partitioning for all scenarios tested.
Cutoff walls were less effective at reducing seawater intrusion for all opening sizes but could reduce the emergence potential
compared to similarly sized subsurface dams. Our results demonstrate the challenging trade-offs in mitigating the coastal
groundwater hazards of seawater intrusion and emergence with sea-level rise, where pumping may be needed to minimize the
potential for groundwater flooding inland of protective infrastructure that could intercept groundwater flow.