Modeling Groundwater Rise Caused by Sea-Level Rise in Coastal New Hampshire

Coastal communities with low topography are vulnerable from sea-level rise (SLR) caused by climate change and glacial isostasy. Coastal groundwater will rise with sea level, affecting water quality, the structural integrity of infrastructure, and natural ecosystem health. SLR-induced groundwater rise has been studied in coastal areas of high aquifer transmissivity. In this regional study, SLR-induced groundwater rise is investigated in a coastal area characterized by shallow unconsolidated deposits overlying fractured bedrock, typical of the glaciated NE. A numerical groundwater-flow model is used with groundwater observations and withdrawals, LIDAR topography, and surface-water hydrology to investigate SLR-induced changes in groundwater levels in New Hampshire's coastal region. The SLR groundwater signal is detected more than three times farther inland than projected tidal flooding from SLR. The projected mean groundwater rise relative to SLR is 66% between 0 and 1 km, 34% between 1 and 2 km, 18% between 2 and 3 km, 7% between 3 and 4 km, and 3% between 4 and 5 km of the coastline, with large variability around the mean. The largest magnitude of SLR-induced groundwater rise occurs in the marine and estuarine deposits and peninsulas with tidal water bodies on three sides. Groundwater rise is dampened near streams. Groundwater inundation is projected to contribute 48% of the total inundated area from both SLR-induced groundwater rise and marine tidal flooding in the city of Portsmouth, with consequences for built and natural resources. Freshwater wetlands are projected to expand 3% by the year 2030, increasing to 25% by the end of the century, coupled with water-depth increases.