Simulated effects of sea-level rise on the shallow, fresh groundwater system of Assateague Island, Maryland and Virginia

Brandon J. Fleming; Jeff P. Raffensperger; Phillip J. Goodling; John P. Masterson

doi:10.3133/sir20205104

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Simulated effects of sea-level rise on the shallow, fresh groundwater system of Assateague Island, Maryland and Virginia

Brandon J. Fleming·Jeff P. Raffensperger·Phillip J. Goodling·John P. Masterson

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Abstract

The U.S. Geological Survey, in cooperation with the National Park Service, developed a three-dimensional groundwater-flow model for Assateague Island in eastern Maryland and Virginia to assess the effects of sea-level rise on the groundwater system. Sea-level rise is expected to increase the altitude of the water table in barrier island aquifer systems, possibly leading to adverse effects to ecosystems on the barrier islands. The potential effects of sea-level rise were evaluated by simulating groundwater conditions under sea-level-rise scenarios of 20 centimeters (cm), 40 cm, and 60 cm. Results show that as sea level rises, low-lying areas of the island originally represented as receiving freshwater recharge in the baseline scenario are inundated by saltwater. This change from freshwater recharge to saltwater decreases the overall amount of freshwater recharging the system. As the water table rises in response to the higher sea levels, freshwater flow out of the system changes, with more freshwater leaving as submarine groundwater discharge and less freshwater leaving as seeps and evapotranspiration. At the current land-surface altitude, as much as 50 percent of the island may be inundated with a 60-cm rise in sea level, and the low-lying marshes may change from freshwater to saltwater. Groundwater levels at 32 wells were monitored for as long as 12 months between October 2014 and September 2015 on Assateague Island. Results from objective classification analysis of 14 shallow monitoring wells show two dominant processes affecting groundwater levels in two different settings on the island. On the western side of the island, between the primary dune and the inland bays, water levels clearly respond to precipitation events. This side of the island is more protected from ocean tides and typically is more vegetated than the eastern side. On the eastern side of the island, between the Atlantic Ocean and the primary dune, water levels clearly respond to tidal events. Specific conductance was measured at four wells, two on the western part of the island and two on the eastern part of the island. Specific conductance values in the two wells west of the primary dune show episodic decreases, coinciding with precipitation events. Specific conductance values in the two wells on the eastern side of the primary dune show episodic increases, coinciding with high-tide events. These high frequency monitoring data are intended to aid in designing a monitoring network that can document both short-term and long-term hydrologic processes on Assateague Island National Seashore. This study uses a modeling approach consistent with models developed for Gateway National Recreation Area, Sandy Hook Unit (New Jersey) and Fire Island National Seashore (New York). Combined, these models are meant to improve the regional capabilities for predicting climate-change effects on barrier islands and provide resource managers with a common set of tools for adaptation and mitigation of potentially adverse effects of sea-level rise.

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Source-reported bounding extent: 37.83473402375478° to 38.29101446582335° latitude; -75.42388916015625° to -75.12039184570312° longitude. This indicates report coverage, not an exact sampling location. View area on OpenStreetMap.

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Brandon J. Fleming, Jeff P. Raffensperger, Phillip J. Goodling, John P. Masterson. 2021-07-16. Simulated effects of sea-level rise on the shallow, fresh groundwater system of Assateague Island, Maryland and Virginia. https://doi.org/10.3133/sir20205104

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