With sea level near Martha’s Vineyard projected to rise between 1 and 6 feet by 2100, and groundwater as the sole-source for public drinking water, this study will directly inform water management. 

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The USGS is developing an island-wide groundwater-flow model, in cooperation with the Massachusetts Department of Environmental Protection, to assess climate-change impacts. Groundwater-flow models represent geologic and hydrologic features and processes, including groundwater and surface-water interactions and changes in both the freshwater and saltwater flow systems. 

“The model incorporates projected climate scenarios, such as sea-level rise and groundwater recharge fluctuations based on changing temperature and precipitation patterns, over the next 100 years,” said hydrologist Tim McCobb, lead researcher on this study. 

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Groundwater is the only source of drinking water on Martha’s Vineyard. The island’s shallow, unconfined aquifer could be susceptible to saltwater intrusion by the surrounding ocean. As sea levels rise, a climbing water table and a higher freshwater-saltwater interface position could affect the potability of water supplies, flood existing infrastructure such as septic systems and basements, impact future development, and disrupt coastal wetland ecosystems and other aquatic habitats. 

Projected increases in air temperature and changes in the magnitude and seasonal distribution of precipitation may also affect aquifers by changing the amount and timing of groundwater recharge in the 21st century compared to the late 20th century. 

In June, the Hydrologic Remote Sensing Branch of the USGS Water Mission Area assisted the New England Water Science Center in determining the depth to bedrock and the depth to the freshwater-saltwater interface by using two surface geophysical techniques at 18 locations on the island. This information will be used in the groundwater-flow model and add to the current understanding of the flow system and its boundaries. 

Hydrologists Carole Johnson and Eric White used a passive seismic method known as the horizontal-to-vertical spectral ratio (HVSR) technique, which estimates surficial material overlying the bedrock surface. Time-domain electromagnetics (TDEM) surveys were also carried out to map the distribution of electrical properties with depth, which relates to aquifer materials and salinity. 

“These new geophysical data will be used with existing data to define the model boundaries and to develop contour maps of the bedrock surface and saltwater and freshwater surfaces across the island,” said McCobb.   

Groundwater-flow models of the island’s fresh groundwater system using long-term averages and changing hydrologic conditions will both be produced. The response to projected climate change effects is not expected to be uniform across the island and over time. Therefore, the model will be a useful tool to evaluate the spatial and temporal impacts to the hydrologic system. 

This study will provide an understanding of how climate change and sea-level rise can influence the hydrology and water resources of Martha’s Vineyard in the 21st century.  This knowledge will inform present-day and future drinking water availability and where management and conservation measures may be needed. 

To learn more about this study, click here.  

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