The main aquifer located in the Outwash Plain supplies most of the Island's drinking water, including for all of the town-owned wells. The entire Island has been designated by the US Environmental Protection Agency as a sole-source aquifer, since groundwater is its only source of drinking water. We currently draw about 1.5 billion gallons per year from the main aquifer, of which about 70% finds its way back into the aquifer after wastewater treatment. Rainfall replenishes the aquifer by about 24.5 billion gallons each year, so even if our use went up to 3.9 billion gallons per year (projected by the USGS), it would still be well below the suggested maximum safe withdrawal level of about 16.7 billion gallons (estimated by the MVC).

Smaller aquifers underneath Chappaquiddick are not connected to the main aquifer and are replenished only by rainfall. In general, the quantity of water recharged to a three-acre lot as required by zoning is more than adequate to meet the water needs of a home and guest house.

In the hilly Western Moraine, the glacial deposits are very different, with sediment types ranging from compact (almost impermeable) clay to porous sand. The sandy deposits make better aquifer materials, since they yield water. As a result of variable deposits Up-Island, there are numerous aquifers that may or may not be connected with others nearby. Finding a good source of well water is sometimes difficult.

The main aquifer is in a state of dynamic equilibrium, swelling and shrinking in response to variations in the amount of precipitation and, to a lesser extent the extraction of water for consumption. The MVC has monitored the water table elevation in a network of wells across the Vineyard since the 1990s, and one since 1978. The data indicate the seasonal and short-term changes in the volume of the aquifer and, as a result, changes in discharge to our fresh and coastal ponds. For example, the water table level in the testing well located in the State Forest in Edgartown indicates that the aquifer at this location expands and contracts over the years by about seven feet, from around 19 or 20 feet to about 12 or 13 feet above sea level. Overall, the rate and path of water flow in the aquifer is about 1-2 feet per day, so rain falling in the State Forest takes decades to travel to a coastal pond and then return to the ocean. The flow paths are the basis for approximating the boundaries of the Island watersheds, or the areas above the water table that contribute groundwater to each of the ponds or harbors.

USGS Groundwater Study on MV 

The US Geological Survey (USGS) and the MA Dept. of Environmental Protection (DEP) are investigating how sea-level rise and climate change will affect groundwater resources on the Vineyard. Shallow, unconfined aquifers, such as the one underlying the Vineyard, are vulnerable to these two processes.

With funding through the USGS and DEP, this project will produce both steady-state and transient groundwater flow models for the Vineyard. The models will then inform water-management decisions related to the Vineyard's drinking water supply. The MVC has provided baseline data and fieldwork to support the project. Final data releases and a journal article are expected in 2026.  

For more info, visit the USGS project website and the USGS New England Water Science Center Facebook page, or contact MVC Water Resource Planner Sheri Caseau.