The Vineyard’s main aquifer greatly exceeds our present-day and projected drinking water needs. Our focus should be on protecting groundwater quality where we draw drinking water and ensuring that public water supply pumping and distribution infrastructure keeps pace with demand.
An aquifer is an underground reservoir of water, residing in the spaces between grains of sand and gravel. The nature of an aquifer is influenced by the geology of the area. The Vineyard's main aquifer is found in the glacial outwash deposits that occupy about 60 percent of the Island, consisting of thick layers of sand and gravel dropped twenty thousand years ago by glacial meltwater streams. The aquifer is replenished or recharged by the excess rainfall that percolates through the soil and seeps into the groundwater. Of the 46 inches of annual precipitation, recharge is estimated to be 22 inches. The Vineyard’s groundwater is abundant and if carefully managed will provide for our foreseeable needs.
Outwash Plain Aquifer: Most of the Island, including all town wells, draws its drinking water from one main aquifer located in the Outwash Plain, where glacial ice deposited layers of sand and gravel as it melted, creating porous deposits that readily absorb rainfall, which percolates down into the water-saturated zone known as an aquifer. The entire Island has been designated by EPA as a Sole-Source Aquifer, since groundwater is the Island’s only source of drinking water. There is a plentiful supply of potable water, provided it is properly protected from contamination. 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).
Chappaquiddick Aquifers: Smaller aquifers lie under Chappaquiddick Island that are not connected to the main aquifer and are replenished only by rainfall. In general, the quantity of water recharged to a 3-acre lot as required by zoning is more than adequate to meet water needs for a home and guest house.
Western Moraine Aquifers: In the hilly Western Moraine, the glacial deposits are very different, displaying a wide range of sediment types ranging from compact, almost impermeable, clay to porous sand. The sandy deposits make good aquifer materials while the clayey deposits may hold some water but do not yield it. As a result, there are numerous aquifers in this area that may or may not be connected with other nearby aquifers. Finding a good source of well water is sometimes difficult.
The MVC’s Aquifer Monitoring Program: The 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, changes in extraction of water for consumption. The Commission monitors the water table elevation in a network of fourteen [map shows more than 14] wells throughout the Vineyard. These wells have been monitored monthly since the 1990s, and one has been measured since late 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 ponds and coastal great ponds. For example, the water table level in the testing well located in Edgartown in the Correllus State Forest indicates that the aquifer at this location expands and contracts over the years by about 7 feet, from around 19 or 20 feet to about 12 or 13 feet above sea level. The data from this monitoring program helped the MVC define the rate and path of the flow of water in the aquifer. The rate is estimated to be 1 to 2 feet per day, so that rain falling in the Correllus 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's watersheds, the areas above the water table that contribute groundwater to each of our coastal ponds.
Public Water Supply: Nearly two thirds of Vineyard homes get their water from a public well and distribution system (Edgartown, Oak Bluffs, Tisbury, Menemsha, and Wampanoag Tribal Housing, the last two being privately owned). The groundwater quality in supply areas, or zones of contribution, of existing public wells is already protected. We also have to make sure that our public water systems have the pumping and distribution capacity to meet future demand.
Private Wells: For the rest of the Island, private wells will be the source of drinking water for the foreseeable future. There is some concern that existing minimum separations between wells and septic fields are not adequate where groundwater flow direction is uncertain.
- Martha's Vineyard Source Water Protection Project (2003): Describes how the water quality of the Island’s critical public drinking water is protected by zoning overlay districts.
- Well Location Map shows the location of wells around the State Forest.
- Water Table Elevation shows how the water table elevation varies throughout the year compared to historic averages at a testing well in the State Forest.
- Suggested Observation Well Specifications provides guidelines for placement, drilling materials, cleaning and maintenance of observation wells and soil borings.
- Groundwater Monitoring gives an example, using the Vineyard Golf Course, of the step-by-step process for monitoring groundwater.
- Environmental Protection Agency – Ground Water and Drinking Water: Resource for contaminants, emergency preparedness, protection and management and ground water and drinking water.