Martha's Vineyard’s groundwater is abundant and if carefully managed will provide for our foreseeable needs. The geological deposits that hold our groundwater supply are very different in the Outwash Plain and in the Moraine.
· 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 very porous deposits that readily absorb rainfall, which then 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 only replenished by rain falling on that island. 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.
We get the water from the aquifer to our taps in two ways.
· 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 latter 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.
Objective W1: Assure a plentiful supply of high quality drinking water.
We need to focus our efforts on protecting the existing recharge areas for public and private wells and those sites that are appropriate for future water supplies.
· Strategy W1-1: Expand public water supply. Expand public supply systems into areas where housing density does not assure water quality protection (such as Ocean Heights, Arbutus Park, Edgartown Meadows, and Mattakeset). Increase public supply system capacity in accordance with projected demand to avoid shortfalls in seasonal delivery of water. We expect the existing withdrawal limits will need to be increased by Massachusetts DEP in order to meet future demand. To provide emergency support, linkages between the three public supply systems are important and we need to update agreements for mutual support in the case of water supply emergencies.
· Strategy W1-2: Plan for and protect future public well sites. Towns should identify and protect future municipal well sites targeting the area down-gradient from the Correllus State Forest in Edgartown and Oak Bluffs. These areas are to the north, south, and east of the Forest and can be protected with an expansion of the existing Zone 2 zoning overlay districts or with a new overlay similar to the Greenlands Water Resource Protection District. A District may limit the housing density, discharge of large volumes of stormwater, the use of hazardous materials, and other land use activities that are likely to impact water quality. Towns should identify future municipal well sites and protect these supply areas by buying land or conservation restrictions, or by enacting zoning overlay districts. As an alternative, the Massachusetts DCR should be brought into discussions about future use of the Correllus State Forest as a drinking water supply site. Current policy is that they will not consider such a use unless there are clearly no other options for needed supply sites.
Strategy W1-3: Strengthen regulation of private wells. In areas where private wells will be the source of drinking water for the future, assure that lot sizes and well placement are protective of water quality by allowing dilution of nitrogen to below the drinking water standard (5 ppm is the suggested goal to assure the standard is met). Well-to-septic separations may need to be increased where groundwater flow direction is unknown. A minimum of 150 feet is suggested for all sites with the possible exception of those sites where groundwater flow direction is known and the well can be sited up-gradient from adjoining septic systems. Adoption of private well resource protection overlays is suggested for all areas identified as long-term private well resource areas. The main goal of the overlay is to assure appropriate land use development for continued high quality drinking water and to regulate or prohibit commercial uses that generate wastes that might impact the aquifer. The West Tisbury Greenlands Water Resource Protection
No comments:
Post a Comment