Groundwater

The principal aquifers on the Cape and Islands are moraines and outwash deposits, which derive their water from local precipitation. The broad outwash plains are mainly composed of sand and gravel, which, in places, is mixed with till and ice-contact deposits, silt, and clay. Yields for 24-in.-diameter wells in outwash deposits generally range from about 200 to 700 gal/min. However, yields of 1,000 to 2,000 gal/min have been reported for some wells on Cape Cod, Martha's Vineyard, and Nantucket. In general, supplies of water for homes, cooling, and small businesses can be developed in most areas of outwash on the Cape and Islands from wells that are 1.5 to 2 in. in diameter with 3 ft of screen set about 10 ft below the water table.

Groundwater flow systems can be identified in areas of outwash on the Cape and Islands from the configuration of the water table. On Cape Cod, bays and streams divide the groundwater system into six areas or cells, each of which has a water-table mound (Guswa and LeBlanc, 1985). The altitude of the water table generally is highest, (about 5 to 60 ft above sea level) near the center of each cell and lowest (0 ft at sea level) near the coast. Groundwater flows in the direction of the greatest hydraulic gradient, which is from the center of the mounds to the ocean. Locally, flow can be towards ponds, swamps, or streams. Under natural conditions, the six cells are hydraulically independent of one another. The steady-state flow through the total Cape Cod aquifer system, as estimated from computer-model analyses (Guswa and LeBlanc, 1985), is about 270 Mgal/d. On Martha's Vineyard, the groundwater flows mainly in one cell, which has a water-table mound that reaches an altitude of 18 to 19 ft above sea level near the center of the island. Several smaller cells are located in Edgartown and on Chappaquiddick Island. On Nantucket, the water table forms several low mounds, the largest of which reaches an altitude of 12 to 14 ft above sea level halfway between Nantucket Harbor and Siasconset.

The moraines are composed of both poorly- and well-sorted sand, silt, and clay that were transported in the glacial ice and left behind when the ice retreated. The textural composition of moraines generally varies more over short distances than does the textural composition of outwash deposits. The Gay Head moraine on Martha's Vineyard consists largely of clay and silt with some sand and lignite. Because the textural variability causes permeability variability and because the unsaturated zone is greater in the moraines than the outwash plains, exploration for water supplies has been discouraged in the moraines. Yields of 2,000 gal/min are obtained from 24-in.-diameter gravel-packed wells in the most permeable zones of the moraine in Barnstable. However, wells drilled in nearby areas have penetrated thick layers of silt and clay and are reported to have been unproductive (Guswa and LeBlanc, 1985, p. 3-4).

In addition to the outwash and moraine deposits, deeper and older preglacial sand and silt are present on Martha's Vineyard and Nantucket. However, these deposits would probably yield less than 100 gal/min and could yield water with elevated iron and (or) chloride concentrations. Bedrock beneath unconsolidated deposits on Cape Cod and the Islands consists of metamorphic rocks, such as schist and gneiss, and igneous rocks, the surface of which generally slopes southeastward from about sea level on the northwestern shore of Buzzard's Bay to as much as 1,600 ft below sea level at Nantucket (Oldale, 1969). The depth to bedrock beneath glacial sediments on Cape Cod ranges from about 80 to 900 ft below sea level. Bedrock is much less permeable than the overlying sediments, commonly contains seawater, and is not considered to be part of the aquifers of Cape Cod or the Islands.