Cape Cod is a hook-shaped peninsula that extends 40 mi into the Atlantic Ocean from southeastern Massachusetts, and is separated from the mainland by a man-made canal. The Cape has an area of about 395 mi2 in Barnstable County, and includes 15 towns.

Martha's Vineyard is situated about five mi south of Cape Cod in Dukes County. The island has an area of about 106 mi2 and includes six towns. Nantucket Island, which is also Nantucket County, is situated about 25 mi south of Cape Cod and about 15 mi east of Martha's Vineyard. Cape Cod, Nantucket and Martha's Vineyard have been designated sole source aquifers by the USEPA. It has an area of about 49.5 mi2 and contains one town, Nantucket. The Elizabeth Islands are a chain of islands that extend about 15 mi off the southwestern corner of Cape Cod. They cover a total of about 15 mi2 and contain one town, Gosnold in Dukes County.

The Cape and Islands are composed of glacial end moraines, which mark the approximate locations of the ice front, and outwash plains, which were formed from sediments deposited by meltwater streams ahead of the ice front. Early glacial advances are recorded only in subsurface deposits, but later advances are recorded in two distinct areas of end moraines and outwash plains. The north sides of Martha's Vineyard and Nantucket contain end moraines related to the South Channel, Cape Cod Bay, and Buzzard's Bay glacial lobes. These moraines are bordered on the south by broad outwash plains. The Cape Cod Bay lobe retreated to the northern side of Cape Cod, where it formed the Sandwich end moraine. A similar retreat of the Buzzard's Bay lobe formed the end moraine now represented by the Elizabeth Islands. Parts of the south sides of these moraines also are bordered by outwash plains.

The following description of the water resources of Cape Cod, Martha's Vineyard, and Nantucket is based on Hydrologic Investigations Atlases 618 (Delaney, 1980), 692 (LeBlanc and others, 1986) and 615 (Walker, 1980).