According to the former Chief of Massachusetts Water Resources Division:

I would like to point out that this area [Greenlands], where the water table is near the highest point in the aquifer, is a most sensitive area with respect to water quality. Waste disposal in this area may pollute a larger amount of ground water than waste disposal in most other locations around the Island. Polluted water entering the aquifer at this location would have one of the longest travel paths through the aquifer before eventual discharge to the sea. Since travel times are expected to be on the order of one foot per day, contaminants entering the ground here might persist in the aquifer for many scores of years. Also, it is highly probable that some groundwater from this area flows to and helps sustain the Lagoon Pond Spring, a 1.5 million gallon per day water supply in Oak Bluffs.

An interesting history of the green lands from the archives of the MVTimes.