Two of Nantucket’s Great Ponds, which are ponds greater than 10 acres in size, Sesachacha Pond and Hummock Pond are maintained as part of an estuarine system by a spring and a fall breaching of the barrier beach. The ponds have breached naturally and receive ocean over wash, but the scheduled breachings are mechanically opened during designated tide conditions and with appropriate weather conditions. The barrier beach separates the brackish pond from the marine waters of the Atlantic Ocean. Brackish water is a blend of freshwater and of saltwater. Each Pond is breached to lower its nitrogen levels and raise its salinity through the exchange of brackish pond waters with the high quality offshore waters.
Pond openings are also to allow the entry of marine species, including herring, blue crab and other species. Studies of Sesachacha Pond in the late 1980's indicated that periodic tidal exchange was required to help stabilize the ecology and these results were reiterated in the recently completed Massachusetts Estuary Plan for each pond. Data has indicated that not conducting the breaching could lead to the salinity levels of the pond dropping, and nutrient levels rising to levels that do not support the species of plants, animals, and invertebrates within the ponds. Even with the openings responsible watershed management is needed, which includes regular inspection of and properly functioning septic systems, responsible use of fertilizers, proper vegetation management and other best management practices associated with watersheds.