The Monomoy Lens: A Primer - Public, Private Wells All Draw Water From The Same Aquifer

CHATHAM – It’s well known that all of the Cape’s drinking water comes from a sole-source aquifer beneath the ground. But there’s a persistent misconception, officials say, that private wells don’t impact groundwater the same as public wells.

 “We all draw from the same aquifer,” Natural Resources Director Greg Berman said. “Private wells and public wells are different but not separate.”

 The main difference between wells drilled privately for residential use and public drinking water wells is their depth and the amount of water they draw. Private wells tend to be shallower than public wells, while public wells pull in a lot more water than a private well, sometimes millions of gallons a day.

 Both, however, draw water from the same resource, said Berman, a certified professional geologist and former U.S. Geological Survey (USGS) hydrologist.

 “I want to clear up the question of if you have a private well, you’re not impacting the water supply,” Select Board Chair Jeffrey Dykens said at the June 16 presentation. “I really want to put that piece of folklore to bed.”

 While the Cape has a single aquifer, it is divided into six “lenses,” the largest of which are the Sagamore lens to the west and the Monomoy lens, which covers most of the Lower Cape and as far west as Yarmouth, Berman said. Chatham has 11 drinking water wells, all of which draw from the Monomoy lens. The aquifer is “unconfined,” he said, in that it is open to the surface. The Cape’s ponds are all reflections of the water table (the upper surface of the aquifer). That’s why, in drier years, the water level of ponds shrink, while in wetter years pond shores can become submerged. Fresh water in the aquifer floats on a layer of salt water.

 The aquifer’s only source of water is surface precipitation, rain and snow. Recent years’ relatively dry conditions mean that the water table is lower than normal, prompting the state to declare drought conditions. The Cape is currently at level two, considered to be a “significant” drought, according to the state drought management task force. For the past several years, Chatham and other Lower Cape towns have imposed outdoor watering restrictions and bans to minimize water use and help the aquifer recover.

 On the Lower Cape, groundwater flows through the sandy substrate toward the ocean. Wells, especially heavily pumped public drinking water supply wells, can create localized drawdown, forcing the flow of groundwater to change in their immediate vicinity.

 Berman could not say how many millions of gallons are in the Monomoy lens, but he noted that, according to the USGS, 69 percent of groundwater discharges at the coast, while 24 percent is trapped in ponds and streams and 7 percent is pumped by public water supply wells. Private irrigation wells use less than 1 percent of the water in the aquifer.

 Pumping too much water out of the aquifer can draw salt water into wells, especially in shallow areas near the coast. Sea level rise plays a part in this, Berman noted. As the level of the ocean rises, so too does the level of the saline water lens beneath the fresh water aquifer. This can create a “bulge” in the fresh water aquifer, pushing the water table level closer to the surface. This can be a problem for deeper wells, as well as septic systems, which are required to have at least five feet between the bottom of a system and the water table. This is likely to be a problem closer to coastal areas, he said; most public water supply wells are farther inland and less likely to be impacted.

 In most Cape towns, the sale of a property triggers a septic system inspection, and a system can be deemed failed if there is less than five feet to the groundwater. 

 “We do have some interesting data coming in on this, and I’m definitely going to keep track of it,” Berman said.

 “Everything we do affects the aquifer,” he said. Drilling wells deeper is not necessarily a solution, he added; that can create other issues, such as higher levels of iron and manganese. Chatham has had to build two multi-million dollar water treatment plants to deal with those chemicals in several public wells.

 While drawing far less water than public wells and not tapping the town’s storage and distribution infrastructure, private wells, either for drinking water or irrigation, still impact the “same shared aquifer that supports public water supplies and ecological resources,” Berman wrote in a memo.

 Berman’s presentation is posted on the town’s website.