Hydrologist Says Cape Has Water To Support Sewer Outfall

By: Elise R. Hugus
Published: 02/14/10

Town officials may be glad to know that Falmouth has a plentiful budget in at least one department: groundwater.

Denis LeBlanc, a hydrologist with the US Geological Survey’s water science center in Northborough, gave a presentation on the town’s “water budget” to members of the nutrient management working group on Thursday, explaining the role of the Sagamore Lens, the sole source aquifer that supplies a majority of the Upper Cape’s fresh water.

Because of the Cape’s geology, the Sagamore Lens is centered on the Upper Cape’s highest point, the Massachusetts Military Reservation. Rainwater and evaporated moisture collect in the groundwater, which flows through the deep kettle ponds dotting Cape Cod, and onward to estuaries, streams, wells, and the ocean. Water flows at a snail’s pace of one to two feet per day, Mr. LeBlanc said, and takes about 100 years to reach the ocean from the MMR.

On average, the Upper Cape discharges 269 million gallons of water per day—nine million gallons through the Coonamessett River alone.

Even at such a tremendous rate, the Sagamore Lens is flush with fresh water, due to plentiful precipitation. As the water is discharged, it is continually being recharged, much like balancing a checkbook, said Mr. LeBlanc.

Estuaries, streams, and coastal waters take up a majority of the “water budget,” Mr. LeBlanc said. Ponds capture about 20 percent of the total, he added, pointing out that warm groundwater flowing through ponds is evident in the winter by the patches that do not freeze.

Wastewater accounts for about five percent of the Upper Cape’s water usage; wells for drinking water take up another seven percent, Mr. LeBlanc said.

“We tend to think of ourselves as having a major impact, but it’s actually very small in non-urban areas. There might be local effects, but this is not a volume that would affect the region,” he told the working group.

In Falmouth, the highest drawdown of the water table is in the densely populated coastal neighborhoods of East Falmouth, said Mr. LeBlanc, showing a map where the areas of high water usage appeared in red.

Department of Public Works Director Raymond A. Jack pointed out that water is piped into these households from the Long Pond reservoir, “water that was never there to begin with.” Installing a sewer system would help “restore the natural balance to the peninsular areas,” he said.

Mr. Jack also noted that irrigation for farms and golf courses is the biggest drain on the water table in Falmouth, not flushing toilets.

Working group member and biologist George R. Hampson asked whether impermeable pavements keep rainfall from reaching the groundwater. Mr. LeBlanc replied that the large areas of asphalt on the MMR might have an impact, but, in general, the Cape does not have enough pavement to require large stormwater drains that would discharge directly to the coast.

In terms of the region’s options for where to release treated wastewater, Mr. LeBlanc said it might seem that the MMR would be the logical place to recharge the aquifer. However, the flow pattern is difficult to predict, he said, referencing issues he and his colleagues are having with the Ashumet Valley sewage plume model.

“The more you stress a system by playing with hydrology, the more difficult it is,” he said. The treated wastewater “might come out in places you hadn’t anticipated,” he said.

An ocean outfall pipe at Nobska Point, proposed as an option in a draft of the comprehensive wastewater management plan, would increase groundwater discharge by an additional three to five million gallons of fresh water per day, said Wastewater Superintendent Gerald C. Potamis.

Asked what the impact of that discharge would be to the local water supply, Mr. LeBlanc said it was “a tiny percentage” of the total volume. “We might see a small decrease in water levels, but it will be hard to pick out from natural variation,” he said.

The working group members said they gained an appreciation for the mechanics of hydrology from Mr. LeBlanc’s presentation; biological impacts or regulatory hurdles are the group’s usual focus.

“Your presentation should be mandatory for the [comprehensive wastewater management plan review committee],” said Mr. Hampson. “What we do here affects the entire system.”