Rising Seas Threaten Cape Cod's Groundwater

Impact of sea-level rise on Cape’s drinking-water supply a concern

By BRIDGET MACDONALD/ecoRI News contributor

CAPE COD, Mass. — From the perspective of a peninsula jutting into the North Atlantic, predictions about sea-level rise and increasing storm severity from climate change mean serious consequences, and policymakers on Cape Cod have been raising concerns about issues from coastal erosion to salt-marsh retreat to rising insurance rates. Problems that will manifest themselves in visible changes to infrastructure, landscapes and wallets.

But a new study is bringing attention to an equally urgent problem that is perhaps more easily overlooked: The impact of sea-level rise on groundwater systems.

“Not only do we have a climate challenge on Cape Cod, we have a wastewater challenge,” said Ed DeWitt, director of the Association to Preserve Cape Cod (APCC). “This is one of the areas where they intersect.”

As the sea level rises, the water table will rise as well. Since fresh water is less dense than salt water, Cape Cod’s groundwater floats on top of the surrounding seawater in a kind of lens. If you looked at a cross-section of the peninsula — from bay to sound, or bay to ocean — this freshwater lens would form a U-shape beneath the surface of the land, sloping downward from the shoreline as it moves inland, and then back up to meet the opposite shore.

Of course, as water levels rise, the level of the land will remain the same, which lends a sense of urgency to conspicuous threats for a coastal landscape, like flooding and erosion. Meanwhile, there could be another serious concern sneaking up on the Cape from below.

If the water table rises, its surface will be closer to the surface of the land. That’s important because, among other things, “It could have implications for septic system placements,” said Peter Weiskel, chief of the USGS MA-RI Office of the New England Water Science Center.

In Massachusetts, Title 5 regulations require a minimum vertical distance between the bottom of a septic system’s leaching area and the top of the water table. It’s typically 4 or 5 feet, but it can be greater in areas where the soil has a higher rate of percolation, because effluent can travel through such soil more quickly.

“We already know there are some septic systems that are tidally influenced, and come in contact with groundwater or seawater, creating unhealthy conditions,” said Jo Ann Muramoto, senior scientist for the Barnstable-based APCC. So in the context of discussions about climate-change impacts on the Cape, she and her colleagues began to connect the dots. “It led us to think about what will happen to septic or wastewater in the coastal zone if the sea level rises.”

That’s where Weiskel’s expertise in hydrology comes into play. Through cutting-edge modeling, Weiskel and colleagues at the USGS are working with the APCC and the Cape Cod Commission to answer the question: How could different sea level-rise scenarios affect mid- and upper-Cape groundwater systems?

“The biggest focus of the work will be to look at changes in the water-table elevation,” Weiskel said. “The other aspect is looking at how stream flows might change.”

Just like the surface of the land, he explained, the bottom of a stream isn’t going to rise — “at least, not in a simple way.”

Fortunately, the ability to capture the complexity of the Cape’s aquifer is what makes the USGS model so powerful.

A unique system
Cape Cod was formed between 10,000 and 15,000 years ago at end of the last ice age, from glacial sediments left at the margins of three separate lobes of ice that converged in the area. These lobes are responsible for sculpting different sections of the Cape, either by mounding up sediments at the leading edge of the ice, dropping sediments underneath during retreat, or releasing sediments in melt water.

“All of this matters because these materials are what make up this aquifer today,” Weiskel said. The soil variations found in different areas of the Cape, from the size of the sand grains to the degree to which they are sorted, are the legacy of this glacial past.

Although the characteristics of the sediment vary, the Cape’s aquifer has a few important overarching features. For one, it’s a sole-source aquifer, meaning 100 percent of the drinking water on Cape Cod comes from the natural store of water underground.

It’s also an unconfined aquifer, replenished directly through the surface by precipitation percolating through the sandy material into a saturated zone.

And throughout the Cape, the patterns of groundwater flow are “strongly affected” by the hundreds of kettle ponds that dot the landscape, because they “offer no effective resistance to flow,” as explained in a 2004 USGS report looking at the effects of pumping in certain areas of the Cape.

“Streams, kettles and groundwater levels are all potentially going to respond to changes in sea level,” Weiskel said. “We want to use existing groundwater models of the mid-Cape area to simulate the changes in the elevation of the water table, and as a result, understand the potential effects on stream flow, groundwater and kettle-pond levels.”

The models are built using two key inputs: recharge record and the boundary conditions.

“Recharge” refers to the amount of water that refills the aquifer — precipitation minus evapotranspiration. So if the Cape receives about 40 inches of rainfall annually, how much of that escapes the forces of atmospheric evaporation and transpiration by plants to reach the water table? Actually, quite a bit.

Weiskel noted that thanks to the climate and permeability of the soil, “We have high recharge rates on the order of 20 to 27 inches per year.” The rates fluctuate naturally with the seasons, plummeting in the summer when plants and trees are leafed out. For the purpose of the model, Weiskel’s team assembled a reliable record based on years of data.

The “boundary conditions” are the constraints placed on the model, determined using the best understanding of the underlying geology and the extent of the aquifer. Coupled with detailed information about land-surface elevations gathered with laser remote-sensing technology, and water-table elevations informed by measurements from a series of monitoring wells, his team put together a carefully calibrated model to simulate changes to the aquifer system.

“Then we can apply various natural and manmade stresses, such as the change in the position of the boundary caused by rising sea level, or other stresses like pumping wells and recharge from septic systems,” Weiskel said. “Even natural recharge is a stress.”

From models to policy
The findings could have valuable applications on the Cape and beyond. Once the methodology has been piloted, the approach could be transferable to other coastal areas with similar geologies, such as southern Rhode Island.

After the USGS has completed the simulations and analyses, the APCC and the Cape Cod Commission will have a framework for developing public outreach and adaptive measures.

“We are hoping the results will be useful and applicable not just for community adaptation with regards to infrastructure, wastewater and stormwater, but also for management of natural resources, like inland fisheries, ponds and lakes, wetlands, and fish runs,” Muramoto said.

This inclusive perspective is important because the Cape’s human and natural communities share many of the same resources. For example, if kettle ponds experience increasing inflow and outflow of groundwater, leading to higher water levels, “It could be a concern to property owners with houses around the ponds, and it could change the hydrology of the ponds,” Weiskel said.

Although Muramoto noted that they probably wouldn’t be able to tease out the effects on specific wetlands, “We are hoping the study will give us enough large-scale information to begin to ask the right questions.”

Will there be a need to change the way septic systems are permitted? Will stormwater systems have to be designed to cope with higher groundwater? Will the overall storage capacity of the Cape’s aquifer change?

Only time will tell, but the models can offer an educated guess, and a starting point for moving forward.