Study: Sea Level Rise Could Bring Localized Aquifer Problems

By: Alan Pollock

Topics: Groundwater protection

In April, U. S. Geological Survey scientists Carol Johnson, Eric White and Tim McCobb installed install water monitoring equipment as part of the Cape Cod study.  USGS PHOTO

CHATHAM — In less than a century, rising sea levels could cause localized basement flooding, septic system failures, roadway deterioration and problems with underground utilities. That's the prediction of a U.S. Geological Survey study carried out in cooperation with regional and state experts.

Released last week, the study used sophisticated computer models to predict the effects on the Cape's freshwater aquifer, given a six-foot rise in sea level. Because salt water is denser than fresh water, the rising ocean would tend to force Cape Cod's groundwater up. Thanks to the surface water drainage allowed by streams and wetlands, the potential rise in the water table would be less than the rise in sea level.

“Near the coast, however, we believe the rise in the water table would probably be larger, suggesting a substantial likelihood for groundwater inundation in some low-lying coastal areas,” wrote USGS hydrologist Donald Walter, the lead author of the study. “Although sea level rise is a global issue, the impacts can be local,” he wrote.

According to the National Climate Assessment performed in 2014, there is a greater than 90 percent chance that sea level will rise between eight inches and 6.6 feet by 2100. A six-foot increase would flood many low-lying coastal areas, and would make many shoreline properties much more vulnerable to storm surge flooding.

But according to the study, the risks to coastal properties aren't all about hurricanes and erosion from storms and flooding tides.
“In some communities, groundwater inundation may result in saltwater intrusion of aquifers, problems with underground utilities and pipes, flooded basements and septic system failures, among other challenges,” a USGS news release reads. “Depending on the severity, it may make areas unsuitable for residential and commercial development.”

The study suggests that while private drinking water wells near the coast may start pumping salty water, municipal wells – which are typically well inland – would not be compromised.

The research focused on the two largest “lenses” of groundwater that make up Cape Cod's aquifer: the Sagamore Lens, underneath the Upper and Mid Cape, and the Monomoy Lens under the Lower Cape. The rise in the water table is expected to be comparatively higher in the Monomoy Lens, because the Lower Cape has fewer large ponds and rivers to drain the rising water table. Even here, the impacts would be expected to be greatest near the shoreline and near creeks and rivers.

“Our study found that extensive saltwater intrusion is unlikely on Cape Cod,” Walter wrote.

The study was supported by the Association to Preserve Cape Cod (APCC), the Cape Cod Commission, the Massachusetts Environmental Trust and the Nature Conservancy.

“This study points the way to several next steps,” APCC Science Director Jo Ann Muramoto said. “In areas where the water table is predicted to rise, communities should plan for impacts on water quality, management of stormwater and wastewater, and infrastructure,” she said.

In the years ahead, it will be critical for Cape Cod officials to monitor changes in groundwater and streams, and to carefully measure sea level rise to see if the model's predictions are verifying.

“The Cape's natural drainage system of streams and connected ponds is important for offsetting the effects of rising groundwater,” Muramoto said. “Protecting and restoring our natural surface drainage system could be an important coastal resilience measure.”