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Effects of Sea Level Rise on Coastal Aquifers

Evaluating the effects of sea level rise on Massachusetts coastal aquifers: a Cape Cod pilot study

The lower Cape studies showed that we cannot simply guess how sea level rise will impact our aquifer system—a regionally-specific numerical model to predict potential changes in space and time has to be developed.

This project addresses one of several Massachusetts Bays Program goals for the Cape Cod region (“Goal E. Prepare for and understand the potential impacts of climate change and sea level rise on coastal ecosystems and communities and enhance education and adaptation efforts”).

(Note: Cited references are indicated by boldface numbers in parentheses. The list of references is provided at the end of this article)

Strong scientific evidence indicates that sea level in the Northeast has been rising and will continue to rise. Recent estimates indicate that sea level in the Northeast is rising three to four times faster than the global rate (1, 2). In the Northeast, sea level rise will likely cause impacts such as increased area of land under water, increased coastal flooding and erosion, increased storm damage, and changes in beaches, dunes, salt marshes, inlets a nd other coastal landforms (3a, 3b). On Cape Cod, we also need to evaluate the effect of sea level rise on the water table and our aquifer.

Figure 1. Cross section of the mid-Cape region from Cape Cod Bay to Nantucket Sound that
shows how groundwater flows through the aquifer from land surface to the shore (4).

How would rising sea level affect the water table? Cape Cod is composed of highly permeable sand and gravel deposits. Permeable means that water can easily pass through these deposits. Rain and melting snow percolate down through permeable soils to recharge the water table at the top of the underlying fresh water aquifer. While the height of the water table depends on recharge, the lowest point of the water table is at sea level.
(Figure 1)

In areas like Cape Cod that are surrounded by the sea, changes in sea level will affect the height of the water table. In other words, as sea level rises, so will the water table. This is a concern in areas where the water table is already close to the surface (Figure 2). Modest changes in water table elevation can result in subsurface flooding that could impact septic systems, infrastructure and property. Water resources, wetlands and ecosystems could also be affected.

Figure 2. Photograph of the New Silver Beach area of Falmouth taken
at high tide. This area was sewered in order to improve water quality.
Before sewering, many septic systems were in contact with high
groundwater which allowed wastewater to leach into coastal waters.

APCC is working with U.S. Geological Survey hydrologists to develop a study that will analyze the effects of sea level rise on groundwater flow in the -mid-Cape region, the most densely populated area of the Cape. The proposed mid-Cape study will build on previous USGS models of the lower Cape aquifer’s response to rising sea level and studies of the mid-Cape aquifer (4, 5, 6). The previous lower Cape studies indicated that rising sea level will cause the water table to rise in areas away from streams, while in other areas drained by groundwater-fed streams the water table could drain faster due to increased streamflow caused by the higher water table, resulting in rejuvenated streams. The lower Cape studies showed that rising sea level can impact coastal aquifers by causing changes in: 1) height of the water table and depth to groundwater, 2) stream base flow, and 3) position of the freshwater/saltwater interface.

On Cape Cod, such changes in hydrology could have important implications for management of wastewater, water and natural resources and for protection of public health and the environment. The proposed mid-Cape study will provide information needed for science-based planning to address these issues. The products will include GIS maps of regional changes in water table elevations, cross-sections showing changes in the saltwater/freshwater interface, and tables of stream-flow changes for different sea level rise scenarios.

In June 2013, APCC was awarded an $80,000 grant from the Massachusetts Environmental Trust. This grant will enable USGS to start modeling the effects of sea level rise on the mid-Cape’s aquifer. APCC and our partners will develop outreach materials and community adaptation measures. Several public workshops are planned during this 3-year program. APCC still needs to raise additional funds to fully support the project.

This is the fourth MET grant that APCC has received. Prior grants were for wastewater education, stormwater utility outreach, and monitoring of invasive species. The Massachusetts Environmental Trust is one of the state's largest sources of funding for water quality initiatives to improve and safeguard the state’s water resources. Funding is from environmental license plate revenues which have funded more than 650 grants totaling over $17 million. Please visit the Massachusetts Environmental Trust website for more information.

APCC’s partners include the USGS, Cape Cod Commission, The Nature Conservancy, Massachusetts Bays National Estuary Program, Barnstable County Coastal Resources Committee. Funding support is provided by grants from the Massachusetts Environmental Trust, the Massachusetts Department of Environmental Protection, The Nature Conservancy, the Cape Cod Five Cents Savings Bank, and by APCC member dues and donations.

APCC feels it is critically important for everyone to understand how sea level rise could affect our aquifer in order to plan for adapting to climate change. To conduct this important study, we need your continued support. For more information, contact Ed DeWitt or Jo Ann Muramoto at (508) 362-4226.