Hydrologic Activity

The freshwater cycle

Outer Cape Cod’s freshwater supply comes solely from an underground aquifer system that is recharged entirely through precipitation.

We all depend on the freshwater cycle, but how does it work? Earth’s freshwater cycles three main pools: precipitation, groundwater, and surface water. As the sun’s energy heats surface water, it evaporates (turns into gas), then condensates, forming clouds. Clouds get heavy and eventually release precipitation.

Groundwater is freshwater that’s stored in saturated zones under the earth’s surface. It is recharged (refilled) when precipitation, like rain, infiltrates (moves slowly) through the ground. The water moves downwards through pores (percolates) in sand, gravel, and rock. Because of its geological history, Cape Cod has sandy soil, which causes increased infiltration.

Cape Cod National Seashore, in collaboration with the United States Geological Survey, has developed a standardized, long-term methodology of monitoring hydrologic activity on the Outer Cape. The program will provide a better understanding of the effects of both natural and human-caused changes in groundwater levels of the Cape Cod aquifer.

The aquifer is bound sideways and below by salt water and is divided into four distinct flow "lenses." These flow lenses are detailed in the figures below.

 

Location of the four flow lenses of the Outer Cape aquifer system

Courtesy USGS

A hydrologic lens is a convex layer of fresh groundwater that floats above the denser saltwater. These “lenses” were formed by geologic processes.

Consider that the entirety of Cape Cod was formed by a glacier over 18,000 years ago. The Lower Cape Cod aquifer was formed by glacial sediments that are under the present-day towns of Eastham, Wellfleet, Truro, and Provincetown. When ice lobes receded, meltwater streams carried sediments from the Atlantic Ocean, creating flat areas known as outwash plains. Because of the position and movement of the South Channel and Cape Cod Bay ice lobes, the Wellfleet outwash plain was created first, then the Truro plain, and finally the Eastham plain (Masterson, 2004).

Glacial melt caused sea-level rise, which is a powerful force of coastal erosion. The outwash plains eroded to become the shoreline. Portions of the outwash plains farthest from the glacier were made of layers of sand and gravel and had high infiltration, creating three of the flow lenses: the Nauset, Chequesset, and Pamet. The Pilgrim flow lens was formed by geological changes of a sand spit (Zeigler and others, 1965; Uchupi and others, 1996).

Groundwater is a hidden but powerful regulator of sensitive coastal ecosystems.

Groundwater plays an important role in coastal ecosystems, including hydrology, biogeochemistry, and stability of wetlands and salt marshes. Changes in groundwater levels can have negative impacts on these systems.

Climate change, sea-level rise, and increased withdrawal rates from groundwater all have the potential to significantly impact kettle ponds, sensitive coastal ecosystems, and freshwater resources critical for humans.

The more raised the lens is on the model below, the closer to the surface the water is. So for example, the raised lenses indicate surface water.

 

The groundwater lenses of Cape Cod

Courtesy USGS

Monitoring Protocol

The monitoring protocol includes collecting data on a monthly basis from 18 observation wells, also known as monitoring wells, and 12 ponds. This approach has been implemented to assess water resources.

The monitoring well locations were chosen based on a variety of factors, including anticipated changes to the hydrology of the Seashore and impacts of development on the groundwater.

Ponds were selected in areas of sensitive ecological importance, representing large areas in the aquifer.

The map below shows our observational wells and pond measuring sites. The data that comes from these sites are used as indicators for groundwater levels in the surrounding areas.

 

Map of observational wells, pond stage sites, and groundwater contours on the Outer Cape.

NPS

 

 

Reports and Publications:

NPS Datastore - Cape Cod National Seashore Hydrology Monitoring Project

 

Works Cited:

John P. Masterson, 2004, Simulated interaction between freshwater and saltwater and effects of ground-water pumping and sea-level change, lower Cape Cod aquifer system, Massachusetts

Zeigler, J.M., Tuttle, S.D., Tasha, H.J., and Giese, G.S., 1965, The age and development of the Provincelands Hook, outer Cape Cod, Massachusetts: Limnology and Oceanography, Redfield Volume, p. R298–R311