Hydrogeology of Lower Cape Cod

The Lower Cape Cod aquifer system is composed of glacially deposited sediments that range in size from clay to boulders. Approximately 15,000 years ago, glacial ice sheets covered much of New England and extended north and east of present-day Lower Cape Cod (Oldale, 1992). Meltwater from these glaciers carried sediments that were originally trapped in the ice, and deposited them in large deltas in adjacent Glacial Lake Cape Cod, the present-day Cape Cod Bay. At that time, global sea levels were lower than current levels because of the large quantity of water contained in the enormous ice sheets. As the ice sheets melted and retreated, sea level rose, and erosion of the glacial Cape Cod shoreline began. The remnants of the once much larger deltaic deposits, or outwash plains, that now constitute Lower Cape Cod continue to be eroded at a retreat rate of about 3 feet (ft) per year along the Atlantic Ocean (Zeigler and others, 1964). These surficial deposits overlie granitic bedrock and range in thickness from about 500 ft in Eastham to over 1,000 ft in Truro (Oldale, 1992).

Figure 3. Freshwater underlain by saltwater forms the flow lenses of the Lower Cape Cod aquifer system (modified from Strahler, 1972) Image enlarged

Kettle-Hole Ponds

Kettle-hole ponds are the most conspicuous freshwater feature found throughout Lower Cape Cod (fig. 4). These ponds are surface-water expressions of the water table because they directly connect with the underlying aquifer system. Kettle-hole ponds are also flow-through ponds with freshwater seeping into and exiting the pond in the direction of ground-water flow. The kettle holes were formed by blocks of ice stranded by the retreating ice sheets and buried by the deltas that expanded into Glacial Lake Cape Cod. These ice blocks were stranded directly atop basal till and bedrock. When the buried ice blocks melted, overlying and surrounding sands and gravels collapsed into the resulting depressions. These depressions eventually became ponds when they filled with ground water as the water table rose in response to the rising seas. These kettle-hole ponds are found throughout north Wellfleet and south Truro within the CCNS (fig. 1).

Figure 4. The kettle-hole ponds of Lower Cape Cod hydraulically connect with the underlying aquifer; therefore, they are surface-water expressions of the water table. Ground water discharges into the ponds along their upgradient shores, flows through the ponds, and then reenters the ground-water system along the downgradient shore. View looking southwest. Image enlarged

Pamets

Valleys, known as pamets, carved the originally flat, outwash-plain surfaces throughout Lower Cape Cod.These pamets formed possibly by spring sapping, a process in which springs migrated inland from the coast by gradually eroding the gently sloping land surface to form a series of east-west trending stream valleys (Oldale, 1992). These pamets sometimes are referred to locally as hollows, and can be found throughout the Wellfleet and Truro areas. The most notable pamet on Lower Cape Cod is the valley that contains the Pamet River in Truro (fig. 1).

Aquifer System

The freshwater-flow lenses of the Lower Cape Cod aquifer system consist of four water-table mounds separated by surface-water discharge areas. For example, the Pamet River is the hydrologic boundary between the Pamet and Chequesset ground-water-flow lenses (fig. 1). Each flow lens is sustained (recharged) by precipitation that percolates through the ground to the water table, the top of the aquifer. Generally, ground water flows radially at a rate of about 1-2 ft per day from the highest point of the water table toward the coast and toward the streams and wetlands that form the boundaries between the lenses. Recharge near the center of the water-table mound travels deeper through the aquifer system than water that recharges near the coast (fig. 3). The water that travels along the deeper flow paths to the coast (fig. 3) can take more than 100 years to reach the coast.

All of the freshwater that flows through the Lower Cape Cod aquifer system is derived from aquifer recharge. Under current (2005) conditions, flow from one flow lens does not discharge to another flow lens; therefore, the four flow lenses are hydraulically independent of one another (Masterson, 2004) (fig. 1).

Water Table

The elevation of the water table is typically referred to in terms of altitude above the mean sea-level elevation of 1929 (NGVD 29). The altitudes of the tops of the water-table mounds (fig. 1) range from about 16 ft above NGVD 29 in the Nauset flow lens to about 6 ft above NGVD 29 in the Pilgrim flow lens. Surface-water bodies, local geology, and changing ground-water pumping and recharge conditions affect the altitude and shape of the water-table mounds. The Chequesset flow lens, which receives the most recharge of the four flow lenses, has a maximum altitude of only about 9 ft above NGVD 29, which is about 7 ft lower than the altitude of the Nauset flow lens; this difference is caused by the large number of streams, such as the Herring River in Wellfleet, that flow in the Chequesset flow lens and by the differences in the permeability of the glacial sediments of the Nauset and Chequesset flow lenses (Masterson, 2004).

The altitudes of the water-table mounds vary seasonally and annually because aquifer recharge rates vary with changes in precipitation and temperature. These rates also may vary from one flow lens to another because of differences in land use, vegetation, depth to the water table, and local rainfall. Water levels at observation well TSW-89 in north Truro (fig. 1) correlated strongly with rainfall (fig. 5) measured at the nearby National Atmospheric Deposition Program site in south Truro (NADP Site, fig. 1). Changes in precipitation, and therefore, aquifer recharge, also result in changes in pond levels, streamflow, and discharge to the coast. The magnitudes of these changes vary across a flow lens and from flow lens to flow lens.

Figure 5. Changes in water levels are directly correlated to changes in precipitation as shown by the change in rainfall at the National Atmospheric Deposition Program site in Truro and water levels at observation well TSW-89 in North Truro, Cape Cod, Massachusetts, from 2000 to 2003. Image enlarged

Water-level fluctuation, in response to changes in recharge, is greatest at the top of a water-table mound away from streams and the coast because water levels change less in areas where ground water discharges (Masterson, 2004). Duck Pond, which is near the top of the southern water-table mound in the Chequesset flow lens (fig. 1), had a mean annual fluctuation of about 1.6 ft, as determined from monthly measurements from 1994 to 1999 (Sobczak and others, 2003). In contrast, Gull Pond, which is close to the Herring River and is connected to nearby kettle ponds by a network of shallow, dug channels, had a mean annual fluctuation of about 0.7 ft during that same period.

Water Budget

Precipitation is the primary source of freshwater to the Lower Cape Cod aquifer system. On average, the rainfall rate is about 42 inches per year (in/yr). It is assumed that about 43 percent of the water (18 in/yr) is removed by evaporation and plant transpiration before reaching the water table; the remaining water (24 in/yr) enters the aquifer as recharge. If a recharge rate of 24 in/yr is assumed over the entire Lower Cape Cod land mass, then freshwater continually flows through and leaves the aquifer at a rate of about 2.5 billion gallons per year, or about 68 million gallons per day (Mgal/d) (fig. 6). Ground-water-flow simulations were made to determine how much of this freshwater discharges directly to the coast through the seabed or reaches the coast as streamflow (Masterson, 2004).

The total water budget for the Lower Cape Cod aquifer system can be subdivided by individual flow lenses (fig. 6). Subdividing the water budget by flow lens provides a better understanding of the distribution of flow to the various hydrologic features than can be obtained from the total water budget for the entire aquifer system. For instance, the total amount of ground-water discharge to streams is about 21 Mgal/d, or 31 percent of the total water budget for the Lower Cape Cod aquifer system, but nearly 60 percent of that streamflow is in the Herring River in Wellfleet, which bisects the Chequesset flow lens (fig. 1).

Ground-water withdrawals for public supply account for only about 1 percent of the total water budget for the aquifer system; however, almost all of the pumped water is from the Pamet flow lens, composing about 7 percent of its total budget (fig. 6). This pumped water is the primary source of drinking water for the town of Provincetown, parts of the town of Truro, and some NPS facilities in the Provincetown area. Most of the wastewater derived from this public supply is discharged into the Pilgrim flow lens as treated sewage at the Provincetown wastewater-treatment facility infiltration beds (fig. 1), with the remainder being discharged through onsite domestic septic systems in unsewered areas.

Figure 6. The hydrologic budget of the Lower Cape Cod aquifer system can be subdivided by individual flow lenses to show the relative importance of the different flow components in each flow lens. Image enlarged

Freshwater/Saltwater Interface

The Lower Cape Cod land mass is small, thereby limiting freshwater recharge, aquifer area, and aquifer depth. Consequently, freshwater in the Lower Cape Cod aquifer system does not extend to the underlying granitic bedrock, but instead is primarily bounded below by saltwater (fig. 3). Depths to the boundary, or interface, between fresh and saltwater within the aquifer system differ among the flow lenses, and are directly proportional to the altitude of the overlying water table. Because of the density difference between fresh and saltwater, the depth to the freshwater/saltwater interface below sea level is about 40 times greater than the altitude of the water table above sea level (Freeze and Cherry, 1979). Therefore, if the water table is 5 ft above sea level, the depth to the freshwater/saltwater interface would be about 200 ft below sea level. Although this is only a general approximation of the actual relation between fresh and saltwater flow, it does provide reasonable estimates of the depths to the freshwater/saltwater interfaces throughout the Lower Cape Cod aquifer system.

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