Figure 1 - Location of Cape Cod (LeBlanc, 1986)
The Cape Cod Glacial Aquifer is a continuous, unconfined aquifer system underlying the Cape Cod peninsula in southeastern Massachusetts. The peninsula extends into the Atlantic Ocean and is separated from the rest of Massachusetts by the Cape Cod Canal. Consisting mainly of highly permeable, glacial sediments, the aquifer provides an excellent source of drinking water for Cape Cod. The permeability of the sediments also makes the aquifer prone to contaminant infiltration. In 1982, the United States Environmental Protection Agency USEPA recognized the Cape Cod aquifer as the principal source of drinking water for the peninsula and designated the aquifer as a sole source aquifer. In recent years,the federal government, the state of Massachusettes, and local governments have been working together to protect and evaluate the recharge areas of the aquifer. Ground water modeling projects have been conducted to study and evaluate how the aquifer's recharge areas contribute to the health of ground-water and surface water bodies of Cape Cod. Due to the absence of major rivers on the peninsula, ground-water flow is the dominant influence on the hydrology of the area.
The Cape Cod Glacial Aquifer underlies the Cape Cod peninsula, an area of 440 square miles in southeastern Massachusetts. Saltwater surrounds the peninsula. Its 586 miles of coastline is bordered by the Atlantic Ocean to the east, Nantucket Sound to the south, Cape Cod Bay to the north and Buzzards Bay to the west (see Figure 1). The east-west portion of the peninsula is called the Inner or Upper Cape, and the north-south portion is referred to as the Outer or Lower cape. Nearly half of the Outer Cape is the Cape Cod National Seashore, managed by the National Park Service. The watershed's permanent population of nearly 250,000 people increases to approximately 500,000 people during the peak summertime tourist season (EOEA, 2004).
Figure 2 - Cape Cod Watershed (EOEA, 2004)
Ground-water dominates the hydrology of the Cape Cod watershed. Surface water accounts for only four percent of the freshwater on the peninsula. Very few large streams and rivers are found. Surface water exists mainly as freshwater marshes, bogs, streams, salt marshes and glacial kettle ponds (see Figure 2). Water from precipitation rapidly infliltrates the aquifer's permeable soil and most of the groundwater discharges into the surrounding coastal waters (EOEA, 2004).
Cape Cod has a north temperate maritime climate. It has well defined winter and summer seasons with high summer temperatures averaging 77.5 degrees Fahrenheit and 38 degree highs in the winter. Precipitation averages about 45 inches a year on the inner cape and 40 inches a year on the the outer cape (Godfrey, 1999).
Precipitation is the Cape Cod aquifer's only source of freshwater recharge. Approximately 55 percent of the yearly precipitation is returned to the atmosphere through evapotranspiration. The other 45 percent goes to ground-water recharge to the aquifer. Surface runoff is less than 1 percent. High water levels occur in the aquifer in the early spring because most ground-water recharge occurs in the late fall, winter and spring. Low water levels occur in the fall. The aquifer's water levels also reflect long term fluctuations such as periods of drought or years of higher than average precipitation. However, data collected over long periods of time have not shown a long term water level decline or rise. These steady water levels indicate a natural balance that ground-water discharge has made to average recharge inflow (LeBlanc, 1986).
Water levels in streams and ponds on Cape Cod are closely related to ground-water discharge. Streamflow has a direct relationship to ground-water levels. The streamflow and ground-water discharge balance is expecially noticable on the Inner cape. Measured flow in streams have been equated to 20 percent of the Inner cape's total ground-water discharge. Ponds also reflect ground-water levels. More than 360 ponds are found on Cape Cod, most having been formed in the glacial outwash plains in depressions called kettle holes. The kettle hole ponds, (Figure 3), are fed by ground-water discharge and are in hydraulic contact with the water table. The pond water level corresponds with the water table around it (LeBlanc, 1986). The ponds are also often locations of through flow for ground-water. Ground-water enters the upgradient end of the pond, and pond water outflow flows out at the down gradient end (Godfrey, 1999).
Figure 3 - Kettle Hole Pond (Oldale, 2001)
The Cape Cod Aquifer is an unconfined unit consisting of mainly glacial sediments. The sediments, deposited by the continental glacier during the Pleistocene Epoch, overlie crystalline bedrock. The glacial deposits cover most of the peninsula and range in thickness from 100 feet in the Inner Cape to to 1000 feet in the Outer Cape. The very tip of the peninsula is covered with post-glacial beach and dune deposits. Consisting of thin layers of fine to coarse sand, the beach and dune sediments are mostly unsaturated and are rarely used for ground-water wells. The major hydrogeologic units found on the rest of the peninsula are outwash plains, moraines and glacial lake sediments (Olcot, 1995a).
Due to their high permeability, sand and gravel outwash plain sediments supply most of the public water on the cape. These stratified deposits are 200 feet thick in some areas and are distributed over nearly the entire length of the peninsula. The outwash sediments are interbedded with layers of clay in some areas. Lacustrine, glacial lake sediments, are found along Cape Cod Bay and consist of gravel and sand. These are sometimes overlain by silt and clay layers, creating small areas of confined, artesian units. Moraines of sandy til form interior ridges of the Inner Cape. The till, poorly sorted boulders, sand, gravel, silt and clay, support low yielding wells because of their low permeability (LeBlanc, 1986).
The hydraulic conductivity of the very permeable outwash sand and gravel deposits was estimated to range from 100 to 500 feet a day during pumping tests in 1985 (LeBlanc, 1986). A more recent aquifer test in the sand and gravel outwash confirmed that the aquifer's hyraulic parameters are consistent with prior estimates. The value for horizontal hyraulic conductivity was 0.23 feet per minute (approximately 300 feet per day), and the vertical hydraulic conductivity was estimated at 200 feet per day. The specific yield estimate was 0.26, the specific storage was 1.3 x 10E-5 per foot and saturated thickness was 170 feet. (Moench, 2001).
The aquifer's upper boundary is the water table, located at the highest level of the saturated zone. The lower boundaries of the aquifer in the Inner Cape are bedrock layers or impermeable layers of clay and silt. In the Outer Cape, the lower boundary is the transition zone separating the fresh ground water and saline water. Ground water flow in the aquifer is mainly horizontal. Vertical flow occurs mainly close to water table divides and at discharge areas at the coast, inlets or streams which lead toward the coast (LeBlanc, 1986).
Figure 4 - Ground Water Flow Lenses of Cape Cod (Eichner, 1993)
As shown in Figure 4, the flow system of the aquifer is actually divided into six separate flow cells or lenses (LeBlanc, 1986). The lenses are hydraulically independent units separated from each other by discharge areas such as bays and streams discharging into the ocean. Each lens is shaped as a water-table mound and ground water flows from the highest water table altitude near the center of the lens and follows the hydraulic gradient to the ocean. This creates a ground water divide running the length of the peninsula. A long term balance between yearly recharge and discharge accounts for stable flow within each lens and their shape remains unchanged.
Figure 5 - Outer Cape Hydrology (LeBlanc, 1986)
Figure 5 shows the hydrology of the Outer Cape, where saline water underlies the fresh water lenses. Thickness of the freshwater is approximatedly 200 feet.
Figure 6 - Inner and Mid Cape Hydrology (Olcot, 1995a)
Inner and Mid Cape lenses are underlain by bedrock or fine-grained deposits. When fine-grained deposits are present, fresh water discharge is restricted and the fresh-saline transition zone is located offshore (LeBlanc, 1986). Figure 6 illustrates the hydrology of the Inner and Mid Cape lenses.
Total flow through the aquifer system as a whole is estimated to be 270 million gallons a day.(LeBlanc, 1986) The largest ground water lenses of the aquifer are the Sagamore and Monomoy lenses located in the Inner Cape. These lenses are approximately 300 feet thick and account for 200 million gallons of water flow per day (EOEA, 2004).
The natural water chemistry of the Cape Cod Aquifer is generally good. The sandy sediments are chemically stable and highly insolluble. Rapid recharge rates, an unconfined setting and short flow paths cause little mineralization of the water. The USEPA recommends a 500 milligram-per-liter limit of dissolved solids in public ground water supplies. The Cape Cod aquifer has a median of 70 milligram-per-liter concentration of dissolved solids. In some place, such as wetland deposits overlain by sand dunes, some higher concentrations of iron, hydrogen sulfide and manganese may be present. Deep wells can exhibit high iron concentrations (Olcot, 1995a).
The Cape Cod Aquifer supplies 96 percent of the fresh water resource on the Cape Cod peninsula. With the exception of one town, all municipalities rely on the aquifer for their public water supply. One fresh water pond in the Inner Cape is used for a public supply and ponds and lakes are also used for irrigation and recreational purposes (EOEA, 2004). Approximately 83.6 percent of the groundwater withdrawn is for domestic and commercial use, 14.8 percent for nondomestic use (mostly for public supply), and 1.6 percent is for agricultural use (Olcot, 1995a).
The entire Cape Cod peninsula is under the jurisdiction of Barnstable County, which has the third fastest population growth rate in the state. The 2000 Census population exceeded 220,000 for the year round population and the summer visitor population more than doubles this figure. The majority of water pumped for public supply systems is in the Inner Cape from the Sagamore and Monomoy lenses. The Pamet lens in the Outer Cape uses 3 percent of the water used in public supply systems. Water supply demand on Cape Cod is projected to increase in the next few decades with an expected increase of 43.6 percent in water usage. A water supply shortage is expected to occur by the year 2020. To meet the increased demand, more than twenty new wells will be needed by 2020 (EOEA, 2004).
On the Outer Cape, only Provincetown and Truro are served by a public water supply system, which withdraws its water from the Pamet lens. The rest of the Outer Cape gets water from shallow, individual wells. An increase in land development and population in the Outer Cape is causing a demand for more centralized water systems. The shift from small capacity systems to large systems could cause ecological problems in the area. Declines in the water table of pond, streams and coastal areas due to decreased discharge could change the ecological balance in these surface waters. Large capacity pumping wells might reverse the flow direction of surface water bodies. Saltwater intrusion is also likely to occur if greater pumping removes enough ground water to cause an upconing of the underlying saline water. Lateral movement of the fresh-saltwater interface might also occur (Masterson, 2005 ).
Solving future water supply shortages is dependent on finding new sites that can support more supply wells and sites which have adequate protection to their recharge areas.The Cape Cod Aquifer is very productive, but pumped water is also returned to the aquifer as wastewater discharge. Usable ground water can be at risk in the vicinity municipal treatment plants and private septic systems due to seepage from these sites (EOEA, 2004).
Federal, state and local govenments work together in Cape Cod to protect the ground water resource. A major milestone was accomplished in 1990 when the state of Massachusetts established the Cape Cod Commision (CCC) as a planning and regulatory agency for the Cape Cod watershed. The commission is a division of the county government and it has been instrumental in coordinating regional cooperation and enforcing regulations to protect ground water resources. The CCC has identified areas that are suitable for wells and wellhead protection areas, and assists towns in prioritzing and acquiring these tracts of land (EOEA, 2004).
On the Outer Cape, the National Park service (NPS), the Massachusettes Office of Environmental Affairs, the United States Geological Survey (USFS), the CCC, and local towns are working together to improve water management strategies. A ground water flow model has been developed to evaluate issues such as ground water withdrawals, effects of seasonal and long-term fluctuations of flow and how these impact supply wells and surface water bodies (EOEA, 2004).
The highly permeable sediments of the Cape Cod Aquifer provide an infiltration pathway for contaminants to enter the aquifer. Contaminants originate and enter the aquifer from specific sources such as hazardous waste spills and also from non-point sources such as septic systems and runoff from roads and yards. Nitrogen contamination to the aquifer is an increasingly significant threat to ground water. As population, development and wastewater discharge increases on the peninsula more nitrogen is introduced into the aquifer. This nutrient is a limiting nutrient which degrades the coastal environment, and maximum levels in drinking water have been established by the USEPA. Nitrogen occurs as nitrate in ground water and usually doesn't decrease over time (Eichner,1993). The CCC has adopted guidelines for wellhead protection areas for public water supply wells. These guidelines include a 5-parts per million nitrogen loading limitation for groundwater, which is below the 10-parts per million drinking water standard set by the USEPA. The CCC, as Cape Cod's planning and regulatory agency has identified four areas of critical concern for Cape Cod's water resouces: wellhead protection areas, recharge areas for potential water supply areas, marine water recharge areas and freshwater recharge areas. The CCC enforces minimal performance standards for development and redevelopment in these areas.(CCC, 2002) CCC
Additional contaminants which threaten the aquifer include compounds introduced by hazardous waste sites, storage tanks, gasoline spills and household chemicals. A major contamination site on Cape Cod is the MMR Superfund Site. It was placed on EPA's National Priority List in 1989. Known in the past as the Massachusetts Millitary Reservation (MMR), cleanup is now coordinated by the Air Force Center for Environmental Excellence (AFCEE). The reservation is currently still functioning as a training area for various branches of the United State military forces (EOEA,2004). Soil contamination and numerous ground water contaminant plumes have been discovered on and in the vicinity of the MMR. The plumes were originally caused by leaking landfills, military training activities, and fuel and chemical spills. The USEPA and the Massachusettes Department of Environmental Protection are working with the AFCEE to complete the cleanup of the site. Remediation projects in place since the late 1990s include treating 18 million gallons of contaminated water a day at treatment plants and treating 100,000 tons of contaminated soil. Residents in the area affected by the plumes have been supplied with newly installed water systems and semi-annual well monitoring. Treatment systems have also been added municipal water supply wells. The USEPA has determined no immediate threat to the public health or the environment exists while final plans for cleanup are carried out (USEPA, 1989).
The Cape Cod Glacial Aquifer is a very productive aquifer which serves as the main source of fresh water to the Cape Cod peninsula. The aquifer is vulnerable to degradation because it is an unconfined unit and consists of very permeable sediments. The biggest challange to the future of the ground water is the development pressures and population growth on the Cape Cod peninsula. Overpumping of the aquifer can lower the water levels of surface water bodies that depend on ground water discharge and may also create salt water intrusion in some areas of the peninsula. Excessive ground water usage creates a wastewater disposal challenge. Contaminants entering the ground water can pose a health risk to the residents and contamination of the aquifer can upset the ecological balance of surface water bodies.
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