Monomoy Lens Acquifer

What Is An Aquifer?

How does the Lens work?
Protecting the Monomoy Lens starts with understanding how it works - its hydrogeology. Lenses can be thought of as mounds of groundwater bordered by marine water at the edge, bedrock on the bottom, and separated from each other by tidal rivers, such as Bass River, that cut across the Cape peninsula. Groundwater refers to subsurface water located beneath the water table, in soils and geologic formations that are fully saturated. The entire layer of fresh groundwater beneath the Cape is referred to as the Cape Cod Sole Source Aquifer, which is made up of the six separate lenses shown in the figure above.

Monomoy Map

Who uses this water?

Monomoy is the second largest lens, and is located under the towns of Dennis, Harwich, Brewster, Chatham, Orleans and a section of Yarmouth. It is approximately 300 feet thick, and is the source of drinking water to over 50,000 homes and businesses that are served by 49 municipal public water supply wells and an estimated 1,000 private wells.

How is the groundwater quality?

The Monomoy Lens supplies generally excellent drinking water from its porous sand and gravel deposits. The water is considered "soft" due to the lack of calcium and magnesium. The pH of the water is naturally low, which can cause blue staining on plumbing fixtures from copper piping. Municipal water supplies are treated to neutralize the pH. Naturally occurring iron and manganese can cause staining, odor and taste problems. Sodium chloride can be elevated in coastal areas due to salt spray or saltwater intrusion.

How do surface waters fit in?

The Monomoy Lens also boasts over 200 freshwater lakes and ponds, 20 streams, and 150 miles of coastal shoreline. The inland surface water bodies are windows on the aquifer that reflect the intersection of low areas in the ground surface with the water table. Groundwater typically discharges into a pond on one side and then pond water recharges the lens on the other side. Streams and rivers act as drains that skim groundwater off the surface of the water table. The large Monomoy ponds (Long, Seymour and Hinkleys) receive groundwater discharge from the lens, which in turn, feeds the Herring River so that groundwater ultimately discharges as stream flow into Nantucket Sound. Where there is only coastal shoreline, groundwater discharges directly into marine water as fresh water seepage. Because of this interconnection, all uses of water- whether for drinking, swimming, boating, clamming, cranberry farming, or wetland habitat - are dependent upon maintaining the quantity and quality of the lens.

Where does the water come from?

AquifierGroundwater of the Monomoy Lens is replenished or recharged from rain and snow that percolate down through the ground. Approximately 60 million gallons per day slowly pass through the aquifer, as pictured above, and are ultimately discharged to the surrounding marine waters. The map below shows how different portions of the aquifer discharge to specific marine embayments. In this respect, groundwater in the aquifer is always moving, keeping a balance of recharge and discharge. Groundwater moves from areas of higher elevation to lower elevation at a fairly slow rate of one foot per day. Flow from the middle of the lens to the shore --a distance of 10,000 feet-takes 10,000 days, or nearly 30 years. This is an extremely long time for the aquifer to flush itself. So contaminants that are introduced to the lens degrade water quality and can ruin our drinking water for generations.

What is a recharge area?

The map above shows the major water resources of the Monomoy Lens and their recharge areas. A recharge area is the portion of the aquifer that contributes groundwater flow to wells, ponds or coastal embayments. The recharge area to a well is commonly called a wellhead protection area or Zone II.

How can land use planning help?

The key mechanism for protecting Cape Cod's sensitive water resources and avoiding the possibility of contamination is proper land use planning in the recharge areas. The Cape Cod Commission, through the Regional Policy Plan, has provided a comprehensive resource based strategy to protect and manage these resources. These are summarized under the resource headings below.

Wellhead Protection Areas

Resource Areas
WellheadEach public supply well is fed by groundwater from a defined recharge area, also referred to as a Zone II, or Wellhead Protection Area. Land use within the recharge area can affect water quality.

Identified Threats

Land Use Controls

What Can I Do?
Know the location of the town water resource areas and what activities could contaminate them. Report illegal waste disposal. Remove underground fuel storage tanks older than 20 years. Read your drinking water consumer confidence reports.


Resource Areas
Nearly 20% of households on Cape Cod depend on private wells for their drinking water. Protection of private wells is a responsibility shared bythe homeowner and bythecommunity at large.

Identified Threats

Land Use Controls

What Can I Do?
Test the water quality of private wells on an annual basis for nitrate, bacteria, and volatile organic compounds. Inform local boards of health of private well location. Minimize use of hazardous materials and dispose of them properly.


Resource Areas
Lakes and ponds on Cape Cod are directly connected to the groundwater. Seasonal fluctuations in the water table cause changes in pond levels.

Identified Threats

Land Use Controls

What Can I Do?
If you have a septic system be sure it is properly located, constructed and maintained. Join or form a lake or pond association. Maintain a buffer strip of native vegetation along shoreline. Minimize lawn fertilization. Do not feed waterfowl.


Resource Areas
A marine recharge area or watershed shown by the dotted lines on the central map is that area of the lens from which groundwater flows into the coastal waters. Marine water quality is affected by nitrogen from land uses in recharge areas, or watersheds.

Identified Threats

Land Use Controls

What Can I Do?
Use a holding tank for boat waste and discharge at pump-out facilities, rather than at sea. Maintain motors to prevent excessive discharge of oil and gas. Properly change and dispose of engine motor oil. Become a water quality monitor volunteer. Maintain vegetative buffers.


Resource Areas
Future drinking water must come from undeveloped land which towns set aside and protect. The total land area left for development of future supplies is rapidly diminishing. The green areas on the central map show where potential water supplies may be sited.

Identified Threats

Land Use Controls

What Can I Do?
Support groundwater protection legislation and education efforts at federal, state, and local levels. Encourage land acquisition or permanent protection of identified future supply sites and adjacent undeveloped land.

What threatens Cape Cod's groundwater?
Although Cape Cod's groundwater is of excellent quality and safe for drinking, sandy permeable soils and shallow depth to the water table make it particularly vulnerable to contamination. Some specific sources of contamination include landfills, hazardous waste spills, underground fuel storage tanks, septic systems and stormwater runoff.

Areas of contaminated groundwater are commonly referred to as plumes. A plume can impact significant areas and is often caused by a release of organic or synthetic chemicals into the water. Identified contamination plumes, shown on the map to the left, are also known as a point source of contamination because they can be traced back to a specific spill or discharge point. The toxic chemicals in these plumes do not easily break down or degrade in groundwater and may move a long way without losing concentration.

The greatest threat to water quality on Cape Cod is the widespread use of on-site septic systems that dispose of human waste into the ground. Although the plume associated with each septic system is small, if located too close to a private well, can contaminate the well with virus and bacteria. In addition, when these plumes are too numerous in an area they contaminate groundwater with high levels of nitrogen. Collectively these onsite septic plumes are referred to as non-point sources of contamination.

How are wells protected from contamination?
Wellhead protection efforts over the last two decades have focused mainly on land use regulations. These have been very effective in protecting public water supply wells from identified point sources and in limiting non-point source contamination. Board of health regulations or zoning bylaws may be created to limit the use of hazardous materials within wellhead protection areas, limit septic system density through minimum lot sizes or establish water quality performance standards for new construction. Other regulatory mechanisms include buffers and set back regulations, hazardous material registration and inspection, and storm drain controls

While use of these regulatory mechanisms are a critical way to protect groundwater they are primarily accomplished through town ordinances that can often be appealed, or do not apply because of a pre-existing use. Similarly, without enforcement of these regulations, the goal of water quality protection may not be met.

What are other ways to protect water quality?
Water quality is best protected by outright purchase and preservation of land in the wellhead protection area. It is also important to protect land that may be needed for future water supply sites. The center map shows tracts of land that may be suitable for new public water supplies. These tracts are referred to as potential wellhead protection areas. The tracts are located both in and out of existing wellhead protection areas.

With the advent of the Cape Cod Land Bank in 1998, Cape towns have a steady stream of revenue with which to buy critical parcels for wellfield protection. However, available funds from this source are often insufficient to pay for the very expensive acreage which Cape Cod has become. The following are some common techniques for protecting these identified potential wellhead protection areas.

Common open space techniques include:

  1. Land donation: Landowners can obtain tax incentives in exchange for donating land to wellfield or wellhead protection use.
  2. Conservation restriction: Property remains in private hands, but owner agrees not to develop all or part of it in order to protect water quality. Cape towns will lower property taxes on the land under restriction. Also useful for income tax deductions and estate planning.
  3. Charitable sale: Seller agrees to take less money than the appraised market price for the land in exchange for tax deductions.
  4. Reserved life estate: An owner continues to live on the property, while conveying the title to a water purveyor or other conservation entity. Income tax deductions accrue to owner, depending on how much longer they can be expected to enjoy use of the property.
  5. Current use assessment: An owner of five acres or more enrolls each year with town, promising to keep the land in its natural or cultivated condition, rather than develop it. Property taxes are reduced significantly and the town acquires the right to buy the property if sold for other uses.


Rules and regulations on well an septic system care
Local Boards of Health
Association for the Preservation of Cape Cod
Water quality testing Barnstable County Department of health & the Environment or Local Water Supplier
Accidental fuel or chemical spills Local Fire Departments
MA Dept. of Environmental Protection
Underground fuel-storage tanks Barnstable County Department of Health & the Environment
Pesticide application and disposal Barnstable County Extension Service
Agricultural management practices US Natural Resource Conservation Service & Barnstable County Extension Service
Water resource assessment, protection and regional land use planning Cape Cod Commission
Marine resource assessment and protection Coastal Zone Management
Barnstable County Agencies
Waquoit Bay National Estuarine Research Reserve

The Priority Land Acquisition Assessment project has been financed in part with federal funds from the Environmental Protection Agency (EPA) to the Massachusetts Department of Environmental Protection (DEP) under a 604(b) Water Quality Management Planning Grant. The contents do not necessarily reflect the views and policies of EPA or DEP, nor does mention of trade names or commercial products constitute endorsement or recommendation for use, Illustrations are used with the permission of the artists: Abigail Tillier, Dana Gaines, Jill Eldredge, Mark Robinson and Heather McElroy. Poster created by Cape Cod Commission.