Nantucket Water Resources Part 1
by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station
Nantucket would not be the beautiful destination place that it is today without constant care and vigilance regarding our freshwater and saltwater resources. I am frequently asked by visitors about the quality of our drinking water and the health of our ponds and harbors, so I thought now would be a good time to discuss some specifics about the liquid gold that surrounds us and lays under our feet. This is part one of a short series of three or four articles over the next few weeks that will address Nantucket’s water quality and hopefully provide some useful information about our surrounding water bodies.
First, we should distinguish between different types of water on Nantucket; we have surface water systems which include ponds, our harbors, vernal pools, intermittent streams, and springs. We don’t have any real rivers on island, which is certainly different from the mainland. We are fortunate in that we have a variety of pond types from kettle hole ponds formed by large chunks of ice from glaciation rattling around in depressions such as Washing and Maxcy’s ponds, great salt ponds like Sesachacha, glacial outwash valleys that have filled in via the surrounding freshwater lens (Hummock Pond), and several man-made ponds. Surface water of importance to Nantucket can be further divided into freshwater, brackish water (i.e., mesohaline or in the range of 5-18 parts per thousand salinity), and salt water. Each type of water body has a range of factors that can affect it in different ways. Anyone who has set up a freshwater or saltwater aquarium immediately learns the variables that may be important such as bacteria and viruses and buffers and the ideal chemistry of the water.
The opposite of surface water is groundwater, which is, as the name implies, water underground. The island is underlain by a huge lens of fresh water that is so large and tends to be so full that it bulges out into the surrounding salt water. In only a few locations around the island does the salt water intrude into our freshwater during periods of drought or extreme localized pumping. In addition, several layers of clay (called aquitards because they stop water) help to prevent sea water from invading the lens. We are also blessed with a very fast recharge rate and enough rainfall (around 43 inches per year) to keep our sole source aquifer full regardless of the amount of water pumped from it. Our Hydrology undergraduate courses and field classes work closely with the Wannacomet water company and it can be quite illuminating to pump wells from around the island and watch them refill to get an idea of the recharge rates and to find out (quickly) where those clay layers are! Although we have some issues with shallow parts of our groundwater due to fertilizer and septic inputs, the water most of us drink from our wells and certainly our town water is quite clean and pristine (www.wannacomet.org). Nantucket has its own source of fresh drinking water created by the glacier 12,000-10,000 years ago. Ground water filters down through layers of sand and clay. Due to the pristine nature of our water, Nantucket does not add chlorine or any other additives to the water supply.
The Wannacomet Water Company pumps groundwater from three different wells that draw the water from two different levels of the aquifer. The Tubular Wellfield is located at Wyer’s Valley off Milestone Road and draws water from the upper level, 40’ below the surface, of the aquifer. Well #12, also located at Wyer’s Valley, and Well #13 located on Ticcoma Way (which is surrounded by the Nantucket State Forest) pumps water from an average depth of 180’ below the surface. The link to their most recent report is www.wannacomet.org/documents/WaterCompany_Report09.pdf.
The Nantucket Field Station has a very useful physical model for educational use that illustrates how water below ground interacts with the surface of the island. These models can be built for the classroom and they do a very good job of showing you how material moves underground. For instance, a spring is any natural occurrence where water flows onto the surface of the earth from below the surface. I tell my students that the springs we see on Nantucket are simply where groundwater intersects with the air. We have a plethora of springs around the island and you can see some of them as you travel around and look at some of our beach bluffs. If you look closely you’ll see water trickling out of some of the cliffs. Our springs made Nantucket a viable place to live for early settlers and for the Wampanoags. According to Nantucket: a History by Robert Alexander Douglas-Lithgow (1914), early settlers were very careful about locating their homesteads near one of the numerous springs such as the one at Shawkemo, Eat Fire Spring by Polpis, Consue Springs off Union Street, or the “Benjamin Franklin” spring which was very close to Peter Folger’s home and now flows to the ornamental fountain/monument to Abiah Folger placed by the Abiah Chapter of the Daughters of the American Revolution which can be seen on Madaket Road and may refresh you on your journey.
Okay, I always try to insert some history mainly for my own edification, but back to some basic science such as learning about the hydrologic cycle which is a fancy way of talking about the constant movement of water above, on, and below the earth's surface. It is a cycle that replenishes ground water supplies. It begins as water vaporizes into the atmosphere from vegetation, soil, lakes, rivers, snowfields and oceans through a process called evapotranspiration. As the water vapor rises it condenses to form clouds that return water to the land through precipitation such as rain, snow, or hail. Precipitation falls on the earth and either percolates into the soil or flows across the ground. Usually it does both. When precipitation percolates into the soil it is called infiltration; when it flows across the ground it is called surface runoff. The amount of precipitation that infiltrates, versus the amount that flows across the surface, varies depending on factors such as the amount of water already in the soil, soil composition, vegetation cover and degree of slope.
On Nantucket we have a variety of factors that effect how much water percolates into our soil and how quickly this water can carry surface contaminants into surrounding ponds and water bodies. Our very sandy soil provides very little resistance or uptake of the water and so water flows very quickly through the ground, from conservative estimates of 1 foot/per minute to even higher flow rates. Surface runoff flows downhill following topographic contours and eventually reaches a surface water body where it is again evaporated into the atmosphere, continuing the cycle. Runoff carries all the things it encounters on its trip, which is why we get concerned when we have impervious surfaces like asphalt roads which can leach materials and provides a sluice way for rain which can be carrying oils and other contaminants. When contaminants are introduced into a water body from runoff, we call this “nonpoint source pollution.” Some of these pollutants occur naturally, such as nutrients from sediments, manure or pet wastes; others are manmade, such as fertilizers or automotive grease. Nonpoint source pollution is a major cause of water quality problems both in Massachusetts and nationwide.
Percolation into the soil, or infiltration, moves under the force of gravity. If soils are dry, water is absorbed by the soil until it is thoroughly wetted. Then excess infiltration begins to move downward to the water table. Once it reaches the water table, it is called ground water. Ground water continues to move downward and laterally through the subsurface. Eventually it discharges through hillside springs or seeps into streams, lakes, and the ocean where it is again evaporated to perpetuate the cycle. Most rock or soil near the earth's surface is composed of solids and voids. The voids are spaces between grains of sand, or cracks in dense rock. All water beneath the land surface occurs within such void spaces and is referred to as underground or subsurface water.
Subsurface water occurs in two different zones. One zone, located immediately beneath the land surface in most areas, contains both water and air in the voids. This zone is referred to as the unsaturated zone. Other names for the unsaturated zone are zone of aeration and vadose zone. The unsaturated zone is almost always underlain by a second zone in which all voids are full of water. This zone is defined as the saturated zone. Water in the saturated zone is referred to as ground water and is the only subsurface water available to supply wells and springs. When you dig down into an area near a wetland, you’ll see a change in the soil based on whether it is under water all the time or whether it is mostly dry.
The term “water quality” is a somewhat ambiguous term that means different things to different groups. Typically when we go out into the field and sample shallow groundwater or pond water, we are looking for a few basic things and then also checking a few items that may be out of whack. It is not all that different from a physical one might have at the doctor’s office, in which basic measurements like weight, temperature, reflexes, and blood pressure are checked. This is one of the reasons that the “health” of a water body is evaluated; unhealthy ponds are full of algae, have very low dissolved oxygen values from an overabundance of organic matter decomposing and may have few fish. Along that same vein, the term “water body” is often used because water quality experts know a pool of water is more complex than it looks and that evaluating factors affecting its health need to be viewed holistically when possible.
When we take samples of a water body, we try to obtain samples from the surface, middle, and near the bottom of a pond or harbor because the water is different in each of those zones. Both salinity and oxygen can really vary between those depth and different fish species also inhabit different levels in a pond. During times of high plant growth, oxygen levels will be supersaturated at the surface of the water (greater than 100%) and will be very low, sometimes dangerously low, near the bottom as plants die and decompose and use up the oxygen. To evaluate what is entering the water, we sometimes set out large buckets to catch rainfall and measure the acid rain and chemicals and gases in that water which is approximately equal to atmospheric inputs. Then we might intercept the groundwater flowing toward the pond by installing wells underground to capture that water. I do that in a variety of places around the island to get an idea of the source of potential contaminants. If you simply measure the water in various locations in a pond, you are essentially taking a “snapshot” of what has persisted in the pond. This is not the whole story though because the plants and sediment on the bottom have absorbed much of excess nutrients that have entered the pond, so you re not really getting an idea of potential harmful inputs. Going back to our pond/body analogy; we would get different information from measuring what is in the food and liquids we ingest versus our blood values for various compounds like mercury or lead. This phenomenon is greatly exacerbated when you talk about a large water body with a lot of inputs like the harbor.
There are a set of standard methods that people use to do water quality measurements and the types of things measured do not change that much regardless of whether the scientist is a volunteer or a professional. Sampling protocols can vary depending on the questions asked and the machines being used to look at various components. We look at nitrogen and phosphorus in its various forms as well as coliforms (bacteria) and pH, salinity, temperature, dissolved oxygen, total dissolved solids, and conductivity among a few other items. Wannacomet Water Company and the Solid Waste Treatment Facility measure organic and inorganic contaminants such as metals and derivatives of pesticides and things like PCBs. As a chemical oceanographer, I have looked for material from hospitals including pharmaceuticals and radioactive contaminants as well as leftover material from bomb fallout and industrial film processing. Lots of scientists are now concentrating on industrial pollution and things called endocrine disruptors (the breakdown products from manmade chemicals such as hand sanitizers and some shampoos) which mimic natural hormones in water and change fish and amphibian sexual characteristics. That’s worth a whole article right there. As part of water quality research, what we find in the water also tells us a bit about its journey, just as hair and blood samples from people can tell forensic scientists a lot about where they lived and what they eat.
The Commonwealth of Massachusetts has a set of guidelines they use to assess the health of a water body and we try to follow some of the same guidelines. In the long run, a localized freshwater (or salt water) management plan would include all these steps and answer all these questions. Information you need include: what types of toxins and excess nutrients are in the water, the fish, the plants, and the sediment. Do these pollutants affect human health (fecal bacteria or other pathogens)? Can you eat the fish? Can you find and reduce the source? What trends do you notice, cleaner, or dirtier? Can changes in behavior or policy help a water body recover? Are there invasive plants or fish present? Are algae blooms overtaking a pond (typically a sign of too much food or nutrients coming into a pond)? Is the pond going anoxic (low oxygen)? Do the frogs have multiple legs (don’t laugh, that is common in the Northern U.S. states)?
The Field Station has a fun “Adopt a Pond” program in which high school students from the island and visiting each summer pick a pond and evaluate its health. This year for the third year in a row we are looking at Tom Nevers Pond. Tune in next week (if you are off island, go to www.yesterdaysisland.com for electronic versions of these articles) to find out what we are seeing in some of our ponds. And for those who may not know, volunteers, interns, and water quality experts (Jim Sutherland and I) associated with the UMB Nantucket Field Station have taken over some of the water quality sampling normally done by the Marine Department when reductions in town finances necessitated discontinuing some of that research. Next week we’ll talk about some of the recent results.
Read Part 2 >
