Nantucket Water Resources Part 2
by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station
Read Part 1 >
Last week‘s article discussed our naturally clean, sand-filtered island aquifer, how water flows underground, and what types of water research is underway on Nantucket. Our deep groundwater, located out of the reach of septic systems and fertilizers at a depth of 40 feet or deeper, is very clean and provides a safe drinking water source. In fact, Nantucket’s location out to sea and away from many mainland sources of pollution like power plants and industrial sites means that our sole source aquifer is safe from most external dangers (except atmospheric inputs like mercury wafting over the US). But water in the surface parts of the saturated zone around the island (between 0-35 feet or so) in some areas around the island contains evidence of what we flush and put on our lawns.
We can find this evidence when we collect samples of groundwater and measure the contaminants in them. We can also see this contamination when we measure ponds that are downstream of sources. Instead of exotic dangers like heavy metals or radioactive pollutants, our primary culprits are garden variety hazards such as nitrate and phosphate from leaking septic systems and fertilizer applications. Wells located only 100-200 feet apart may have very different amounts of contaminants. Out in the field, we measure nitrate and orthophosphate, which are the more bioavailable (easier to use) nutrient species of nitrogen and phosphorus. Nutrients are exactly what they sound like: minerals and elements that are needed for growth and normal plant and animal processes in small amounts that, when administered in large amounts, can “overstuff” a pond. Some of our ponds are having Thanksgiving dinner almost every day! Pond health is very similar to our health and you can overfeed a pond or harbor by supplying excess “food” in the form of nutrients. What we try to do is measure where the excess “calories” are coming from and then put the pond on a diet to help it slim down. Some ponds, if they are really “fat” and have stored way too much food, must be liposuctioned by removing the packaged food or fat hanging out in the form of huge algal mats. But just like removing fat without stopping the intake that got you there in the first place, “liposuction” should be done after the diet is successfully (and permanently) in place. Not that I am one to be lecturing a chubby pond...who knows, it could be genetic.
Back to our forensic science and determination of what is getting into the ponds and harbors. We measure a variety of different chemical and physical parameters such as temperature, dissolved oxygen, turbidity, pH, conductivity, total dissolved solids, and salinity, etc. when we evaluate ground or surface water conditions. Low dissolved oxygen levels indicate contact with either a marsh or peaty area or close contact with a septic leach field. Very high dissolved oxygen levels, especially in the harbor or ponds, indicate lots of excess plant growth. Salinity in a water body that should be fresh shows us that some salt water intrusion is occurring. Salt water intrusion is relatively rare on Nantucket considering we are an island; even in Madaket by Hither Creek, the groundwater is very fresh. Each piece of information is another piece of the puzzle that lets us know where that water has been and whether it is in good shape or not. For a person you might take hair and blood samples and samples of what we excrete to learn the same kind of things. Hair sampling would be more like sediment sampling or perhaps taking fish or submerged aquatic vegetations samples because they are integrating what has entered the water over time. Once in a while we will also do coliform (bacteria) samples in groundwater if we suspect septic influences and of course we usually do that for our ponds. Birds, dogs, and people all contribute fecal coliforms and there is an entire suite of (mostly harmless) natural bacteria, not unlike the many types that live in us (I know, I hope you aren’t eating as you read this, remember how good our water is!).
Many people depend on water clarity (how clear or transparent the water is) to tell if a pond is healthy. This can be helpful in some instances if quick growing algae is causing the water clarity to diminish, but it is of no help on some of our smaller ponds which are very “tannic” due to the presence of decaying leaves and natural plant life which cause the water to turn dark orange. In fact, some ponds like the field station pond, are very hard to evaluate for nitrate and phosphate concentrations because the colored compounds or tannins block our ability to measure nutrients which use a range of color changes that occur after simple chemical manipulation and correspond with concentration. But this orange color is as natural as tea; in fact tea is tannins, or colored compounds made by steeping tea leaves and allowing them to impart this colored material to hot water.
So, we now know that tannins are a natural organic material that is created as water passes through peaty soil and decaying vegetation. The presence of tannins tints water yellow to orange to brownish “tea color,” and can cause yellow staining on fabrics, fixtures, china, and laundry. Tannins may give a tangy or tart aftertaste to water. They may also cause water to have a musty or earthy odor. Tannins are composed of a variety of compounds such as fulvic or humic acid, and they are more common in surface water supplies and shallow wells than in deep wells. Water in marshy, low-lying, or coastal areas is also more susceptible to tannins. We can even use the combination of tannic acids in water to learn a great deal about where the water has been and it is theorized that these tannins help fish identify where they were born. A simple test for tannins involves filling a clear glass with water and letting it sit overnight. If the color settles to the bottom of the glass, the discoloration is most likely caused by iron and/or manganese and not tannins. If the intensity of the color remains intact, it is most likely caused by tannins. Tannins make water filtration systems work harder and can restrict iron removal, so if you believe you have them in your well water coming to your house, the best thing to do is dig a deeper well, or let your filtration specialist know you have tannins so that they can set up a system appropriate for your home. Tannins are simply an aesthetic issue, and are not a heath hazard, except for your clothing items. Excess iron, which is relatively common in some parts of the island, can also cause the discoloration you might see in the wash.
Scientists needed to develop a system to determine pond health and the “fatness” or amount of biological productivity of a pond. This is a measurement of how much microscopic plant life is being generated which would become food for high trophic level organisms such as fish or invertebrates. This measurement is important, because some ponds (and parts of the ocean) can be naturally (or non-naturally) devoid of life and non-productive. Just like the ideal body ranges for humans, you can be too skinny or too fat. These terms were developed for categorizing biological productivity: oligotrophic waterbodies have the lowest level of biological productivity; mesotrophic waterbodies have a moderate level of biological productivity; eutrophic waterbodies have a high level of biological productivity; hypereutrophic waterbodies have the highest level of biological productivity. We measure a pond’s trophic level based on its nitrogen and phosphorus concentrations and by the amount of floating and attached algae present, but other groups may use chlorophyll concentrations or Secchi depth (depth of light penetration measured by viewing a disk underwater) to evaluate water clarity.
Thanks to historical water chemistry data, scientists noticed certain patterns when comparing chlorophyll and water clarity data. After looking at hundreds of lakes, it became clear that, in most lakes and ponds, as chlorophyll concentrations from phytoplankton growth increase, water clarity decreases. This led them to believe that, for the most part, they could begin to predict how biologically productive a lake is based on its water clarity. They hypothesized that if lake water is not very clear, it’s more than likely due to an abundance of algae. The presence of large amounts of algae suggests that the lake is a productive system, providing an abundance of food for aquatic life. But hypereutrophic water bodies have way too much algae growth occurring and very low water clarity and this is especially noticeable when all the algae dies in the winter and sinks to the bottom causing anoxic (low or no oxygen) dead zones. And as we discussed a bit above, tannins and also suspended sediments like clay or peat particles or decaying plants can greatly reduce water clarity in systems that are perfectly fine. In the harbor, clear water is very important for the successful growth of eelgrass and reduction of light in the water column due to the growth of excess phytoplankton from nutrients contributed by humans (anthropogenic inputs) is one of the primary factors contributing to serious eelgrass declines.
My research crew at the UMass Boston Nantucket Field Station has been measuring shallow groundwater for quite some time and high school students have been helping us appraise the condition of some of our ponds such as Gibbs, Sesachaca, Maxcy, Tom Nevers, and Washing. This work is in addition to the long term research done by the Marine Department on many of our ponds and the harbors which can be accessed by going to http://www.nantucket-ma.gov/Pages/NantucketMA_Marine/bioreports.
We have been conducting larger scale and more frequent sampling of groundwater and pond water in the Shimmo area (Pest House Pond) and for Hummock and Miacomet Ponds in addition to sites of concern relative to marshes and the harbors such as in Madaket and Monomoy. This week, we’ll discuss two of the smaller ponds that have been analyzed by students. Tom Nevers Pond contains a significant amount of tannic material and is dark orange at times. Phosphate levels are quite elevated in the pond (and have been consistently elevated for the past 3 years). This phosphate isn’t “packaged” and sequestered into large algal mats like other ponds because the low water clarity prevents plants from taking up this excess food. This pond is extremely shallow—on average between 45-55 inches deep. For the past four years, the levels of phosphate have been approximately 100 times natural background levels (2.5 mg/L versus 0.025 mg/L) while nitrate values have been exceptionally low as best as we can determine although they are extremely difficult to measure due to the presence of tannins. The nitrate test depends on the creation of an amber color to evaluate nitrate levels, so a natural amber color will mask that color development. This year, samples will be sent off to see if a total nitrogen test can circumvent that problem. The high phosphate levels indicate that fertilizers are the likely culprit for the extremely high phosphate numbers (septic leaks contribute both phosphate and nitrogen) and further determination of those sources and their input pathways is underway this year.
Students from Ms. Davidson’s 8th grade class at Cyrus Pierce Middle School completed a groundwater transport project in which they used a normal amount of name brand fertilizer on a 15 x 5 section of lawn, watered that section and then measured the amount of phosphate and nitrate that reached a well 10 feet away within 24 hours. They found levels of phosphate were highly elevated in that short of a time and just a bit elevated two weeks later. Clearly, lots of the fertilizers that we are putting on our lawns are instead feeding the pond and harbor algae. Look for these super students to help me in a campaign to inform people about fertilizer “dos and don’ts.”
Washing Pond was sampled by our students in 2006 and was found to have excellent water clarity (photic depth of 12 feet), healthy amounts of dissolved oxygen, and no obvious factors indicating compromised pond health. Despite the proximity of houses to the pond, the nitrogen content of Washing Pond (measured in nitrate form) was on average 0.1 mg/L and the phosphorous (orthophosphate) content averaged 0.08 mg/L, suggesting no outside input into the pond system. For comparison, in their 2005 report, the Wannacomet Water Company reported 0.54 mg/L of nitrate as the highest amount measured in their wells (and negligible amounts of copper, lead, and coliforms).
All over the island, groundwater that has been contaminated from septics or fertilizers can be 100-1000 time background levels. But in many areas, groundwater levels are refreshingly low, and can be in the range of 0.01-0.1 mg/L for nitrate and phosphate in areas near conservation land or in soil that is effectively filtered the groundwater. Many homeowners are being more diligent about pumping and inspecting their septic tanks (thanks to the efforts of the Health Department) and reducing unnecessary fertilizer applications so we are seeing reductions in nutrient load in a few areas on island. In later articles we’ll discuss some of the other factors that effect harbor water quality and talk more about how we can protector groundwater and ponds. Keep thinking lean when it comes to overfeeding your local neighborhood water body.
