What would a good outcome to the Nagog Pond controversy look like?

There has been a lot of negativity around Concord’s proposal to expand its water treatment plant at Nagog Pond and Acton’s reaction to the same. In this post, let’s take a step back and try to envision what a good outcome might look like. A good outcome would safeguard the ecosystems of Nagog Pond and Nagog Brook, and would be a win-win-win for the three towns that share legal rights to the waters of Nagog Pond.

A good — and possible — outcome would:

  1. scale the water treatment capacity to the size of the pond
  2. develop a protocol for timing water withdrawals and releases so as to minimize harm to the downstream ecosystem and aquifer
  3. collaborate on data collection and hydrologic modeling to provide decision-makers with answers to “what if” questions
  4. construct and administer the water treatment plant as a regional facility with costs and water shared among the three towns

Kim Kastens floated some of these ideas in her talks at the First Parish of Concord on February 25 and at the Acton Senior Center on April 5. (Thanks to the attendees for their enthusiastic reception and insightful suggestions.) This post is not offered as a complete and final answer to the question posed in the title, but rather, as an invitation to consider a wider range of possibilities.

1. scale the treatment capacity to the size of the pond

There is a reason that this body of water is called Nagog Pond rather than Nagog Lake. It’s a small body of water nestled into a very small watershed. The entire watershed for Nagog Pond is only 1.25 square miles (Concord Water Department, 2016).

The U.S. Geological Survey (USGS) and the Massachusetts Department of Environmental Protection (DEP) have modeled the rate at which water can safely be withdrawn from each reservoir in the Commonwealth (Levin, et al, 2011). For Nagog Pond, the model says that water can be withdrawn at 0.86 million gallons per day (MGD); this is known as the “firm yield.” If water is consistently withdrawn at a higher rate, the reservoir will fail in a severe drought. “Reservoir failure” means that the water level in the reservoir is drawn down to the level of the intake pipe.

The capacity of Concord’s proposed Nagog treatment plant is 1.5 MGD. This is far more than the firm yield, and far more than the historic rate of water withdrawal (see graph below).

Water providers need some flexibility, and so it would be reasonable to have some days with withdrawals a bit above the firm yield balanced out by some days below. One million gallons per day might be a plant capacity that would provide some flexibility and would be more appropriately scaled to the size of the resource.

2. manage withdrawals so as to minimize harm to the downstream ecosystems and aquifer

Management tools could include: concentrating Nagog Pond water withdrawals in seasons when the downstream ecosystem is less vulnerable, allowing releases through the lower outlet of the dam when Nagog Brook streamflow is low, and setting a threshold on how far down the pond level can be drawn.

The Commonwealth of Massachusetts recognizes Cold Water Fishery Resources as important indicators of ecosystem health. There are few Cold Water Fishery Resources in the entire SuAsCo watershed (labelled as Concord watershed on the map below), and only one in Acton: Nagog Brook.

The fishery is most vulnerable in the summer, when flow rates in the brook are low and water temperature is high. Concord’s stated reason for needing additional water from Nagog is to allow them to rest their wells, rather than to increase their total town-wide water usage. This is a good  practice and an understandable goal. But it should be possible to develop a reservoir management protocol that concentrates well-resting and water withdrawals from Nagog Pond in the non-summer months.

In addition to the spillway, the dam at Nagog Pond has a 24-inch-square lower outlet at the base of the dam that’s operated by a slide gate (Environmental Partners, 2016, p. 21). During the hottest months, it should be possible to release a continuous trickle of water through this lower outlet. Water from the bottom of a pond is colder than surface water, so a controlled release from the lower outlet will help with both stream flow and temperature.

A final protective strategy could be to limit the level to which the surface of the pond could be drawn down. Maintaining the pond depth protects the ecosystems of both the pond itself and the downstream brook. As water level drops, sunlight penetrates to greater depth in the pond and the water becomes more stagnant; both changes increase the likelihood of pond eutrophication. Less obviously, the water level in the pond also impacts the amount flowing in Nagog Brook.  Nagog  is a “gaining stream,”  a stream that gains water through seepage from groundwater–which is why the water stays cool in summer and warm in winter. The weight of the water in the pond (the “hydraulic head”) helps to drive  this ground water flow (OARS, 2017).

Expert guidance would be needed to decide on a threshold depth. One suggestion would be 3 feet below the spillway. The water level has been pulled down to this level approximately ten times over Nagog’s history as a reservoir.  A more drastic suggestion would be 7 feet below the spillway, which  is the greatest depth to which the water level has been drawn down in the 100+ years of water withdrawals from Nagog Pond.

Concord’s current proposal would extend the intake pipe deeper into the pond, all the way to the Acton–Littleton town line. The proposed pipe is designed with an intake to be used “in the event of a low water level,” which is 25 feet below the level of the spillway (Concord, 2016). Drawing down the pond to this level would leave just a small stagnant pool, 15 feet deep, in the center  of the Nagog basin.

A management protocol that protects the fishery and the pond should also help to recharge the aquifer around Acton’s Conant wells.  As shown on the map below, Nagog Pond flows into Nagog Brook, which flows into Nashoba Brook; then the combined flow traverses the recharge area for Acton’s Conant 1 and 2 wells.

3.  data collection and modeling

Throughout the Nagog Pond water treatment hearings before the Acton Board of Selectmen, there were a number of questions that arose about our regional hydrologic systems to which there were no definitive answers. For example: would increasing water withdrawals from Nagog Pond lead to decreased water availability in Acton’s Conant wells, which lie downstream from Nagog Pond along the Nagog/Nashoba Brook system? This discussion degenerated into “You can’t prove that it would hurt the wells” versus “You can’t prove that it would be safe for the wells,” and finally, a statement that it’s not possible to be sure  one way or the other with the evidence in hand. Our Board of Selectmen were forced to make a decision with incomplete information.

But this is the kind of question that science knows how to handle. There is a regional hydrologic model for the Assabet River Basin that was developed by the USGS to support decision making about water management alternatives (DeSimone, et al, 2004). The lead author of this report, Leslie DeSimone, offered assurance that the model is still in use, open source, with technical support available, and well suited to answer “what if” questions such as the one about the Conant wells (DeSimone, 2017). What would be needed would be to localize the model by feeding it more detailed data on Acton’s ground and surface waters. Developing the capacity to do fine-scale hydrologic modeling for our section of the Assabet Basin would be a great project to undertake in collaboration with one of our local universities.

If we were to design an environmental monitoring program to provide decision support for water management and safeguard the aquifer and ecosystems, what data types might it include? A starting list:

  • pond level of Nagog Pond
  • water withdrawals from Nagog Pond
  • downstream releases from Nagog Pond (a) over the spillway, and (b) through the lower outlet at the base of the dam
  • water temperature in Nagog Brook (continuous measure)
  • Nagog Brook stream depth (continuously measured) with calibration  to convert depth to discharge
  • water level and withdrawals from Acton’s Conant wells

All of these data types are routinely measured across the country and well understood. Water temperature and stream depth at Nagog Brook were monitored for a time by the Quail Ridge Golf Course as a condition of their permit to withdraw water for irrigation. Water level and withdrawals from Acton’s wells are measured and used for real-time decision making, but are not logged in a form that allows easy access for analysis of time series data.

There seems to have been little research on the  ecology of Nagog Pond.   Investigating pond ecology isn’t something that water managers would normally undertake, and it takes multiple years of data collection to understand a pond’s ecology and limnology through the seasons and across the years.  This would be a fabulous project for a high school Environmental Science class. Both  Concord-Carlisle and  Acton-Boxborough High Schools offer Advanced Placement Environmental Science, which includes a required field research component. Either class (or both in collaboration!) could measure the pond’s temperature structure and water chemistry, and sample for plankton, algae, invertebrates, fish, and plant species, looking for changes through time and variation across space. As the enlarged water treatment plant comes on line and water level in the pond draws down, the question for the environmental science students will  be if and how the pond ecology is impacted. Carrying out this field research would require the water managers to permit non-motorized boat and/or wading access to the pond for research purposes. To see inspiring photographs of AP Environmental Science students doing aquatic field research, look here. 

4. construct and administer the water treatment plant as a regional facility

According to the Massachusetts Acts of 1884, Concord, Acton, and Littleton all have rights to water from Nagog Pond. With goodwill and careful planning, it should be possible to work out an arrangement in which all three towns share the costs of building and operating the facility, and all three towns use the water.

We manage the myriad large and small decisions required to collaborate across town lines to build and operate regional high schools: how much should each town pay? How many representatives should each town have on the school board? How should the school buses be routed? All these questions and more are hotly debated, but in the end agreement is reached and the children of both towns benefit. Surely we could manage this level of neighborly collaboration for a water treatment plant.

Final thoughts

There may be no easy path forward from where we sit now, in Land Court, to achieve this vision. But if we can’t envision what a good outcome might look like, we surely won’t get one. We, the people, need to speak up and tell our leaders we want a win-win-win solution that works for our entire region — not a winner-take-all outcome that leaves neighbors estranged and the environment damaged.


Concord Water Department (2016). Public Water Supply Annual Statistical Report, available by request from the Massachusetts Department of Environmental Protection, Bureau of Water Resources.

Concord Water Department (2016).  Design Plans, Appendix L, sheet sheet C-6. 

DeSimone, L.A. (2004). “Simulation of ground-water flow and evaluation of water-managment alternatives in the Assabet River Basin, eastern Massachusetts,” U.S. Geological Survey.

DeSimone, L.A. (2017).  Phone and email with Kim Kastens.

Environmental Partners (2016). “Nagog Pond Water Treatment Plant, Acton Massachusetts, Draft Environmental Impact Report.” http://www.concordnet.org/DocumentCenter/View/4851.

Levin, S.B., S.A. Archfield, and A.J. Massey (2011). “Refinement and evaluation of the Massachusetts Firm-Yield Estimator Model version 2.0,” Vol. Scientific Investigations Report 2011-5125), U.S. Geological Survey (prepared in cooperation with the Massachusetts Department of Environmental Protection).

OARS (2017). Testimony of hydrologist Peter Shanahan to Acton Board of Selectmen and Letter to Acton Board of Selectman for Remand Hearing, Nov 2017.

3 responses to “What would a good outcome to the Nagog Pond controversy look like?

  1. I’ve had a few questions on this post. Rather than make the post even longer than it is, I’ll answer them in the comments field.
    Question 1: how would the costs be divided up among the 3 towns?
    A: This would have to be negotiated. But here are some ideas:
    * divide up the costs 1/3 : 1/3 : 1/3 among the three towns.
    * split the costs in proportion to the populations of the three towns, or the population of the water service areas of the 3 water departments/districts.
    * agree in advance on a different ratio, for example 50%:25%:25%, for both the costs and the water.
    Costs would include both the upfront costs for bringing the water treatment plant up to current standards, and ongoing operational costs.

  2. Question 2: Would the water coming out of the lower outlet in summer be cold enough to be safe for the fish? The dam is at the end of a shallow embayment where the water may get pretty warm in summer.
    A: Good point. Groundwater, which seeps into a gaining stream like Nagog Brook, stays at near constant temperature all year, so it could be colder than than the water from the bottom of the dam in summer. Water temperature at various depths in the pond would be an easy thing to measure, but we don’t now have that data. A fisheries expert would have more insight into the trade-offs between more versus less water, and warmer versus cooler water.

    I think the bigger point I was trying to make is that there is an array of management tools that could be used to balance the needs of the ecosystems and the humans, and that some combination of these should be deployed. All these management tools would benefit from more data on the pond/brook/groundwater/climate system.

  3. Another question: Wouldn’t this be a big unanticipated expense for the Acton Water District?

    A: : It would certainly be a cost for the AWD and thus for the rate-payers. Concord is planning on paying $13M to upgrade the water treatment capacity to 1.5 MGD. For a smaller 1 MGD capacity plant there should be some upfront cost savings. So let’s say $12M split 3 ways, would give $4M as Acton’s share if the split were even.

    At the March 5, 2018 Acton Board of Selectmen meeting, the Water Resources Advisory Committee (WRAC) presented some preliminary estimates of what the future water demand in Acton would be with various increments of planned and possible future development. WRAC is in the early stages of the town Water Study, but if those early estimates are correct, then Acton will either have to find additional water sources or curtail development substantially.

    Any new water sources are likely to be expensive. The possibility of connecting the the Massachusetts Water Resource Authority (MWRA, Quabbin Reservoir) was considered a few years back and rejected as too expensive. At the March 26, 2018, AWD Commissioners meeting, Chairman Len Phillips asked what would be the next water sources tapped in the case that more water was needed. Environmental manager Matt Mostoller listed several potential sources, of which the Assabet 3 well was the first and most productive. But Assabet 3 has high levels of 1,4-dioxane, a probable human carcinogen. Adding treatment for 1,4-dioxane to the South Acton Treatment Plant has not been costed out. But as a possibly-relevant point of comparison, recent news reports out of Michigan and Minnesota say that towns there are putting in systems to treat their water for 1,4-dioxane with $10M to $30M price tags and $1M -$2M annual operating costs. See: http://www.mlive.com/news/ann-arbor/index.ssf/2016/10/what_would_it_cost_to_equip_an.html and

    The Nagog water is of high quality, and the needed treatment is a mature technology. The cost and benefits of this option should be placed alongside the pros and cons of other options, rather than being rejected out of hand as being too expensive.

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