Watershed-Based Plan


The Foster's Pond watershed occupies about 1.5 square miles. About half the watershed (northeast of the Pond) drains into Frye's Brook, which flows into the Pond near Azalea Drive. Click on the map above to open a larger image.

After treating Foster’s Pond repeatedly for toxic blue-green algae, the Corporation decided to get an answer to a persistent question: Can’t something be done to prevent algae blooms?

In 2017, the Corporation commissioned a study to find an answer. We learned that there are things we can do to lessen the problem, but there are no silver bullets. In the coming months, we’ll be rolling out more information and asking for volunteers to coordinate specific initiatives that will contribute to preserving and protecting the Pond.

The key to controlling algae is phosphorous. Phosphorous is an essential nutrient for most plants, including algae. It is also a “limiting nutrient,” about the only one that is practical to reduce to the point where algae growth can be affected. If its availability in a water body can be lowered, algae blooms can be controlled.

We’ve known for years that Foster’s Pond develops, over the course of a summer, too much phosphorous in the water - enough to stimulate algae growth. But where does it come from? Until recently, there was no economical way to find out.


This map depicts land uses in the Foster's Pond watershed. The two largest contributions to phosphorous loading come from forested areas and low-density residential neighborhoods. Click on the map above to open a larger image.

In the past, detailed sampling of every possible source, from septic systems to intermittent streams, would have been required, and the costs would have been astronomical. Even worse, the experts who proposed such studies said they could probably not reach useful conclusions and were likely to tell us, after we spent tens of thousands of dollars collecting data, that they would need to conduct further studies.

In 2017, however, a new tool became available. The State commissioned Geosyntec Consultants, a world-wide environmental consulting firm, to develop a computer model that can use data pertaining to any watershed to estimate phosphorous loading from a variety of sources. We hired Geosyntec to apply the model to Foster’s Pond.

The result is our very own “Watershed-Based Plan” (or WBP) that provides us hard numbers on how much phosphorous is coming into the Pond each year, where it’s coming from, how much we need to target to see a reduction in algae, and what are the most cost-effective targets.

Geosyntec looked at the entire Foster’s Pond watershed, an area of 957 acres or 1.5 square miles, where surface water flows directly into the Pond or flows into streams or intermittent tributaries that find their way to the Pond. A little less than half of the Pond’s watershed consists of a “sub-watershed” draining into the Pond through Frye’s Brook, which flows under County Road and enters the Pond at the Old Mill Reservoir, between Azalea Drive and the Goldsmith Woodlands.

Using both data from the model and information specific to Foster’s Pond, Geosyntec was able to estimate that 255 pounds of phosphorous is entering the Pond each year. If we could reduce that amount by 10%, just 25 pounds per year, our algae problem would be significantly reduced.

That’s a reasonable goal. But it’s easier said than done.


Of the 255 pounds of phosphorous entering the Pond each year, relatively little comes from rain falling directly into the Pond or from nearby septic systems. Most is carried in from stormwater run-off.

To cut phosphorous flowing into the Pond, we need to understand where it’s coming from. Of the 255 pounds, 26 pounds fall from the sky, raining directly into the Pond from storms. A mere 21 pounds seep into the Pond from the 74 homes - 67 in Andover and 7 in Wilmington - that have septic systems within 200 feet of the Pond.

The remaining 208 pounds per year come from stormwater run-off, washing phosphorous from land in the watershed into streams and gullies that lead to the Pond.

The biggest single contributor is not, however, what you might think. Forested areas - like the Goldsmith Woodlands and the Wilkinson Reservation - contribute 88 pounds (42%) of the watershed’s land-based phosphorous loading. The phosphorous comes from natural processes, like the decomposition of leaf litter and fallen trees.

The next biggest contributor, however, is less surprising: low-density residential areas (read: big lawns) throughout the watershed, especially in the Frye’s Brook subwatershed. Low-density residential land use contributes an estimated 72 pounds of phosphorous per year. The Frye’s Brook subwatershed, even though comprising less than half the entire watershed, contributes about 60% of the land-use based phosphorous loading to Foster’s Pond.

So how do we reduce the phosphorous flow? We aren’t about to cut down the forests, and we can’t stop the rain. We can remind near-by residents who rely on septic systems to use only phosphate free cleaning products, but that's a relatively small part of the problem.

To make a real difference, other approaches are needed. The WBP suggests several. Some are for individuals. Others will take volunteers to organize a coordinated effort.

For individuals, here’s a handy checklist:


Here is a list of things you can do to protect the Pond. Remember, if you live in the Foster's Pond watershed, your actions can affect the health of the Pond. Click on the image to open a printable pdf version.

For efforts that will require the coordinated work of volunteers, the WBP offers a menu of possibilities: