The scheduled Zoom meeting with consultant Ed Eichner was held on February 16 at 6:00 PM via Zoom. Ed made an excellent, comprehensive presentation and he and David Gould answered questions at the end. You can view a recording of the program on the HPWA Website Events page here:
The report has taught us a lot about how our watershed works and what we can do near-, interim-, and long-term to make significant improvements in watershed water quality. If we can limit the entry of phosphorus into our ponds, we will be able to control algae blooms and toxic cyanobacteria blooms. As Ed pointed out, the best long-term solution is to provide sewer systems for some 400 of the homes on the north and west sides of Little Herring Pond and Great Herring Pond. There are several problems with doing this:
- It is expensive and disruptive for residents.
- The Plymouth sewer treatment plant is not close to our watershed.
- It will take a long time to plan, permit, and execute this option.
Of these problems, the worst is the cost of sewering for residents. A major education program will have to be undertaken in order to objectively present the facts to residents and to the Town of Plymouth and explain the consequences (water quality, property values) of inaction on a sewering program.
All is not lost, however, since:
- Redbrook has a sewer treatment plant that is much closer; we are already investigating that possibility.
- Ed listed short- and interim-term actions that may be effective to remove significant amounts of phosphorus.
Ed pointed out that Plymouth is now experiencing the effects of a burst of growth in the 1980s and 1990s that has contributed to the appearance of increased amounts of phosphorus in our ground water. Iron in our soil initially slowed the migration of phosphorus from septic systems to the ponds but with time (30 to 40 years), the iron was all bonded to phosphorus and any additional phosphorus from the septic systems could flow more quickly into the groundwater and eventually, into the ponds. The pollution situation, not only in the Herring Ponds Watershed but also throughout Plymouth, is likely to get worse with time.
Near-term, it is important for us to:
- Monitor the phosphorus content of Great Herring Pond.
- Research some of the interim-term solutions for permitting, cost, feasibility, effectiveness.
Kim is already looking into the purchase of flow meters needed for the former task. We are implementing some of the short-term management options this year with stream flow (with Sara Grady’s help) starting in April with the town funding testing of water quality samples at the stream sites. The Town is also paying for the additional recommended sampling water quality profiles – so instead of just doing 0.5M, 3M, 9M and 1M from bottom we are doing 0.5M, 3M, 8M, 9M, 10M and 1M from bottom. Plymouth will also be funding the additional cyanobacteria sampling to be collected April-Oct during the sampling noted above. An Article to implement the short-term monitoring goal of installation of two continuous monitoring meters as well as having SMAST conduct the more robust stream flow measurements and staff gauge starting in 2024 will be introduced by Plymouth at Fall Town Meeting.
The good news is that Little Herring Pond is responsible for 47% (the single largest contributor) of the phosphorus that eventually ends up in Great Herring Pond. The bad news is that 87% of this phosphorus comes from septic. Why then, doesn’t LHP have cyanobacteria blooms? There are several reasons:
- LHP spring water is colder than GHP water; cyanobacteria prefer warmer water.
- LHP has a short (13 days) residence time vs. GHP (7.2 months) and can flush out its phosphorus more quickly.
We are fortunate that many interim-term solutions need only to deal with the much smaller LHP than the larger and deeper GHP.
Some of the interim-term solutions are:
- Floating gardens that also remove phosphorus (LHP).
- Treatment of all pond water with alum (LHP and GHP).
- Treatment of deep water with alum (GHP).
- Runoff remediation (LHP and GHP).
- Programs to encourage septic system monitoring, pumping (LHP and GJP).
- Permeable Reactive Barriers that remove phosphorus (LHP).
There are some major disadvantages to some of these options:
- Floating gardens only remove inorganic phosphorus; most of pond phosphorus is not inorganic.
- Alum treatment of pond water must be done every year; it would be expensive.
- Deep water treatment lasts a long time but only provides removal of a small amount of the phosphorus that needs to be removed.
- Runoff is also not considered a major source of the phosphorus that needs to be removed.
- Plymouth would need to provide some education and rebate dollars.
That leaves the Permeable Reactive Barriers. These have not been tested on a large scale and it may be difficult to secure permits. However, we do know that if the water runs through them slowly enough, the active ingredient can remove a lot of phosphorus. The discussion has thus far centered around installing a barrier on the Carters River between LHP and GHP. Some excavation would be necessary on Carters River in order to slow the river water speed down at the barrier site. As yet, we don’t know what installation and maintenance costs would be.
Our Water Quality Plan Review Committee (all members have agreed to continue as a project steering committee) will soon start sorting through the permitting, cost, feasibility, effectiveness details.