By Don Williams
Co-Chair, Water Quality Committee

Great Herring Pond (GHP) has witnessed cyanobacteria blooms for 3 of the last 5 summers. After the first and worst cyanobacteria bloom in 2020, the Herring Ponds Watershed Association (HPWA), partnering with the Town of Plymouth, commissioned a Water Quality Study by Ed Eicher and SMAST to determine the cause of this toxic bloom and to chart an action plan to prevent future blooms.

The Study, received in early 2022, reported:

• Phosphorus was responsible for the bloom
• Little Herring Pond (LHP) provided 47% of Great Herring Pond’s phosphorus
• Septic systems from the Ponds of Plymouth development were most likely the source of much of the phosphorus
• The best long-term solution to the problem is to sewer the Ponds of Plymouth, and, if possible, the area in the northwest corner of Great Herring Pond.

Since sewers are expensive and the nearest sewer treatment plant is miles away, it was unlikely that they would be built soon. The Water Quality Committee, along with its Plymouth and SMAST partners, met and considered several medium-term solutions and eventually settled on installing a Permeable Reactive Barrier (PRB) in the Carters River to capture the phosphorus from LHP before it reached GHP.

The PRB was in place for 11 weeks from June through September 2024 but, unfortunately, did not remove enough phosphorus to be used again without some modification. Undeterred, The Water Quality Committee (WQC), consisting of Brian Harrington, Tom O’Brien, Geri Williams, Lee Pulis, Jim Smith, Joe Solimini, Bob McDonald, Jerry Levine and Paul Denoncourt and its Plymouth and SMAST partners are currently investigating other approaches. We are also reaching out to local experts to find new approaches.

We remain optimistic that a solution will be found and that the water quality of GHP will be improved by a “Plan B” that will capture more of the phosphorus pollutant. We are considering several approaches involving:

• Using native mussels to collect phosphorus.
• Mapping the groundwater plumes into LHP so that in-ground PRBs can be used where the plumes enter LHP.
• Finding a better adsorbent of phosphorus.

The Water Quality Committee volunteers will again collect water samples, temperature and dissolved oxygen readings each month April – October in GHP for analysis by SMAST, and additional samples in Little Herring Pond which add more data for our understanding of pond dynamics. Water samples will be collected several times this summer from 19 near shore locations to be analyzed for e.coli to ensure that swimming areas are safe. We’ll monitor GHP for cyanobacteria and notify you if there is a health problem, which we hope we will avoid this summer.

Stay tuned!