Great Herring Pond is Safe Again!

The good news has come in bunches this week for Great Herring Pond!

On Monday, the Town of Plymouth removed the advisory warning about the presence of cyanobacteria.  This came as a result of:  1) a visual inspection of Great Herring Pond by the Town of Plymouth, 2) a history of fluorometer testing by Geri that showed static low and moderate levels of cyanobacteria between July 3 and July 22, and 3) a cyanobacteria-specific test on a sample taken on July 15 that showed both a low level of cyanobacteria and no microcystin toxin present.  The appearance of the cyanobacteria never managed to morph into a full-blown bloom, indicating that there was never enough phosphorus to fuel one even though hot hot, sunny, calm weather provided ideal conditions for a bloom to occur, given enough phosphorus.  It was encouraging to hear that residents were staying out of the water and keeping small children and dogs away from the pond.  We have learned a lot about cyanobacteria since the monster bloom of 2020.

It is hoped that the installation of the Permeable Reactive Barrier (PRB) beneath the Carters Bridge Road bridge will remove enough phosphorus to keep the level low enough in Great Herring Pond to prevent future blooms.  At the end of the two-month installment period (dictated by the herring), we will know if this is true.  Meanwhile, we are actively working on a “Plan B” involving capturing the phosphorus entering Little Herring Pond from the old cranberry bog north of the pond.  We need to find out if there is enough phosphorus present in this location to justify installation of an in-ground PRB that can be left in place year-round and should work well given the slow speed of groundwater (long contact time with the adsorbent polonite).

Finally, we have just received the results from sampling 19 sites around Great Herring Pond (including common beaches) for harmful E. coli bacteria.  I am pleased to report that none of the sites have levels that approach the 80 colonies per liter deemed to be the maximum allowed for swimming.

Apparently, we are doing many of the right things for the safety of our residents. 

Enjoy the rest of your summer in Great Herring Pond.  We will be continuing to monitor both cyanobacteria and E. coli for your safety.

Don Williams, President
Chair, Water Quality Committee
Herring Ponds Watershed Association