Testing Overview And Time Periods
Overview of Water Quality Testing
The Six Ponds Improvement Association, which was formed in the 1950’s, has a long history of being concerned with the health of the water in the ponds that provide a beautiful environment for those who live in the neighborhood and critical habitat for a number of protected species.
In the 1970s, the Town of Plymouth commissioned baseline surveys of water conditions and flora and fauna in 41 Plymouth ponds. All of the ponds in the Six Ponds area were included in these detailed studies. Two of our ponds were included in studies conducted in the early 1990s. It was found that the health of our six ponds ranged from excellent to poor with the worst two being Halfway Pond and Little Long Pond (highly eutrophic since they were degraded by an abundance of nutrients).
Over the intervening years, Six Ponds association members have regularly volunteered one or more times a year to collect water samples from all six ponds. Sometimes this water quality testing has been funded wholly or partly by the Town, sometimes partly by grants, and sometimes entirely by the association itself. The water sampling has varied from just one top and one bottom sample from each pond just once time a year to top and bottom samples from several locations in each pond several times a year. Water sample analyses have ranged from just a few basic chemical parameters some years to a broad array of nearly 25 parameters, including salts and other contaminants, which were analyzed by a private environmental laboratory for about 10 years. Likewise, the equipment used over the years has ranged from very basic to highly sophisticated electronic recording devices.
The condition of the water in some ponds has been found to vary considerably from year to year, and the ponds generally differ from one another depending upon weather conditions. For example, there have been years when phosphorus levels have exceeded eutrophic levels in Little Long Pond and in Halfway Pond. In these years, algae blooms and pond weed growths have been likely to occur, and sometimes these have occurred simultaneously and both have been extensive. At the same time, the other ponds have not exhibited similar problems in these same years. On the other hand, there are times when other ponds have been negatively impacted by chemicals in road and lawn runoff and by septic systems. In any case, the association has found it important and necessary to keep watch on the condition of these ponds and to report problems to the Town.
Since 2014, most of the lab analysis has been conducted for the Town of Plymouth at the UMass Dartmouth School for Marine Science and Technology (SMAST) lab in New Bedford. The Town has used this lab ever since the Town's Department of Marine and Environmental Affairs began the Ponds and Lakes Stewardship program (PALS) in 2014 which has lead to the publication of an atlas tracking the health of 39 Plymouth ponds which includes our six ponds. Prior to 2014 pond sample analysis was partly funded by the Six Ponds membership and the samples were delivered to Envirotech Laboratories, Inc. in Sandwich for chemical analysis. We continue to use Envirotech Labs on an as-needed basis.
With the advent of the PALS-related testing the Six Ponds volunteers started to collect data related to pond health more frequently; however, water samples are collected only once a year and the analyses include fewer chemicals. The data are typically collected 2-4 times each year and include the Secchi depth and dissolved oxygen (DO) and temperature profiles with depth in the ponds.
The DO-temperature profile is collected with an instrument provided by the Town. The general procedure used in collection is to anchor the boat at the deepest point in the pond as determined by a depth sounding and bathymetric maps. The depth finder used also records the pond surface temperature. Once anchored at or near the deepest spot in the pond, the DO-temperature sensor is lowered to 0.5m depth on the cable, then 1.0m, 2.0m, 3.0m, etc. until the bottom is near or the cable length is reached (30m or about 100ft). The deepest point in Long Pond is 110ft, making it the deepest naturally occurring pond or lake in the Commonwealth, about 6ft deeper than Walden Pond.
The Secchi depth uses a weighted disk suspended into the pond on a marked measuring tape to get a visual measurement of the water clarity. The disk has black and white quadrants and it is lowered into the pond until it disappears from view and the depth of disappearance is recorded. The disk is then raised back up and the depth of reappearance is recorded. The average of the two depths is the Secchi depth. Usually the Secchi depth decreases during the spring and summer seasons due the increase of phytoplankton (algae/cyanobacteria and other plant matter) growing in the ponds. As these organisms grow and multiply the clarity of the water is decreased.
At the same time, these organisms consume pond nutrients and release oxygen from photosynthesis into the ponds. If there is abundant sunlight, warm temperatures, and plenty of nutrients these microscopic organisms can undergo explosive “blooms.” Some of the organisms can be detrimental to human health and the 2020 bloom of Cyanobacteria (aka blue-green algae) at Halfway Pond resulted in its closing by the State DPH and Town Environmental Affairs. In addition to eliminating swimming in the Pond, no pets should be allowed to drink from the Pond, or humans take fish from the Pond for consumption.
Please recognize that we can all contribute to improving the health of the ponds by reducing nutrient inputs through minimizing or eliminating lawn and garden fertilizers and properly maintaining and pumping our septic systems. Every bit of effort helps in keeping our ponds healthy and naturally beautiful.
Written by Leighton Price and Hampton Watkins, September 2020
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