Mill Brook watershed committee wants answers about phosphorus

The West Tisbury group will query their environmental consulting firm about results and methodology.

Matt Pelikan

The Mill Brook Watershed Management Planning Committee (MBWMPC) in West Tisbury continues to work on a report that will describe the challenges faced by bodies of water in the 3,400-acre drainage. The brook heads up in Chilmark and flows east and south through West Tisbury, emptying into Tisbury Great Pond.

The primary focus of discussion on Monday was the quality of testing for phosphorus (P) levels carried out by environmental consultants ESS, a Waltham-based company. The watershed study was begun over a year ago to assess the environmental health of the drainage and produce a plan to manage it. The study is ongoing.

In addition to the members of the committee, West Tisbury resident Bill Wilcox attended the meeting. Mr. Wilcox, who was described in correspondence with ESS as a “consultant to the committee,” was present to analyze answers provided by ESS to three questions the committee had asked ESS staff via email. Mr. Wilcox, who grew up Edgartown, had been a teacher, an extension agent, and a water-resource planner with the Martha’s Vineyard Commission (MVC) before his retirement in 2011.

Mr. Wilcox, who had come up with the questions for the committee to ask ESS, questioned the methodology used for collecting samples, and generally found the consulting firm’s answers to be inadequate.

For instance, Mr. Wilcox questioned what seemed to be an anomaly in the reporting of phosphorus test results. In the test results returned to the MBWMPC, “total phosphorus” values at Site 1 for March 1, 2015, and Feb. 2, 2016, were lower than values for “dissolved phosphorus.” Mr. Wilcox had asked, “Wouldn’t dissolved P be a subset of total P?”

The ESS staff person responded, “Without looking at the data, I can tell you that the results should be that dissolved P is a subset of total P. However, when we bring this issue up with the lab, they insist that this is the best that they can do and that the variability in the instruments does not allow them to always guarantee that the dissolved phosphorus levels will be less than the total. Not all labs work this way, but this one did.” MBWMPC members confirmed that a state-certified lab had been used.

Mr. Wilcox’s second question was, “Is the dissolved P inorganic/soluble reactive P?” ESS replied that the dissolved P was a mixture of inorganic and organic phosphorus. They then added two paragraphs that explained the various origins and forms of inorganic and organic phosphorus. Wilcox said he did not feel that they had answered his question.

Mr. Wilcox’s final question went to sample-collection methodology. “Were the dissolved P samples filtered in the field — within 15 minutes of collection?” Here ESS gave a clear answer, but one that disappointed Wilcox and the committee members.

“ESS and MVC did not field-filter the dissolved P samples, but we did strive to meet 24-hr holding times to allow for the lab to filter and analyze samples immediately upon receipt.”

Mr. Wilcox explained that this was unsatisfactory because microbes in the sampled water would harvest the dissolved P and alter the results. Departures from standard procedure lead to results that can’t be compared with samples collected using the standard methodology.

Committee member Selena Roman made a motion to allow Mr. Wilcox to speak directly with ESS staff in order to get more satisfactory answers. This was seconded and passed unanimously.

Committee member Kristen Fauteux then presented data from farmer Simon Athearn, whose family owns Morning Glory Farm, and who has been withdrawing water from Mill Brook to irrigate some of his West Tisbury fields. The committee needs to know the total amount of water that farmers are removing from the brook in order to regulate removals in a sustainable way. This is important to the management plan, as streams and rivers must maintain a minimum flow to protect the integrity of their ecosystems. Ms. Fauteux read the numbers to the committee, and Mr. Wilcox and other members of the committee made calculations in an attempt to determine the total amount of water that was being withdrawn from the brook during each watering event. They also tried to arrive at a total figure for the growing season. Because of the complexity of the watering schedule for four different fields, the committee members decided that more data — the frequency of watering for each field — were needed. Ms. Fauteux will contact Mr. Athearn again.

The Mill Brook watershed study was begun in March 2015. It has included data collection from specific sites to measure rainfall, and to analyze nutrients and chemicals in the water and water quality. It has monitored water sources, diversions, and withdrawals, water flow and temperatures, and will record seasonal weather impacts and the impact of existing and potential sources of threats to the health of the watershed. The sum of $30,000 was approved for the study at the April 2014 West Tisbury town meeting, but the sole bid came in from ESS at $45,000. The approach was then scaled back in order to go forward with the approved amount. An additional $6,600 was requested in April 2015, reduced from $15,000 because of a donation from the MVC, and volunteer help.

At least as far back as March 2014 there had been concern about the health of the brook. A study that recorded water temperature at nine locations along the course of the stream documented peaks that exceeded the tolerance of brook trout. There are 10 dams along the length of Mill Brook, and the water impounded behind them warmed, for example, to greater than 87°F below Priesters Pond during three days in July 2013. Data have continued to be collected, and newer numbers confirm earlier ones.

At their most recent meeting, the committee members were continuing to work on the watershed study. Co-chairman Chuck Hodgkinson urged everyone to remember to document methodologies used during data collection so that they could be replicated in future watershed studies. Dam caretaker and civil engineer Kent Healy urged the collection of rainfall data from multiple sites within the watershed in order to get a more accurate picture of the total. Committee member Prudy Burt told the rest of the committee that she was arranging to have an invertebrate zoology expert examine photographs of collected specimens in order to identify them more exactly.

The next committee meeting is scheduled for Dec. 12 at 5 pm in the West Tisbury library.