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Mirror Lake water quality good, watershed study says

August 25, 2011
WOLFEBORO — On Tuesday evening, Aug. 16, Mirror Lake Protective Association (MLPA) President Dusty Davies introduced the results of the watershed management study to an audience at the Kingswood Arts Center. Davies had set the study in motion in 2010.

While the village of Mirror Lake is in Tuftonboro, 51 percent of its watershed area is in Tuftonboro and 49 percent is in Wolfeboro.

The report, presented by Bob Hartzel of Geosytech Consultants, included the results of a year-long study of Mirror Lake's water quality, especially in regard to its phosphorus (P) load, for high P levels contribute to algal growth, which reduces water clarity and can produce conditions that encourage the growth of cyanobacteria, which emit toxins.

A blue/green algal bloom, spotted in Mirror Lake in 2008, resulted in its designation on the list of New Hampshire Threatened or Impaired Waters in respect to contact recreation. It therefore qualified for financial support for the watershed management project. Davies sought and received a grant on behalf of the MLPA from the New Hampshire Department of Environmental Services (NHDES) with funding from the U.S. Environmental Protective Agency under Section 319 of the Clean Water Act.

In the meantime, the World Health Organization's standards on the toxicity of cyanobacteria have changed, establishing a lower number than estimated previously. Hartzel elaborated, "Only certain species [of cyanobacteria] produce toxins and they don't produce them all the time."

The goals of the study were to analyze the lake's water quality, identify and quantify specific sources of phosphorus and provide a plan for maintaining water quality now and into the future, which is certain to bring continued development.

Mirror Lake's water quality is good, said Hartzel. According to the NHDES trophic classification system, its average total phosphorus (TP) concentration of 10.4 μg/L (micrograms per liter, which means one millionth of a gram per liter) "places Mirror Lake on the cusp of the 'Ideal' and 'Average' categories for this water quality parameter."

Levels above ten raise a lake's eutrophication (aging) rate and the toxicity of phytoplankton increase markedly, says Dr. James Haney of the University of New Hampshire. That is what the MLPA wants to guard against.

Hartzel provided a pie chart on the projector screen that denoted wedges, or percentages, of the sources contributing to the lake's P content. The highest percentage (25) comes from the air. Others sources identified along with their percentages are residential lawns and storm water runoff for instance (25); internal, i.e., already in the lake, (15); forest (17.5); roads (7.5); and septic systems (7). The open/pasture/recreation categories together total 3.5 percent. The Wolfeboro spray fields and wetlands contribute 1.1 percent combined, at .6 and .5 respectively.

Wolfeboro has a permit from NHDES to spray effluent (clear treated water) on its spray fields as needed to reduce the volume of water going through its Rapid Infiltration Basin system, but the town removed the pipes that irrigated the 22 acres within the Mirror Lake watershed (1.5 percent of the watershed area) in 2010.

The town has a forestry plan for the acreage, which is reverting to its natural state. "Don't rest on your laurels because the spray fields are no longer in operation," said Hartzel, who pointed out that, looking ahead, the firm estimated population increases by 2030 translating to approximately 19 and 25 additional homes for Tuftonboro and Wolfeboro, respectively. That means a projected increase in P as the MLPA attempts to reduce the load.

Homeowners on every parcel will need to find opportunities to reduce storm water runoff, he added.

Davies declared herself "delighted at the outcome" of the report and shared her plan to write a grant for several of the remediation steps outlined in the report. Phosphorus reductions could be achieved with ditch stabilization along Lang Pond Road; residences on Mirror Lake Drive and the area comprised of Church Lane, Steeple Lane Oak Hill Road and Chipmunk Lane with the highest density of development in the watershed, are natural targets for storm water retrofits and rain garden demonstration sites; and in Wolfeboro, resurfacing the Abenaki Ski Area parking lot with porous pavement strips and installing bioretention cells was suggested.

Further down the line, maybe within 30 years, community septic systems are a likely consideration as density increases.

Davies said that the report, which includes all the data and recommendations along with a field guide to the aquatic plants of Mirror Lake, will be available to all on the MLPA Web site.

Wolfeboro and Tuftonboro selectmen were among those who attended the meeting. Public Works Director Dave Ford said that he has read the draft report and looks forward to an opportunity for further discussion to discuss the Best Management Practices recommended in Wolfeboro environs.

Martin Lord Osman
Salmon Press
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