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Herbicide treatment on Lake Thoreau prompts restriction on irrigation

A major Hydrilla bloom prompted water restrictions in 2020

Reston Association is urging residents to avoid using water from Lake Thoreau for irrigation until mid-October.

The guidance comes after Aquatic Environment Consultants treated Hydrilla, an invasive aquatic species, on the water. Conducted monthly and typically around the middle of the month, treatments will continue throughout the summer to maintain low levels of herbicide in the water.

The company was contracted to treat Hydrilla when it became a “nuisance” two years ago, according to Ben Rhoades, RA’s watershed manager.

“There are no use restriction associated with the herbicide, Fluridone, other than irrigation,” Rhoades wrote in a statement to FFXnow.

In 2020, RA encouraged residents to avoid contact with the lake after a major algae bloom took over the surface of the lake. The incident piqued conversations about RA’s maintenance of its lakes and the most appropriate timing of lake treatment to avoid similar issues in the future.

The bloom was later ruled non-toxic.