A 5.5-acre solar project would neighbor Cambridge’s drinking water supply in Lexington and put an access road into Waltham. (Image: randymajors.org)

Cambridge is suing over a 5.5-acre solar panel project in Lexington that would cut down more than 1,000 mature trees protecting the Hobbs Brook Reservoir, the main source of Cantabrigians’ drinking water.

The appeal of a Lexington Planning Board decision was filed June 8 in Superior Court, hoping to overturn a May 17 approval for a project by a Marlboro limited liability corporation called Tracer Lane II Realty.

Tracer Lane’s application in November for a solar panel project calls for a roughly 1-megawatt ground-mounted system of solar panels in woods abutting the reservoir in Lexington, as well as a 20-foot-wide access lane for maintenance and emergencies that would go through a residential neighborhood in Waltham.

There were five public hearings for the project, ending with a 4-1 vote in approval by the Lexington Planning Board that has led to four appeals, according to the board. The City of Waltham has filed one, and a neighborhood that abuts the project has hired an attorney to file one, according to a resident familiar with the project. Tracer Lane has an action against Waltham.

The solar project would bring the state closer to a goal of switching to renewable sources of energy by 2050.

But the legal filing by Cambridge, where legislators have also been moving aggressively toward renewable electrification and away from carbon-emitting power sources, says the city is “aggrieved by the decision” and called for its annulment as “factually unsupported, unreasonable, arbitrary and capricious.”

The deforestation could lead to pollution of the reservoir, which is in a protection watershed zone, Cambridge says in its filing. The trees that Tracer Lane II Realty expects to remove slow the speed of stormwater and protect against erosion; support surface and groundwater quality; provide shade that shields the water from thermal impacts; promote the water body’s resiliency; and serve as flood storage, the city says.

Cutting of these trees will cause “permanent loss of the water quality,” affecting more than 100,000 people who use the reservoir as their main drinking supply, the Cambridge filing says. Beyond the negative effect on the water supply, the project increases the threat of invasive species.

A spokesperson for the Lexington Planning Board said it had no comment.