The Pennsylvania Public Utility Commission ventured into the legal weeds this week by blocking an electric company from using herbicides to clear trees under a power line, a decision that could force the state's utilities to rethink their vegetation-control practices.

The head of the state's utility-industry association called it a "troubling decision."

On Wednesday, the PUC ordered West Penn Power Co. to forgo using herbicides on a property about 25 miles northeast of Pittsburgh after a homeowner complained the chemicals would contaminate his family's drinking water.

By a 3-2 vote, the commission rejected a hearing examiner's recommended decision that the homeowner failed to prove the utility had violated the law. West Penn's proposed use of herbicides to treat tree stumps to prevent their regrowth was consistent with PUC precedent and the utility's own approved vegetation-management plan, the commission acknowledged.

"Simply finding that the planned method of clearing the right-of-way is consistent with a plan is insufficient to provide a fair result in the present case," Commissioners David W. Sweet and Gladys M. Brown, the commission chair, acknowledged in their joint motion to block the herbicide application. They were joined in the vote by Andrew G. Place, the vice chair.

Commissioners John F. Coleman Jr. and Robert F. Powelson, in a seven-page dissent, said the decision created uncertainty for utilities and put the PUC in a position of micromanaging the companies' day-to-day operations.

"The record for this case is devoid of any evidence that the health and safety of Pennsylvania's citizens, or its environment, has been harmed by the commission's current regulatory scheme and [utilities'] use of herbicides as part of their vegetation management obligations," Coleman and Powelson said in their statement.

Utilities' methods of managing vegetation are an endless source of public friction. Regulators press companies to step up tree-trimming practices to reduce power outages caused by falling limbs, but customers often push back about aggressive and unsightly pruning methods.

The West Penn case brought by Robert M. Mattu, a retired brickmason in Leechburg, had attracted little attention because it was filed as an individual complaint against the utility. In March, Administrative Law Judge Katrina L. Dunderdale recommended dismissing the complaint.

But with the PUC's ruling, it now takes on larger policy implications. On Wednesday, the commission gave consumer and business advocates 30 days to comment on the ruling.

West Penn Power could appeal the case to the courts. "We are still digesting the Public Utility Commission's recent decision and evaluating our options," the utility's spokesman, Todd Meyers, said in an email Thursday.

Mattu, 64, said he feared that West Penn's proposal to treat tree stumps with Garlon and two other herbicides to prevent regrowth would cause the chemicals to leak into the two drinking-water wells of his house, located about 70 feet from the company's right of way across his property.

"I've lost a lot of sleep over this thing, and now I'm happy," Mattu said in a telephone interview Thursday. "I've got a quarter-million-dollar home here on 10 acres of land, and if they take my water, what am I going to do?" He lives with his wife, and his son and two grandsons live next door.

West Penn Power has operated a 138,000-volt transmission line across the land since 1968. Mattu bought the property in 1978. He said the utility had previously cleared vegetation beneath the power line without the use of herbicides.

Mattu said he first took his concerns to local and state elected officials, whom he said were unresponsive, so he filed a complaint with the PUC. Later, he hired a local lawyer to argue his case. "This is just costing me money that I shouldn't have to pay," he said.

West Penn said that its use of the herbicide was consistent with best industry practice, and that without it cut tree stumps quickly re-sprout. The company said it attempted to address Mattu's concerns by sending a forestry specialist and a representative of Dow, the herbicide's manufacturer, to the property. It also offered to test his water before and after applying the herbicide.

The utility association said that it is still studying the decision, but that it injected uncertainty into the ways utilities manage the thousands of miles of power lines they operate. West Penn alone is spending about $40 million this year to trim trees and maintain right-of-ways on about a fifth of its 26,000 miles of power lines.

"We're concerned because we need predictability in these things, and the regulations say you need to follow industry best practices," said Terrance J. Fitzpatrick, president of the Energy Association of Pennsylvania. "I think it's clear that was what was done here."

The commissioners who voted to block the use of herbicides said it was a "fact-specific" decision and not intended to create a "bright line test" by which future cases should be evaluated.

"Under appropriate circumstances, a landowner should be able to seek an exception to the utility's proposed use of herbicides, and if the utility still refuses, the landowner should be able to seek relief from the commission," they said.