Operator error caused the Dec. 10 fluoride spill that contaminated the water supplied to 34,000 homes and businesses in Cumberland and York counties, according to a state Public Utility Commission report.

An employee at Pennsylvania American Water Co.’s Yellow Breeches treatment plant in Fairview Twp. used a piece of plastic to hold a manually operated, spring-loaded switch open, the report said.

The PUC investigation also concluded that the incident wasn’t reported to commission personnel in a timely fashion, that the public wasn’t adequately notified that a “do not consume” advisory was in effect, and that the public wasn’t notified of the possible dangers of drinking water with elevated levels of fluoride.

Pennsylvania American Water Co. “has taken swift and positive steps to ensure that this type of chemical spill does not occur again,” and improved its notification and operating procedures, the report said.

Joi Corrado, a water company spokeswoman, said employees have received additional training in operating procedures, mock drills are being held to take employees through various types of emergencies and a spill-detection alarm system has been installed at the plant where the leak occurred.

“The spill was caused by operator error,” Corrado said. “If they had been following proper procedures, it wouldn’t have occurred.

“We are cooperating fully with the PUC, and we have made improvements to ensure that this doesn’t happen again.”

The PUC also directed other water companies across the state to implement better procedures for notifying consumers of emergencies and to update emergency procedures.

The Yellow Breeches water treatment plant will be replaced this year with a $36 million plant equipped with equipment that will eliminate the possibility of such errors, Corrado said.

The operator used a piece of plastic to hold a switch open, allowing fluoride to be pumped continuously into tanks, and then left the area to perform other duties. The operator failed to remove the plastic before his shift ended at 8:30 a.m., the report said

An operator who came on duty at 8 a.m. was notified later by an alarm that the system was malfunctioning, but strong hydroflurosilic acid fumes kept him from entering the building to turn off the switch, the report said. As a result, the pump kept pumping fluoride until 1:30 p.m., when it was deactivated by a hazardous material response team.