The North Texas Municipal Water District’s annual chlorine maintenance began Feb. 26 with little mention from residents and city officials. Then in mid-March, the maintenance process was brought into the spotlight when well-known environmental activist Erin Brockovich openly questioned the district’s methods in a series of Facebook posts.

Since then area residents have been seeking answers, and the NTMWD maintains that the water was safe to drink and use during the chlorine maintenance process, which ended March 26.

In a news release sent out after Brockovich’s post, the district said it uses three common disinfectants for water treatment: chlorine, ozone and chloramine, which is a combination of chlorine and ammonia. These disinfectants are intended to eliminate bacteria, parasites and viruses, according to the district.

“Ozone is the most powerful disinfection process, and chlorine is used to ensure the water remains safe as it moves through the pipes throughout the regional and local system,” the news release stated.

During the 28-day chlorine treatment, the district used free chlorine, which temporarily removes ammonia from the process. NTMWD has performed this free chlorine treatment since 2007 and claims it is a preventive measure, not a response to an issue in the water system.

In her Facebook posts Brockovich criticized the district’s practice of using chlorine and chloramine. In the past she has raised questions about the same methods in water districts supplying other Texas cities.

Brockovich claims the practice of using chlorine produces byproducts, which she did not name, that are toxic but “just not yet regulated.” One toxic group of chemicals she mentioned, trihalomethanes, is regulated by the federal government.

Prolonged exposure to trihalomethanes at levels well above the federal limit is known to be linked to liver, kidney or central nervous system problems as well as an increased risk of cancer in animals, according to the Environmental Protection Agency.

The district released the results of an independent lab report March 21 showing the levels of trihalomethanes rose in March but remained within the federal regulatory standards.

The report, conducted by Ana-Lab Corp. in Kilgore, Texas, showed a sample of water collected March 15 from the NTMWD’s northern system, which serves Plano, contained levels of trihalomethanes that were 33 percent below the federal regulatory limit of 80 parts per billion. The trihalomethane levels in the water typically average closer to 23 parts per billion, said Mike Rickman, deputy director of operations and maintenance for the water district. The levels of trihalomethanes in the water are expected to rise during the free chlorine treatment process, he said.

Another sample from the water district’s southern system, which serves Forney, among other cities, contained trihalomethane levels that were 11 percent below the federal limit.

Rickman said the district made ozone its primary disinfectant in 2014.

“In the past we used chlorine or chloramine as our primary disinfectant,” he said. “Now that we use ozone [as our primary disinfectant], our trihalomethane level is much lower initially coming out of the plant than it was when we used chlorine in both places.”

On March 26, Frisco Mayor Jeff Cheney and City Manager George Purefoy published a letter to residents explaining that the city also conducts its own water quality tests “to ensure we’re delivering a safe product to our consumers ... .”

The water district made a presentation to Frisco City Council and residents during Council's April 3 meeting.