The local agency that supplies water to Plano, Frisco, McKinney and other North Texas suburbs on Wednesday released the results of an independent lab report showing the levels of a potentially toxic byproduct of an ongoing water-treatment process rose this month, but remained within the federal regulatory standards.

The report, conducted by Ana-Lab Corp. in Kilgore, Texas, showed a sample of water collected on March 15 from the North Texas Municipal Water District's northern system, which serves Plano, contained levels of trihalomethanes that were 33 percent below the federal regulatory limit of 80 parts per billion. Another sample from the water district's southern system serving Forney, among other cities, contained trihalomethane levels that were 11 percent below the federal limit.

The results of the report were released less than a week after the water district came under fire from California-based environmental activist Erin Brockovich, who criticized the district's annual practice of running chlorine through its pipelines in a series of Facebook posts widely shared and discussed by local residents.

Prolonged exposure to trihalomethanes at levels 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, according to the Environmental Protection Agency's website.

The levels of trihalomethanes in the water are expected to rise during the free chlorine treatment process, said Mike Rickman, deputy director of operations and maintenance for the water district. The district has maintained its water is safe to drink during the treatment period, which began in late February and is expected to wrap up on March 26. The trihalomethanes levels in the water typically average closer to 23 parts per billion, Rickman said.