The Texas Commission on Environmental Quality has notified the Port O'Connor Improvement District water system that the drinking water being supplied to customers had exceeded the maximum contaminant level for total trihalomethanes. The U.S. Environmental Protection Agency has established the maximum contaminant level for total trihalomethanes at 0.080 milligrams per liter based on locational running annual average, and has determined that it is a health concern at levels above the containment level, according to a notice the Port O'Connor Improvement District sent to residents this week.