City officials said that a frozen valve in an old municipal water pipe could have caused the higher than normal levels of total coliform bacteria recently found in the public water supply. A trace amount of the bacteria had been detected in tests on several samples taken on Trapelo Road, officials announced Monday. City workers flushed the water and added chlorine to combat the bacteria, and the system received a clean bill of health last weekend, said Stephen Casazza, city engineer. Although there was no public health threat, the high coliform levels meant the water supply did not meet state Department of Environmental Protection requirements, Casazza said. He said engineers were inspecting an old 8-inch pipe with a broken valve that he thinks caused the alarming test results. The pipe may have to be taken out of the system and water rerouted through newer mains, he said. The city will likely have to send a notice to all Waltham residents explaining the mishap, since six of last month’s 92 water samples came back positive for coliform, which is above the state’s requirement that only up to 5 percent of samples can test positive for the bacteria.