Only 12 of America’s 100 largest water systems share information about the pipe that connects the water main to homes, despite federal officials recommending more transparency.

While some large systems—such as those in Boston and Cincinnati—publicly disclose their data, many don’t. A Sept. 21 report from the Government Accountability Office analyzing the websites of the systems found that few disclose the data, leaving residents and regulators in the dark.

The study was a followup to see how states are responding to a nationwide 2016 Environmental Protection Agency request to post the pipes’ data...