SOUTH PORTLAND, Maine — The Portland Water District supervisor who investigated nine broken water mains in the city since the end of last month on Monday reported the causes remain unclear in six of the episodes.

“I am unable to determine the direct cause of the majority of these leaks,” James Wallace said. “There appears to be no negligence on the part of the District that contributed to any of these leaks.”

Wallace, who oversees district plant and systems operations, issued his report to water district General Manager Ronald Miller.

Causes of three breaks were determined by Wallace. He attributed an Aug. 28 break to fire hydrant use on High Street. A Sept. 12 break at 59 Main St. and one on Sept. 13 on Evans Street were attributed to excavation work by Public Works crews from the city and New England Utility Contractors.

The report details a wide span of ages in the broken mains, with the cast-iron High Street main installed in 1892 and a main that burst on Hinckley Drive on Sept. 6 installed in 1999. Wallace said newer ductile iron pipes are sometimes corroding because of soil conditions, so the district wraps new mains before installation and is looking for methods to address corrosion in already installed mains.

Wallace said his findings were based on reviews of water pressure surges or drops, construction activity in affected areas and discussions with city officials.

He said pressure readings from a meter station on Broadway near Mill Creek Park provided key data for his report.

“I was unable to identify any significant pressure surges measured at this location in the minutes or hours leading up to each leak that could be identified as a direct cause of the leaks,” Wallace wrote.

Water district crews were flushing mains about six times on the night before water main breaks. Wallace said the work is a normal activity done Monday through Thursday nights, and the metering station showed water pressure remained stable during and after the work.

Wallace said the continuing infrastructure work in Knightville, where a new water main was installed on Cottage Street, played no role in other water main breaks from Aug. 24 to Sept. 14.

The series of water main breaks forced street closures and traffic diversions, and the Sept. 6 break on Broadway near Anthoine Street led to business closures at 740 Broadway.