The Portland Water District released a report Monday saying it was not negligent in a series of water main breaks in South Portland but it couldn’t determine the cause of most of the leaks, including one that flooded a restaurant and a yoga studio.

Water district officials investigated after nine leaks occurred in the city from Aug. 25 to Sept. 14.

They found that a leak on High Street on Aug. 28 was caused by hydrant use by the city, that a break on Main Street on Sept. 12 was caused by excavation by the city’s Public Services Department, and that a break on Evans Street on Sept. 13 was caused by excavation by New England Utility Contractors.

The cause of the other leaks — including a water main break on Sept. 6 that damaged J.P. Thornton’s and Greener Postures Yoga at 740 Broadway Plaza — remains unknown.

Investigators looked for significant changes in water system pressure, which can cause pipes to break; construction projects and fire-hydrant use; and the acidity of soil, which can cause pipes to corrode more quickly than normal.

In his report, James Wallace, director of water services for the Portland Water District, said “there were no system-wide pressure surges immediately preceding the leaks.”

He also found there was no unusual activity at the water treatment facility in Portland that would have contributed to the leaks, according to the report.

The failures occurred in both new and old pipes, ranging from an 1892 cast iron pipe on High Street to a 1999 ductile iron pipe on Hinckley Drive, according to the report.

The water district is addressing corrosion issues for newer ductile iron pipes by wrapping the pipes before they are installed, Wallace said.

Michelle Clements, spokeswoman for the water district, said the water system dates to the 1800s and some of its piping is more than 100 years old. Portland and South Portland “in particular have a lot of old pipe because they were part of the original system,” she said.

In the past year, the water district has spent $1.5 million in water main upgrades in South Portland.

The water district had 88 water main breaks last year, down more than 30 percent from 10 years ago, Clements said.

“Water main breaks are decreasing due to our steady investments in a water main replacement program. Currently we invest about $3 million a year to replace water pipe and expect by 2016 we will spend $5 million,” she said.

Staff Writer Gillian Graham can be contacted at 791-6315 or at:

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