Washington Gas has repaired four natural gas leaks in manholes deemed potentially explosive by university researchers who tested the District’s aging pipes.

The leaks posed an immediate threat, according to the company, which has also scheduled repairs of six other manhole leaks that the research team detected, according to the utility company.

The research team, which has tested city lines since 2013 , revisited the city at the end of April and reported the latest findings of 10 potentially explosive leaks to the company May 4.

The team, which includes Robert B. Jackson, a professor of environmental sciences at Stanford University, used a car-mounted monitoring device to survey the city street-by-street last year and found that aging cast iron mains over more than 400 miles of lines are showing wear and tear.

Washington Gas has received conditional approval from D.C. utility regulators to speed up pipe replacement over the next five years. But replacing all the pipes could take as long as 40 years, according to Washington Gas’s filings with the District’s Public Service Commission, and cost about $800 million. Regulators have asked to see both a 25-year and a 40-year scenario for the work.

Although the timetable is not set, regulators have given Washington Gas conditional approval for the first five years of a plan. The opening phase would start in June, take 16 months and cost the average residential heating customer a total of $6.22 during that time, the company’s filings show.

The estimated added costs to the average customer for the full replacement program would swing from $150 a year to cover a 40-year plan to about $231 a year if the project is compressed into 25 years.

The research team’s latest leak report found potentially explosive methane buildup at 10 manhole sites. That includes six sites that also showed high readings during checks in 2013, which the team previously reported to Washington Gas, Jackson said.

“I did not expect to see those still there,” he said.

During its 2013 tests, the team discovered nearly 6,000 natural gas leaks but reported that the vast majority posed no direct health or safety risk.

After checking the research team’s report from May, Washington Gas crews found only four sites that it said required immediate repairs. They had not been previously identified in 2013 as having suspected leaks, company spokesman Ruben Rodriguez said. He declined to identify the repair locations because of security concerns.

Washington Gas declined to discuss the criteria it used to determine which leaks require immediate fixes, making it impossible to independently compare levels it may have found with those the researchers said they detected.

Leak detection and repair procedures are guided and enforced by federal agencies and local utility regulators. Washington Gas practices “exceed” what is required by code, the company said in a written statement. It declined a request for an interview about the researchers’ report.

The company’s statement also said that the researchers’ technology has “limitations in detecting and qualifying gas leaks.”

Jackson said that “if the company is saying they are not finding these [additional six] leaks where we say they are, I would strongly disagree, since we have pinpointed them three times now” — twice in 2013 and again in May.

But, he said, “they must have a reason, but we don’t know what those reasons are if they aren’t saying what criteria they are using to say what concentrations are dangerous and what are not.” He said his team uses thresholds “that are pretty standard in the industry.”

The leaks posing a potential for explosion showed that the level of methane rose to more than 50,000 parts per million, or 5 percent, which is the “lower explosive limit” of methane in air, according to the researchers.

During the team’s most recent visit, Jackson said, there was evidence of repairs on some other leaks at manholes that it had reported to Washington Gas in 2013.

In addition to studying health and safety risk from leaks, the team has looked at how gas emissions affect the cost to consumers because of gas lost on transmission lines and how emissions affect climate by trapping heat and the effect on ground-level ozone.