Nearly 5,900 leaks from aging natural gas pipelines were found beneath the streets of Washington, researchers from Duke and Boston universities said Thursday. A dozen of the 5,893 leaks found during the survey conducted in January and February 2013 could have posed explosion risks, the researchers said in a report made public Thursday. Some manholes had methane concentrations as high as 500,000 parts per million of natural gas, well above the threshold at which explosions can occur. Four months after alerting Washington authorities about the dozen leaks, the research team returned and found nine still were discharging dangerous levels of methane. "Finding the leaks a second time, four months after we first reported them, was really surprising," said Robert B. Jackson, a professor of environmental sciences at Duke who led the study. "Repairing these leaks will improve air quality, increase consumer health and safety, and save money," Jackson said. "Pipeline safety has been improving over the last two decades. Now is the time to make it even better." Across the nation, natural gas pipeline failures are responsible for an average of 17 fatalities, 68 injuries, and $133 million in property damage annually, information provided by the U.S. Pipeline and Hazardous Materials Safety Administration indicated. Jackson's team collaborated with researchers from Boston University and Gas Safety, Inc., on the study. The team mapped gas leaks under all 1,500 road miles within Washington using a high-precision spectrometer installed in a GPS-equipped car. Laboratory analyses confirmed isotopic chemical signatures of the methane and ethane found in the survey closely matched that of pipeline gas. The findings were published this week in Environmental Science & Technology journal.