That's got to change, says Tom Curtis, deputy executive director of the American Water Works Association.

"Now we're entering an era where pipes are starting to break down," Curtis said. "We're seeing increasing breakage rates all across the country just because the pipe network is older and more pipelines are approaching the end of their useful life."

Nationwide, replacing all the aging pipes is a staggering undertaking that is estimated to cost more than $500 billion over the next 25 years, according to a recent report by the American Waterworks Association.

Yet spending on line overhaul is lagging badly, according to the American Society of Civil Engineers, with a cumulative gap of $84 billion projected by the end of the decade.

About six years ago, Baltimore began to do what other cities experiencing similar problems had already embarked upon — a thorough analysis of its water distribution network to identify the pipes most at risk of rupturing and wreaking the greatest havoc.

More than half of the 1,583 miles of water lines serving the city were put in the ground more than 80 years ago, according to public works figures, and 54 miles were installed before 1900.

Age alone, though, does not mean a pipe is about to break, engineers say. Some lines dating from the 1960s and early 1970s, made from pre-stressed concrete, have proven to be particularly rupture-prone. The 6-foot pipe that flooded Dundalk three years ago was concrete. The cast-iron pipes installed in the late 19th and early 20th century were much thicker than today's lines, engineers say, so they have tended to last.

But the longer a pipe is in the ground, engineers say, the more chances there are for something to go wrong — acidic water in the ground corroding the pipe, vibrations and pressure from traffic overhead, the seasonal swelling and contracting of the soil as the ground freezes and thaws.

Many iron pipes leak before they break, engineers note. In Baltimore, roughly 20 percent of the 225 million gallons of water drawn from reservoirs daily gets "lost," some through fire hydrants illegally opened and other unaccounted-for uses, but much of it from leaks.

The city's analysis, completed last year, has identified nearly 47 miles' worth of smaller mains — 20 inches or less in diameter — that are prone to breaking and need replacement now, according to Kocher. Another 17 miles of pre-stressed concrete lines, which are generally much larger, also are considered at high risk of failure.

The Light Street line was identified as high-risk three years ago, Foxx, the public works director, said Tuesday. But its replacement wasn't deemed as urgent as some. Design work wasn't scheduled to begin until 2014, according to Chow, the water and wastewater chief.

This year, the city plans to replace just 2.2 miles of those high-priority mains. But public works officials note that each restoration project takes up to three years, half for design and half for construction.

"It's impractical and unrealistic to think we can basically start tearing up the city and replacing pipelines in a big way," Chow said. The department lacks the staff and budget to tackle all the high-priority replacement projects at once, he noted, not to mention the disruption to traffic that would cause.

Next year the agency hopes to replace 20 miles of pipe, and then to increase that by five miles in each of the next four years. Meanwhile, it has begun a high-stakes juggling act, trying to resolve the biggest threats while watching — or rather, listening — for indications of imminent failure in others. Acoustic fiber-optic cables help engineers pick up audible "pings" that indicate that wires wrapped around the concrete are breaking.

This year, the city acquired a new gadget to check its largest mains without having to cut off the flow of water so their interiors can be inspected. The robotic "PipeDiver," as it's known, helped detect evidence of deterioration in a 54-inch water transmission line in Southwest Baltimore. Officials began emergency work to replace three 16-foot sections of concrete pipe the week before the Light Street main broke.

Kocher said he didn't know how many people would have lost water if the 54-inch main — nearly as high as a Volkswagen Beetle — had burst. But city and county officials issued a call to 1.2 million of the system's customers to cut back on watering lawns and filling pools while repairs are made.

The city's drinking water infrastructure earned a grade of C-minus last year from the Maryland chapter of the American Society of Civil Engineers, which found that public works officials have begun to grapple with their aging pipes but need to increase staffing and funds to do more. Many systems are doing worse — the society gave the nation's overall water infrastructure a D-minus in its most recent report card.

"The big thing is, they have to spend more money to fix up their water system," said Frank Kaul, a Baltimore-area private engineer who is on the board of the engineering society and who oversaw the city's report card. "Ultimately, Baltimore is doing a better job with its water system than the national average, but it's still not a good enough grade. There's just too much that's got to be done."

Mayor Stephanie Rawlings-Blake and other local officials have appealed for federal aid to help overhaul aging water systems. But Kaul doubts that much if any will be forthcoming in the current political climate.

"We already have a huge deficit, and I don't see the political winds pushing for that," he said

Without state or federal funds, the only way to pay for line replacement is to raise water rates, public works officials say. Residential customers' bills have more than doubled in the past 10 years, and the city recently raised the rate 9 percent, with similar increases likely in future years.

"I understand the 9 percent rate increase is a heartburn for everybody," said Chow, adding that he hopes the public understands that the reason is to try to get ahead of the breakdown of aging pipes. He cautioned, however, that there will be continuing needs for pipe replacement and that the planned rate increase might not be enough.

"We're playing catch-up," he said.

Baltimore Sun reporter Candus Thomson and news researcher Paul McCardell contributed to this article.

twheeler@baltsun.com

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