by Carl Etnier
On January 29, a broken water main hemorrhaging 5,000 gallons of water a minute made parts of Route 12 in Montpelier unusable, causing northbound traffic to be rerouted through the hills of Middlesex and East Montpelier. Four days later, a main on Nelson Street sent a torrent of water down the frozen street, turning right on Barre Street and spilling into Main Street, where it created a thick layer of ice greeting Saturday visitors to downtown.
These were just the most dramatic of the leaks in the capital city’s water infrastructure this winter. Eighteen leaks of various sizes have sprung in the city’s water mains since the beginning of November, the city engineer and public works director told city councilors at their February 13 meeting. In fact, one leak on East State Street was still ongoing at that time—the frozen ground made it difficult to determine the location of the break.
Public Works Director Tom McArdle and City Engineer Kurt Motyka explained in a memo and oral report to the Council that the city’s water system is a complicated network, with some cast-iron pipes over 100 years old and some newly installed, all-plastic pipes. And yet, as City Manager William Fraser put it at the Council meeting, “It’s not necessarily the oldest pipes that are failing. It’s a particular type of pipe, some of which were put in 30 years ago, thinking at that time that was the best thing to use.”
He was referring to “ductile iron” pipes. Ductile iron pipes are thinner walled, lighter, and more flexible than earlier cast iron pipes. The memo to the City Council says many city water mains installed starting in the 1970s were made of ductile iron, which “was marketed as a nearly indestructible pipe with a high operating pressure range.”
The ductile iron pipes installed for decades work well—until they corrode. The memo to the Council describes Montpelier’s soils as “highly corrosive” and says “many of the ‘newer’ water mains . . . are experiencing premature failure.”
In an interview with The Bridge, McArdle said the practice of grounding buildings’ electrical systems to cold water pipes also creates electrical current in the mains that leads to corrosion—much as one pole inside a battery corrodes as the battery discharges. “The failures are almost cancerous,” he said. “The pipe will pit, and it will develop a hole.”
McArdle said pipes can be protected from this type of corrosion by adding sections of “cathodic protection,” attachments to the pipe designed to corrode instead of the pipe itself, and he wonders whether Montpelier needs to systematically add more to the city’s water mains.
A water system needs to withstand high operating pressure when it serves a city built in a river valley, where the water treatment plant sits 400 feet above the downtown. The combination of head (pressure from gravity) from the treatment plant and the treatment plant’s pumps makes the usual pressure in Montpelier’s mains 200 pounds per square inch, or psi. (Normal household water pipes have a pressure of around 50 psi.)
Transient pressure surges, or “water hammer,” can cause pressure spikes well above the usual level. Water hammer occurs when flow is quickly reduced in the system, as when a valve is closed to shut off water to a broken main. The tremendous force of the water flowing through the system dissipates in the form of a pressure wave. The pressure wave, in turn, can cause new breaks in weakened sections of mains elsewhere in the system.
For example, as water gushed out of the broken main on Nelson Street, flooding the downtown, city workers scrambled to shut off the flow quickly. They managed to stanch the flow so quickly that a pressure wave of 10 psi was felt all the way up at the water treatment plant on Paine Turnpike. That amount of pressure is equivalent to what you’d feel in your ears if you swam down to a depth of 20 feet in a lake.
In addition to pressure waves and corrosion, city officials blame breaks on winter weather. Although pipes are buried—the Elm Street main was six feet down, McArdle said—cold snaps can still subject the ground around them to the freezing and thawing that moves the soil and stresses buried pipe. The memo to the City Council included a chart showing the number of leaks per month since 2012, and there’s a clear trend of more leaks in the winter. The highest number of monthly leaks occurred in February 2013 and February 2015, when nine leaks occurred in the shortest month of the year.
Frustratingly, the cause of a leak isn’t always apparent. “Oddly, the Elm Street water main was perfect,” McArdle told the Council, and then admitted, to laughter, “There was a long crack in it.” But he explained, “There was no pitting on it, no corrosion. The cement lining was intact. I believe that may have been related to a joint it was connected to, that caused a joint to radiate 11 feet down the pipe.”
If ductile iron was the new, “nearly indestructible” wonder material of the 1970s, all-plastic HDPE piping is the great hope of this decade. It doesn’t corrode. It doesn’t need regularly spaced joints like the one blamed for the Elm Street leak; it’s a seamless, fused material. It can expand and contract with pressure surges, absorbing water hammer effects. City Councilor Glen Coburn Hutcheson asked about plastic growing brittle over time, and McArdle replied that sunlight is the main cause of plastic going brittle. Since water mains are buried, he said it’s not an issue.
Like a lot of things that promise less maintenance over their lifetimes, HDPE piping is more expensive up front. It’s also very hard to purchase it now. McArdle said the city couldn’t procure it for the water and sewer work it did on Northfield Street over two years, so PVC was used instead. They did use HDPE on Lague Drive last year and plan to use it this summer for work on Clarendon Avenue.
The city is also working to reduce pressure waves and their damage. The system’s water pumps can cause pressure spikes as they cycle on and off, and the city is changing how the pumps are controlled to reduce sudden pressure changes. Where booster pumps are installed for water mains, pressure relief valves are put in to open and relieve pressure surges. The city now plans to dial them down from a triggering pressure of 20 psi to 5 psi above normal system pressure, so they’ll respond more often to pressure surges.
McArdle said the city has been following various iterations of a master plan for the water system since the 1970s, with all 52 miles of the city’s mains now scheduled to be replaced every century. However, as reality intrudes in the form of “multiple and repeated failures,” the Council was recommended to accelerate replacement of the mains under East State Street, School Street, and Nelson Street. The next time the city is scheduled to issue a bond for additional water system funding is fiscal year 2023, when a $4.5 million bond is planned.
When water mains break, state regulations mandate what triggers a notice to users to boil water. An elaborate system of redundant loops and closely spaced valves usually allows the city to keep water flowing to most areas while isolating a leak. However, the catastrophic leak at Elm Street led to a drop in system pressure that was so large that a boil-water order was issued for the entire city.
McArdle told <I>Montpelier Grapples with Broken Water MainsThe Bridge </I>this was the first time a city-wide notice had been issued since the treatment plant opened in 2001. He emphasized the boil-water orders are initially issued on the basis of the potential for contamination from breaks, not actual contamination. McArdle said that in recent years, “we have not had a failed sample” from the system during a boil-water order.