Marcin Wójcik parks the van at midnight, cuts the engine, and waits thirty seconds for the street to go quiet. Not silent, there is no such thing as silent in a city, but quiet enough that the microphone bolted to the end of his listening rod can pick out one sound from the rest: the hiss of treated water escaping a cast-iron main laid in 1961.
“Traffic is the enemy,” he says. “A bus goes past and I lose the whole read. Rain is worse. Rain sounds like a leak everywhere, which is another way of saying it sounds like a leak nowhere.”
Wójcik works nights for the water utility in a city of 1.3 million people, and his job has a name that sounds almost old-fashioned for something done with digital correlators and satellite data: leak detection. Every water network loses some fraction of what it treats and pumps before that water reaches a customer’s tap. The industry term is non-revenue water, and in this city it currently sits at nineteen percent. Some of that is metering error and theft. Most of it is leaking straight into the ground, through joints that have shifted, through corrosion nobody can see, through pipe that has simply outlived the century it was built for.
Listening for a sound that shouldn’t be there
The work starts before anyone touches a shovel. Acoustic loggers, small waterproof devices about the size of a hockey puck, clamp onto valves and hydrants across the network and record sound continuously overnight, when the system is quiet enough that a leak’s signature isn’t drowned out by demand. In the morning, software flags sections where the noise pattern looks wrong. That narrows a leak’s location to a block, sometimes two.
Narrowing it to a spot you can actually dig requires a person. Wójcik and his partner, Halina Dabrowski, walk the flagged section with a ground microphone, listening at valve boxes and along the pipe route itself, moving the mic a few metres at a time and comparing volume. Where the sound peaks, they mark the pavement with orange paint. It is unglamorous, repetitive, and it is also the only method that works reliably on pipe this old, because correlator readings, which compare the timing of a leak’s sound between two sensors to calculate distance, assume a pipe material and diameter that a century of patch repairs may no longer match exactly.
That gap between where a leak sounds loudest and where it actually is costs money every time a crew digs in the wrong spot. A single excavation in this city, permits, traffic control, asphalt restoration included, runs to roughly four thousand euros. Dig wrong and the utility pays for two holes to fix one leak.
The arithmetic of an old system
The case for the work is not sentimental, it’s arithmetic. Every cubic metre of water that leaks out of the ground has already been through a treatment plant, chemically dosed, pumped uphill through the system at pressure. It represents electricity, chemicals, and labor, spent on water nobody will ever drink. Utilities in older cities have been running the numbers for decades and arrived at the same conclusion from different directions: it is usually cheaper to hunt leaks than to replace an entire network at once, because full replacement of a city’s water mains, at current construction costs, would take longer than the pipe itself would last if left alone.
So the hunting continues, block by block, night by night. The utility budgets for roughly six kilometres of proactive leak survey a week, on top of leaks reported by residents who notice a wet patch on their street or a sudden drop in water pressure. Wójcik has been doing this for eleven years. He says the work has gotten harder, not because the pipes have gotten worse, but because the city has gotten louder, later. Delivery vans now run through the small hours. Nightclubs let out at four. The windows of true quiet, the ones his instruments need, have gotten shorter.
“Fifteen years ago I had from midnight to five,” he says. “Now some nights I get two good hours. You learn to work fast in them.”
By 5 a.m. the marks are down, six or seven of them along tonight’s route, and a day crew will come by later in the week to open the ground and see whether the sound led them to the right place. Most of the time, Wójcik says, it does. The rest of the time, they dig again.