How do you find a water leak underground?

Buried leaks are expensive in a specific way: the water is gone before anyone sees it. On a service line between the street and a building, weeks can pass with no visible sign beyond a bill that looks wrong.

Here is the order that actually works.

Step 1: Confirm there is a leak

Before anyone brings equipment, prove the leak exists.

Shut every fixture and watch the water meter. If it still registers flow, water is going somewhere. If the meter has a low-flow indicator, that is the fastest confirmation available. Repeat overnight, when legitimate use should be near zero.

Buildings with flow sensing skip this step, because the answer is already in the data. Minute-by-minute readings show a base flow that never returns to zero, which is the signature of a continuous leak, and they show the night it started. They also show it for leaks that were already running the day the sensor went in, which is how most systems find their first leak.

Step 2: Narrow the run before you pay anyone

Isolation is free and it saves survey time. Close the valve between the street and the building. If flow stops at the meter, the leak is inside. If it continues, it is on the service line.

Repeat inwards: irrigation zone valves, wings, risers. Every valve you close rules out a section. A specialist arriving at a narrowed run costs far less than one arriving at a whole site.

Flow data accelerates this too. The flow rate tells you roughly how big, and the time-of-day pattern tells you what kind. An overnight constant is a fixture or a buried line. A block that repeats at the same hour every day is almost always irrigation. That is often enough to skip straight to the right valve.

Step 3: Match the locating method to the pipe

This is where most people go wrong, by booking a method that cannot work on their pipe.

• Metal pipe under pressure: acoustic correlation. More on acoustic leak detection.
• Plastic pipe, or low pressure: tracer gas. Plastic absorbs sound, so acoustic often finds nothing.
• Hot water lines and slab leaks: thermal imaging, with a moisture meter to confirm. The camera sees temperature, not water.
• Unknown pipe route: ground-penetrating radar or pipe locating first. You cannot survey a line nobody can find.
• Irrigation: zone-by-zone isolation before anything else. Irrigation leaks are common, seasonal, and often solved without a specialist.

Step 4: Dig once

The whole point of a survey is a single excavation. Mark the location, confirm with a second method if the first was ambiguous, and have utilities located before breaking ground.

What does this cost?

Locating is a per-visit service, priced by site, access, and survey length. It ends when the leak is found.

That is the part worth planning around: the survey finds this leak. It does nothing about the next one, and nothing about the running toilet on the third floor. Buildings that repeat the exercise every couple of years are paying a recurring cost with no accumulating benefit.

How do you avoid needing this again?

Flow sensing on the main line. A sensor like the Flowie O water flow sensor reads the water minute by minute and learns the building's normal pattern on the AlertAQ™ water intelligence platform, so a buried leak that starts overnight raises an alert overnight instead of surfacing on a quarterly bill.

The financial case is usually the waste, not the drama. The 2026 State of Water Management Report flagged 3.4 billion gallons of water waste in a single year across monitored buildings. At Fanshawe College, flow data showed one building running an abnormally high base flow during the day; fixing it saved more than $100,000 a year. At an apartment community in Mayfield, Ohio, flow data confirmed a pre-existing leak, and the team's fixes cut water expenses 31% year over year in 2021-2022 and recovered a $72,000 refund for wasted water, plus a sewer credit.

Flow sensing does not replace a specialist for the final few feet on buried pipe. It changes when you call them, what you can tell them, and how much water has left the pipe by the time you do.

See how the detection methods compare, water use monitoring, or what an always-on system costs.