PORTLAND, Maine — Brendan Felix’s yellow metal detector chirped and warbled as he ran it over an exposed tree root in front of a house on Kenwood Street last week. Felix let out a sigh, reached for a long metallic probe and started poking it into the ground below the tenacious root, listening for a telltale metal-on-metal sound. He was searching for one of the long-buried valves that controls the flow of drinking water throughout Greater Portland.

“I’ve got 20 minutes, max,” Felix said, setting himself a time limit before moving on. The deadline was important, as there were still more than 42,000 water service valves to find. The Portland Water District has a total of 64,000 valves it needs to find in order to bring its records up to date.

To help complete the already decade-old project, it recently hired South Portland engineering firm Sebago Technics, which is using sophisticated GPS mapping techniques, as well as shovel-wielding, boots-on-the-ground workers. With Sebago Technics on board, the District hopes to complete the project in fewer than two-and-a-half years. The Portland Water District supplies water to 16 percent of Maine’s population, including the communities of Falmouth, Raymond, Scarborough, South Portland, Standish, Windham, Cape Elizabeth, Cumberland, Gorham, Portland and Westbrook.

Until the mapping project started, all the service valve locations, for homes as well as businesses, were recorded on paper cards with hand-drawn diagrams. “We’re.