Zach Henderson of Woodard and Curran Engineers said crews used the food-grade green dye to differentiate a Court Street storm sewer line from a sanitary sewer line Thursday morning.

Henderson said a subcontractor is using remote-controlled robots with closed-circuit cameras to inspect the city’s storm sewer system, and they wanted to make sure they were inspecting the correct storm line.

“We had not anticipated needing to use any dyes,” Henderson said. “When you look at the maps, everything looks easily nice and clean. But when you get out in the field, there may be four or five manholes in one intersection. We want to make sure they are surveying the right one.”

Henderson said the dye was put into a storm drain along Court Street, draining into the Androscoggin River at an outfall just below the Great Falls and upstream from Festival Plaza.

The City Public Services Department and the Sewer District said they took numerous calls from concerned residents Thursday and Friday.

Public Services Operations Manager Kevin Doyle said the testing is part of a federal- and state-mandated Municipal Separate Storm Sewer System program, which requires the city to create a distinct storm water system that’s separate from the sanitary sewer system. Storm water is designed to flow into the river, while the sewer system ends at the Lewiston Auburn Water Pollution Control Authority plant in Lewiston.

“It’s good that people are calling,” Doyle said. “But this is why we do testing like this. We want to make sure we don’t have illicit connections between the two. If there is, we can find it and we can fix it before there’s a problem.”

Henderson said crews would be back in Auburn with their cameras the week of Aug. 24.

“The study should take about two weeks, and we are looking at critical drain networks in the city,” he said. “These are drain lines that are under major roadways, schools, hospitals and other places that if they failed, it would be a bigger problem than others.”

staylor@sunjournal.com

UPDATED 9:50 A.M.:  AUBURN — Residents of the Twin Cities seeing green shouldn’t be alarmed.

City officials say the bright green plume reported Thursday from a storm drain under the Auburn side of the Longley Bridge is harmless dye used to test the city’s storm drain system.

The city issued a notice Friday morning after residents  became alarmed by the “bright neon green substance” in the river.

“Please DO NOT BE ALARMED, this substance is nontoxic and is essentially the food coloring that was used to test the storm drains,” the city’s notice states. “We sincerely apologize for any alarm that this may have caused.”

They will be testing more drains Friday and Aug. 24.

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