The waters of a wildlife refuge near Reagan National Airport may turn a bright peach color Thursday as Dominion Virginia Resources attempts to figure out how mineral oil leaked from its Arlington substation into the Potomac River in January.

A dye-tracer test, ordered by the Virginia Department of Environmental Quality, will use nontoxic dye like that used to color the Chicago River each year on St. Patrick’s Day, Dominion spokesman Charles Penn said.

The test, planned for Thursday morning, may result in visible dye between the Crystal City substation, at 18th and Fern streets, and Roaches Run Waterfowl Sanctuary, where the county’s storm water drains run off. The sanctuary is connected to the Potomac via a small outlet under George Washington Memorial Parkway.

A Jan. 24 transformer failure resulted in the leak of 13,500 gallons of mineral oil. An unknown amount spread into the sanctuary, which is overseen by the National Park Service, and the river, which is under the control of the D.C. Department of Energy and Environment. The sheen of oil spread as far south as the Dyke Marsh Wildlife Preserve, south of the Beltway’s Woodrow Wilson Bridge, before dissipating about two weeks later.

Twenty-one birds, mostly Canada geese, died after the oil coated their feathers. The dye that will be used Thursday does not threaten animal or aquatic life, Dominion said, and is approved for this use by state and federal environmental agencies. The dye may be present for up to a year in soils and groundwater, but it breaks down in sunlight.