City staff notified Calumet Montana Refinery just before 3 p.m. on Thursday of a sheen on the river south of the refinery.

Upon examination of the city storm water outfall, oily liquid was found among the rip raff under the drain, according to Calumet.

Employees from Calumet collected a sample and are testing it to identify the material, according to Calumet. The refinery is working with the city on the investigation.

The amount of liquid is estimated to be less than a gallon.

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To contain the sheen, Calumet has deployed an oil-absorbent boom at the site and is deploying booms in the river.

This story will be updated as more information becomes available.