The Coast Guard reports an Eversource electrical conduit under I-93 near the Somerville/Charlestown line was the source of oil showing up in the Mystic River.
After three weeks of sleuthing, the Coast Guard says:
An electrical pipe running underground, under the I-93 bypass in Somerville, was identified as being the source of the product. After excavation, it was discovered that a crack in the line allowed the oil to enter the soil. The product leaked into the storm drain system and then into the Mystic River.
A temporary clamp was fitted around the crack to stop the leak. Crews are scheduled to weld a permanent seal on Wednesday.