A federal judge in Bangor has approved the second phase of a study looking at mercury pollution in the Penobscot River.

One issue is whether it would be better to remove toxic mercury downstream from the former HoltraChem factory in Orrington or let the Penobscot River flush the pollution naturally.

Earlier this year, Senior U.S. District Judge Gene Carter agreed with a team of researchers who said mercury pollution in the lower Penobscot posed a threat to the health of wildlife and humans.

From the 1960s until it closed in 2000, the HoltraChem factory used mercury to produce chemicals for paper mills and other industries.

The study is being paid for by Mallinckrodt Inc., the sole surviving former corporate owner of the chlor-alkali factory.