A notice Thursday of intent to file a lawsuit in the cleanup of mercury in the Penobscot River may help this slow-moving project along or stall it through legal wrangling. That’s the risk the groups that intend to sue take by turning to the court. One aspect of their suit that ought to prove helpful no matter what the legal outcome is the demand for a large-scale, independent assessment of mercury’s effect on the river.

The state of Maine and the current operator of the chlor-alkali facility in Orrington, HoltraChem, have been working for over a year to reduce mercury emissions and are said to be making progress. Mercury is known to cause behavioral and physiological disorders, including birth defects and reproductive disorders. It was said to be responsible for the mad hatters in the mercury-using hat industry of Danbury, Conn., during the last century.

HoltraChem, likes its predecessors at the site, makes chlorine, caustic soda and bleach. The problem identified in the planned suit by the Maine People’s Alliance and the Natural Resources Defense Council rests in the river itself, where mercury hot spots” are a real and serious problem in addition to the spread of mercury downstream. Some of the hot spots in the cove near the plant measure between 200 and 400 parts per million, where typical mercury problem areas elsewhere in the state register 2 or 3 parts per million.

The hot spots, however, precede the ownership by HoltraChem, which acquired the plant in 1994. The facility was opened in 1967 and operated for years by International Minerals and Chemical Corp. (IMC), which dumped large quantities of mercury into the air and water. IMC has undergone broad changes since its days along the Penobscot. It is now called Mallinckrodt and is, interestingly, a health-care company with net sales last year of $2.4 billion. (HoltraChem has made its own, far lesser contributions of mercury to the river, and has runoff problems, which it continues to work on.)

Carefully removing high levels of mercury from the Penobscot is in everyone’s interest, and if a lawsuit can fairly move this process along, it should be pursued. The current and previous owners have been working with the DEP and the U.S. Environmental Protection Agency for the last year to achieve this end. The negotiations have not been swift, but at least all parties have been willing. A broad independent assessment that looks well downstream could help matters along. It might address what the group sees as shortcomings in a recently completed 10-year site investigation paid for by the current and previous owners.

Knowing what the effects and dangers are to humans and wildlife along the river would demonstrate why cleaning it up is so important and would direct resources most effectively.