Competing plans to test the Penobscot River for mercury from the former HoltraChem site are at odds over what areas specifically should be tested. The plans are before U.S. District Judge Gene Carter, who is expected to choose one this fall. But the judge’s own opinion on the need for testing, offered in July, makes clear that one plan is clearly superior.

Former plant owner Mallinckrodt Inc. years ago was supposed to test the Penobscot River to see whether the thousands of pounds of mercury the company dumped there posed a health hazard but has not. It now proposes to concentrate its mercury study within a dozen miles south of the discharge site. The Maine People’s Alliance’s plan would go as far as “mid-bay” on the Penobscot and test many more sites. Taking Judge Carter’s opinion as a guide for testing should lead both parties in the following direction.

First, “dangerously high levels of mercury may be present in Penobscot fish and other sea food consumed by the public. These elevated body burdens of mercury may also present an imminent and substantial endangerment to the environment.”

Second, “the Court concludes that the methylmercury downriver of the plant, resulting, in part, from Mallinckrodt’s actions at the plant site, may present an imminent and substantial endangerment to public health and the environment.”

Third, “the evidence clearly demonstrated that the Penobscot River is contaminated with mercury through the mouth of the River and into the Bay.”

Fourth, “the scientific literature concerning the effects of mercury in an aquatic system teaches that methylation is a continuous process that can go on for decades or longer, creating the most severe adverse impacts downstream from the original mercury source.”

The judge then ordered the two parties to figure out how far the harm extended. Because the judge mentioned the bay specifically, it is reasonable to assume that he meant to determine the level of harm in the bay. Mallinckrodt’s proposal, however, fails to do this, going approximately as far as Fort Point Cove. More important, it sets specific areas for testing rather than committing to following the trail of mercury south as far as necessary.

Mallinckrodt understandably is concerned about eventually being forced to clean up mercury that it did not put in the Penobscot, that instead was carried there on the wind. Their study just last year, however, found that mercury levels in the Kennebec River were approximately one-fifth of those in the Penobscot. The company may have an argument heading out Penobscot Bay when mercury levels drop to background levels, whether measured by the Kennebec or another reasonable sample.

Until then, however, the judge was clear in his order: Find the mercury downstream, with downstream meaning the places where the high levels of mercury continue to pollute.