At what point do people collectively realize that a common practice, like dumping municipal sewerage in a river or burying trash over an aquifer, must end?

Sometimes, the timing is about right. Getting lead out of gasoline and paint in the 1970s came relatively soon on the heels of the science demonstrating their health threats. Other times, we are horrified as we look back; did a majority of people really think spraying untested insecticides like DDT was not a health risk?

Last month, the lobster industry had to absorb the gut punch that mercury was found in some fish in the mouth of the Penobscot River. The heavy metal was routinely dumped into the river by the HoltraChem plant in Orrington from the late 1960s through the early 1980s, well after its threat as a neurotoxin was known.

Harvesters, already reeling from low prices in the last two years, worry that the news could give summer visitors a reason to not buy that lobster dinner, even though the river area where the metal was found in fish now is closed to fishing and represents a tiny fraction of Maine’s harvesting grounds.

A spokesman for the Maine People’s Alliance, one of two groups fighting to get the parent company that inherited the chemical plant site to clean it up, said HoltraChem had to be pressured way back when to sound a siren before it dumped mercury in the river each month. That this was a concession boggles the mind today.

Hindsight, they say, is 20/20. But something can be learned from this ongoing environmental debacle. If time travel were possible, a visit to the meeting at which the mercury dumping was approved would be instructive. No doubt the company argued that its profitability—and, therefore, the jobs it provided—was tied to having easy disposal of the mercury.

No one anticipated then that the price we would pay for that profitability and those jobs would continue through 2014 and beyond.

So what decision will we make today that will have folks shaking their heads in disbelief 40 years from now? That’s the challenge. Is it dredging around Mack Point and Sears Island in Searsport and dumping the spoils off Islesboro? Is it building the Keystone XL pipeline? Expanding wind power in the Gulf of Maine?

Knowing future outcomes isn’t easy. And often, less-than-ideal compromises must be made. But if that hypothetical time machine is available, maybe we should use it to go 40 years into the future, or at least imagine we can, as we weigh these decisions.