Environmentalists who seek stricter governmental regulations controlling water pollution should be certain to have their facts straight.
Anticipate
When we're trying to pull out a principle for a parallel argument, we don't think in terms of a formal, algebraic representation of the argument.
Think more holistically, like a slogan or piece of advice.
This argument seems to feel like the old Boy Who Cried Wolf story, in which a boy cries wolf even though there isn't a wolf two different times and the concerned townsfolk fooled by his ruse learn to distrust him. Then, when there really is a wolf, they don't come running to his aide, because he's lost credibility.
Similarly, this argument is saying that environmentalists better be sure there's really a wolf there (with water pollution). If not, people won't listen to them some time in the future when there really is a wolf (dire threat) there.