Really? THIS is the terrible truth, that Duck Duck Goose is responsible for zebra mussel infestation!? And what river pipes is Cliff referring to: Runoff pipes from companies and large farms pumping waste into the river? Clogging them might be a good thing, I think.
This story would have more plausibility if Mark and Diana admitted up front that the zebra mussel problem was a national (or international) issue. And their focus on a single source of contamination is obviously not to solve zebra mussel infestation, but to show an example of how zebra mussels can spread through inattention or indifference.
But the plot twist here—which I think is good—is that Duck Duck Goose is actually concerned about an entirely different issue, and that is something they do not want Mark to discover. We await the Duck Duck Goose enforcers to get this story moving along.