who knows how their's doing it but the water does seem cleanier around the bog wench
You worked long and hard, building wetlands; finding plants that could bear the poisoned flow of the rivers and cultivating them, turning desolate river mouths into deltas full of fierce and hardy greenery, coaxing detritovores to take up home amid the roots and introducing engineered organisms where needed to break down the most stubborn pollutants. Eventually, over time, the water flowing into the Sea was clearer and less sludgy, and your work was being copied by others, anywhere rivers flowed into sufficiently large bodies of water, slowly and painstakingly scrubbing the rivers of the poison leeched from the land.