Charleston concentrates on stormwater drainage as an anti-flooding system. But that’s not the whole story. Stormwater picks up all sorts of materials, big and small that can hurt the environment and removes it.
One of the ways that’s done is by using trees to either transform pollution (excess fertilizer in stormwater runoff) or to draw it up into the tissue of the trees, concentrating the toxic material into an easily removed form.
Here’s an article covering the basics: