Regarding the Potomac, there may be some unconventional pollutants that spilled from the pipe, but otherwise I think it just was fecal matter. Some turbidity plus fecal coliform (I only did a little water work permitting treatment works in my career). I assume the conventional stuff washes out and eventually the surviving oysters have a normal habitat and perform their filtering function. The thing to watch for might be algae blooms, which can lead to toxics (e.g., red tide). timing -wise, maybe the high levels wash out before that becomes an issue?
