Part of the problem with this year's Nation's Tri was the overflow of water from the storm sewers. The excess runoff flooded into the regular sewers and into the Potomac River. So even though the river was relatively calm on race day, there was still a potential health problem because of the millions of gallons of sewage that spilled into the river a few days before.
D.C. is finally building out a solution for this ongoing problem, of untreated sewage flowing into local rivers whenever it rains. They are digging out two new tunnels to store water from heavy rainstorms. That water can then be treated before being released into the Potomac and Anacostia rivers. The current system cannot hold all of the extra water. To avoid backflow into homes and businesses, the water (from the storm sewers and the regular sewers) is dumped into the rivers untreated. Untreated sewage is released into the rivers about 75 times a year.
Once the two tunnels are complete, the D.C. area should be able to avoid this problem during future rainstorms. The Chesapeake Bay should also benefit from lower amounts of nitrogen and phosphorus being dumped into the rivers and bay.
The only problem is that the first part of the project won't be finished until 2018, and it won't be completed until 2025. Be patient until then.