Here's the thing realistic population demographics don't work in video games, its best to assume for every civilian you see there are ten you don't, and every ten raiders is only one raider as its a video game and there needs to be lots of guys to kill. Would you prefer an even more unplayable game by being over populated by pointless NPC's, or one with only a handful of combats available? Even going back to the original fallout games they used representave population, a dozen NPC's was all you saw of the one thousand residents of Vault 13.
One thing that bugged me about fallout 3 and that New Vegas did right was vegetation. The DC ruins looked like the bombs had just dropped a year or two previous, not the 160 years that it was supposed to be. I know a blasted empty wasteland is part of the feel, but its silly. After all, we know what areas look like long after atomic disasters.... and they pretty much match what they looked like before the disasters. I like how in New vegas, the Colorado river is NOT irradiated. I mean, you would think the ocean around DC wouldnt be irrradiated 160 years after a disaster unless there was a nuke buried in a sandbank.
The reason New Vegas is in such good shape radiation wise is because House either disabled or shot down most of the missiles heading for the area, so more or less the entire region was spared a nuclear strike. DC as the nations capital would have been the hardest hit, you also can't compare nuclear disasters to a nuclear war. When the entire world is contaminated its really hard to get clean, try cleaning a dirty floor with a dirty mop and dirty water. It just doesn't work to well
Also the Potomac River[like most rivers] flows into the ocean. Rivers pick up and carry pollution and stay polluted all the time, why should it be any different when they pick up irradiated particles from the ground, the air.