Other sources of trouble without clear-cut present solutions are big ones. Surface runoff from both cities and rural areas, as we have seen, causes much pollution. In the country, soil conservation measures can slow it somewhat and strain out some pollutants, and augmentation of streams' flow can enhance their capacity to oxidize the wastes. But neither of these seem likely to do much to ease the longrun buildup and diffusion of persistent pollutants like pesticides, or to avert the possibility of disastrous spills. Public education and wiser restrictive legislation may help, but the only real hope in terms of these poisons appears to be that more selective and less indestructible substitutes will be found, and all promising means of biological pest control explored. Continuing programs are focused on the problem, but it continues to be serious. Pollutive runoff from urban areas merges with the whole question of urban sewer systems, for most of it gets to the river through storm sewers. We have seen that the old-style combined sewers of the District of Columbia and Alexandria cause gross pollution when storms force open their overflow gates, and we have seen too why the approach to this problem that formerly prevailed--the arduous, hugely expensive digging up of sewers and their replacement with dual pipes to carry storm runoff and sewage separately--is no longer considered satisfactory. For the more modern dual systems also contribute much trouble through the filthy rainwater that pours out into streams from the storm system and through the accidental or illegal channeling of sanitary wastes into storm sewers. A wholly satisfactory answer would allow runoff water as well as all sanitary wastes to be held for full treatment at a standard plant. But when we consider that at the Washington metropolis the dirty local runoff from a single storm may amount to billions of gallons, the question of where to hold it grows a bit complex, and is leading toward experimentation with such ideas as vast subterranean networks of tunnels for storage. Partial answers might come from subjecting storm and mixed flows to different and lesser kinds of treatment by micro-screens at sewer outfalls, detention and settling tanks, and filtration beds. These possibilities and others need much investigation and testing.