In the last three years alone, more than 9,400 of the nation’s 25,000 sewage systems — including those in major cities — have reported violating the law by dumping untreated or partly treated human waste, chemicals and other hazardous materials into rivers and lakes and elsewhere, according to data from state environmental agencies and the Environmental Protection Agency … It is not clear whether the sewage systems that have not reported such dumping are doing any better, because data on overflows and spillage are often incomplete.