The federal Clean Water Act might be a great thing in theory, but how’s it doing in practice? Not so well, it turns out, due to the failure of the U.S. EPA to adequately enforce it. At any given moment, roughly 25 percent of all large industrial plants and water-treatment facilities are in violation of federal pollution standards — but the EPA generally fails to take action against all but a handful of them. According to an internal study by the agency, half of the serious offenders exceed pollution limits for hazardous substances by more than 100 percent; 13 percent exceed limits by 1,000 percent. Yet on average, the EPA takes formal disciplinary action in no more than 15 percent of cases, and fewer than half of those actions result in fines. (And small ones, at that: The average fine was just $6,000.) The data from the study is drawn largely from the years 1999-2001, but represents a decade-long trend, officials said. The rampant polluting can devastate fisheries, poison waterways, and threaten human health.