LIKE many of the nation's other great rivers, the Hudson has been looking cleaner over the last decade or so. ``Fishermen used to be afraid to wade in the Hudson because it was full of oil slicks and raw sewage,'' says John Cronin, river keeper of the Hudson River Fishermen's Association in Garrison, N.Y. ``But they're not afraid anymore. Instead of sewage, you see pleasure boats and swimmers. And these days shad fishing has been the best on record.'' But two recent studies suggest that rivers in the United States are not as clean as they may seem. Although the federal government has spent more than $100 billion under the Clean Water Act of 1972 and its 1977 amendment to build sewage treatment plants and restrict the discharge of industrial effluents, levels of several toxic chemicals are increasing and, in some cases, exceeding government standards.
For the most part, these pollutants don't come from factories or other so-called ``point sources'' that are being regulated under the act. Instead, they flow from disparate ``nonpoint sources'' - such as chemical-laced storm water runoff from farms and city streets and from acid rain - that are largely unregulated.
Unless something is done, according to a study of 380 rivers released in March by the United States Geological Survey (USGS), nonpoint source pollution can prevent the federal government from achieving its water quality goals.
The USGS study - the first nationwide study of river water quality - found widespread increases in nitrate, salt, arsenic, and cadmium, particularly in the Midwest and Northeast.
The researchers attributed increases in arsenic and cadmium directly to acid rain.
They traced elevated nitrate levels to acid rain as well as fertilizers, whose usage rose 68 percent between 1970 and 1981. And the scientists found the main cause of river salinity to be salt used for melting ice on highways.
Although none of these chemicals exceeds federal standards in water, ``they are a source of concern,'' says USGS hydrologist Richard Smith. He notes that arsenic has been found to be dangerous to human health at any level and that nitrates in water have been associated with cancer in humans. He adds that about half the rivers studied are tapped for drinking water.
The Hudson River, a source of drinking water for five New York state communities, was the focus of a study by Inform, a New York environmental research group. The Inform report, to be published this month, finds that in ``hot spots'' throughout the river, four toxic chemicals - PCBs, lead, mercury, and cadmium - violate state and federal standards.
Although PCB discharges declined from more than 30 pounds a day to about one pound a day after being restricted under federal and state law in 1976, the chemicals persist in the river sediment and fish because they take many years to break down. ``To get rid of PCBs, you have to dredge the sediment,'' says Steven Rohmann, who directed Inform's study. ``But the next problem is, what do you do with the contaminated soil?''
Unlike PCBs, which have been traced to specific industrial plants, according to Dr. Rohmann, the lead, mercury, and cadmium in the river originate at thousands of uncharted sources where metals break down, from abandoned automobiles to mines that leach acid.
The Clean Water Act amendment passed earlier this year should help stem the flow of nonpoint source pollution from older cities such as New York, where storm water is purified by sewage treatment plants, because the amendment has allocated $400 million to improving the plants. In other areas, however, where storm water drains untreated into rivers, the amendment will have no effect.
The solution, as Rohmann sees it, is for states to develop their own strategies for controlling nonpoint source pollution, as about a dozen states have already done.
In Wisconsin, for example, the Department of Natural Resources teaches farmers to restrict the pollution they generate by tilling their land less often, reducing pesticide and fertilizer applications, and by building fences to keep dairy cows away from rivers. ``Nothing will chew up a river bank and thereby increase erosion faster than a 1,400-pound Holstein,'' says John Konrad, who started Wisconsin's nonpoint source pollution program almost 10 years ago.
Although the program is voluntary, Mr. Konrad estimates that half the state's farmers participate and that they have reduced soil infiltration into waterways by 50 to 75 percent. Over the past few years, he has documented that trout, which are sensitive to pollution, have increased in various streams.
Maryland was one of the first states to control nonpoint source pollution in urban areas. Since 1970, Maryland has required construction workers to keep excavated soil, a source of lead and other metals, off city streets. Strategies include piling bales of straw or plastic garbage bags around the perimeter of construction sites. State inspectors make regular visits to the sites to see that the law is enforced.
Maryland enacted further legislation in 1982 to restrict pollution from storm and water runoff. Under this law, if a new building or subdivision increases the rate at which storm water drains, the builder must install a detention pond to catch the rain, thereby preventing floods from sweeping pollutants off city streets into waterways.
But nonpoint source pollution controls don't come cheap. Maryland, for example, spent $32 million between 1984 and 1986. It was worth it, according to Kenneth McElroy, director of planning and analysis for the Maryland Office of Environmental Programs. ``To maintain water quality we've got to control nonpoint source pollution,'' he says. ``Otherwise, we'll see setbacks.''