Twenty-five years ago, President Lyndon Johnson declared the Potomac River a national disgrace. Today, the river is an environmental success story, the result of dedicated effort from several key groups, well-placed funding and a concerned and motivated public. But despite this progress, there's no guarantee the next generation will have a Potomac clean enough to fish and swim in.

By the late 1960s, the impact of human habitation along the river since the 1600s, coupled with a surge of population growth in the District during World War II, had taken its toll. Untreated sewage poured directly into the river resulted in oxygen depletion and massive fish kills.

Today, almost 20 years after the Clean Water Act, the Potomac is home to many species of fish that weren't there 20 years ago. And submerged aquatic vegetation, one sign of a healthy river, is making a strong comeback. The river is used for drinking water, recreation and the disposal of sewage treatment plant effluent that is safe for the environment.

This success is a result of investing roughly $1 billion from federal, state and local sources since 1972 to improve waste-water treatment facilities. And local municipalities spend $100 million annually to operate these plants. The success is also a result of an unusually high level of cooperation among federal, state and local governments -- especially considering that the Potomac watershed includes parts of West Virginia, Virginia, Pennsylvania and Maryland. The Interstate Commission on the Potomac River Basin, which researches and monitors the Potomac's water quality, and the Metropolitan Washington Council of Governments, the regional planning agency for local governments, have played important roles in the river's improved quality.

Additionally, Blue Plains Sewage Treatment Plant, though it has experienced some recent problems, continues to be a vital part of the Potomac cleanup and provides advanced sewage treatment for about 2 million area residents.

There are other forces behind the improved water quality: people are recycling, buying lead-free gasoline and limiting the use of fertilizers on their lawns, all of which have a positive impact on the Potomac.

So why is the river still in jeopardy? A growing population in the metro area is placing pressure on water-treatment facilities. But there are other issues, and they affect not just the Potomac but the water quality around the nation.

Pollution from nonspecific sources, such as agricultural and urban runoff, is one example. According to the Environmental Protection Agency, such sources represent more than 50 percent of pollutants in lakes and rivers. It will take a strong and coordinated effort to successfully address the challenge presented by this form of pollution.

Contamination by toxic substances from municipal, agricultural and industrial sources is another example. In 1986, EPA estimated that 37 percent of the toxic industrial compounds entering surface water did so by passing through treatment plants, which are not designed to remove these compounds. Industry must utilize pollution prevention and pre-treatment techniques to abate this problem.

Overflowing sewers, which can result during periods of wet weather, occur in about 1,100 older cities across the nation. Cost-effective strategies must be developed for the management of this pollution source.

Groundwater, which represents 95 percent of the country's freshwater supply and is the drinking water source for 50 percent of all Americans and for 97 percent of our rural population, is another big concern. Contamination already threatens 50 percent of the groundwater sources used as drinking water supplies.

Both point and nonpoint pollution sources must be controlled to protect this vital resource. Septic tanks, leaking underground storage tanks, hazardous wastes dumped in landfills and agrichemicals are threats to groundwater.

It's time to secure funding for new or upgraded waste-water treatment plants to meet the requirements of the Clean Water Act. Smaller communities already struggling to comply with ever-stricter water quality standards will be hard-pressed financially. Residents in this region can almost certainly expect to pay higher user fees.

The price to protect the Potomac is going up. For too long, the public has paid artificially low prices for water resources because of federal subsidies that will soon expire. The average American community pays 0.1 cent per gallon of treated water -- which does not reflect the true cost of the service.

People have worked hard to make the Potomac cleaner than it was 20 years ago. The quality can continue to improve -- but there's a price to pay. Clean water is not a gift. -- Arthur W. Saarinen Jr. is president of the Water Pollution Control Federation.