Several years ago John Murto was out before dawn fishing the stretch of fast Potomac River water just above Chain Bridge.

He couldn't get a bite and he couldn't understand why.

Then the sun came up.

The river was moderately clear for good fishing. But along the D.C. shore where he was anchored a stream of murky, cloudy water was pouring down right where his line went in.

"It looked like the stuff that comes out when you flush your car radiator," Murto said. "We couldn't even get eels to bite."

He went looking for a reason and found one. Workers at the Dalecarlia water treatment plant were cleaning out a sedimentation tank that day. The murky mess was discharging directly into the river.

Washingtonians drink Potomac River water, which may come as a surprise to many. What came as more of a surprise to Murto is what happens to all the crud that has to be removed from river water before it runs through Washington taps.

Dalecarlia takes the impurities out and sends the clean water on its way. When the sediments build up to a certain point in settling tanks, workers pull the plug and send up to 800 tons of jellied mud, including additives used to treat it, back out into the main stream.

It doesn't do the river any good.

Aluminium sulfate is the chemical added to river water as it enters Dalecarlia-15 tons of it a day. The aluminium combines with mud to settle it out. This mixture of aluminium and mud is what gets discharged into the river above Chain Bridge, and What ruined Murto's fishing that day.

Enter the Environmental Protection Agency, the federal pollution watch-dog. In 1977 EPA ordered Dalecarlia officials to stop violating federal laws against discharged of suspended solids and aluminum into the river.

Harry Ways, the plant supervisior, immediately ordered studies on how to dispose of the sludge. About once a month the big clots of up to 800 tons are discharged. Every day there is a smaller discharge of about 2 1/2 tons, according to Ways' assistant, Perry Costa.

The cleanup proposal involves collecting the sludge, processing out most of the water, compressing it, then hauling it away in trucks.

The cost to D.C. and its water users will be $26 million.

Ways, who grew up on the river and has worked with it all his life, thinks that's an excessive amount to spend for what he views as a minor problem. He pointed out that the total capital investment in the D.C. water system to date is only $70 million, and he feels that $26 million could be used far better to update the aging system.

"Within the context of cost-benefit ratio," said Ways, "I'd say we weren't doing anywhere near $26 million worth of damage.

"In my view the environmental impact is negligible. However, EPA doesn't agree. We're under orders from them to correct this situation and by 1983 we'll have done it."

Steve Bugbee, an EPA aquatic biologist looks at it another way.

"I'd say it probably screws up fishing for a little way downstream," said bugbee, who pursues shad and small-mouth bass on the river himself.

"This sludge will do the same thing in the river that it does in the settling ponds. It'll settle out and it could smother fish food and fish eggs on the bottom.

"It can increase turbidity, cutting down light penetraion, and affect the food chain. And we're also finding that aluminum, a metal, is toxic to certain life stages of fish."

Could the aluminum have direct effect on life in the Potomac, where the discharge is almost immediately diluted in millions of gallons of water rushing downstream?

"Frankly, I doubt it," said Bugbee. "There's a high dilution rate and the discharge is intermittent. I suspect the biggest effect is probably sedimentation in the immedediate area and some smothering."

Nonetheless, said Bugbee, the aluminum is "something they've added in the treatment and I consider it potentially harmful. The fact that it's small doesn't give them the right to treat this stuff and then send it back in the river."

In four years it won't be happenning that way. The District and its water users will pay the price.

Trucks will line up to haul away the new, solidified effluent. That system won't be perfect either.

"We don't know where those muddy trucks are going to go," said Ways.

"But they'll have to rumble by somebody's house."