The Atran River, tumbling lazily into the North Sea, was so vinegary from acid rain 10 years ago that its famous salmon were nearly gone.
But experts since have treated the Atran with heavy doses of lime and Swedish environmentalists have declared the river "cured."Salmon are back in greater numbers than ever.
The use of large amounts of lime is proving successful in other areas of Sweden as well.
Over the last few years helicopters and boats have dumped thousands of tons of powdered lime into rivers and lakes to counteract acid rain driven by wind from Europe's industrial areas.
The results have been stunning, fisheries inspector Brodde Almer said of the Atran.
"There were so many salmon in the river that we decided to extend the season by two weeks," he said in an interview.
By 1978, catches had dropped to 200 salmon, or 1,550 pounds. This year the river gave up 2,993 salmon for a total weight of more than 10 tons, making it one of the best salmon rivers in all of Scandinavia.
Liming was used by Greek farmers 5,000 years ago to neutralize acidity in their fields after harvesting.
Sweden followed their example after acid rain began killing its rivers, and until this year it was the only country engaged in large-scale, state- funded liming. Norway has started a similar project but on a smaller scale.
Part of the reason for Sweden's success is the low cost of liming. Mr. Almer said it costs just $50 a ton to mine lime and distribute it.
Since 1976, the government has spent $83 million to clean Sweden's water resources. This year it set aside $19 million for liming.
Farmers spend a similar amount to lime their fields.
Results have been most dramatic in rivers like the Atran and its tiny tributary, the Hogvadsan, which once had a PH value of 4.5. PH value is a measure of acidity, and pure vinegar measures a 3. The higher the PH, the less the acid.
The PH value is up to 7.1 now in the Atran and 6.0 in the Hogvadsan.
Mr. Almer said 10,000 tons of lime were poured into the Atran system over the last decade. "That has turned the tide."
Acid rain is caused by burning oil and coal spewing sulfur dioxide and nitric oxides into the air, where they are transformed into volatile acids.
About half the acid rain in Sweden falls as snow. Pollutants build up during the winter months and wash out with the spring thaw, producing a sudden peak of acidity in lakes and rivers at a time of the hatching and spawning of the fish.
In most of Scandinavia the bedrock is acidic granite, with a topsoil layer so thin that it holds nothing to neutralize acid rains.
Acidification of the water or soil can poison the food chain and endanger human health, said William Dickson, a researcher of the Natural Protection Board.
"Fish that don't die become poisonous to predators that eat them, including humans, because of accumulations of heavy metals in the fish's tissues," Mr. Dickson said.
He cited as one example the discovery of high concentrations of cadmium in the kidneys of moose, which become contaminated by eating leaves and grass.
"More alarming is the fact that groundwater in western parts of Sweden has been contaminated. In some parts of the country, high levels of aluminum and copper have been found in drinking water," he said.