In its landmark 1983 study of the Chesapeake, the Environmental Protection Agency put much of the blame for the bay's pollution on about 24,000 farmers from Maryland, Virginia and Pennsylvania, whose chemical fertilizers, animal manure and pesticides wash into the Chesapeake and its tributaries with the rain. Aaron Stauffer, a successful Lancaster, Pa., farmer for the past 40 years, believes that the criticism was misplaced.

"The only reason the farmers' waste gets into the stream in most cases is when there's a severe storm, and that's an act of God," said Stauffer, echoing the feelings of many other farmers.

Nonetheless, Stauffer has dramatically altered his farming techniques in the past two years to reduce pollution from his 67-acre farm near Ephrata, where he raises corn, cattle, poultry and hogs. He is using fewer pesticides and substituting animal manure for chemically based commercial fertilizer. He has restructured his land, building terraces to stop fertile farmland from washing away into streams and rivers.

Since farm wastes are not regulated, there is no way of knowing how much farmers contribute to pollution in the bay.

But the bay study attributed much of the runoff problem to hundreds of farmers along the Susquehanna River in Pennsylvania, the Chesapeake's largest source of fresh water. The study found that while Pennsylvania had placed strict controls on industrial and sewage polluters, it still had problems with farm pollution.

Such problems stem, in part, from the farming traditions of Pennsylvania's many people of German and Dutch ancestry. Many of those families have divided their land among their children, generation after generation, and the result has been hundreds of increasingly smaller farms. Struggling harder each year to make a profit by increasing yields, the farmers douse their land with costly chemical fertilizers and pesticides.

To supplement their income, some farmers crowd their land with livestock.

While animal manure is arguably not as bad a pollutant as chemical fertilizer, large quantities of it are detrimental to the growth of underwater grasses that are essential to aquatic life.

Stauffer said that since he started using his new conservation techniques, his corn yields have increased to 220 bushels an acre, the best ever. His farm has become a U.S. Geological Survey field experimental station, where the runoff is controlled and the nutrients in it are measured automatically every 15 minutes.