Few national rules control the pollution that runs off farms nationwide - pollution suspected in the outbreak of a mysterious fish-killing microbe in Maryland and currently the most widespread source of contamination in America's rivers.
Only the largest livestock operations, holding thousands of animals in pens, require permits from the Environmental Protection Agency for the storage of manure. And although 16 states require other, smaller farms to adhere to plans on how to manage manure and fertilizers, most states have purely voluntary programs."It is really not regulated," said Scott Faber of the American Rivers conservation group.
Pesticides and soil erosion from farms also pollute waterways, but the nutrients in manure and other fertilizers - mainly nitrogen and phosporous - upset nature's balance, with sometimes unforeseen results.
On Maryland's Eastern Shore, officials are trying to determine if poultry farms contributed to an outbreak of pfiesteria piscicida, a bacteria that killed scores of fish in Chesapeake Bay tributaries and may have sickened people who came in contact with the rivers.
A concentration of huge hog farms is the main suspect in a similar appearance of pfiesteria that killed millions of fish two years ago in North Carolina. And some scientists believe dairy herds could have been the source of the parasite cryptosporidia in Wisconsin in 1993 - which caused 100 deaths and made 400,000 sick - and in Georgia in 1987.
Nutrients are beneficial when used in proper amounts to help crops grow, and many farmers use cattle and chicken manure on fields. It is a low-cost alternative to chemicals and helps farmers produce more food.
But these nutrients also feed undesirable organisms, sometimes making them grow out of control. In Florida's Everglades, for example, years of phosphorous runoff from nearby sugar cane fields has led to huge stands of cattails that choked out other native grasses.
A recent study by the Ecological Society of America found that human activity including farming has doubled the amount of nitrogen entering natural systems, resulting in areas of extremely low oxygen in estuaries and coastal waters.
This condition has created a 7,200-square-mile-and-deep "dead zone" in the Gulf of Mexico near the Mississippi River's mouth. Scientists believe decaying algae that grew dramatically because of farm runoff have robbed the area of oxygen needed for marine life.
EPA's 1994 survey of state water quality identified farm runoff pollution as the biggest problem in 60 percent of rivers and streams included on its "impaired" list.
Yet for the most part, government's response to this so-called "non-point" pollution has been to recommend that farmers voluntarily adopt nutrient management plans to prevent too much nitrogen and phosphorous in water. The Agriculture Department provides some technical and financial assistance.