BOSTON — DURING the recent Mississippi flooding, atrazine, the most popular weed-killing herbicide in the Midwest, was flowing past Thebes, Ill., at the rate of 12,000 pounds a day during peak river flows.
``This is not an incredible amount when you consider the total amounts of agricultural chemicals used by farmers,'' says Don Goolsby, a hydrologist with the US Geological Survey (USGS) in Denver. ``The total amount is 200 [million] to 300 million pounds of chemicals each year.''
But the atrazine contamination points to a larger danger: Record levels of herbicides and pesticides from flooded farmlands have seeped into the swollen river. According to USGS, as these chemicals move downstream one result may be widespread biological damage where the river's muddy water plumes into the salty waters of the Gulf of Mexico.
In addition, most water treatment plants in the Midwest are not equipped to eliminate atrazine and other chemicals, such as cyanazine, used on farms, from drinking water. Mr. Goolsby, who led the USGS team sampling river water, says that in order to remove atrazine, a suspected carcinogen, from water, a treatment plant needs activated carbon filtration. Few water-treatment plants in the Midwest have this process.
``We don't have a health problem with the levels, but we are surprised that the levels of agricultural chemicals in the river are as high as they are,'' Goolsby says. ``We thought the high flow of water would mean dilution [of the chemicals].''
The Environmental Protection Agency (EPA) says the maximum safe contaminant level in drinking water for atrazine is 3 parts per billion, measured on an annual basis. The amounts detected by USGS are not calculated on an annual basis, and will likely decrease as the floodwaters recede.
But as the water flow in the river increased, the amount of chemical concentrations stayed the same as measurements last year. ``The result,'' says Goolsby, ``is that daily loads of chemicals in the river increased by 50 percent.''
``That's an incredible amount,'' says Scott Faber, director of floodplain programs for American Rivers, an environmental group in Washington, D.C. ``And dilution of the chemicals is not occurring. People don't usually think of farms as sources of pollution, but we know now that farms are probably the most difficult sources of pollution to regulate.''
In addition to the herbicides and pesticides, the amount of organic and inorganic compounds such as combinations of nitrates, phosphates, sewage wastes, and grasses and leaves, is also high as the floodwaters reach Baton Rouge, La., and the Mississippi Delta. More than 830,000 cubic feet of water per second carried chemicals and other pollutants past Baton Rouge at peak on Aug. 13, three times the normal flow.
``Preliminary results from a nutrient productivity study in mid-July,'' says Michael Dowgiallo, an oceanographer with the National Oceanographic and Atmospheric Administration, ``indicate that nitrate concentrations [in the river] were 20 to 25 percent higher than would be expected.''
Where the Mississippi flows into the Gulf, the nutrient-rich freshwater acts like a fertilizer to the algae or phytoplankton, the microscopic plants at the base of the coastal food web. When the algae decomposes at the bottom, and depletes the oxygen, it creates an abnormal hypoxic condition and reduces aquatic life.
``For years there has been a huge dead zone at the mouth of the river,'' Mr. Faber says. ``Estimates are that it is 2.4 million acres in size'' - or almost double the size of Rhode Island.
This hypoxic condition occurs off the coast of Louisiana every summer, often lasting into early October until the waters are stirred by heavy winds.
Scientists now estimate that, because of the flood, the hypoxic zone may be twice its usual size, and may last much longer than usual. But it is so large, and often not visible from the surface of the water, that the exact size is unknown.
``The fresh-water flow has been so high,'' Mr. Dowgiallo says, ``it has exacerbated the hypoxia this year, and appears to be the most extensive since monitoring began in 1985.
``The key thing is that there are parts of the food chain that are being affected. We don't know the full range now, but we are seeing a pattern of high temperature in the water and low salinity.
Divers have observed that organisms that cannot move out of the affected area become stressed or die.''
Adding to the chemical pollution of the river are the unknown amounts of gasoline, engine oil, industrial pollutants, sewage, and hazardous waste that spilled into the river during the floods. The EPA says that 27 Superfund hazardous sites were either flooded or temporarily inundated.