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Where do chemicals go when it rains?

A farmer's oft-heard lament is that it always seems to rain at the wrong times. When plants need moisture, the sky is clear. And as soon as fertilizers and pesticides are applied, a downpour comes--just in time to wash them away before they can be effective.

But Don Wauchope and a team of seven researchers in Tifton, Georgia, want it to rain hard soon after they've applied agrichemicals to corn plants on their research plots. In fact, they have set up an overhead irrigation system that can simulate an inch of rain an hour--the type of hard shower that farmers hope to avoid after they've applied pesticides and fertilizers to their crops.

Over a 15-week period last spring from planting to harvest, the researchers created 12 of those heavy rains in the first part of a 2-year study to measure agrichemical runoff. Those rains are yielding a downpour of data for Wauchope and fellow researchers with the U.S. Department of Agnculture and the University of Georgia Coastal Plain Experiment Station in Tifton. The team is analyzing first-year data, and Wauchope says, "It's already clear that we are getting unique and very useful results."

Researchers cooperating in the study are Ben Burgoa, Clyde C. Dowler, A. William Johnson, Laurence D. Chandler, Harold R. Sumner, and Clint Truman, all of USDA's Agricultural Research Service in Tifton; and Jessica Davis-Carter, Gary J. Gascho, and James E. Hook of the University of Georgia Coastal Plain Experiment Station there. Overall, more than 30 people are involved in the project, one of the most intensive runoff experiments ever performed.

The purpose of the research, partially funded by a grant authorized by the USDA National Water Quality Initiative, is to simultaneously measure the movement of water, chemicals, and soil caused by heavy rains. They want to track that movement off crop leaves, as well as into the soil and off the field. While many of the applied chemicals break down and degrade before such rains occur, some can pollute ground- or surface-water supplies if it rains too hard too soon after they are applied.

"The goal is to help water quality agencies better predict agrichemical runoff trouble spots," says Wauchope, an ARS chemist in Tifton. "Once they determine those spots, they can develop strategies for controlling runoff."

Those strategies are important, given the volume of agrichemicals used in American agriculture. In the late 1980' s, farmers used an estimated 460 million pounds of herbicides alone each year in the lower 48 states, according to a 1991 report by Resources for the Future, Inc., a private research firm in Washington, D.C. About two-thirds of that amount is applied to corn and soybeans, according to the report, which was financially supported by USDA, the U.S. Environmental Protection Agency (EPA), the National Oceanic and Atmospheric Administration, and four chemical companies. Annual fertilizer use is about 100 billion pounds.

Previous rainfall studies have measured chemical residues in runoff. But, except for small-plot experiments, they have relied on natural rainfall, which can be sporadic. "in our study," Wauchope says, "we're controlling the amount and timing of the rain we apply, so our results are more consistent."

The researchers are copying a technique developed by Pete Coody at Miles Corp., a pesticide manufacturer. The technique allows researchers to simulate rain on a 50- by 150-foot area--big enough to really look at tillage and crop effects.

In the study, seven agricultural chemicals were applied at recommended rates and times, ranging from before planting to crop maturity. They were the nematicide fenamiphos, the herbicides alachlor and atrazine, the insecticide chlorpyrifos, a copper fungicide, and nitrogen, phosphorns, and zinc fertilizers. Also, potassium bromide was used as a tracer.

The researchers planted corn on the plots, which were on a loamy sand with a slope of 3 percent. They measured water moving into the soil and running off the plots for sediment, the four pesticides, pesticide degradation products, and other chemicals and tracers.

The experiment was done simultaneously on large plots, called mesoplots (about 7,500 square feet), and on microplots (about 65 square feet). Wauchope says both were used to determine the effect the size of the plots may have on runoff or erosion.

The rainfall simulator consists of 46 overhead sprinklers spaced 10 feet apart in two rows 48 feet apart and running the length of the plots. Runoff water and sediment were collected in v-shaped troughs at the bottom of the slope.

The two herbicides, alachlor and atrazine, are widely used to control weeds, primarily in corn and soybeans. In the late 1980's, 64 million pounds of atrazine and 55 million pounds of alachlor were applied annually on all cropland in the lower 48 states, according to the Resources for the Future report. Wauchope says both herbicides are of special interest because they are being found in trace amounts in rivers, especially in the U.S. Corn Belt.

Fenamiphos, one of the few remaining nematicides allowed for use, is applied to the soil to control nematodes. The insecticide, chlorpyrifos, is applied to crop foliage.

After each chemical treatment, the scientists created a "worst-case scenario" for runoff and erosion--a heavy rainfall 1 day later. Each rainfall poured 2 inches of rain on the test plots over a 2-hour period. Researchers collected the water and soil samples--more than 3,000---and are analyzing the data. The preliminary findings so far show:

* Maximum runoff losses of about one-half inch of water per rainfall event from the mesoplots, decreasing to one-fifth of an inch when the corn was mature. Runoff from the microplots was about twice as much per square foot because the water had less time to infiltrate before reaching the collector troughs than on the larger plots.

* Soil erosion rates of 1 ton per acre on the mesoplots and 1.5 tons on the microplots, in the most erosive rains.

* Herbicide losses of up to 5 percent from the mesoplots and 9 percent from the smaller plots; nematicide losses of 1 percent or less (because it was incorporated into the soil).

* Dissolved phosphorus losses of about 2 pounds per acre for the mesoplots and twice that for the smaller plots for all the runoff events combined.

* Indications that runoff losses for all the other chemicals were much smaller than for the herbicides.

Wauchope says the results for atrazine and alachlor losses are higher than many previous studies indicate, but "we're manufacturing worst-case conditions by making it rain hard only 24 hours after application."

Overall, he says, the study has "demonstrated the ability of these methods to simulate runoff conditions that are not likely to happen, but on which pesticide regulations are based."

He also says EPA and pesticide manufacturers may use the rainfall simulation technology as pan of their programs to assess the risk of pesticide movement to water sources. If they do, it could save millions of dollars compared to current field methods, where researchers, like farmers, don't always get the rain when it's needed.--By Scan Adams, ARS.

Don Wauchope is in the USDA-ARS Nematodes, Weeds, and Crops Research Unit, Georgia Coastal Plain Experiment Station, P.O. Box 748, Tifton, GA 31793. Phone (912) 386-3892, fax number (912) 386-7225.
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Title Annotation:agricultural chemicals
Author:Adams, Sean
Publication:Agricultural Research
Date:May 1, 1993
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