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Occurrence and Fate of Pesticides in Four Contrasting Agricultural Settings in the United States

^a U.S. Geological Survey, 5231 South 19th St., Lincoln, NE 68512
^b U.S. Geological Survey, 2130 SW 5th Ave., Portland, OR 97201
^c U.S. Geological Survey, Box 25046 Denver Federal Center, Lakewood, CO 80225
^d U.S. Geological Survey, 122 Civil Engineering Building, 500 Pillsbury Drive, SE, Minneapolis, MN 55455
^e U.S. Geological Survey, 934 Broadway, Suite 300, Tacoma, WA 98402

^* Corresponding author (gvsteele{at}usgs.gov).

Received for publication April 3, 2007. Occurrence and fate of 45 pesticides and 40 pesticide degradates were investigated in four contrasting agricultural settings—in Maryland, Nebraska, California, and Washington. Primary crops included corn at all sites, soybeans in Maryland, orchards in California and Washington, and vineyards in Washington. Pesticides and pesticide degradates detected in water samples from all four areas were predominantly from two classes of herbicides—triazines and chloroacetanilides; insecticides and fungicides were not present in the shallow ground water. In most samples, pesticide degradates greatly exceeded the concentrations of parent pesticide. In samples from Nebraska, the parent pesticide atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine] was about the same concentration as the degradate, but in samples from Maryland and California atrazine concentrations were substantially smaller than its degradate. Simazine [6-chloro-N,N'-diethyl-1,3,5-triazine-2,4-diamine], the second most detected triazine, was detected in ground water from Maryland, California, and Washington. Metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] rarely was detected without its degradates, and when they were detected in the same sample metolachlor always had smaller concentrations. The Root-Zone Water-Quality Model was used to examine the occurrence and fate of metolachlor at the Maryland site. Simulations accurately predicted which metolachlor degradate would be predominant in the unsaturated zone. In analyses of relations among redox indicators and pesticide variance, apparent age, concentrations of dissolved oxygen, and excess nitrogen gas (from denitrification) were important indicators of the presence and concentration of pesticides in these ground water systems.

Abbreviations: ESA, ethanesulfonic acid • f_oc, mass fraction of organic carbon in soil (dimensionless) • K_oc, soil organic carbon-water partition coefficient • OXA, oxanilic acid • RZWQM, root-zone water-quality model • SAA, sufinylacetic acid • t_{1/2, soil}, soil dissipation half-life • UZ, unsaturated zone

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