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TECHNICAL REPORT
Ground Water Quality

Major Herbicides in Ground Water

Results from the National Water-Quality Assessment

^a U.S. Geological Survey (USGS), 1201 Pacific Ave., Suite 600, Tacoma, WA 98402
^b USGS, Placer Hall, 6000 J Street, Sacramento, CA 95819-6129
^c USGS, 400 S. Clinton St., Box 1230, Iowa City, IA 52244

Corresponding author (jbarbash{at}usgs.gov)

Received for publication January 4, 2000. To improve understanding of the factors affecting pesticide occurrence in ground water, patterns of detection were examined for selected herbicides, based primarily on results from the National Water-Quality Assessment (NAWQA) program. The NAWQA data were derived from 2227 sites (wells and springs) sampled in 20 major hydrologic basins across the USA from 1993 to 1995. Results are presented for six high-use herbicides—atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine), cyanazine (2-[4-chloro-6-ethylamino-1,3,5-triazin-2-yl]amino]-2-methylpropionitrile), simazine (2-chloro-4,6-bis[ethylamino]-s-triazine), alachlor (2-chloro-N-[2,6-diethylphenyl]-N-[methoxymethyl]acetamide), acetochlor (2-chloro-N-[ethoxymethyl]- N-[2-ethyl-6-methylphenyl]acetamide), and metolachlor (2-chloro-N-[2-ethyl-6-methylphenyl]-N-[2-methoxy-1-methylethyl]acetamide)— as well as for prometon (2,4-bis[isopropylamino]-6-methoxy-s-triazine), a nonagricultural herbicide detected frequently during the study. Concentrations were <1 µg L^-1 at 98% of the sites with detections, but exceeded drinking-water criteria (for atrazine) at two sites. In urban areas, frequencies of detection (at or above 0.01 µg L^-1) of atrazine, cyanazine, simazine, alachlor, and metolachlor in shallow ground water were positively correlated with their nonagricultural use nationwide (P < 0.05). Among different agricultural areas, frequencies of detection were positively correlated with nearby agricultural use for atrazine, cyanazine, alachlor, and metolachlor, but not simazine. Multivariate analysis demonstrated that for these five herbicides, frequencies of detection beneath agricultural areas were positively correlated with their agricultural use and persistence in aerobic soil. Acetochlor, an agricultural herbicide first registered in 1994 for use in the USA, was detected in shallow ground water by 1995, consistent with previous field-scale studies indicating that some pesticides may be detected in ground water within 1 yr following application. The NAWQA results agreed closely with those from other multistate studies with similar designs.

Abbreviations: a.i., active ingredient • CGAS, Ciba-Geigy Atrazine Study • DRASTIC, Depth to water, net Recharge, Aquifer media, Soil media, Topography, Impact of the unsaturated zone, and hydraulic Conductivity of the aquifer • HAL, lifetime health advisory level • LUS, land-use study (NAWQA study component) • MCL, maximum contaminant level • MDL, method detection limit • MMS, Metolachlor Monitoring Study • MWPS, Midwest Pesticide Study • NAWQA, National Water-Quality Assessment • NAWWS, National Alachlor Well-Water Survey • NPS, National Pesticide Survey • PMP, pesticide management plan • SUS, subunit survey (NAWQA study component) • t_1/2, half-life for transformation in aerobic soil • USDA-ARS, U.S. Department of Agriculture–Agricultural Research Service • USEPA, U.S. Environmental Protection Agency • USGS, U.S. Geological Survey

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