Major herbicides in ground water: Results from the National Water-Quality Assessment

To improve understanding of the factors affecting pesticide occurrence in g round water, patterns of detection were examined for selected herbicides, b ased 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. Res ults are presented for six high use herbicides-atrazine (2-chloro-4-ethylam ino-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[eth ylamino]-s-triazine), acetochlor (2-chloro-N-[2,6-diethylphenyl]-N-[methoxy methyl]acetamide), acetochlor (2-chloro-N-[ethoxymethyl]-[2-ethyl-6-methylp henyl]-N- [2-methoxy-1-methylethyl]acetamide)-as well as for prometon (2,4- bis[isopropylamino]-6-methoxy-s-triazine), a nonagricultural herbicide dete cted frequently during the study. Concentrations,were <1 mug L-1 at 98% of the sites with detections, but exceeded drinking-water criteria (for atrazi ne) at two sites. In urban areas, frequencies of detection (at or above 0.0 1 mug L-1) of atrazine, cyanazine, simazine, alachlor, and metolachlor in s hallow ground water were positively correlated with their nonagricultural u se nationwide (P < 0.05). Among different agricultural areas, fre quencies of detection were positively correlated with nearby agricultural use for at razine, cyanazine, alachlor, and metolachlor, but not simazine. Multivariat e analysis demonstrated that for these five herbi cides, frequencies of det ection beneath agricultural areas were positively correlated,vith their agr icultural use and persistence in aerobic soil. Acetochlor, an agricultural herbicide first registered in 1994 for use in the USA, was detected in shal low ground water by 1995, consistent with previous held-scale studies indic ating that some pesticides may be detected in ground water within 1 yr foll owing application. The NAWQA results agreed closely,vith those from other m ultistate studies with similar designs.