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! Introduction ! Methods ^ Sampling design 3 Data-collection techniques 3 Analytical procedures 3 Quality assurance 10 Discussion 11 References cited 17 ILLUSTRATIONS Figures 1-5. ~ Map showing: 1. Location of study area, hydrologic units, and sites sampled during 1989 and 1990 in the midwestern United States 2 2. Location of sites sampled in South Dakota, Nebraska, and Kansas, map numbers 1 through 30 8 3. Location of sites sampled in Minnesota, Iowa, and Missouri, map numbers 31 through 78 9 4. Location of sites sampled in Wisconsin and Illinois, map numbers 79 through 115 10 5. Location of sites sampled in Indiana and Ohio, map numbers 116 through 147 11 Figures 6-9. ~ Graph showing: 6. Concentrations of alachlor during 1989-90 reconnaissance study 18 7. Concentrations of atrazine during 1989-90 reconnaissance study 19 8. Concentrations of cyanazine during 1989-90 reconnaissance study 20 9. Concentrations of metolachlor during 1989-90 reconnaissance study 21 TABLES Table 1. Quantities of four major herbicides applied in 10 agricultural Midwestern States 1987-89 2 2. Sampling-site names, locations, and drainage areas 4 3. Concentrations of selected herbicides and two atrazine metabolites in qualityassurance samples, 1989-90 12

Reconnaissance data for selected herbicides, two atrazine metabolities, and nitrate in surface water of the Midwestern United States, 1989-90