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A radial‐arm collector well was at more risk than vertical wells, but MCLs were not exceeded in any well. A 175‐mi (282‐km) stretch of the Illinois River was studied to assess the potential effects of flooding on the quality of pumped groundwater. Water pumped from four municipal water supply wells (pumping rates, 140‐500 gpm [0.009‐0.03 m3/s]) was analyzed. Three of these wells were vertical wells located 175‐220 ft (53‐67 m) from the river's edge at normal pool stages. The fourth well was a radial‐arm collector well with laterals extending beneath the river bed. Although concentrations of atrazine in the Illinois River during peak flows of 1995 and 1996 were as high as 6 and 12 μg/L, respectively, the concentration of atrazine in groundwater pumped at the three vertical‐well sites remained below the 0.1‐μg/L detection limit. In 1996 a small breakthrough of atrazine was observed in the collector well water. The high concentration of nitrate in river water appeared to have slightly increased the nitrate in pumped groundwater at two locations. Dilution and other factors may have affected the transport of river water contaminants in the alluvial aquifer between the river and the pumping wells.

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