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During spring runoff events, herbicides in the Platte River are transported through an alluvial aquifer into collector wells located on an island in the river in 6 to 7 d. During two spring runoff events in1995 and 1996, atrazine [2-chloro-4-ethylamino-6-isopropylaminos-triazinel concentrations in water from these wells reached approximately7 mug/L, 70 times more than the background concentration in ground water. Concentrations of herbicides and metabolites in the collector wells generally were one-half to one-fifth the concentrations of herbicides in the river for atrazine, alachlor [2-chloro-2'-6'-diethylN-(methoxymethyl)-acetanifi del, alachlor ethane-sulfonic add (ESA) [2-((2,6-diethylphenyt) (methoxymethyl)amino)-2-oxoethane-sulfonic acidl, metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy- l-methylethyl)acetamidel, cyanazine [2-((4-chloro-6-(ethylamino)-1,3,5 triazin-2-yl)-amino)-2-methylpropionitrilel, and acetochlor [2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6methyl-phen yl) acetamidel, suggesting that20 to 50% river water could be present in the water from the collector wells, assuming no degradation. The effect of the river on the quality of water from the collector wells can be reduced through selective management of horizontal laterals of the collector wells. The quality of the water from the collector wells is dependent on the (i) selection of the collector well used, (ii) number and selection of laterals used, (iii) chemical characteristics of the contaminant, and (iv)relative mixing of the Platte River and a major upstream tributary. [ABSTRACT FROM AUTHOR]