GROUNDWATER AS A NONPOINT-SOURCE OF ATRAZINE AND DEETHYLATRAZINE IN ARIVER DURING BASE-FLOW CONDITIONS

Alluvial groundwater adjacent to the main stem river is the principal nonpoint source of atrazine and deethylatrazine in the Cedar River of Iowa after the river has been in base flow conditions for 5 days. Betw een two sites along a 116-km reach of the Cedar River, tributaries con tributed about 25% of the increase in the atrazine and deethylatrazine load, whereas groundwater from the alluvial aquifer contributed at le ast 75% of the increase in load. Within the study area, tributaries ag gregate almost all of the discharge from tile drains, and yet the trib utaries still only contribute 25% of the increase in loads in the main stem river. At an unfarmed study site adjacent to the Cedar River, th e sources of atrazine and deethylatrazine in the alluvial groundwater are bank storage of river water and groundwater recharge from areas di stant from the river. Atrazine and deethylatrazine associated with ban k storage water will provide larger concentrations to the river during early base flow conditions. After the depletion of bank storage, stab le and smaller concentrations of atrazine and deethylatrazine, origina ting from groundwater recharge, continue to be discharged from the all uvial aquifer to the river; thus these results indicate that alluvial aquifers are an important nonpoint source of atrazine and deethylatraz ine in rivers during base flow.