Diel variation of trace metals in the upper Clark Fork River, Montana

Hourly sampling of an oxic, slightly alkaline river with high concentrations of trace metals stored in bed and flood plain sediments reveals diel cycles in dissolved Mn and Zn and acid-soluble particulate Al, Fe, Mn, Cu, and Zn. Metal concentrations increase 2-3-fold at night as pH and dissolved oxygen decrease. Dissolved Win and Zn cycles may be the result of redox reactions in river-bed sediments or variations in influx of water from the hyporheic zone as a result of evapotranspiration. Increased particulate metal concentrations result from an increase in total suspended matter at night. Particulate metal concentrations decrease several hours before the decline of dissolved concentrations in the morning. Major elements and ions including Ca, Mg, Na, Si, Cl-, SO42-, and total alkalinity show no evidence of diel variation. The results have implications for monitoring and assessment of reclamation because they show that daytime sampling underestimates the flux of metals in the river.