Temporal variability in metal concentrations in a mine-impacted stream: Implications for metal bioavailability

Variability in total and dissolved metal concentrations and dissolved organic carbon (DOC) in a stream impacted by a cobalt/copper mine were determined on several temporal scales, including hourly, daily, weekly, seasonally, and annually. Stream samples were collected during 1993 and 1994 spring runoff and 1993 low-flow conditions. Concentrations of mine-released metals varied from approximately one order of magnitude within a 24-hour period to several orders of magnitude seasonally. During spring runoff, dissolved metal concentrations peaked before total metal concentrations. Total metal concentrations tended to vary more widely on all temporal scales than did dissolved concentrations. Temporal changes in DOC concentrations did not follow those of metals: when metal concentrations were highest (early spring runoff), generally DOC concentrations were lowest. DOC concentrations increased as metal concentrations decreased during later spring runoff. The results show the temporal variability in metals bioavailability and demonstrate the importance of accounting for temporal variability in surface water sampling design and toxicity evaluations.