Geochemical response to variable streamflow conditions in contaminated and uncontaminated streams

[1] Seasonal variations in stream inorganic geochemistry are not well documented or understood. We sampled two mining-impacted and two relatively pristine streams in western Montana over a 12-month period, collecting samples every 4 weeks, with supplemental sampling (at least weekly) during spring runoff. We analyzed all samples for dissolved (operationally defined as <0.2 μm) and total recoverable concentrations. Generally, the trace elements (Al, As, Cu, Fe, Mn, and Zn) did not correlate linearly with streamflow, while the major elements (e.g., Ca, K, and Mg) did. Suspended sediment, total recoverable metals, and H+ followed clockwise hysteresis rotations, driven by short-term flushing events during the very early stages of spring runoff. Mining-impacted sites had higher concentrations of many trace elements than did relatively pristine sites. One of the mining impacted sites exhibited strong geochemical responses to spring rain events in the basin. The results underscore the need to sample streams frequently during changing hydrologic and climatic conditions in order to accurately monitor surface water quality and to determine solute and particulate loads (both contaminant and noncontaminant).