Seasonal and storm-scale variations in heavy metal concentrations of two mining-contaminated streams, Montana, U.S.A.

Abstract

The impacts of spring runoff and short-term storm events on water quality are poorly documented. We used ultraclean methods to examine the filterable (<0.2μm) and total recoverable geochemistry of one small and one large mining-impacted stream in Montana, USA, over 2 years. At the small stream, filterable Fe, Mn, S, and Zn were higher during spring runoff than during the rest of the year. The highest concentrations of As, Cu, Fe, and Mn in both years occurred following a particularly large summer rainstorm. At the large stream, there were concentration spikes for Al, As, Cu, Fe, Mn, and Zn during an early snowmelt episode and on the rising limb of the main runoff discharge peak in 1999. In 2000 there were large increases in Al, As, Cu, Fe, Mn, P, and Ti concentrations after spring precipitation events. During the late summer rains in 2000, there were 3-8 fold increases in total recoverable Al, Cu, Fe, Mn, and Zn. We show that some of the strongest geochemical variations in mining-impacted streams can occur during spring runoff and summer rain events, both of which can be short-lived and easily missed in monitoring programs based on quarterly or monthly sampling designs.