Annual maxima in Zn concentrations during spring snowmelt in streams impacted by mine drainage

Long-term hydrochemical monitoring and 2 years of intensive sampling were u sed to identify annual patterns in Zn export from three neighboring catchme nts in Summit County, Colorado. These catchments are characterized by a sno wmelt-dominated hydrologic cycle, but range in the level of mining impact f rom little to severe. Zn concentrations increased during snowmelt along str eam reaches with a history of mining, but were diluted by snowmelt where me tals originated in widely disseminated pyrite in the host rock of the catch ment. Inter-site differences in the relationship between Zn and sulfate, to gether with inter-annual variability in the timing and magnitude of peak Zn concentrations suggest that a portion of the Zn flush is retarded, perhaps through interaction with cation exchange sites in soil. Although Dissolved Organic Carbon (DOC) concentrations also increase during snowmelt, there w as no indication that the export of Zn was facilitated by the flush of orga nic carbon.