Spatial and temporal dynamics of mercury in Precambrian Shield upland runoff

Methylated and total Hg, and TOC concentrations were measured in precipitat ion and runoff in a first order Precambrian Shield watershed, and in precip itation, throughfall, shallow groundwater and runoff in a zero Precambrian Shield watershed. Plots dominated by open lichen-covered bedrock and anothe r containing small patches of conifer forest and thin discontinuous surfici al deposits were monitored within the zero order catchment. Methyl (3-10 fo ld) and non-methyl (1.4-2.8 fold) Hg concentrations changed irregularly dur ing rainfall and snowmelt runoff events in all catchments. Temporal pattern s of Hg concentration in runoff included flushing and subsequent dilution a s well as peak concentrations coinciding with peak or recession flow. Mercu ry export was highest from lichen-covered bedrock surfaces as a result of h igh runoff yields and minimal opportunity for physical retention and in the case of MeHg demethylation. Forest canopy and lichen/bedrock surfaces were often net sources for Hg while forest soils were mostly sinks. However, up land soils undergoing periodic reducing conditions appear to be sites for t he in situ production of MeHg.