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Waste rock and ore associated with Hg, precious and base metal mining, and their surrounding host rocks are typically enriched in mercury relative to natural background concentrations (<0.1 lg Hg g -1 ). Mercury fluxes to the atmosphere from mineralized areas can range from background rates (0-15 ng m -2 h -1 ) to tens of thousands of ng m -2 h -1 . Mercury enriched substrate constitutes a long-term source of mercury to the global atmospheric mercury pool. Mercury emissions from substrate are influenced by light, temperature, precipitation, and substrate mercury concentration, and occur during the day and night. Light-enhanced emissions are driven by two processes: desorption of elemental mercury accumulated at the soil:air interface, and photo reduction of mercury containing phases. To determine the need for and effectiveness of regulatory controls on short-lived anthropogenic point sources the contribution of mercury from geologic non-point sources to the atmospheric mercury pool needs to be quantified. The atmospheric mercury contribution from small areas of mining disturbance with relatively high mercury concentrations are, in general, less than that from surrounding large areas of low levels of mercury enrichment. In the arid to semi-arid western United States volatilization is the primary means by which mercury is released from enriched sites.

Air/surface exchange of mercury was investigated over background soils at five sites in the Tahquamenon River watershed in the Michigan Upper Peninsula in the summer of 1998. Measurements of Hg fluxes were performed during midday periods using the ORNL Teflon dynamic flux chamber. Mean Hg emission fluxes were 1.4±0.3-2.4±1.0 ng m −2 hr −1 for three shaded forest sites and 7.6±1.7 ng m −2 hr −1 for an open field site. Hg dry deposition was observed at a heavily shaded forest site over wet soils (mean =-0.3±0.2 ng m −2 hr −1). The overall mean Hg flux was 1.4±1.4 ng m −2 hr −1 for the four shaded forest sites. The Hg fluxes observed at these sites are similar to those found at other northern background sites. Significant, rapid response of Hg emission to solar radiation was observed over these background soils. Artificial irrigation over these soils induced immediate and measurable increases in Hg emission. Soil temperature was found to be less influential to Hg air/surface exchange over these heavily shaded forest background soils than we have seen elsewhere.

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