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Daily event precipitation samples were collected from 1995 to 2006 in Underhill, VT, USA and analyzed for mercury (Hg) and trace elements. Over the 12-year period, annual Hg deposition levels at Underhill did not decline significantly despite regulatory efforts to reduce Hg emissions in the United States. Mercury and trace element wet deposition data were examined using the multivariate receptor model EPA PMF 3.0 to identify the sources contributing to Hg deposition at the Underhill site. Results indicate that coal combustion, a mixture of incineration and non-ferrous metal smelting, and a phosphorus source contributed to the observed Hg wet deposition. The model consistently indicated that coal combustion contributed ∼60% of the total Hg measured in wet deposition and the contribution from this source did not change significantly over time. Mercury and trace element deposition were further analyzed using a hybrid-receptor model, quantitative transport bias analysis (QTBA), to identify the likely source locations contributing to Hg deposition. Results from QTBA indicate that the majority of Hg deposition at Underhill is due to transport from the Midwestern United States, where the density of coal-fired utility boilers in the U.S. is largest. |
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