Investigation of the Accuracy of Monomethyl Mercury Measurements in Rainwater in the
Presence of Increased Inorganic Mercury

Kate McPeek, Lucas Hawkins
Corresponding author - Frontier Geosciences, 414 Pontius Avenue North, Seattle WA 98109

Eric Prestbo
Tekran Canada R&D Facility, 330 Nantucket Blvd., Toronto, ON M1P2P4, Canada.

Measurements of mercury in rainwater are an important tool used to assess deposition rates into watersheds and other sensitive environments. Monomethyl mercury (MMHg) is of special concern due to its known toxicity to living organisms. For eight years MMHg has been measured at many National Atmospheric Deposition Program/Mercury Deposition Network (NADP/MDN) sites. This study investigates standard NADP/MDN sampling protocol for potential enrichment of wet MMHg deposition. For this experiment, inorganic mercury (Hg2+) was added to rain collectors prior to deployment to test for a positive MMHg bias. Each collection event consisted of three samples spiked with 10 ng of Hg2+ and three control samples containing no Hg2+ spike. All samples were analyzed for total and monomethyl mercury via cold vapor atomic fluorescence spectrometry (CVAFS). A total of five weeklong sampling events resulting in 30 samples were collected in Seattle, WA from May 2006 to November 2006. The outside temperature during collection events ranged from 40-70°F. Results were evaluated using three statistical tests: paired t-test (mean delta=0.008ng/L, p=0.4579), linear regression (slope=1.0843), and linear regression with intercept forced through zero (slope=0.960). Preliminary results indicate that there is not enrichment of MMHg.