Measurements of lead isotope compositions in unfiltered San

[1] Because of mercury's (Hg) relatively high vapor pressure and long (0.5-2 years) atmospheric residence, there is the potential for long-range transport of contaminant Hg. Many studies have focused on that transport and deposition in central and eastern North America, Europe, and the Arctic, but there has been little research on the cycling of Hg in the western coast of North America. That deficiency is addressed in this preliminary study, which indicates there is long-range transport of Hg across the North Pacific. This transport is evidenced by the elevated (relative to equatorial and theoretical baseline) Hg concentrations in rainwater collected on the coast of California, as well as by the positive correlation between North Pacific storm tracks and Hg concentrations, with maximum concentrations associated with storms from 20°-40°latitude. Those tracks trace air masses containing industrial emissions with peak O 3 concentrations moving eastward off the Asian continent. The Asian fluxes appear to enhance Hg concentrations both directly, through the emission of particle-bound Hg and reactive Hg 2+ , and indirectly, by increasing the rate of oxidation of Hg 0 in the atmosphere. Superimposed on the trans-Pacific background of industrial Hg is a local signal, with elevated concentrations at the urban site relative to the more pristine coastal site in California. This secondary enrichment is tentatively attributed to elevated local emissions of redox species, including O 3 and its precursors, which increase oxidation rates of Hg 0 in the atmosphere and Hg concentrations in precipitation.

The New Almaden mercury mines in California (USA), which collectively represent the largest historic producers of mercury in North America, are a persistent source of mercury contamination to the San Francisco Bay estuary. An estimate based on total mercury concentration (Hg TOT ) and provisional stream flow data measured at a gauging station in the Guadalupe River during base flow conditions yields a base flow flux of 30 g of mercury for the month of October 2000. In contrast to this base flow estimate, one 2-day rain event in October 2000 resulted in a flux of 22 g of mercury past this site. An estimate of mercury transport from the entire Guadalupe River watershed based on a sediment transport model and our measured suspended particulate Hg TOT (0.5–4 μg g −1 ) results in a total of 4–30 kg year −1 transported to the southern reach of the estuary. Sediments in the southern reach have lower Hg TOT (most ≤0.4 μg g −1 dry wt) and monomethylmercury (MMHg, c. 1 ng g −1 dry wt) concentrations than those in the Guadalupe River (Hg TOT , 0.41–33 μg g −1 dry wt; MMHg, 1–10 ng g −1 dry wt). Because the most elevated methylmercury concentrations (8–12 ng g −1 dry wt) were found in sediments deposited immediately upstream of hydraulic structures (e.g. diversion dams and weirs) within the river, it is proposed that such physical structures may represent important zones of MMHg production and fluxes to San Francisco Bay.

No abstract

No abstract