MP12 Organic, Metallic, Organometallic Pollutants
Monday, 14 November 2005: 8:00 AM - 6:30 PM in Exhibit Hall

(MCK-1117-742801) Mercury and methyl mercury loads entering San Francisco Bay from the New Almaden historic mercury mines.

McKee, L¹, Leatherbarrow, J¹, ¹ San Francisco Estuary Institue, Oakland, CA, United States

ABSTRACT- San Francisco Bay is listed by the State of California as impaired for mercury (Hg). The largest proportion of current Hg loads to the Bay is derived either directly or indirectly from the historic New Almaden Mining District, the largest producer of Hg in North America between 1850 and 1975. This paper presents Hg monitoring data collected from autumn 2002 to spring 2005 during intensive sampling during floods on the Guadalupe River downstream from the historic mines and greater San Jose. During the first two years of sampling, total Hg (HgT) was measured along with suspended sediment concentration (SSC), other trace metals, and organic carbon. The third year included total dissolved Hg (HgD), total methyl Hg (MeHgT), and dissolved methyl Hg (MeHgD). Since Hg is mostly transported with sediment particles, a turbidity sensor was used to derive 15-minute SSC data to help interpret Hg release and transport processes. Hg measured in bed load accounted for <5% of the total load. To date, 105 Hg samples have been collected during the full range of flow conditions. Maximum concentrations of HgT and MeHgT were 19,000 ng/L and 2 ng/L, respectively. These elevated concentrations reflect the dominant influence of mercury release from the mining area and exceed previously observed concentrations in river systems polluted from urbanization alone. Due to differences in climatic forcing and mercury release, annual water column HgT loads over the period of study have ranged between approximately 9 kg and 116 kg. We suggest that maximum water column HgT loads might be 10 times greater during the wettest years. These new HgT load estimates have helped refine existing perceptions about the supply of mercury to the Bay and prioritize management actions aimed at reducing water quality impairment.

Key words: mercury, transport, mining, San Francisco Bay