MERCURY DEPOSITION IN A TIDAL MARSH DOWNSTREAM OF THE HISTORIC NEW ALMADEN MINING DISTRICT, CALIFORNIA

A record of mercury deposition was provided by sediment recovered from piston cores of a south San Francisco Bay tidal marsh that is 30-km downstream of the New Almaden mining district, formerly the largest mercury mining district in North America. Pre-mining sediment mercury concentrations were 80±30 ng/g, which are similar to pre-mining concentrations in cores taken from other parts of San Francisco Estuary. Concentrations in the core increase to a maximum of about 1200 ng/g, corresponding to a period in the mid-20th century, nearly 50 years after the peak in mercury production at the mines. The extent of contamination from upstream mining activity appears to reflect the amount of processed ore disposed of at the surface, and also periods when mercury was recovered from reworking these surface ore dumps and open cuts. Transport of this contaminant mercury to the tidal marsh appears to be influenced by hydrologic modifications in the watershed, including dam building and subsidence related to groundwater withdrawal. Although San Francisco Estuary is contaminated with mercury from numerous historic mining sources, including late 19th century hydraulic gold mining in the Sierra Nevada, there is little evidence of pre-mining contamination from natural mercury sources in the southern reach of San Francisco Estuary.