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 San Francisco Bay tidal marsh that is 30-km downstream of the New Almaden Quicksilver mining district, formerly the largest mercury mining district in North America. Pre-mining sediment concentrations were 80±30 ng g^-1, which are similar to pre-mining concentrations in cores taken from other parts of San Francisco Bay. Concentrations in the core increase to about 1200 ng g^-1, after a peak of mining activity in the early to mid 20th century. The extent of contamination from upstream mining activity appears to reflect the amount of processed ore disposed of at the surface and also from periods when mercury was recovered from reworking these surface ore dumps and open cuts. Although San Francisco estuary is contaminated with mercury from numerous historic mining sources, including late 19th century hydraulic gold mining in the Sierra Nevadas, the contamination in the southern reach of the estuary is predominantly from mining at New Almaden.