Historical trends of metals in the sediments of San Francisco Bay, California

Concentrations of Ag, Al, Cr, Cu, Fe, Hg, Mn, Ni, Pb, V and Zn were determi ned in six sediment cores from San Francisco Bay (SFB) and one sediment cor e in Tomales Bay (TB), a reference estuary. SFB cores were collected from b etween the head of the estuary and its mouth (Grizzly Bay, GB; San Pablo Ba y, SP; Central Bay, CB; Richardson Bay, RE, respectively) and ranged in len gth from 150 to 250 cm. Concentrations of Cr, V and Ni are greater than mea n crustal content in SEE and TB sediments, and greater than found in many o ther coastal sediments. However, erosion of ultramafic rock formations in t he watershed appears to be the predominant source. Baseline concentrations of other metals were determined from horizons deposited before sediments we re influenced by human activities and by comparing concentrations to those in TB. Baseline concentrations of Cu co-varied with Al in the SFB sediments and ranged from 23.7 +/- 1.2 mu g/g to 41.4 +/- 2.4 mu g/g. Baseline conce ntrations of other metals were less variable: Ag, 0.09 +/- 0.02 mu g/g; Pb, 5.2 +/- 0.7 mu g/g; Hg, 0.06 +/- 0.01 mu g/g; Zn, 78 +/- 7 mu g/g. The ear liest anthropogenic influence on metal concentrations appeared as Hg contam ination (0.3-0.4 mu g/g) in sediments deposited at SP between 1850 and 1880 , apparently associated with debris from hydraulic gold mining. Maximum con centrations of Hg within the cores were 20 times baseline. Greater inventor ies of Hg at SP and GB than at RE verified the importance of mining in the watershed as a source. Enrichment of Ag, Pb, Cu and Zn first appeared after 1910 in the RE core, later than is observed in Europe or eastern North Ame rica. Maximum concentrations of Ag and Pb were 5-10 times baseline and Cu a nd Zn concentrations were less than three times baseline. Large inventories of Pb to the sediments in the GB and SP cores appeared to be the result of the proximity to a large Pb smelter. Inventories of Pb at RE are similar t o those typical of atmospheric inputs, although influence from the Pb smelt er is also suspected. Concentrations of Hg and Pb have decreased since the 1970s (to 0.30 mu g/g and 25 mu g/g, respectively) and were similar among a ll cores in 1990. Early Ag contamination was perhaps a byproduct of the Pb smelting process, but a modern source of Ag is also indicated, especially a t RE and CB. (C) 1999 Elsevier Science B.V. All rights reserved.