New isotopic evidence for chronic lead contamination in the San Francisco Bay estuary system: implications for the persistence of past industrial lead emissions in the biosphere

Abstract

Measurements of lead isotope compositions in unfiltered San Francisco Bay waters from 1989 to 1998 have brought new insights into the cycling of anthropogenic lead in estuaries. Isotopic compositions of lead in the shallow (<2 m) southern reach were essentially invariant ( approximately 90% derived from 1960s-1970s leaded gasoline) during the study period because of limited hydraulic flushing and the remobilization of lead from bottom sediments. In contrast, in the northern reach freshwater flushing from the San Joaquin and Sacramento rivers produced seasonal and decadal variations in lead isotope compositions. The seasonal shifts are attributed to advection of soils containing late 1980s gasoline lead into the bay during winter rains. Mass balance calculations indicate that only a small fraction (1-10%) of this leaded gasoline fallout from the late 1980s has been washed out of the San Joaquin and Sacramento rivers' drainage basin by 1995. Superimposed on this seasonal cycling was a long-term systematic shift in the component of gasoline lead expressed in the river systems, with a small ( approximately 5-10%) decrease in the amount of 1960s-1970s gasoline lead in river and North Bay waters. The retention of gasoline lead in the river systems draining into the bay as well as San Francisco Bay sediments indicates that historic gasoline deposits may remain in the combined riparian/estuarine system for decades. Such a persistence is in contraindication to recent reports of rapid (annual) decreases in lead contamination in other environments, and the link between climate and contaminant transport suggests local or global climate change will have an impact on contaminant distribution and fate.

Figure 1

Sampling sites in San Francisco Bay. The seven sites in the northern reach are shown as white circles; the three sites in the South Bay are shown as yellow circles; and the two fluvial input sites are shown as green circles. The Central Valley covers 40% of the land surface area of California and is the drainage basin of the Sacramento and San Joaquin rivers; it is shown in green on the Inset.

Figure 2

Historic leaded gasoline consumption and lead concentrations in northern reach waters. The blue line charts leaded gasoline consumption in liters for the state of California from 1960 to 1992 (personal communication, J. Cogan, Energy Information Administration). Consumption is estimated for the years 1960–1970 based on the available data from 1960 and 1970. Concentrations of dissolved lead for the northern reach are given for 1979 (23), 1989, 1993, and 1995. Concentrations for each year are the average of three stations in the northern reach during the summer when the waters at each station had comparable salinities. Error bars are one standard deviation of the three station averages.

Figure 3

Daily total discharge of the Sacramento and San Joaquin rivers from 1989 to 1999. Sampling events in this study are plotted as red lines.

Figure 4

Lead isotopic compositions of bay waters from 1989 to 1998. Fields are drawn around waters from the northern reach (white), South Bay (yellow), and rivers (green). Lead isotopic compositions in all waters sampled in 1989 fall within a field occupied by South Bay samples during the entire study period (see text for discussion).

Figure 5

Seasonal shifts in lead isotope compositions in bay waters compared with the generalized flow rate of the Sacramento and San Joaquin rivers. A field is drawn around the isotopic compositions of waters sampled in each collection period. The highest ²⁰⁶Pb/²⁰⁷Pb values recorded in each collection period are found in river waters, whereas the lowest ²⁰⁶Pb/²⁰⁷Pb values are found in South Bay water samples (compare Fig. 4).

Figure 6

Lead isotopic compositions of San Francisco Bay waters and potential lead sources. Bay waters are shown as fields that encompass the waters collected during low flow (brown outline) and high flow (blue outline). A third field (black outline) surrounds all San Francisco Bay waters collected in 1989 and waters in the South Bay throughout the study period. Possible sources of lead to the bay include 1960s–1970s leaded gasoline (dark red), 1980s leaded gasoline (light red) (, –44), preindustrial bay sediment as recorded in the bottoms of sediment cores (19), and a component representing hydraulic mine sediments (24).