NEW ISOTOPIC EVIDENCE FOR CHRONIC LEAD CONTAMINATION IN THE SAN FRANCISCO BAY AND IMPLICATIONS FOR PERSISTENCE OF PAST INDUSTRIAL LEAD EMISSIONS IN THE BIOSPHERE*

Douglas J. Steding¹, Charles E. Dunlap, and A. Russell Flegal

Earth Sciences and Environmental Toxicology

University of California, Santa Cruz

Santa Cruz, CA 95064

*Excerpted from an articlebeing published in the Proceedings of the National Academy of Sciences

¹Corresponding author: dsteding@es.ucsc.edu

Abstract

New insights into the cycling and long-term movement of anthropogenic lead in estuaries have been gained by monitoring lead concentrations and isotopic compositions in San Francisco Bay water from 1989 to 1998. Isotopic compositions of lead in the shallow (<5 m) southern reach (South Bay), which has limited hydraulic flushing, are invariant within a small range over the study period. Those compositions appear to be due to the diagenetic remobilization of historic lead deposits, and suggest that ~90% of lead in the water column is redistributed from past leaded gasoline emissions that are most characteristic of aeolian fluxes in the 1970s. Conversely, isotopic compositions of lead in the northern reach (Suisun and San Pablo bays) are variable on both seasonal and long-term time scales. Those variations appear to be primarily due to fluctuations in fluvial inputs of natural and anthropogenic lead from the Sacramento and San Joaquin rivers, which commingle with the benthic fluxes of lead within the estuary. During high-flow periods in the winter and spring, isotopic compositions within the northern reach shift towards that of the riparian signature of drainage from the Central Valley, discharged into the estuary from the San Joaquin and Sacramento rivers. The isotopic composition of this modern riparian flux is offset from that of the preindustrial (<1840, riparian flux to the bay as represented in pre-industrial Bay sediment cores), and represents a continuing mixture of hydraulic mining sediments and anthropogenic lead in the estuarine drainage basin. Long-term variations of lead isotopic compositions in the northern reach are correlated with ENSO associated variations in fluvial discharges to the estuary. Following the late 1980s drought in California, the early 1990s wet period (El Niño years) flushed a detectable pulse of 1980s leaded gasoline emissions into the system. This suggests that lead was initially accumulated in soils within the drainage basin during the drought and was then mobilized during the subsequent wet years. The isotopic composition of the northern reach during periods of high fluvial inputs is noticeably offset towards the composition of late 80s gasoline when compared to river samples during low flow periods.

Mass balance calculations suggest that only small fractions of the leaded gasoline fallout from the late 1980s (5-10%) have been washed out of the Sacramento and San Joaquin valleys drainage basin. The retardation of lead removal indicates that historic gasoline deposits from the drainage basin may be retained in the basin for decades. It is, also, in contraindication to the much more rapid decrease in lead contamination in other environmental matrices (e.g., oceanic surface waters, arctic snows, and human blood) that have been directly correlated with reductions in aeolian emissions from leaded gasoline.

Background

San Francisco Bay, which receives drainage from a watershed encompassing 40% of the land surface area of California, is heavily urbanized (Nichols et al., 1986) with elevated trace metal concentrations in sediments and surface waters arising from past industrial emissions and from present reductions in freshwater inflow due to water diversions (Flegal et al., 1996). The distribution of contaminants within the Bay corresponds with its two biogeochemical regions: a northern reach dominated by freshwater flushing from the inflow of the San Joaquin and Sacramento rivers and a shallower (< 2 m) southern reach with limited freshwater flushing. Both northern and southern reaches contain elevated surface water dissolved lead concentrations (» 0.050-2.0 nM) that have remained relatively constant over the past two decades (Gordon, 1980;Flegal et al., 1996; Kozelka et al., 1997). That consistency contrasts with the marked reduction in atmospheric emissions of leaded gasoline aerosols during the same period, with the phase-out of leaded gasoline in California initiated in the mid 1970s and completed by 1992. This decline is shown in records of the consumption of leaded gasoline in California, which peaked at over 50 billion liters in 1976 and then declined throughout the 1980s.

Methods

Samples of unfiltered surface waters were collected from throughout the Bay, using trace metal clean techniques (Patterson and Settle, 1976), during low-flow (summer) and high-flow (winter) hydraulic regimes from 1989 to 1998. Sampling sites were chosen to represent the widest range of hydrographic regimes: the major freshwater inputs to the Bay, seven sites in the North Bay, and four sites in the South Bay. Unfiltered samples were used since the isotopic compositions of filtered (0.45 mm) and unfiltered waters are indistinguishable in the Bay, a result of the effective scavenging of lead by suspended particulates (Dunlap et al., 2000).

Aliquots of trace metal samples extracted using the ammonium1- pyrrolidine dithiocarbamate / diethylammonium diethyldithiocarbamate (APDC/DDDC) (Bruland et al., 1985) were processed for lead isotopic composition analysis using high-purity reagents. The aliquots were dried and digested in aqua regia (prepared from quartz distilled acids) and hydrogen peroxide (Baker Ultrex 30%) to remove refractory organic compounds. The samples were then passed through anion exchange columns (Dowex 100) using hydrobromic acid (Seastar) to further purify the lead. Lead isotopic compositions were analyzed by thermal ionization mass spectrometry (VG 54-Warp).

Results and Discussion

The results of this study provide insight into the cycling of metals in a complex aquatic environment. In the southern reach of San Francisco Bay, elevated concentrations of trace metals exist despite recent efforts to eliminate emissions of those metals. This continuance is demonstrated in the cycling of lead in the South Bay, where the maximum flux of lead occurred during the peak in leaded gasoline consumption in the mid-1970s, and where that lead is still predominant in surface sediments and overlaying waters more than two decades later.

Moreover, 1970s gasoline lead is still predominant in the northern reach of the estuary, even though it is continually being flushed by freshwater discharges from the Sacramento and San Joaquin rivers. This persistence of industrial lead attests to its relatively long (decadal) residence time in the Bay. It also reflects the sensitivity of the Bay to pollution in California.

The cycling of lead and other particle reactive contaminants within San Francisco Bay is dominated by a complex interplay between benthic remobilization, freshwater inputs and Bay hydraulic dynamics. Since transport of particle-reactive pollutants through the Sacramento and San Joaquin rivers has been demonstrated to be a slow process, inputs of “clean” sediments to the Bay will only occur after the rivers themselves are cleaned. In the case of lead, this has not occurred despite the twenty years that have passed since the first efforts to phase out leaded gasoline (Dunlap et al., 2000).

The delayed transport of particle reactive contaminants in the Sacramento and San Joaquin rivers is further demonstrated by the addition of late 80s gasoline lead during high flow in the 90s: a small (5-10%) portion of the total lead emitted in the late 80s has entered the Bay during high flow events in the 1990s. Therefore, in addition to the over 2 decade old reservoir of 1970s gasoline lead, a large reservoir of late 1980s gasoline lead still resides in the river basins, waiting to be advected into the Bay during winter runoff. This late 1980s gasoline has also served to trace nonpoint source fluxes from the rivers’ drainage basin. Furthermore, in the future, when these inputs cease, there will still be considerable elevated lead levels within the Bay as a result of the internal recycling of lead.

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