Temporal and Spatial Variations in Trace Element Cycles in San Francisco Bay


The "Queen of Clean", Genine Scelfo, collecting water for trace metal analyses in San Francisco Bay. The water is sucked up through acid (sub-boiling quartz distilled) cleaned Teflon tubing using a peristaltic pump (to preclude exposure to other, less clean surfaces), and then forced through an acid cleaned filter (0.45 m) cartridge to remove particulate material (actually some particulate material and colloidal material pass through the filter). Since the filtered water also includes dissolved complexes, the concentrations that we measure are operationally termed "total dissolved". This includes small (<0.45 m) particulate, colloidal, complexes, and ionic concentrations. For a detailed discussion on the distribution of lead among all of those phases see: Kozelka, P.B., S. Saudo-Wilhelmy, A.R. Flegal, and K.W. Bruland. 1997. Physico-chemical speciation of lead in South San Francisco Bay. Estuarine, Coastal and Shelf Science 44: 649-658.


The San Francisco Bay Program has established a systematic, and relatively comprehensive, survey of temporal and spatial distributions of trace elements in the estuary. The program has replaced a hodgepodge of disparate surveys that used a variety of methods to measure different trace element concentrations in different parts of the estuary during different periods. With the new program, it is now possible to identify areas with anomalously high trace element concentrations in the estuary, determine the factors causing that pollution, and develop means to control it. As such, the San Francisco Bay Program now serves as the national model, promulgated the United States Environmental Protection Agency Office of Water, for monitoring trace element contamination in aquatic systems.

Establishing A Systematic Program of Sampling and Analyses

Previous programs to investigate trace element contamination in San Francisco Bay were characterized by their limited scope and analytical inconsistencies. Without a system-wide perspective, the studies were usually limited to a few measurements in a specific region of the estuary within a small time period. The parochialism of those sampling designs precluded a comprehensive assessment of the state of the estuary. Previous reports of trace element concentrations in the estuary also varied by orders of magnitude, because of inconsistencies among the sampling and analytical techniques employed in different studies. Consequently, it was impossible to derive even a superficial perspective of the distribution of trace elements in the estuary through a composite analysis of data collected in those previous programs.

Those problems have been resolved with the creation of a systematic sampling program for the entire estuary. Since samples are collected from the freshwater confluence of the Sacramento and San Joaquin rivers at the northernmost reach of the estuary down to sloughs in the southernmost reach of the estuary, pronounced spatial differences in some trace element concentrations are readily apparent in plots of those data. Since three sets of samples are collected each year and the program has been in place for several years, seasonal and annual variations in the spatial gradients are also readily apparent. Finally, since rigorous sampling and analytical protocols have been utilized in the generation of all of those data, comparisons of spatial and temporal variations in the trace element data are not circumspect.

Identification of Areas of Concern

The program has identified two principal problems with trace element contamination in the estuary. Both copper and nickel concentrations exceed water quality criteria in the southern reaches of the estuary during some periods in summers when freshwater discharges to the estuary are lowest. These seasonal increases have been highest during drought years when freshwater discharges to the estuary have been minimal.

Consequently, trace element contamination in the southern reach of the estuary appears to be caused by a combination of local and system-wide factors (Flegal et al., 1996). These include (1) local inputs from wastewater outfalls into the South Bay, (2) temporal reductions in freshwater discharges from the Sacramento and San Joaquin rivers, (3) surface runoff from areas surrounding the South Bay, and (4) seasonal releases from contaminated sediments within the South Bay. The following sections briefly summarize the impacts of each of those factors.

Wastewater discharges

While a cursory analysis of the monitoring data supports the common public perception that wastewater discharges are responsible for pollution in the San Francisco Bay estuary, a more detailed analysis shows that inputs from those discharges only account for some of the elevated trace element concentrations within the estuary (Flegal et al., 1991; 1996). For example, the seasonally high concentrations of copper and nickel in the southern reach of the estuary fit a simple dilution mixing line between the concentrations of those elements in sea water and in wastewater discharges into the South Bay (Flegal et al., 1991), but more rigorous mass balance calculations and geochemical analyses demonstrate that inputs of trace elements, including copper and nickel, from other sources may be equal to or in some cases exceed inputs from wastewater discharges into that area. Those analyses are corroborated by similarities in trace element excesses in the northern reach of the estuary for the past two decades. Similarly, internal excesses of trace element concentrations in the northern reach of the estuary Moreover, comparisons

Temporal reductions in fluvial discharges from the northern reach of the estuary

Many trace element concentrations in the South Bay vary inversely with the volume of freshwater discharges into the Sacramento and San Joaquin rivers (Flegal et al., 1991; Smith and Flegal, 1993. Notably, copper and nickel concentrations often exceed water quality criteria in the South Bay during summer periods when riverine flows to the estuary are minimal. This pattern corresponds with the hydraulic flushing model proposed by Peterson et al. (19xx) to account for the similar temporal variations of nutrient concentrations in the estuary three decades ago. Consequently, elevated trace metal concentrations in the South Bay may be controlled by fresh water discharges to the northern reach of the estuary.

As a result, problems of copper and nickel contamination in the South Bay may be diminished by actions that address each or all of those factors. This creates a series of options that may be evaluated in terms of their relative efficacy and cost. For example, the feasibility of reducing surface runoff of copper by controlling its use in brake pads is being considered as a cost-effective means of decreasing copper concentrations in the South Bay, because waste water discharges of copper to the South Bay have already been decreased by 95% and additional reductions may be prohibitively expensive (Brake Pad Forum, 1996).

Catalysis of Complementary Studies

By identifying areas of concern, the monitoring program has catalyzed additional studies that specifically address those concerns. Notable among these are studies to determine whether the water quality criteria for copper in the South Bay are appropriate. These include measurements of the chemical speciation of copper, which show that a large fraction of the dissolved copper in the South Bay is in forms that are not readily available to the biota (Donat and Bruland, 1993 ; Sanudo-Wilhelmy et al., 1996). There have also been studies with estimates of the relative importance of inputs of copper to the South Bay from waste water discharges and contaminated sediments, which indicate that both contribute to the elevated levels observed during the summer period (Flegal et al., 1996; Kuwabara et al., 1996; Rivera-Duarte and Flegal, 1996). These, and similar studies (e.g., Abu-Saba and Flegal, 1995, 1997; Rivera-Duarte and Flegal, 1995, 1996, 1997; Smith and Flegal, 1995; Woods et al., 1993), have facilitated the development of appropriate actions to address those areas of concern.

The complementary studies have benefited from the monitoring in three other ways. First, the program has provided an efficient means to collect additional samples and conduct complementary analyses. Second, the programs sampling cruises have often been coordinated with those of other programs (e.g., United States Geological Survey) to expand the breadth and scope of the collections. Third, the data from the program have been provided for evaluation and modeling by other groups (e.g., Stanford, University of California, United States Geological Survey) interested in the cycling of trace elements in the estuary. Consequently, the data base for trace elements in the estuary is now much greater than one generated by the regional program and numerous groups are involved in analyses of that expanded data base.

The National Model for Monitoring Trace Elements in Aquatic Systems

The success of the San Francisco Program has been recognized on a national level. The Office of Water of the United States Environmental Protection Agency has developed new protocols for measuring trace elements in aquatic systems that is based, in large part, on the San Francisco Bay Program. These include ten new methods for sampling, processing, measuring, and reporting trace element concentrations using techniques employed in the San Francisco Bay program (USEPA, 1995a-j). That methodology has been presented at two USEPA national meetings in Norfolk, VA (1994 and 1995) and five USEPA Trace Metal Workshops (1995 - 1996) around the United States (Boston, MA; Chicago, IL; Denver, CO; San Antonio, TX; Seattle, WA). Presentations at each of those meetings have included both a video of the sampling program in San Francisco Bay and a discussion of the success of that program. As a result, the San Francisco Bay Program is now serving as the model for the national program for monitoring trace elements in aquatic systems.

Judicial Confirmation of the Methodology in Civil Litigation

The judicial credibility of the methodology incorporated in the monitoring program has been substantiated in litigation. This occurred in a Proposition 65 class action suit in California, which was based on analyses of elevated lead concentrations in solutions in some lead crystal glassware. Since the analyses used the same protocols as the monitoring program, which had been approved by both the California State Water Resources Control Board and the United States Environmental Protection Agency, the judge ruled the analyses were valid. As a consequence, the resulting settlement favored the complaints of the citizens of California.


See SFEI 1996 Annual Report and Website



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