Speciation of Nickel in South San Francisco Bay David L. Sedlak and William W. Bedsworth University of California, Berkeley Introduction Materials and Methods Results Discussion Acknowledgments References

Introduction

Data collected as part of the RMP indicate that nickel concentrations in South San Francisco Bay occasionally exceed the U.S. EPA's proposed water quality criterion for marine waters of 8.2 mg/L (i.e., 140 nM). The two main sources of freshwater in South San Francisco Bay are wastewater effluent and surface runoff. Dissolved nickel concentrations are typically highest in South San Francisco Bay during summer, when the input of surface runoff is small relative to wastewater discharges. Dissolved nickel concentrations decrease during winter, when surface runoff greatly exceeds wastewater discharges.

The U.S. EPA's water quality criteria have been criticized because they do not consider site-specific factors that affect pollutant toxicity. Attempts to consider site-specific factors in regulations, such as the water effects ratio (WER) and the total maximum daily load (TMDL), must consider the different forms of nickel discharged by each source, because the toxicity of metals to aquatic organisms is positively correlated with the concentration of dissolved, uncomplexed metal (Sunda and Lewis, 1978; Sunda et al., 1978; Anderson and Morel, 1978).

Results of previous studies (Sedlak et al., 1997; Bedsworth and Sedlak, 1998) indicate that most of the nickel discharged by wastewater treatment plants consists of a strongly complexed form of nickel, NiEDTA^2-. In contrast, nickel in surface runoff in South San Francisco Bay consists mostly of uncomplexed nickel or weak nickel-organo complexes. The stronger complexes are thought to be significantly less toxic to aquatic organisms than uncomplexed nickel or weak nickel-organo complexes. Therefore, an assessment of the effects of nickel in South San Francisco Bay needs to consider the relationship between source speciation and nickel fate, transport, and toxicity in San Francisco Bay.

To assess the effect of speciation on nickel toxicity in South San Francisco Bay, data are needed on nickel speciation in the water column. The only available data on nickel speciation in South San Francisco Bay consist of two measurements made near the Dumbarton Bridge that indicated that approximately 60% of the dissolved nickel was complexed by a strong ligand (Donat et al., 1994). To further assess temporal and spatial variability in the speciation of nickel in San Francisco Bay and the relationship between nickel sources and speciation, samples collected during the 1997 RMP were analyzed for nickel speciation. The purpose of the study was to assess seasonal patterns in nickel speciation resulting from varying contributions from different nickel sources and to evaluate the stability of NiEDTA^2- complexes discharged to San Francisco Bay.

Materials and Methods

Surface water samples were collected during the 1997 RMP. After collection, filtered samples were stored on ice and transported to the University of California at Berkeley, where they were frozen until analysis. Samples were analyzed for nickel speciation using competitive ligand exchange/cathodic stripping voltammetry (CSV) as described elsewhere (Donat et al., 1994; Bedsworth and Sedlak, 1998). Direct measurements of NiEDTA^2- were not performed because the concentrations of the complex were expected to be near or below the limit of quantification in all samples, except those collected immediately proximate to the outfall of the San Jose/Santa Clara Water Pollution Control Plant (SJSC WPCP).

Results

Concentrations of dissolved nickel in San Francisco Bay ranged from 7 to 160 nM (i.e., 0.4 to 9.4 mg/L; Figure 7.3 and Table 7.2). Highest dissolved nickel concentrations were observed in sample C3-0, which is located near the outfall of SJSC WPCP. During summer, when the contribution of surface runoff was small, the concentration of nickel measured at this location (i.e., 160 nM) was similar to the concentration measured in the effluent of the SJSC WPCP (dissolved nickel concentrations in the effluent of the SJSC WPCP typically range from 70 to 160 nM or 4.1 to 9.4mg/L). Given the low salinity of the sample (i.e., 11) and the absence of other sources of freshwater, it may be concluded that this sample is approximately 2/3 wastewater effluent (based on a wastewater effluent salinity of 0 and seawater salinity of 34). During winter, when the volume of surface runoff was equal to or greater than the volume of wastewater effluent, the concentration of dissolved nickel measured at this location decreased to 56 nM (i.e., 3.3 mg/L), which is similar to the concentration of dissolved nickel detected in runoff samples from Coyote Creek and the Guadalupe River (dissolved nickel in surface runoff during dry weather range from approximately 15 to 40 nM or 0.9 to 2.3 mg/L as reported by Sedlak et al., 1997). The speciation of nickel in samples from South San Francisco Bay was consistent with the expected seasonal contributions from wastewater effluent and surface runoff: most of the nickel in samples collected from sites in South San Francisco Bay during summer was complexed, whereas complexed nickel generally accounted for less than half of the dissolved nickel in samples collected during winter.

Samples collected in other sections of San Francisco Bay generally contained lower concentrations of dissolved nickel. The lowest concentration of dissolved nickel was usually observed in the sample collected near the Golden Gate. As the high salinity water from the Golden Gate area mixed with the freshwater coming from South San Francisco Bay, nickel concentrations decreased.

Discussion

Measurements of nickel speciation in wastewater effluents and in surface runoff can be used along with equilibrium predictions and the results of the laboratory experiments to predict the effect of different sources on nickel speciation in South San Francisco Bay. As discussed elsewhere (Bedsworth and Sedlak, 1998), most of the dissolved nickel in wastewater effluent consists of NiEDTA^2-, while the dissolved nickel in surface runoff consists of weaker nickel-organo complexes. Furthermore, results of equilibrium predictions and laboratory experiments (Sedlak et al., 1997) indicate that NiEDTA^2- in the effluent of the SJSC WPCP will not dissociate after it mixes with water from San Francisco Bay. The speciation of nickel in South San Francisco Bay should therefore exhibit seasonal differences: in summer, when most of the nickel entering the system consists of stable NiEDTA^2- complexes, high concentrations of strong nickel complexes should be present. During winter, when nickel entering South San Francisco Bay consist of approximately equal amounts of strongly complexed nickel from wastewater effluent and weaker nickel complexes from surface runoff, the percentage of complexed nickel should decrease.

Analysis of data collected during 1997 are consistent with this hypothesis: the highest concentrations of complexed nickel are observed during summer and approximately equal concentrations of complexed and uncomplexed nickel are observed during winter. During winter, the concentrations of strongly complexed nickel are approximately equal throughout South San Francisco Bay. During all three seasons, concentrations of complexed nickel reach a level that is approximately constant north of the Dumbarton Bridge. The decrease in complexed nickel concentrations with distance from the wastewater outfalls suggests that the wastewater treatment plants that discharge to South San Francisco Bay are responsible for the complexed nickel.

The complexed nickel in the wastewater effluent appears to follow conservative behavior as it mixes with seawater. As discussed by Flegal et al. (1991), the concentration of a conservative pollutant should exhibit a linear decrease with increasing salinity. Although the data exhibit considerable scatter, the concentration of complexed nickel is consistent with our expectations. The trend is most evident during July, when the highest concentrations of complexed nickel are present. The relationship is less pronounced in April and is absent in January because the concentrations of complexed nickel decrease.

In contrast to the complexed nickel data, concentrations of uncomplexed nickel are approximately equal throughout San Francisco Bay. This suggests that there is an internal source of uncomplexed nickel in San Francisco Bay. A likely source of uncomplexed nickel are the bay sediments, which range from approximately 50 to 100mg/kg. Although these sediments are likely to be strongly bound by sulfides and organic matter, resuspension of sediments could be responsible for the low concentrations of uncomplexed nickel observed in the surface waters.

Acknowledgments

This research was funded by the City of San Jose, California. Additional support was received from the UC Toxic Substances Research and Teaching Program.

References

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