Environ. Sci. Technol. 1997, 31, 3455-3460
Temporally Variable Freshwater Sources of Dissolved Chromium to the San Francisco Bay Estuary KHALIL E. ABU-SABA* Department of Chemistry, University of California, Santa Cruz, California 95064 A. RUSSELL FLEGAL Environmental Toxicology, University of California, Santa Cruz, California 95064
Chromium is a common contaminant in estuarine environments, originating from chemical manufacturing, steel refining, and leather tanning. However, it is also relatively abundant in the earth’s crust, resulting in substantial natural sources from upland watersheds. The objective of this study is to quantify the relative magnitudes of natural and anthropogenic chromium fluxes to San Francisco Bay, an important estuary in the National Estuaries Program. Chromium speciation was determined throughout the estuary during the wet and dry seasons of 1992-1995. Those measurements, hydrographs of freshwater endmembers, and silicate-salinity plots reveal the complexity of chromium sources to the highly modified estuary. Episodic flushing of an agricultural drain transports chromium leached from alluvial sediments to the northern reach of the estuary. In contrast, the principal dissolved chromium source to the southern reach appears to be diagenic remobilization from sediments within the estuary. In both the northern and southern reaches, dissolved chromium fluxes are scavenged back onto particles, showing that sediments serve as both a source and a sink in the geochemical cycle of chromium. Direct anthropogenic chromium inputs are dwarfed by high-flow fluvial inputs but are comparable to low-flow dissolved inputs.
Introduction Chromium is a widespread contaminant in estuarine environments (1). Recent research has demonstrated the bioavailability of both the trivalent and hexavalent forms (2). In San Francisco Bay (Figure 1), chromium concentrations are elevated in bivalves collected near the head of the estuary relative to the rest of the estuary (3). However, it is unknown whether this increased bioaccumulation results from direct anthropogenic inputs of chromium or inputs from the surrounding watershed. Estuarine cycling of chromium is complicated by the contrasting geochemistry of its two dominant redox states, chromium(III) and chromium(VI). The hydrolysis species of cationic chromium(III) can be rapidly scavenged within the turbidity maximum zone, whereas the tetrahedral oxydianion, chromate, has a very low particle affinity (4). Although chromium(VI) is thermodynamically favored in oxic waters, reduction in-situ produces chromium(III) (5, 6), which can persist metastably due to its sluggish oxidation kinetics; solution oxidation half-times are days to months (7, 8). These chemical properties lead to complex spatial distributions of both redox species in estuaries (9, 10). * Corresponding author e-mail:
[email protected]; phone: 408-459-5336; fax: 408-459-2088.
S0013-936X(97)00208-3 CCC: $14.00
1997 American Chemical Society
Variable freshwater sources to San Francisco Bay further complicate evaluation of chromium-salinity distributions. Previous research has demonstrated that agricultural drainage increases concentrations of dissolved solids and silica in the estuary’s source waters (11, 12). Therefore, shifts in the freshwater sources to the estuary may also lead to seemingly non-conservative concentrations of trace metals, as suggested for chromium distributions in the Mississippi River Delta (13). Source water hydrographs can help resolve complexity introduced by time-variable estuarine freshwater sources. Flow data are important because source water changes can send a chemical pulse down the estuary. Spatial concentration gradients can result from temporal variations in the source water (14). Nutrient data can also be used to identify time-variability in freshwater end-members. Dissolved silica, for example, provides an ancillary reference for comparison with other more complex non-conservative components in San Francisco Bay (15). Biological silica removal is insignificant as compared to fluvial inputs during high-flow periods, making silica a marker for different freshwater sources. In this study, we use dissolved silica-salinity distributions to identify spatial gradients in northern San Francisco Bay that are derived from time-variable freshwater sources. San Francisco Bay receives freshwater from a 163 000 km2 watershed, which covers 40% of California (16). Essentially all of this runoff (>90%) enters the bay through the Sacramento River, much of it during the winter and spring highflow periods (17). During peak flow periods, water is diverted through the Yolo and Sutter Bypasses to prevent flooding in the Sacramento Valley. That overland flow rejoins the Sacramento River just north of Rio Vista. The agricultural lands of the bypasses become fluvial systems, with flows episodically exceeding those of the Sacramento and San Joaquin Rivers. These sporadic changes in freshwater sources may have substantial impacts on contaminant transport to the estuary.
Experimental Section Samples were collected using trace metal clean techniques, as previously described (10, 18). Analytical procedures for determination of total dissolved (10 nM) concentrations are found in the northern reach of the estuary during the winter and spring high-flow periods (>1000 m3 s-1) (Figure 2A). In contrast, the relatively low (2-4 nM) dissolved chromium concentrations observed during the summer were comparable to oceanic surface waters (4, 9, 20). Episodic increases (27-60 nM) of dissolved chromium concentrations also occurred in the sloughs of the southern reach (BA05 and BA06) in April 1994 (Figure 2B). Those inputs were scavenged between the sloughs and the Dumbarton Bridge (BA30). Discharge records appear to rule out a direct anthropogenic source for that episodic event (21). Concurrent
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elevations in lead and silver concentrations and suboxic conditions suggest that sediment diagenesis contributed to the observed fluxes of dissolved chromium (22). This has also been suggested as the cause of seasonal fluxes of dissolved silver and lead in the southern reach (23, 24), and may be linked to the decay of organic matter following the spring diatom bloom in the South Bay (25). In addition to the two episodic events in sloughs of the extreme southern reach (BA05 and BA06), these sites had consistently higher dissolved chromium concentrations (610 nM) than the rest of the southern reach (2-5 nM). However, the relatively low freshwater flow into the southern reach (
