Sediment Monitoring Discussion Patterns in Sediment Contamination in 1997 Comparisons to Sediment Quality Guidelines   Effects of Sediment Contamination Summary of Sediment Conditions in the Estuary References

Sediment contaminant concentrations measured in the San Francisco Estuary exhibit considerable variability depending on the sites and times at which they were sampled. That variability reflects the proximity to the sources of contamination, the biogeochemical interactions between the dissolved and particulate phases in water and bedded sediments of the Estuary, and sediment transport mechanisms such as deposition and resuspension. Because all of those factors affect sediment concentrations measured by the RMP, the concentrations reported only provide information about the status of sediments at the times and locations collected. Understanding the differences in concentrations among the stations and Estuary reaches, or between sampling periods and over several years, requires some knowledge about the factors listed above, but much of that information is not known. Sediment transport mechanisms are well illustrated in the 1997 RMP sediment data: flood flows in January (see article by Cloern et al. in Chapter 3: Water Monitoring) produced observable changes in RMP sediment concentrations and trends. RMP sediment monitoring does provide reliable measurements of sediment contamination that reflect the most recently deposited sediments and may be used to track trends in the concentrations over time

Patterns in Sediment Contamination in 1997

As in past years, concentrations of most contaminants were highest in the Southern Sloughs and South Bay than in the other Estuary reaches. Average concentrations of chromium, cadmium, lead, mercury, nickel, selenium, zinc, and chlordanes were highest in sediments of the Southern Sloughs, PAHs were highest in the South Bay, and PCBs were highest in the Southern Sloughs and South Bay. In contrast, arsenic was highest in the Central Bay, and copper and total DDTs were highest in the Northern Estuary. Concentrations at the sandy sediment sites were generally lower than at the muddy sediment sites.

The February samples were collected following the flood flows of January 1997. Only mercury concentrations were generally higher throughout the Estuary in February than in August. However, several contaminants (e.g., copper, lead, mercury, selenium, and PAHs) had obviously elevated concentrations at the San Joaquin River (BG30) in February. Conversely, arsenic, chromium, cadmium, and selenium were usually higher in August than in February, whereas all other contaminants showed no obvious seasonal trends.

Another way to examine the effects of the flood flows on sediment concentrations is by examining the RMP trends plots (Figures 4.17 and 4.18). Flood flow effects may be observed as either changes in the average, or increases in the range of values plotted for each contaminant in each reach. Copper and PAHs at the River sites were most obviously increased from past values. Cadmium, chromium, nickel, chlordanes, and DDTs showed increases in the range of values for the South Bay (includes data from the Southern Sloughs). In contrast, arsenic appeared to be below previous values in the Northern Estuary and Central Bay.

The patterns described above indicate that flood flows may elevate some contaminants, but not others. Those patterns are most obvious at sites nearest the major tributaries to the Estuary. Where concentrations were elevated, it is assumed that sediment-associated contaminants were flushed into the Bay by the flows. Conversely, lower concentrations following the floods suggest that those contaminants were not associated with the sediments that came in with the flood. The possible role of resuspension and mixing of existing sediments during flood flows is not known.

Comparisons to Sediment Quality Guidelines

Sediment quality guidelines (SQGs) are concentration values that help interpret RMP results. Since there are no formal regulatory sediment contaminant guidelines, several different sets of guidelines may be used to evaluate monitoring results from several perspectives (Table 4.8).

The USGS's sediment coring data provide historic concentrations of several trace elements. Prior to the gold rush and subsequent industrialization in the region, sediments reflected natural concentrations of trace elements present in the earth's crust. There were no synthetic pesticides or chlorinated hydrocarbons (Venkatesan et al., 1999), and very low levels of petroleum hydrocarbons from combustion of natural materials and peat degradation (Pereira et al., 1999). Obviously, Bay concentrations will never return to those levels, but this knowledge provides an important historical perspective against which to evaluate current Bay conditions.

RMP sediment data have been compared to the Effects-Range guidelines (see Sediment Introduction). Those guidelines are effects-based and may be used to evaluate the potential for biological effects. New information about interpreting the Effects-Range guidelines show that for amphipod bioassays, when one or more contaminants exceed their ERL values, 38% of the samples were toxic. When more than 14 ERLs were exceeded, or more than 4 ERMs were exceeded, more than half of the tests were toxic (Long et al., 1998).

Most of the 1997 RMP sediment samples had multiple ERL exceedances, and at least 1 ERM exceedance (Table 4.9a and 4.9b), which suggests a potential for ecological effects. Arsenic, chromium, copper, mercury, nickel, and HPAHs most frequently exceeded ERLs. Nickel always exceeded its ERM and mercury exceeded the ERM at Guadalupe River in the Estuary Interface Study in February. Horseshoe Bay (BC21) and San Pablo Bay (BD22) exceeded numerous PAH ERLs in February.

Another set of SQGs was recently developed by the SFBRWQCB for the San Francisco Estuary (see the article by Gandesbery et al. in this Chapter). Ambient Sediment Concentration (ASC) values were based on the 85^th percentile of reference or ambient Bay concentrations. Therefore, they reflect an upper limit for ambient or current "background" concentrations. As shown on Table 4.10a and 4.10b, most 1997 RMP samples exceeded at least some of the ASC values. Samples from San Bruno Shoal (BB15), Point Isabel (BB41), and Davis Point (BD41) in February, and Oyster Point (BB30) in August were all within the ASC guidelines. Interestingly, the San Bruno Shoal (BB15) and Yerba Buena Island (BC11) samples from August had numerous ASC exceedances. Sites in the Southern Sloughs, Coyote Creek (BA10), Horseshoe Bay (BC21), San Pablo Bay (BD22), Pinole Point (BD31), and San Joaquin River (BG30) also had numerous concentrations above the ASC guidelines. Chromium and nickel were the most frequently exceeded contaminants, but several individual PAH compounds also exceeded the ASC guidelines.

Effects of Sediment Contamination

The effects of sediment contamination are monitored by the RMP using sediment bioassays and through the Benthic Pilot Study. Sediments may also affect contaminant concentrations in fish (see article by Davis et al. in Chapter 6: Pilot and Special Studies) and bivalve tissues. There was no toxicity from the sandy sediments at Davis Point (BD41) and Red Rock (BC60), and none at Horseshoe Bay (BC21) and San Bruno Shoal (BB15). However as in previous years, many sediment samples were toxic to amphipods and bivalve embryos. Half of the amphipod bioassays indicated toxicity. Toxicity occurred in samples from both seasons at Napa River (BD50), Yerba Buena Island (BC11), Alameda (BB70), Redwood Creek (BA41), South Bay (BA21), and San Jose (C-3-0). Those results differed from previous years in that more of the South Bay samples were toxic. There was also toxicity to bivalve embryos at 36% of the sites. In 1997, both samples from Coyote Creek (BA10) were toxic, and as in past years, all samples from the Rivers (BG20, BG30), Grizzly Bay (BF21), and Napa River (BD50) were toxic as well.

RMP investigators continue to study the cause of the observed toxicity. For the amphipod test, mixtures of contaminants in sediments was shown to be highly associated with toxicity in most samples (Thompson et al., 1996). The mean ERM quotient (mERMq) is a cumulative ERM index that reflects additive concentrations of mixtures of contaminants. In the 1997 results, mERMq values below about 0.2 were usually not toxic, and mERM quotients above about 0.25 usually were toxic (Table 4.11), although samples from Coyote Creek (BA10) and Grizzly Bay (BF21) in August, 1997 had mERMq values above 0.3 and were not toxic. While they were not tested, samples from the Estuary Interface Study in Coyote Creek (BA10) and elsewhere had mERMq values that would suggest they were toxic.

For the bivalve embryos, dissolved metals (divalent cations) in sediment elutriates at the Rivers (BG20, BG30) and Grizzly Bay (BF21) were probably responsible for the observed toxicity. At the Sacramento River site (BG20), organics were also implicated.

Investigations into sediment contaminant effects on benthos are continuing under the RMP Benthic Pilot Study, but analysis is not yet complete. Our preliminary results, however, indicate that most RMP sites are inhabited by many species characteristic of unimpacted conditions. In order to demonstrate a benthic response to contamination, the Bay Protection and Toxic Clean-up Program samples, which included several impacted sites (Hunt et al., 1998) has been added to the RMP database. Next year's Annual Report will include complete benthic assessments.

Summary of Sediment Conditions in the Estuary

One of the most commonly used methods of assessing sediment condition is to consider information about sediment contamination, toxicity, and benthos together: the Sediment Quality Triad (Long and Chapman, 1985; Chapman et al., 1997). While each of the three components individually provide information about sediments, it is the "weight of evidence" using all three components that creates an overall assessment of sediment condition. At this point, the RMP has good information on the first two components, although bioassays are not conducted at all sites; benthic assessments are being developed. Summary information about sediment contamination and sediment toxicity for each site is shown in Table 4.11.

According to the information presented in this Chapter, sediment contaminant concentrations in the San Francisco Estuary were often above levels known to cause effects at most of the RMP sites. The highest concentrations of most contaminants were at the Estuary Interface sites, the Southern Sloughs, and in the South Bay. Sediments at many sites were toxic to either amphipods or bivalve embryos. Toxicity was most pronounced and occurred most frequently in the Suisun Bay (BF10, BF20, BF30) and Rivers (BG20, BG30) sites, and in the South Bay, although Redwood Creek (BA41) was most toxic to amphipods. The flood flows of January 1997 appeared to generally elevate sediment concentrations in the February samples, especially near the major tributaries, and the incidence of toxicity was also greater than in August.

The RMP sites are monitored to provide information on background or ambient Bay condition, and do not provide comprehensive information about all Bay sediments. Several other non-RMP studies have shown that sediment concentrations and toxicity are even higher at many locations around the Bay's margins (e.g., closing military bases, toxic hot spots). The RMP's new objective for information synthesis (see article by Hoenicke and Bernstein in Chapter 2: Review Implementation) encourages the summarization and integration of such information, and could be accomplished through future Special Studies.

References

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