Sediment Trends Plots of Sediment Trends References

Sediment contaminant concentrations have been measured at most of the RMP sites since 1991. Samples were collected by the Bay Protection and Toxic Clean-up Program (BPTCP) Pilot Studies in 1991­1992 (Flegal et al., 1994) and by the RMP since 1993. Combining data from those two programs provides a time-series of 12 sampling periods over 7 years. Averages and ranges of concentrations for trace elements and trace organics, over time, are shown for each Estuary reach (Figures 4.17a, b, c, d, e, f, g, h, i, & j and 4.18a, b, c, d, & e).

Except for the Rivers, plots for the various Estuary reaches include only muddy sediment samples (<60% sand). At the River stations, one or both stations had coarse sediments in each sampling period. A separate plot is presented showing trends for all samples with coarse (>60% sand) sediments, including the Rivers when sandy.

For the trace elements, chromium concentrations appear to have increased in most reaches (except South Bay) between 1991 and 1997, and nickel appears to have increased in the northern Estuary (Figure 4.17). Although silver in the Northern Estuary, South Bay, and Central Bay was higher in 1991 and 1992 than in subsequent years, only concentrations in the South Bay appear to have continuously decreased. There were no obvious seasonal (wet, dry) patterns in any of the trace element trends. However, the January 1997 floods noticeably changed concentrations of some trace elements measured the following month. Cadmium, mercury, nickel, and zinc were obviously elevated in some South Bay samples as evidenced by the large range bars. Conversely, arsenic appeared to decrease in the South Bay, and in the Central Bay and Northern Estuary as well. Similarly, copper and mercury were elevated in the River samples. Aside from occasional "spikes" in lead and selenium concentrations, those two elements showed no obvious trends or influence of the 1997 floods. However, both trace elements exhibited elevated concentrations at the San Joaquin River station (BG30) in February (Figures 4.5 and 4.8).

For the trace organic contaminants, total DDTs in the Northern Estuary appear to have increased since 1991, but total PCBs, total chlordanes, and dieldrin in the Rivers appear to have decreased (Figure 4.18). PCBs in the Central Bay and dieldrins in the South and Central bays have also decreased in concentration. Total PAHs at the Rivers and other reaches are often higher in concentration in the wet-sampling period than in the dry-sampling period, but there was no other obvious seasonal pattern in the trace organic contaminants. As with some of the trace elements, the 1997 flood apparently caused increases in some trace organic contaminants. The most obvious increases were in total PAHs and total DDTs at the River stations. Total chlordanes and total DDTs also increased at the South Bay sites in February, 1997.

In considering the trends in these plots, it is important to recognize that concentrations may be influenced by physical sediment factors as well as proximity to sources. In general, sediments with more silt and clay (percent fines) and higher total organic carbon (TOC) have higher concentrations than sediments with more sand and low TOC. Therefore, some of the variation represented in the plots could be attributable to spatial and temporal variations in sediment type rather than in changes in concentrations per se. Additionally, rigorous time-series analysis generally requires more than the 10 to 12 samples available. Further study of the relationships between concentrations and other sediment factors, and over time, are good candidates for future RMP Special Studies.

Plots of Sediment Trends