Characterization of Sediment Contamination in South Bay Margin Areas

The Bay margins (i.e., mudflats and adjacent shallow areas of the Bay) are important habitats where there is high potential for wildlife to be exposed to contaminants. However, until recently, these areas had not been routinely sampled by the Regional Monitoring Program for Water Quality in San Francisco Bay (RMP) due to logistical considerations. In 2015, the RMP conducted a spatially-distributed characterization of surface sediment contamination and ancillary characteristics within the RMP-defined Central San Francisco Bay margin areas. This was repeated in 2017 within South Bay, which for this report refers to the area collectively encompassing Upper South Bay (usually just called the “South Bay” segment in the Bay RMP, “Upper” added here to distinguish from the combined area), Lower South Bay, and “Extreme” Lower South Bay (previously named “Southern Sloughs”) margin areas. 

Findings from the 2015 survey of margin areas in Central Bay (Yee et al., 2017) showed contamination in the margin areas accounted for 20% of the PCB mass in Central Bay, which is disproportionately high compared to margin area (5% of Central Bay). Central Bay watersheds contains many of the oldest and most industrialized urban environments within the Bay Area. There was a significant difference in mean contaminant concentrations in sediment from margins and nearby open water areas. In contrast, for South Bay, margins are more proximate to open water areas and represent a greater percentage of the overall Bay surface area. Therefore, as hypothesized, in the South Bay more uniform distributions were found, likely in part due to these physical factors. Average concentrations in South Bay margins were lower than in Central Bay margins, which was also hypothesized, because South Bay watersheds contain more recent industrial development and relatively less older industrial areas as a percentage of total area (~2% vs 5% for Central Bay watersheds). Although combined loads of PCBs from the South Bay watersheds are about 40% higher than from Central Bay areas, their total area is over 5 times larger, and their total runoff volume is more than double. 

Ambient margins data in South Bay provide a context against which the severity of contamination at specific sites can be compared. The baseline data could also be useful in setting targets and tracking improvements in watershed loads and their nearfield receiving waters, or for appropriate assessment of re-use or disposal of dredged sediment. These spatially distributed data also provide improved estimates of mean concentrations and contaminant inventories in margins. Based on data from this study, contamination in the margin areas accounts for 35% of PCB mass in the upper 15 cm of surface sediments in South Bay, which is approximately proportional to the relative area of the margin (34% of the region). In contrast, margins only contain 30% of the mercury mass in South Bay, somewhat less than their proportional area. 

Given the large inventory of contaminants in the open Bay compared to annual loads, changes may be difficult to see in the open Bay in the short- and mid-term. However, due to their smaller inventories and closer proximity to likely sources and loading pathways, improvements in margin sediments may potentially be detected more easily or sooner as loads are reduced, even if South Bay margins are better connected and more similar in concentrations to adjacent open water areas than in Central Bay.