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.

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.