Sediment recontamination potential and biological impacts of hydrophobic organics from stormwater in a mixed-use watershed

This study focused on understanding the influence of stormwater on sediment recontamination and biota bioaccumulation. Stormwater and sediment sampling was conducted at Paleta Creek San Diego, CA, a mixed-use urban watershed. Stormwater samples were size fractioned into clay, silt, sand, and filtered fractions to better understand the association of PAHs and PCBs with different particle sizes. Settling traps were deployed during storm seasons to capture solid particles deposited by stormwater. Bioaccumulation studies of the receiving water sediments were conducted on bent-nosed clams (Macoma nasuta) both in-situ and ex-situ. The results indicated that the majority of PAHs and PCBs were discharged during the early stages of the storm and were primarily associated with coarser particles (>20 μm). Sediment near the discharge exhibited higher levels of contaminants, consistent with the findings from the settling traps and stormwater discharges. Ex-situ bioaccumulation studies demonstrated that sediment contamination did not correlate with bioaccumulation in Macoma nasuta, whereas porewater accurately reflected bioaccumulation trends. This study highlights the importance of considering stormwater discharge patterns and particle sizes when assessing sediment recontamination but also that the resulting bulk sediment contamination may not reflect bioavailability as measured by organism bioaccumulation. These insights contribute to a better understanding of the impacts of stormwater runoff on sediment and biota in southern California coastal watersheds, aiding in the development of effective management strategies. [Display omitted] •Stormwater contamination in a mixed-use watershed was evaluated.•The initial stages of storm contained larger particulates and more PAH and PCBs.•Sediment traps rather than bulk sediment indicated receiving water recontamination.•Bioaccumulation in receiving waters was not related to bulk sediment concentration.•Freely dissolved porewater concentrations (Cfree) correlated well with bioaccumulation in receiving waters.