Effects of polycyclic aromatic hydrocarbons in urban stormwater on receiving sediment

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Urban stormwater runoff has long been identified as a major influence to the contamination of receiving water bodies and sediment. The episodic nature of storms combined with the imperviousness of urban surfaces, lead to stormwater discharges laden with high levels of solids-associated polycyclic aromatic hydrocarbons (PAHs). These compounds pose a concern due to their toxicity, mutagenicity and carcinogenicity, and many of them have been placed on USEPA Priority Pollutant List. The core objective of this study was to determine the physical, chemical and biological characteristics of stormwater runoff form a mixed use urban watershed and determine the distribution and bioaccumulation potential of its effects on the receiving sediment. Historically, stormwater assessment has been focused on loads rather than impacts on sediments and different sampling approaches were needed to characterize those impacts. The experimental approach involved a 2-year sampling plan in Paleta creek at Naval Base San Diego (NBSD) involving a variety of sampling approaches including intensive sampling of individual storms, water and sediment collection before and after the winter rainy season and settling traps collecting depositing sediments throughout the storm season. Storm runoff samples from 2 storms in January 2016 were collected and size fractionated. Receiving sediments were monitored with water column, sediment and sediment trap measurements. Porewater passive samplers and both in-situ and ex-situ bioaccumulation studies using bent-nose clams (Macoma Nasuta) were conducted in cooperation with US Navy personnel to assess the response of the receiving benthos. Total Organic Carbon (TOC) and Black Carbon (BC) contents were measured to better understand the source of the depositing solids as well as to link PAHs in sediments to their bioaccumulation potential. Sediment and tissue was extracted by pressurized liquid extraction (PLE), storm samples were liquid-liquid extracted (LLE) and final analysis was carried out by HPLC-FLD and GC-TQMS. In preparation for the sediment sampling, a study of PLE was conducted in order to develop an in-house method that would allow us to process large amounts of sediment samples in an efficient and accurate way and, in particular, extract PAHs effectively from weathered and high BC sediment samples. The combination of size fractionated stormwater loads with sediment traps were identified as the most effective monitoring tools to assess sediment recontamination. Analysis of stormwater samples showed most of the PAHs were associated with large particles in runoff and led to rapid near field deposition and sediment recontamination. SEM imaging confirmed the presence of large BC-rich particles in the near field traps. However, bioavailability was limited as indicated by bioaccumulation studies suggesting that sediment recontamination assessment should also be coupled with assessment of bioavailability. Porewater concentrations were also shown to correlate well with the observed bioaccumulation suggesting that either bioassays or porewater assessment could characterize bioavailability for PAHs. Parent and alkylated PAH ratios allowed stormwater from this watershed to be separated from sediments settling in areas away from the stormwater discharges.

Stormwater, Sediments, Polycyclic aromatic hydrocarbons (PAH), Urban runoff, Macoma nasuta, Porewater, Bioassays