Distribution of Trialkylamines and Coprostanol in San Pedro Shelf Sediments Adjacent to a Sewage Outfall

The fate of sewage from the Orange County Sanitation District (OCSD) ocean outfall was assessed by measurements of trialkylamines (TAMs) and coprostanol (COP) in bottom sediments from the San Pedro Shelf (southern California). The wastewater discharge is an important source for steroidal sewage markers, as deduced from ratios of COP to other sterols (COP/epicoprostanol, COP/dinosterol and COP/total sterols) and from the very high concentrations of TAMs (35 μg g−1) and COP (8.3 μg g−1) in sediments in the immediate vicinity of the outfall. Decreasing concentrations of both molecular markers with increased distance from the outfall were observed, except in the north-west, which corresponds with the predominant bottom current direction in the region. The offshore portions of Newport and San Gabriel submarine canyons act as efficient traps for fine-grained sediments containing high concentrations of TAMs, which have a high clay affinity characteristic of their amphiphilic chemical properties. The TAM/COP ratio in sediments (0.3–11.2) was larger than in the effluent (0.07), indicating a relatively higher stability of TAMs than COP in sediments under prevailing oxic conditions. Because the two markers exhibit different distributions and fates, an integrated use of both markers provides an effective tool to assess the effects of wastewater discharges to the marine environment.