Streamlining long-term trends detection in benthic contamination: A Santa Monica Bay (USA) test case

Understanding trends in benthic contamination provides environmental managers with insights into emerging challenges and a basis for developing risk mitigation strategies. It also provides opportunities to implement sustainable practices that align with evolving expectations. Commonly, most trends are obtained from time series plots by intuition, the outcome of which, more often than not, lacks clarity. This shortcoming could be circumvented by suppressing the seasonality and irregularity components of a time series as shown in this study. With hundreds of sediments collected over 33 years from a canyon (E6), a coastline (C1), a wastewater treatment outfall station (Z2) in Santa Monica Bay, California (USA) and wastewater effluents collected concurrently from the point source discharging at Z2, this approach is also shown to accommodate gaps in data, changes in analytical instrumentation and a choice of reporting PCBs as congeners or as Aroclors. By the proposed method, levels of DDTs, PCBs, Cr, Cu, As, Zn, Pb and Ni contamination, and proportions of fine grains in sediments were all found to be trending unambiguously downward at all three sediment stations (linear regression: all r² = 1.00). The rates of decrease were similar at the outfall and coastline stations (implying the outfall did not affect sediment quality more than storm water perhaps), but faster in the canyon station. Elsewhere, significant correlation were expected between proportions of fine grain sediment with contaminant levels at C1 and E6, both having higher proportions of fine grain sediments than Z2. None was observed but at Z2 which had the lowest fine grain proportions. Not observed as well were correlations between contaminant levels in HWRP effluent and in HWRP outfall sediment.