Sediment-associated organopollutants, metals and nutrients in the Nechako River, British Columbia: a current study with a synthesis of historical data

The flow-diverted Nechako River is the second largest tributary of the Fraser River which drains 25% of mainland British Columbia, Canada. The watershed provides critical reproductive habitats for anadromous chinook salmon (Oncorhynchus tshawytscha) and sockeye salmon (O. nerka) and year-round habitat for the endangered Nechako white sturgeon (Acipenser transmontanus) population. In this study, the chemical quality of sediment in the upper Nechako River and its major tributaries was examined for polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), legacy organochlorine (OC) pesticides, polycyclic aromatic hydrocarbons (PAHs), organophosphorus and phenoxy acid herbicides, polychlorinated dibenzo-p-dioxins (PCDDs) and furans (PCDFs), labile (exchangeable) metals/metalloids, and major nutrients (N and P). sigma PCB and sigma PBDE were detected in channel bottom sediments at all Nechako River and tributary sites; hexachlorobenzene (HCB) and 4,4 '-DDE (dichlorodiphenyldichloroethylene) were also detected at most sites. Organophosphorus and phenoxy acid herbicides were not detected in any samples. While sigma PCB, sigma PBDE, HCB, and 4,4 '-DDE concentrations in bottom sediments were always well below provincial and federal guidelines for the protection of aquatic life, concentrations of total labile As (As-T), Cr-T, Cu-T, Fe-T, Mn-T, Ni-T, and Zn-T in suspended and/or bottom sediments often exceeded provincial threshold effect levels (TEL) and, in some cases, approached or exceeded probable effect levels. Distinct Cu-T, Ni-T, Pb-T and Zn-T concentration maxima in suspended sediments were linked to road and urban densities. Historical PCDD, PCDF and PAH data show that concentrations in bottom sediment samples were highest at a site in the lower Nechako River between Miworth and the City of Prince George where PCDD and PCDF concentrations exceeded the interim federal TEL. Nitrogen and P concentrations in bottom sediments were negatively correlated with water velocity and sediment texture; the evidence suggests that reductions in velocity resulting from the 1952 diversion of the Nechako River have increased the nutrient content of bottom substrates.