Sources, timing, and fate of sediment and contaminants in the nearshore: insights from geochemistry

Abstract

Rivers in Cascade watersheds carry sediment with a volcanic composition that is distinct from the plutonic composition of the Puget lowlands. Compositional properties (signatures) allow discrimination of river-sourced Cascade from lowland sediment, and inferences about transport pathways. Surface sediment on land contains atmospheric radionuclides whose known decay rates define monthly (7Be) and decadal (210Pb) timescales of sediment inputs from land to nearshore regions. We used geochemical signatures to source river-borne sediment in two urban embayments in Cascade watersheds: Commencement Bay (CB) and Bellingham Bay (BB). We concurrently determined sediment contaminant levels and, in CB, used geochemical aging to distinguish contaminants in recent winter outflow from those that were pre-existing. Methods are described in Takesue et al. (2017). Geochemical signatures showed that Puyallup River (PU)-sourced fine sediment (<63 µm) accumulated more along the northeast (NE) shore of CB than the southwest: median 91% and 69%, respectively. River-sourced sediment from recent winter storms, containing higher 7Be activities (open circles, Fig. 1A), also accumulated on the NE shore and had lower contents of PAHs, fecal sterols, PCBs, and potentially toxic metals (TM) compared to the south shore; only PBDEs were higher (column graphs, Fig. 1A). Lower 7Be and 210Pb activities in south shore sediment indicated that contaminants there were associated with older sediment. Existing sediment in CB contained higher levels of urban contaminants than new PU material. No PAH, PCB, or TM levels in CB exceeded Washington State marine sediment quality standards (WAMSQS). Geochemical signatures were not distinct in the Nooksack River watershed and lowlands, precluding sediment sourcing in BB. PAH ratios in BB sediments were ubiquitous (open circles, Fig. 1B), suggesting atmospheric rather than riverine transport, and had values indicating biomass/coal combustion sources, except for three sites along the urban waterfront that had values indicative of vehicle emissions (closed circles, Fig. 1B). Elevated TM occurred offshore of Fairhaven (cadmium, Cd; copper, Cu; lead, Pb; zinc, Zn), Whatcom Creek Waterway (Pb, Zn), and the marina (Cu, Zn). No PAH or TM concentrations in BB exceeded WAMSQS. Insights gained from sediment geochemistry about the sources, timing, transport, and fate of riverborne fine sediment and contaminants in nearshore regions are valuable components of monitoring programs that can help guide habitat restoration and resource management decisions toward effective and sustainable outcomes.