Persistent Organic Pollutant Sources and Pathways to Juvenile Steelhead Trout in the Nisqually River

A 2014 study on contaminant exposure in outmigrating steelhead trout (Oncorhynchus mykiss) from in-river and the estuary habitats of Skagit, Green/Duwamish and Nisqually rivers and their associated nearshore marine habitats documented that polybrominated diphenyl ethers (PBDEs) were highest in the Nisqually River system. Moreover, PBDEs concentrations in steelhead trout were above critical body resides (CBRs) for increased disease susceptibility throughout the Nisqually river system: 33% of fish in-river at the smolt traps, 33% of fish caught in the estuary and 50% of fish in the associated marine basin. Subsequent sampling of steelhead trout at the Nisqually River smolt trap in 2015 also confirmed that approximately one third of the fish had PBDE at levels known to increase disease susceptibility in salmonids. PBDEs were only detected in a portion of the samples, and PBDE contaminant exposure was hypothesized to be limited to a subset of the watershed. The purpose of this study was to conduct a source assessment study to identify and prioritize potential sources of PBDE to the Nisqually River so that corrective actions may be implemented. Specific objectives were to 1) conduct a synoptic survey to assess the spatial distribution of PBDEs in the main stem Nisqually River and its tributaries, and 2) to identify and characterize potential sources of PBDEs to the Nisqually River system, based on the results of the synoptic survey. PBDE concentrations were measured in water samples (via semi-permeable Membrane SPMDs) and in biofilms (i.e., algae and microbial biomass).

The results of the 2017 PBDE source assessment suggested that PBDEs were primarily entering the Nisqually River system via the three major tributaries of the Nisqually River: the Mashel River, Muck Creek, and Ohop Creek, however, limited spatial sampling within the tributaries restricted our ability to definitively conclude the major sources. Generally, PBDEs were measured in low concentrations in both water and biofilm samples with the exception sites in the three major tributaries. Elevated PBDEs concentrations were only measured in water samples at the upper Mashel River site whereas the highest total PBDE concentrations were measured in biofilm samples from Muck Creek, followed by the upper Mashel River site, and then Ohop Creek. Biofilms were shown to bioconcentrate PBDEs approximately 1000 times and generally well correlated at higher concentrations. At the upper Mashel River site, the biofilm PBDE congeners had a different pattern of PBDE congeners than all other sites and had an enriched signal of nitrogen stable isotope, ẟ15N. The combination of elevated PBDE levels, a different pattern of PBDE congeners and an enrichment of ẟ15N strongly suggested exposure to a wastewater source. The outfall for the Eatonville WWTP outfall is located slightly upstream from the site, suggesting that that it is potentially the source of PBDEs in this tributary. The elevated concentrations return to near background concentrations just five miles downstream where the Mashel River meets the mainstem Nisqually River, suggesting that the PBDEs were diluted as they flow away from the source, possibly by input from the Little Mashel River that enters downstream of the Eatonville outfall but upstream of the confluence with the Nisqually River, or from groundwater sources. Additional sampling would be necessary to document the extent of the Mashel River that is affected by PBDE inputs from the Eatonville wastewater treatment plant outfall and how this relates to Steelhead rearing habitat.

The pattern of PBDE congeners measured in Muck and Ohop creeks were distinct from those in the upper Mashel. River site. Heavier PBDE congeners were detected in both Muck Creek (i.e., 203, -206, -207, -208 and -209) and, Ohop Creek (i.e., 203, -206, and -208), however, there is some uncertainty regarding the true concentration due to laboratory blank contamination. Although, Muck and Ohop creeks biofilms have a heavier PBDE congener pattern, the source of these PBDEs was not likely from a wastewater source as their nitrogen stable isotopes, ẟ15N, which are often altered when exposed to wastewater, are not altered compared to other biofilm samples, and there are no wastewater treatment plant discharges directly to these creeks. Additional sampling would be necessary to document the source PBDEs to Muck and Ohop creeks.