Stormwater Action Monitoring 2017/18 Mussel Monitoring Survey


Published: October 2020

Pages: 90

Publication number: FPT 20-13

Author(s): Mariko Langness and James West

Executive Summary

Toxic contaminants enter the Puget Sound from a variety of pathways including non-point sources such as stormwater runoff, groundwater releases, air deposition, and point sources like marinas, industrial and wastewater treatment plant outfalls, and combined sewer overflows. Contaminated stormwater is considered one of the biggest contributors to water pollution in the urban areas of Washington State because it is ongoing and damages habitat, degrades aquatic environments, and can have serious impacts on the health of the Puget Sound. Monitoring pollutants and their effects on the marine biota of Puget Sound is critical to inform best management practices and remediation efforts in this large and diverse estuary.

In the winter of 2017/18 the Washington Department of Fish and Wildlife (WDFW), with the help of citizen science volunteers, other agencies, tribes, and non-governmental organizations, conducted the second of a series of biennial, nearshore mussel monitoring efforts under the Stormwater Action Monitoring (SAM) program. The first SAM Mussel Monitoring survey was conducted in the winter of 2015/16 (Lanksbury et al., 2017).

SAM is a collaborative stormwater monitoring program funded by municipal stormwater permit holders in western Washington. This monitoring survey for SAM was intended to characterize the spatial extent of tissue contamination in nearshore biota residing inside the urban growth areas (UGAs) of Puget Sound, using mussels as the primary indicator organism. Future biennial SAM surveys will continue to track mussel tissue contamination in the Puget Sound nearshore to answer the question: “Is the health of biota in the urban nearshore improving, deteriorating, or remaining the same related to stormwater management?” Although the primary focus of this document was to report on SAM program data, we included data for additional sampling conducted by WDFW and its partner organizations, and note the benefits of this larger, cooperative monitoring effort.

In this study we used native mussels (Mytilus trossulus) as indicators of the degree of contamination of nearshore habitats. We transplanted relatively uncontaminated mussels from a local aquaculture source to over ninety locations along the Puget Sound shoreline, covering a broad range of upland land-use types from rural to highly urban. At the end of the study, after approximately three months of exposure, we measured the concentration of several major contaminants in the mussels’ soft tissues including several classes of organic chemicals, such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs, or flame retardants), and chlorinated pesticides (including dichlorodiphenyltrichloroethane compounds, or DDTs) and seven metals (lead, copper, zinc, mercury, arsenic, cadmium, and aluminum).

WDFW staff, volunteers, and partners deployed mussel cages to 94 monitoring sites; 41 SAM sites (38 repeated from 2015/16 survey, three new sites), one new SAM reference site, eight Pierce County (Option 2) sites, and 44 Partner sites. Mussel cages were recovered from 92 of those sites (i.e., 98%), with cages lost at one SAM and one Partner site. Similar to the 2015/16 survey results, Σ42PAHs , TPCBs, Σ11PBDEs, and Σ6DDTs were the most abundant organic contaminants detected in mussels at all sites (SAM, Pierce County, Partner). When compared to the 2015/16 survey results, TPCBs and Σ6DDTs in SAM site mussels had significantly higher median concentrations, indicating those contaminants should be closely monitored in future surveys to track whether there is an increasing trend. Similar to the 2015/16 survey results, all metals were frequently detected in mussels at all sites. Due to a change regarding the laboratory analysis methodology of the metal analytes, no temporal comparisons were made between survey years for the metals data.

The distribution of mussel tissue contaminant concentrations along the Puget Sound UGA was examined using cumulative frequency distribution (CFD) plots. The CFD plots revealed similar patterns for ΣR42PAHs, Σ11PBDEs, and Σ6DDTs, with all skewed toward the lower concentrations, suggesting that the majority of Puget Sound UGA shorelines have relatively low concentrations of these contaminants and that only a few sites have much higher concentrations, perhaps from locally high non-point sources, or site specific point sources. The CFD pattern for TPCBs was unlike the other organic contaminants in that it had a more gradual contaminant accumulation as the shoreline length increased, suggesting sources of this contaminant is more widely dispersed within the Puget Sound UGAs. The CFD patterns for most of the metals (arsenic, cadmium, lead, mercury, and zinc) had a more gradual contaminant accumulation as the shoreline increased, suggesting these contaminants are more widely dispersed within the Puget Sound UGA shoreline. The CFD pattern for copper was unlike the other metals, having a pattern more skewed to the lower concentrations, with only a few sites with much higher concentrations.

Sites with the highest concentrations of organic contaminants were located mainly in the more urbanized and industrialized south-central Puget Sound basin and sites with lowest concentrations were mainly in the remote and least developed Hood Canal basin. Similar to the organic contaminants, sites with the highest concentrations of metals were located in the urbanized south-central Puget Sound basin. However, low metal concentration sites occurred within the same urban south-central basin; a pattern not observed with the organic contaminants where all the sites had high or intermediate concentrations within the south-central basin. Further, continued positive correlations between the concentration of key organic contaminants (ΣR42PAHs, TPCBs, Σ11PBDEs, and Σ6DDTs) and metals (lead and zinc) in mussels and the percent of impervious surface in adjacent watersheds is evidence that this characteristic of urbanization provides a transport pathway for toxic chemicals from terrestrial to aquatic habitats.