Stormwater Action Monitoring Program Puget Sound Nearshore Mussel Monitoring: 2021/2022 Status Report

The Stormwater Action Monitoring (SAM) Status and Trends in Receiving Waters program conducts monitoring in Puget Sound nearshore marine waters to provide a regional assessment of whether collective stormwater management actions are leading to improved receiving water conditions. The SAM Puget Sound Nearshore Mussel Monitoring studies focuses on the bioaccumulation of pollutants in caged native bay mussels (Mytilus trossulus) to evaluate the current status and trends of nearshore conditions. The 2021/2022 survey was the first conducted under the new SAM study design, whereby the sampling area expanded to the entire nearshore of the Puget Lowland ecoregion and the study sites were stratified into four categories by estimates of average percentage of total impervious area (TIA%) of the contributing upland watersheds. In this survey report we provide information on the status of contamination in the Puget Sound nearshore. Where appropriate, results from this survey are compared to prior surveys conducted under the original study design, without additional analyses conducted to measure statistical significance. The statistical significance of temporal trends in the survey data will be evaluated in the next survey report, and will consider the study design changes that occurred between survey years. Key findings from the 2021/2022 survey include:

• Similar to prior survey years, Σ16PAHs, TPCBs, Σ11PBDEs, and Σ6DDTs continue to be the most abundant organic contaminants detected in mussels of the Puget Sound nearshore. Though, the detection frequency for PBDEs noticeably reduced in this survey. All the metals (arsenic, cadmium, copper, lead, mercury, zinc) also continue to be frequently detected in mussels.
• The central tendency concentrations of most of the measured organic and metal contaminants in mussels at survey sites across the Puget Sound shoreline were similar or lower when compared to concentrations from prior surveys with sites within the urban growth areas only.
• Estimates of the spatial extent of mussel tissue contamination in the Puget Lowland ecoregion indicate that most of the Puget Sound nearshore length (approximately 50-90%) had low concentrations of Σ16PAHs, TPCBs, Σ11PBDEs, and Σ6DDTs based on project-specific thresholds, and less than approximately 5% of the nearshore length had high concentrations.
• Most of the Puget Sound nearshore length (approximately 50-75%) had low concentrations of cadmium, lead, and mercury, and intermediate concentrations of zinc and copper. For the majority of the metal analytes (cadmium, copper, lead, mercury, zinc), a small proportion (approximately 0-10%) of the nearshore length had values exceeding the high concentration threshold. Arsenic was the only measured contaminant where the majority of the nearshore length (approximately 75%) had concentrations exceeding the high concentration threshold, with sites located across all TIA% categories.
• Most of the organic and metal contaminant distribution patterns shifted toward lower concentrations across the nearshore when the sampling area expanded to the entire Puget Lowland ecoregion. A higher proportion of the nearshore length had values below the low concentration threshold, and a lower proportion of the nearshore length had values above the high concentration threshold.
• Sites located adjacent to the upland watersheds with a higher percentage of impervious area had greater exposure to Σ16PAHs, TPCBs, Σ11PBDEs, Σ6DDTs, and zinc. Results were congruent with prior surveys, and support the hypothesis that impervious surface continues to provide a transport pathway for several toxic chemicals from terrestrial to aquatic habitats in Puget Sound.