Stormwater Action Monitoring Program Puget Sound Nearshore Mussel Monitoring: 2023/2024 Status and Trends Report

The Stormwater Action Monitoring (SAM) Status and Trends in Receiving Waters program conducts long-term contaminant monitoring in Puget Sound nearshore marine waters. Monitoring Is intended to provide a regional assessment of whether and how stormwater discharge and the collective stormwater management actions implemented in the region are affecting nearshore biota contaminant levels. SAM Puget Sound Nearshore Mussel Monitoring studies focus on the bioaccumulation of pollutants in caged native bay mussels (Mytilus trossulus) to evaluate the status and trends of nearshore conditions. The 2023/2024 Puget Sound Nearshore Mussel Monitoring survey represents the fifth successful deployment of mussels for the SAM program study and the second under the new study design. The new study design samples across the entire nearshore of the Puget Lowland ecoregion and stratifies study sites into four categories by estimates of average percentage of total impervious area (TIA%) of the contributing upland watersheds. Results from this study characterize the current spatial extent of contamination in the nearshore and the magnitude of concentrations for the most frequently detected contaminants monitored in the 2024 survey. This study further evaluates changes occurring between the 2022 and 2024 surveys conducted under the new study design and tests for trends in the larger monitoring dataset across the eight-year monitoring period (2016-2024) using a subset of sites within the urban growth area. Additionally, contaminant concentrations related to nearshore development based on total impervious area (TIA%) in adjacent watersheds are described. Results are further compared against established thresholds for low, intermediate, and high relative concentrations.

Similar to prior surveys, four organic analytes (Σ16PAHs, TPCBs, Σ11PBDEs, Σ6DDTs) and all six metal analytes (arsenic, cadmium, copper, lead, mercury, zinc) continued to be the most abundant contaminants detected in mussels of the Puget Sound nearshore, with Σ16PAHs and TPCBs having the highest concentrations. Estimates of the spatial extent of mussel tissue contamination in the Puget Lowland ecoregion during the 2024 survey showed that most of the Puget Sound nearshore length had low concentrations of Σ16PAHs, Σ11PBDEs, and Σ6DDTs based on project-specific thresholds, and that a very small proportion of the nearshore length had high concentrations. The nearshore length with high concentrations of these three organic contaminants occurred only within the most developed shoreline category with a total percent impervious area in upland watersheds above 40% (high TIA% strata). Estimates of the spatial extent of mussel tissue contamination by TPCBs in the 2024 survey differed, showing that most of the Puget Sound nearshore length had intermediate concentrations of TPCBs, and that a quarter of the total nearshore length had high concentrations. The nearshore length with high TPCB concentrations were spread across the four TIA% strata; however, the most developed shoreline category had the greatest proportion of nearshore length with high concentrations. These results continued to show how higher concentrations of organic contaminants in mussels were associated with higher levels of impervious surface in the adjacent watershed. The spatial extent of mussel tissue contamination by metals also showed that most of the Puget Sound nearshore length had intermediate or high concentrations of all six metal analytes.

When comparing the spatial extent of mussel tissue contamination in the nearshore in the 2022 and 2024 surveys, the distribution patterns for Σ16PAHs, Σ11PBDEs, and Σ6DDTs were very similar, and showed small changes with either the percentage of nearshore length with high concentration values having reduced or the percentage of nearshore length with low concentration values having increased.

The spatial distribution patterns for TPCBs and all six metal analytes differed and showed that concentrations across the nearshore increased in the 2024 survey. When examining the organic contaminant distribution changes between the 2022 and 2024 surveys within the four nearshore development categories (least, low, medium, and high TIA% strata), a decrease within the low and medium development strata and an increase in the high-development strata was observed in the concentrations of Σ16PAHs across the nearshore. Concentrations of Σ11PBDEs decreased in the high TIA% strata, Σ6DDTs decreased in the medium TIA% strata, and TPCBs increased across all four TIA% strata.

When comparing the concentrations in mussel tissues collected in the nearshore during the 2022 and 2024 surveys, no changes were observed in the geometric mean concentrations of Σ16PAHs, Σ11PBDEs, and Σ6DDTs. In contrast, an increase in TPCB concentrations was observed in mussel tissue in 2024, at 2.2x higher than 2022. For metal analytes, mercury, cadmium, arsenic, copper, and zinc, concentrations also significantly increased. However, no meaningful differences in concentrations were observed for lead. These early monitoring results should consider the unplanned extended exposure period in the 2024 survey, possibly leading to additional contaminant uptake. Additional sampling years under the new study design will help infer conclusions regarding any significant trends for contaminants across the nearshore of the broader Puget Lowland ecoregion. Future temporal trend analyses should focus on data collected under the new design (2022 onward), with occasional supplemental analyses spanning the full monitoring period for urban growth area sites.

When testing for trends in the larger eight-year, five survey monitoring period within the urban growth area, a moderate decline in Σ11PBDEs and a weak but plausible decline in Σ16PAHs was observed. No meaningful trends were observed for Σ6DDTs or TPCBs over the monitoring period. For the metal analytes, there was a moderate increase in cadmium and arsenic and a weak but plausible increase in zinc concentrations. These results were similar to the prior temporal change analysis conducted for the first three SAM monitoring surveys conducted between 2016 and 2020. Further, the declining Σ11PBDEs concentrations but stable TPCB concentrations were congruent with the temporal patterns seen in two other WDFW-TBiOS indicator species, English sole and Pacific herring monitored for over 20 years and reported in the Toxics in Aquatic Life Vital Sign.

Water quality in the Puget Sound is influenced by numerous natural and anthropogenic drivers, including river flow, tidal mixing, oceanographic conditions, climate variability, seasonal patterns, pointsource discharges, and land-use change. These factors can mask or mimic potential effects of stormwater management, making it challenging to isolate management signals. With only two surveys available for analysis under the new Puget Sound-wide sampling design, this dataset has limited power to detect trends or attribute observed differences to stormwater management actions. Given the complexity of Puget Sound ecosystem and the incremental nature of stormwater improvements, observable water quality responses are expected to emerge slowly and may require multiple years of data and multiple lines of evidence to confidently distinguish management effects from background variability.