PNNL

Abstract

The classification of shellfish growing areas in the Salish Sea is influenced by water quality, with high fecal bacteria levels leading to closures due to public health risks. To improve understanding of pathogen transport near shellfish beds and support regulatory monitoring and restoration, we developed a three-dimensional numerical model to simulate fecal bacteria dynamics. Using the Portage Bay region—historically impacted by fecal contamination—as a testbed, we incorporated a fecal bacteria module into the FVCOM-ICM-based Salish Sea Model (SSM-FB). The refined SSM-FB successfully reproduced the magnitude and seasonal variation of fecal bacteria concentrations in 2021. The simplified fecal bacteria kinetics with first-order decay due to mortality was demonstrated to be sufficient in dynamic coastal environment with short flushing time. Model results confirmed that Nooksack River discharge is the dominant source of fecal bacteria in Portage Bay, with peak concentrations over shellfish beds closely following high riverine bacteria loads. However, model predictive skill was constrained by limited fecal bacteria loading data from the river and coastal stormwater outfalls. During dry periods, stormwater outfalls likely contributed localized contamination, particularly following rainfall events. Increased field surveys in the watershed and outfalls—especially during storm events—could enhance early contamination detection. Integrating watershed-scale models to improve fecal bacteria loading estimates at finer temporal and spatial scales could significantly enhance nearshore predictions. Such advancements would support more effective monitoring, shellfish bed management, and harvesting schedule optimization.