South Puget Sound dissolved oxygen study: water quality model calibration and scenarios

Portions of South and Central Puget Sound are on the Clean Water Act Section 303(d) list of impaired waters because observed dissolved oxygen measurements do not meet the Washington State water quality standards. Human sources of nutrients can increase algae growth, which can decrease oxygen as the additional organic matter decays. Low oxygen can impair fish and other marine life. Computer modeling tools are needed to isolate the impacts of human contributions. The purpose of this study is to identify how much human contributions are contributing to low dissolved oxygen (DO) in South Puget Sound. Previous reports summarize data collection, nutrient load estimates for marine point sources and watershed inflows that include point and nonpoint sources, and the circulation model. This report summarizes the calibration and application of the water quality model to isolate the impacts from groups of sources. The model predicts the regional and seasonal patterns of chlorophyll, DO, and nitrogen throughout South and Central Puget Sound. The model predicts that internal (inside the model domain) current human nutrient loads from marine point sources and watersheds as well as external (north of model domain) current anthropogenic loads are causing DO to decline by as much as 0.4 mg/L in portions of Totten, Eld, Budd, Carr, and Case inlets, and East Passage, which violates the standards. There are not violations across the entire South or Central Puget Sound. While keeping the external anthropogenic load constant, internal marine point sources exert a greater impact than human sources within watershed inflows. Reducing the internal human nutrient load would decrease the magnitude and extent of DO depletion. Additional scenarios are needed to isolate the effects of individual sources