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Puget Sound Georgia Basin Model

Nutrient Loads to the Salish Sea (Puget Sound and Georgia Basin) - Rivers, Outfalls, and Ocean Boundaries

Nutrient measurements from a variety of sources were used to develop watershed-loading estimates for the 64 watersheds of interest shown in the figure below. Hydrologic analysis used to develop the associated stream flows is provided in Hydrodynamics/Freshwater Inflow and by Mohamedali et al. (2011). The primary source of the data was measurements conducted as part of the Washington State Department of Ecology's South Puget Sound Dissolved Oxygen (DO) study, which monitored 33 rivers between 2006 and 2007 (Roberts et al. 2008). Multiple linear regression was used to predict daily nutrient concentrations for the rivers and streams from more coarsely sampled (e.g., monthly) data sets.

The primary area of interest includes the watersheds that drain into Puget Sound, but watersheds that drain into the Straits of Georgia and Juan de Fuca have also been included.

Source: Mohamedali T, M Roberts, B Sackmann, and A Kolosseus. 2011. Puget Sound Dissolved Oxygen Model Nutrient Load Summary for 1999-2008. Publication No. 11-03-057, Washington State Department of Ecology, Olympia, Washington.

A total of 99 municipal wastewater treatment plants (WWTPs) or industrial facilities, 23 major rivers, and 116 watershed streams discharge to the model domain, either directly into the marine waters of Puget Sound or into rivers downstream of monitoring locations. This includes 78 U.S. municipal WWTPs, 9 Canadian municipal WWTPs, 5 oil refineries, 4 active pulp/paper mills, and 1 aluminum facility within the Puget Sound study area. Of all the forms of nitrogen, dissolved inorganic nitrogen (DIN; sum of nitrate+nitrite and ammonium) is of greatest interest because this form of nitrogen often limits the growth of marine algae. The figure below is an example of loads distributed around the Salish Sea domain that were computed using this technique. Median DIN concentrations in rivers and WWTPs discharging directly into the Salish Sea between 1999 and 2008 are shown.

Freshwater inflow - DIN loads.

Municipal WWTP and industrial discharge - DIN loads

 

Ocean Boundary Conditions

Since the 1950s, researchers have pointed out the importance of the process by which water trapped behind the sills in fjords such as Puget Sound is replaced by water of greater density that flows in over the sills. This inflow in the summer months is fed by coastal upwelling, which supplies most of the macronutrients available for production. For Year 2014 calibration using the current version of the model with expanded grid, the ocean boundary values at the edge of the continental shelf for nutrients, DO, and algal biomass were derived from quarterly monitoring data collected from various locations over the continental shelf by the Department of Fish and Oceans, Canada (DFO) and interpolated to the model ocean boundary in time and space (Khangaonkar et al. 2018).

Water-quality (T/S, nutrients, DO, and algae) monitoring stations from the Department of Fisheries and Oceans (DFO) over the continental shelf, along with SSM boundary. The quarterly monitoring data from DFO stations were interpolated in space and time to provide water quality ocean boundary conditions.

Example SSM OBC water quality profile at a representative boundary node in Year 2015 June, generated from DFO quarterly monitoring. Red dots are selected DFO data within the particular month.

Salish Sea Model Overview

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