Nutrient pollution impact reduction assessment in a deep estuary, euphotic zone avoidance/bypass considerations

The feasibility of reducing nutrient pollution impacts by redirecting excess nutrient flux away from the photic zone is investigated. Alternate effluent discharge strategies to avoid or bypass the euphotic zone were tested under the hypothesis that in deep estuaries, depth of the surface exchange outflow layer may be greater than euphotic zone depth, providing opportunity for a fraction of the nutrient pollution to be exported out passively. We used the Salish Sea region in the Pacific Northwest as a test bed for this assessment. Euphotic zone depth in the Puget Sound basin of Salish Sea in U.S waters varies from 8 m to 25 m while the depth of outflow layer is approximately 60m. Sensitivity of biological response and water quality impact were quantified using an established biophysical model of the system, using exposure to low DO levels as the metric (< 2 mg/L hypoxia and < 5 mg/L impairment). Opportunity to reduce nutrient pollution impact was tested through outfall relocation strategies, applied to 99% of the anthropogenic loads currently delivered to the Puget Sound. The results show that relative to natural impairment levels, marine wastewater outfalls are responsible for 36% of increase, while loads from upstream watersheds that enter Puget Sound via river flows, are responsible for 70% of increase in impairment. Results were consistent with the hypothesis in that moving the outfalls to deeper waters resulted in reduced primary production. However, in some basins, the benefits of lower water column respiration were offset by reduced DO production and were accompanied by some loss in the strength of circulation. Puget Sound basin results indicate worsening of DO impairment hours (average +3.0%), while Whidbey Basin showed improvement in DO impairment hours (-6.8%) relative to existing conditions. The results indicate that presence of multiple sills and the associated reflux flows / circulation obstruct the export of nutrients out of the system. The efforts to relocate outfalls to achieve euphotic zone bypass and improve DO impairment were therefore not as effective as hypothesized.