Estuarine nutrient pollution impact reduction assessment through euphotic zone avoidance/bypass considerations

The feasibility of reducing nutrient pollution impact by redirecting the effluent to depths below euphotic zone was investigated for deep estuarine Puget Sound region Salish Sea in Pacific Northwest America. hypothesis tested that thickness outflow layer estuaries may be greater than depth, allowing a fraction nutrients exported out passively through layers immediately below. depth varies from 8 25 m while can reach up ≈ 60 m. Outfall relocation strategies were on 99% anthropogenic loads currently delivered Sound. quantified using previously established biophysical Model , gross primary production and exposure low dissolved oxygen (DO) levels as metric (< 2 mg/L hypoxia < 5 impairment). Eliminating (above natural) rivers wastewater reduced 8.1% 11.2%, respectively. Relocating outfalls deeper waters resulted improvements, but only sill-less sub-basins such Whidbey, where DO impairment decreased (7.9% 6.8%, respectively). presence multiple sills circulation cells increased rendered bypass goals unfeasible originally envisioned. However, an alternate export pathway identified bottom exchange flow via Whidbey Basin Deception Pass. An unexpected reduction magnitude (≈ 4%) due additional (22%) freshwater discharged estuary also noted. potential loss strength rerouting natural surface submerged deep-water is new unforeseen impact.