A study of the nutrients in the main basin of Puget Sound
This study compared recent and historical data to determine the presence of any significant changes in nutrient and oxygen concentrations subsequent to METRO discharge, examined seasonal cycles in water properties, and examined the flux of nutrients within the study area.
Abstract
In 1973, the council of the Municipality of Metropolitan Seattle (METRO) implemented a program of environmental planning, pilot plant studies, Puget Sound water research projects, etc. One of the tasks proposed was to determine what effects on water properties within the main basin of Puget Sound are attributable to discharge from METRO's sewers. This study compared recent and historical data to determine the presence of any significant changes in nutrient and oxygen concentrations subsequent to METRO discharge, examined seasonal cycles in water properties, and examined the flux of nutrients within the study area. The study area included those waters south of Whidbey Island and north of The Narrows including Colvos Passage but excluding the Port Orchard system.
Observations of water properties were made during 22 cruises beginning 5 November 1974 and ending 19 November 1975. Water properties observed at three week intervals included salinity, dissolved inorganic orthophosphate, nitrate, nitrite, ammonia, silicate, and dissolved oxygen. Two time studies were made at the end of the West Point sewer. Samples for the uptake of 15N-labeled compounds were made at selected locations and times.
A comparison of the new data (1974-75) with the historical data (1932-1963) indicated about a 10% increase in phosphate and no measureable change in the dissolved oxygen. This increase in phosphate was well within the normal variability observed in the historical data. Long-term changes in both dissolved oxygen and phosphate have been observed that are related to changes in upwelling off the Washington coast.
Ammonia originating at a sewer may be used as a tracer of the effluent very close to an outfall, but concentrations generally decrease rapidly to the usual low background level in a short distance and time. Uptake of nitrate and ammonia by phytoplankton showed a definite seasonal trend and were high in comparison to open ocean values.
Observations made in the physical model of Puget Sound indicated that the effluent plume from the West Point outfall were filamentous, fractured, and generally of low concentrations outside of the "near field". This was confirmed by limited field observations.
Transport calculations for the main basin showed an average flux of 60,000 m3sec-1 into or out of the basin. In comparison, the average combined discharge of all sewers entering Puget Sound was 225 m3sec-1. Calculations of the flux of phosphate (as phosphate ion) showed an average transfer of 1,210 metric ton per day while the average phosphate contribution from sewers was only 12.6 tons per day. Also at any given time the amount of phosphate in the main basin is about 18,400 tons.
