RECENT SALINITY CHANGES
by MARK ROSENKRANZ
Garrison Lake has gone through some dramatic changes over the past ten years. From the surface the lake does not look much different. However, starting about three meters down, the lake is very unique.
Prior to 1992 outflow from the wastewater treatment plant was discharged into the lake. Accelerated eutrophication of the lake led the city of Port Orford to explore options for wastewater disposal. In 1991 a dune disposal system was approved and built in the foredune separating Garrison Lake from the ocean. The wastewater disposal system was effective in keeping approximately two million gallons of effluent from the lake per day.
An El Nino storm event during the winter of 1997-1998 brought unusually high seas and strong southerly winds that eroded the foredune separating Garrison Lake and the ocean. Along with washing most of the waste disposal piping out to sea, the ocean washed over the now reduced dune and filled the bottom of Garrison Lake with seawater. A study by Richard Petersen from Portland State University in September 2000 found that the conductivity of Garrison Lake increased from 6597 µS/cm at three meters to 38,056 µS/cm at four meters (figure 1). Ocean water conductivity is generally around 43,000 µS/cm. Based on a bathymetric survey in 2001 by Mark Rosenkranz at Portland State University the total volume of Garrison Lake is 1,879,000 m3. The volume of salt water starting at three meters is 637,000 m3 or roughly a third of the lake volume (figure 2).
The consequences of such a large amount of salt water in the lake is a very stable chemolimnion that will not easily mix with the surface water. Extended stratification may lead to hydrogen sulfide production in the chemolimnion that could be released if the stratification were destabilized by a large wave from the ocean or another high-energy event.