The Cause of Continued Hypolimnetic Anoxia in Long Lake, WA, Following Advanced Wastewater Treatment by the City of Spokane

Abstract

ABSTRACT Long Lake, an impoundment of the Spokane River, was eutrophic prior to 1978. Extensive hypolimnetic anoxia, excessive summer algal growth and limited water clarity existed as a result of nutrient loading from the City of Spokane's primary sewage treatment plant. Advanced wastewater treatment (Am) began in the fall of 1977 with a process for 85 percent phosphorus removal. Investigations during post-AWT years (1978–1981) have shown significant reductions in phosphorus loading with corresponding declines in chlorophyll a, phytoplankton biovolume, primary productivity and increased water clarity to levels characteristic of mesotrophy. Hokever, anoxia has continued to exist, although the degree of anoxia has been considerably less. The purpose of this study was to investigate the relationship between various measures of oxygen demand and the phytoplankton community in Long Lake with AWT “on-line”. Biochemical oxygen demand, chemical oxygen demand and sediment oxygen demand were determined. Sediment ox...