Response of a Shallow, Polymictic Lake to Buffered Alum Treatment

Abstract

ABSTRACT The first aluminum sulfate (alum) and sodium aluminate lake treatment in Washington State was performed in Green Lake, a shallow (mean depth = 3.9 m), highly eutrophic, polymictic lake in Seattle. During October 1991, 181 tons of alum and 76.5 tons of sodium aluminate were applied to attain a total aluminum dose of 8.6 mg L−1. The use of sodium aluminate as a buffering agent allowed a higher dose of alum to be used, and maintained lake pH (6.7) and alkalinity (28 mg CaCO3 L−1) during treatment Secchi disk transparency increased from 1.9 to 6.1 m following treatment and remained higher throughout 1992 and 1993 than in pretreatment years with summer means of 3.5 and 2.9 m, respectively. Total phosphorus (TP) concentrations decreased from a mean of 40 μg L−1 before treatment to 14 μg L−1 immediately following treatment and remained below 29 μg L−1 during 1992 and below 35 μg L−1 during 1993. The 1992 and 1993 whole-lake, mean summer TP concentrations of 20 and 26 μg L−1. respectively, met the lake management goal of 30 μg L−1. Blue-green algal blooms were greatly reduced, despite the absence of summer dilution water inputs. Mean summer 1992 and 1993 chlorophyll a concentrations were 7.1 and 12.6 μg L−1; these concentrations were a marked improvement from the 1981 summer mean concentration of 28 μg L−1. The effectiveness of the alum/sodium aluminate treatment was critical in determining that the proposed $4.5 million, 11400 m3day−1 water treatment plant, which would have provided low-nutrient dilution water to Green Lake, would not yield the desired improvement in lake water quality.