Variations in the aluminum:phosphorus binding ratio and alum dosage considerations for Half Moon Lake, Wisconsin

Abstract

The aluminum:phosphorus binding ratio (Al:P) is an important variable for estimating the Al dosage required to inactivate loosely bound and iron-bound P (redox-P) in sediment for internal P loading control in lakes. For shallow Half Moon Lake, Wisconsin, the Al:P ratio varied in a negative exponential pattern as a function of increasing redox-P concentration. While more Al was needed to inactivate higher concentrations of redox-P, inactivation was more efficient at higher redox-P. The Al:P ratio needed to bind 90% of the redox-P exceeded 150:1 for redox-P concentrations <0.2 mg/g and approached 20:1 for concentrations >2.0 mg/g. Competition for binding sites by other constituents in relation to redox-P concentration may be responsible for this pattern. Although organically bound P was not important in Half Moon Lake, it may be in other cases, and lake specific assays are recommended to determine the most appropriate Al dosage. Even then, slower processes of P release from labile organic P and vertical diffusion may not be addressed by higher Al dosages, and more research is warranted. Because redox-P varied horizontally as a result of lake bathymetry, variations in the Al:P ratio were considered for lake-wide alum dose calculation for Half Moon Lake. The estimated lake-wide average dosage of 115 g Al/m2 was high but similar to other recent effective treatments reported in the literature.