Application of Lime and Alum to Stormwater Retention Lakes to Improve Water Quality

Abstract

Abstract Urban stormwater retention lakes receive enormous amounts of nutrients from point (sewers) and non-point (domestic runoff) sources. Water quality in these lakes is poor and characterized by: 1) high phosphorus concentrations and algal biomass, 2) poor buffering capacity (alkalinity as low as 60 mg/L as CaCO3); and 3) high pH (often greater than 9). Thus chemical treatments which alter pH will have to be carefully considered. Recently, we used lime (Ca(OH)2) and/or alum (Al2(SO4)3·14H2O) to reduce phosphorus concentrations in the water column and precipitate out particulate matter. Of the two chemical treatments, we found that a lime/alum mixture was better at controlling macrophytes and shoreline filamentous algae, but alum was better at controlling planktonic algal growth and turbidity. A combination of both chemicals, lime which elevates pH and alum which lowers pH, is used to maintain pH within a desirable range (6-10). Overall water quality can be improved through the application of alum/lime mixtures, however, these applications will have to be applied routinely throughout the open-water season due to continuous inputs of nutrients from point- and non-point sources.