Alum treatment effectiveness and longevity in shallow lakes

Abstract

An evaluation of six shallow Washington State (USA) lakes, treated with alum in the 1980s, has shown that treatment effectiveness ranged from 50 to 80% and lasted for at least five years. Such effectiveness/ longevity makes alum a highly cost-effective, in-lake treatment for shallow lakes. However, alum may be completely ineffective, or effectiveness may be short-lived, if much of the lake is covered with macrophytes that senesce during summer and contribute phosphorus (P) to the water. The sharp reduction in blue-green algae following alum treatments suggests that inhibition of the sediment-to-water migration rate of blue-greens may be an important mechanism for alum’s effect in oxic shallow lakes. That is in contrast to the primary mechanism in stratified anoxic lakes, which involves the complexation of P in the alum floc layer under reducing conditions.