Sediment aluminium content of Lakes Rotorua and Rotoehu: 2024 monitoring survey

Abstract

Continuous alum (aluminium sulphate) dosing of the Utuhina Stream was initiated in 2006, with the objective of reducing dissolved reactive phosphate (DRP) loading of Lake Rotorua, thereby reducing lake primary productivity. A second continuous alum dosing station began operation on the Puarenga Stream in 2010. The water quality of Lake Rotorua has improved, with the Trophic Lake Index (TLI) declining from 5.0 in 2004 to 4.3 in 2024. Similarly, continuous alum dosing of the Waitangi Stream which flows into Lake Rotoehu began in 2011. However, alum dosing at this site was not as effective as anticipated, as invasive aquatic weed inhibited alum from reaching the main basin. As of 31 December 2023, a total of 993 tonnes of aluminium had been dosed to Lake Rotorua and 128 tonnes to Lake Rotoehu.