First use of whole lake alum treatment in New Zealand
Lake quality criteria in Environment Bay of Plenty’s Water and Land Plan indicate that five of the region’s lakes need improvement. A number of remedial methods are being investigated and trialled. Lake Okaro, near Rainbow Mountain, has the poorest quality of the Rotorua lakes and is small enough (33 hectares) for alum treatment to be tried to precipitate and inactivate phosphorus. Land management options, such as artificial wetlands, are also being implemented, and these will be the main long-term methods for sustainable lake quality management.
Lake Okaro suffers blue-green algal blooms that are favoured by high nutrient levels and a low ratio of nitrogen to phosphorus. Adding alum aims to reduce the phosphorus concentration of the surface waters so that blue-green algae are no longer encouraged. In December, alum was applied to a concentration of 0.5 g/m3 aluminium in the surface waters. The effectiveness of this initial low dosing has yet to be proven, but it is hoped that when the aluminium has been incorporated in the lake sediment it will act as a barrier to phosphorus release and improve lake health in the longer term.
NIWA contact: John Quinn [[email protected]] John McIntosh, Environment Bay of Plenty [[email protected]]