To help reduce the impacts of phosphorus in certain lakes we treat incoming streams with alum.
The continual low dose of alum binds the phosphorus in the incoming streams so when the streams reach the lake the plants cannot use the phosphorus to grow.
Currently there are three phosphorus plants in the Rotorua district:
The P-locking plants have been very successful in improving water in Lake Rotorua. In 2012 Lake Rotorua had the best water quality since the 1990s and this was attributed to the alum dosing in conjunction with favourable climate conditions and on-farm changes.
It was originally estimated that alum dosing would remove two tonnes of phosphorus from Lake Rotorua. Our scientists now think we are actually getting 10 tonnes of phosphorus out of the Lake Rotorua. More modelling work is underway to better understand the effect alum dosing is having on water quality.
We are conscious of any effects our interventions may have on the lakes ecosystems and environment. As such we are working with the University of Waikato to monitor the streams where alum dosing occurs. To date there is no evidence that the alum dosing has any adverse effects. For more information see our science report on Concentrated Alum Trials.