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Abstract
The application of alum (aluminium sulphate) is widely used for lake restoration. Aluminium binds dissolved reactive phosphate (DRP), reducing its availability to phytoplankton and inhibiting algal blooms. The Bay of Plenty Regional Council has conducted continuous alum dosing of the Utuhina Stream since 2006 and the Puarenga Stream from 2010 to 2018. This has resulted in the addition of more than 730 tonnes of aluminium to Lake Rotorua. Improvements in Lake Rotorua water quality occurred concurrently with peak alum dosing, although sediment surveys conducted in 2012 and 2015 were unable to locate significant depositional areas of aluminium floc in the main basin of Lake Rotorua.
Elevated amorphous aluminium (i.e., recently deposited aluminium) was reported at sites closest to the Utuhina and Puarenga discharge points in the 2015 sediment survey, indicating that aluminium was potentially accumulating in the nearshore zone. The University of Waikato was contracted by the Bay of Plenty Regional Council to conduct a follow-up survey of near-shore sites along the southern edge of Lake Rotorua. In addition to sediment total and amorphous aluminium content, total phosphorus content was also determined. Amorphous aluminium is recognised as the fraction of total aluminium able to adsorb dissolved phosphorus, sequestering it from the water column. For this study it was assumed that increased proportions of amorphous aluminium were primarily derived from alum dosing.
Type
Report
Type of thesis
Series
Environmental Research Institute Reports
Citation
Date
2019
Publisher
Environmental Research Institute, The University of Waikato