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Lake Rotorua and Lake Rotoehu: Total and non-crystalline aluminium content in bottom sediments

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 treatment of the Puarenga and Utuhina Streams discharging to Lake Rotorua and the Waitangi Soda Stream entering Lake Rotoehu for nearly 10 years. This has resulted in the addition of more than 722.3 tonnes of aluminium to Lake Rotorua and 80.8 tonnes to Lake Rotoehu. Dissolved reactive phosphorus levels within the streams have been markedly reduced due to alum dosing. This appears to support long-term trends of reducing Trophic Level Index (TLI) levels in both lakes, indicative of water quality improvement, with greater success in Lake Rotorua. In addition, secondary sediment capping effects from the dosing have been suggested as a potential process explaining recent improvements in lake water quality. Accumulation of alum-derived aluminium in lake sediments potentially provides the additional benefit of adsorbing dissolved phosphorus which might otherwise be released from lake sediments under anoxic conditions. The University of Waikato was contracted by the Bay of Plenty Regional Council to conduct on-going monitoring of sediment aluminium and phosphorus concentrations in lakes Rotorua and Rotoehu. In addition to sediment total aluminium content, the proportion of amorphous (non-crystalline) aluminium 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 derived from alum dosing. Fifteen sediment cores were taken from Lake Rotorua and the findings from the analysis of these cores was compared to data collected in 2006 and 2012. Sites were chosen to represent three different depositional zones: zone 1 (Sites Ru1-4) to represent the immediate depositional area of the treated streams, zone 2 (Sites Ru5-11) to represent the main sediment accumulation basin (<20 m water depth) and zone 3 (Sites Ru12-15) to represent an area of low sediment accumulation (< 20 m water depth). Analysis of the sediment showed little or no accumulation of aluminium in either zone 2 or zone 3 compared to similar surveys conducted in 2006 and 2012. There was also no change in the mean sediment total aluminium content for zone 1, however, two zone 1 sites (Ru2 and Ru3) contained notably higher proportions of amorphous aluminium in their surface sediments. In association with these two sites, surface sediment pore water dissolved reactive phosphate (DRP) concentrations were reduced compared to other sampled sites. These findings suggest that alum-derived aluminium is only accumulating in an area close to the discharge points of the Utuhina and Puarenga Streams, although further survey work will be needed for confirmation. The lack of aluminium accumulation in the main lake basin indicates that secondary sediment capping effects are unlikely to be substantial, and recent improvements in the water quality of Lake3 Rotorua are driven by other factors, such as a positive feed-back loop between reductions in organic matter sedimentation and nutrient regeneration. A total of seven sediment cores were taken from Lake Rotoehu in a transect line running from the mouth of the Waitangi Stream out into the main basin. Significant accumulation of alum derived aluminium (>25,000 mg kg⁻¹) was found in the sediment of Te Weta Bay, however, no observable accumulation of aluminium (i.e., <5000 mg kg⁻¹) was found in the main basin of Lake Rotoehu. Impedance of flow from the large biomass of Ceratophyllum demersum in Te Weta Bay is likely preventing alum flocs from reaching the main lake basin, negating any hoped for benefits of DRP sequestration through sediment capping. In addition, large diel shifts in pH between 6 and 9.5 caused by photosynthesis-respiration cycles of C. demersum are causing dissolution of the aluminium-phosphorus bound precipitate, reducing the performance of the alum dosing programme. It is recommended that an additional sediment survey be conducted around Kawaha Point and the area north of Sulphur Bay in Lake Rotorua to determine if these areas are accumulating aluminium derived from alum flocculent. Given improved understanding of processes though which alum dosing improves lake water quality, alum dosing rates for Lake Rotorua should be reviewed with the aim of testing if lower alum dose rates can achieve the TLI target. The Waitangi Soda Stream alum treatment programme should be reviewed in order to assess whether a more effective strategy can be implemented in the presence of potentially confounding factors such as lake weed and high iron levels in stream inflows.
Type of thesis
ERI report
Tempero, G. W., & Hamilton, D. P. (2016). Lake Rotorua and Lake Rotoehu: Total and non-crystalline aluminium content in bottom sediments (ERI report). Hamilton, New Zealand: Environmental Research Institute, Faculty of Science and Engineering, The University of Waikato.
Environmental Research Institute, Faculty of Science and Engineering, The University of Waikato
© 2016 copyright with the authors.