Sediment aluminium content of Lakes Rotorua and Rotoehu: 2020 monitoring survey

The application of aluminium sulphate (alum) to freshwater systems is commonly undertaken in order to restrict the availability of dissolved reactive phosphate (DRP), thereby reducing phytoplankton growth. Alum treatment of inflows to Lake Rotorua was initiated in 2006 and to Lake Rotoehu in 2011. As of December 2020, a total of 773 tonnes of aluminium had been dosed to Lake Rotorua and 124 tonnes to Lake Rotoehu. Improvements in water quality have been subsequently observed in Lake Rotorua with values of the Trophic Lake Index (TLI) decreasing from 4.8 to 4.2. In contrast, water quality has not significantly improved in Lake Rotoehu and alum dosing was halted from July 2018 to December 2020 while a review was conducted. The University of Waikato was contracted by the Bay of Plenty Regional Council to conduct on-going monitoring of sediment aluminium concentrations in lakes Rotorua and Rotoehu as part of their resource consent conditions for alum dosing of inflows to these lakes. In addition to sediment total aluminium content, the proportion of amorphous (non-crystalline) aluminium was assessed. Amorphous aluminium is recognised as the fraction of total aluminium able to adsorb dissolved phosphorus, sequestering it from the water column. It was assumed that increased proportions of amorphous aluminium were primarily derived from alum dosing. Fifteen sediment cores were taken from Lake Rotorua in December 2020. Previous surveys had reported little aluminium accumulation in the main basin of Lake Rotorua, with aluminium accumulation primarily occurring near the outlet to the Utuhina Stream and the 45 m deep crater north of Motutara Point. Based on this information, coring locations were modified from previous surveys with seven near-shore and crater sites sampled and the number of main basin sites sampled reduced to eight. The background sediment aluminium content for the main basin of Lake Rotorua is approximately 5 g Al kg-1 dry weight. The current survey supports the findings of previous sediment surveys with the main depositional areas for alum derived aluminium being the surface (1–4 cm depth) sediments of Te Ruapeka Bay, which can reach 12 g Al kg⁻¹ and the crater north of Motutara Point at ~8 g Al kg⁻¹. There was no apparent accumulation of aluminium beyond background levels to the east of Sulphur Bay or in the main basin of Lake Rotorua. In Lake Rotoehu, seven sites were surveyed in December 2020, extending from the mouth of the Waitangi Stream out into the main basin of the lake. Results were similar to those reported by Tempero and Hamilton (2016), with background sediment aluminium content approximately 5-6 g Al kg⁻¹. Alum derived aluminium was primarily accumulating in Te Wairoa Bay close to the discharge point of the Waitangi Stream with levels exceeding 25 g Al kg⁻¹ However, the two-year cessation of alum dosing resulted in a small decline of ~5 g Al kg⁻¹ in the surface sediments. Despite a five-fold increase in total aluminium content in Te Wairoa Bay over background levels, amorphous aluminium content appears to have declined due to the 2-year halt in alum dosing and aging of alum floc to the mineral gibbsite. There was no evidence of aluminium accumulation in the main basin of Lake Rotoehu.