|dc.description.abstract||This report is intended to supplement the original 2015 report “Ecotoxicological Review of Alum Applications to the Rotorua Lakes” published as Environmental Institute Report Number 52 by the University of Waikato. A number of relevant studies published since 2015 regarding toxicological effects of aluminium (Al) are reviewed, and aspects specific to Lake Rotorua which were identified during the Plan Change 10 science review conducted by Professor Warwick Vincent for the Bay of Plenty Regional Council in 2018 including;
1) Are the aluminium concentrations used in toxicological testing relevant to the low concentration of aluminium measured in the water of Lake Rotorua?
2) Given Lake Rotorua has a low buffering capacity, are pH shifts driven by algal bloom photosynthesis likely to result in toxic aluminium complexes?
3) What is the potential for long-term chronic toxicological effects, particularly on downstream receiving ecosystems?
4) What is the effect of natural geothermal sources of aluminium in Sulphur Bay?
Since 2015, significant advances have been made in determining toxic thresholds for Al, particularly chronic effects. The United States Environmental Protection Agency (USEPA) has recently published as series of studies (i.e., Cardwell et al. 2018, DeForest et al. 2018, Gensemer et al. 2018, Wang et al. 2018) covering acute and chronic toxicological testing on eight aquatic species under a wide range of environmental conditions and total aluminium concentrations, including those common to Lake Rotorua (i.e., water hardness 14 mg L⁻¹ (as CaCO₃); dissolved organic matter 2 mg L⁻¹, pH 6.8; total Al ~0.020 mg L⁻¹. This work resulted in the publication of a multiple linear regression (MLR) model developed to predict chronic Al toxicity under varying conditions of dissolved organic matter (DOM), pH and water hardness (. The model predicts the likely total aluminium concentrations hazardous to 5% (HC5) of species or genera for a range of pH’s. The HC5 value is a statistically derived value from sensitivity data of multiple species to a single toxicant, and is similar to the Predicted No-Effect Concentration (PNEC) reported for single species toxicological tests. The mean total Al concentration in Lake Rotorua (0.020 mg L⁻¹) was found to be well below the calculated threshold for chronic toxicity effects based on the modelled HC5 estimates (from 0.045 mg L1 at pH 6.5 to 0.644 mg L⁻¹ at pH 8, under typical DOM and hardness conditions). Therefore, chronic toxicological effects from Al exposure are unlikely to occur under typical pH conditions in Lake Rotorua. However, caution should be used in applying these values as the data is not based on New Zealand species.
In poorly buffered eutrophic systems such as Lake Rotorua, diel pH cycling of up to 3 pH units (i.e., pH 6.5 - 9.5) is common. When pH >8, Al(OH)₃ converts to the more toxic Al(OH)₄⁻ species, resulting in a lowering of the toxic threshold concentration at pH levels >8. Compared to acidic conditions there is relatively little research examining toxic effects of Al under alkaline conditions. Most research indicates that toxicity increases above pH 8, with acute effects becoming more prevalent above pH 9. However, given the low total Al concentrations in Lake Rotorua and comparatively short exposure times to high pH during algal blooms, acute Al toxicological effects resulting from phytoplankton driven diel shifts in pH are unlikely. However, it is recommended that chronic effects of combined total aluminium and shortduration alkaline pH shifts be further investigated to determine the potential osmoregulatory impacts on key species for Lake Rotorua.
Currently, there is no requirement for monitoring of potential downstream impacts in the Kaituna River or Ōngātoro/Maketu Estuary from alum dosing to Lake Rotorua. A total Al concentration of 0.08 mg L⁻¹ measured at the half-way point of the Kaituna River in May 2008 (following dosing to the Utuhina Stream but prior to Puarenga Stream dosing) has been reported, well below predicted HC5 toxicity threshold. Notably, tributaries to the Lower Kaituna River appear to make greater contributions to total Al loading of the Kaituna River, with inflow concentrations ranging from 0.14 - 0.52 mg L⁻¹, presumably associated with increased suspended sediment levels. The ultimate discharge point of Lake Rotorua is the Ōngātoro/Maketu Estuary. Golding et al. (2015) proposed a guideline value of 0.024 mg total Al L⁻¹ for 95% species protection, based on chronic effect values from 11 species covering six taxonomic groups. This value is well above the mean total Al concentration for Lake Rotorua, therefore direct impacts of alum dosing on organisms in the downstream receiving environment are unlikely, however toxicological impacts may be occurring due to the contributing high total Al in some tributaries of the Kaituna River.
Concentrations of total aluminium in the Sulphur Bay area of Lake Rotorua are naturally high (>1 mg L⁻¹) due to geothermal fluid infiltration. This has produced an area of naturally low aquatic biodiversity due to the low pH (~3) and prevalence of highly toxic Al³⁺. However, these effects are negated at the mouth of bay where the geothermally influenced waters mix with water from the main lake body. At this point normal lake diversity can be observed. No adverse effects have been observed from alum dosing of the Puarenga Stream.
The cautious use of continuous low level alum dosing remains the preferred option for reducing phosphorus loading to Lake Rotorua. However, the low buffering capacity, soft water and low dissolved organic matter preclude bulk or widespread application of alum for phosphorus management. Under the present alum dosing regimen, impacts on downstream biota are unlikely due to low water column aluminium concentration. However, further evaluation of potential toxic effects during phytoplankton driven diel pH cycling is recommended.||