Ecotoxicological review of alum applications to the Rotorua Lakes

Abstract

The use of alum (aluminium sulphate) has become a recognised technique for the restoration of freshwater systems. When added to water, alum dissociates and dissolved aluminium undergoes a series of hydrolysis reactions resulting in the formation of aluminium hydroxide (Al(OH)₃) which adsorbs dissolved phosphorus and coagulates suspended solids. The resulting flocculent sequesters dissolved and particulate phosphorus, reducing primary production, thereby improving water clarity. Aluminium hydroxide is a relatively benign substance with peak abundance occurring at pH 6.3; above and below this point, soluble, more toxic aluminium species predominate. For example, under alkaline conditions (>pH 8.5) toxic Al(OH4)⁻ forms, while below pH 4.5 free monomeric aluminium (Al³⁺) becomes prevalent. The hydrolysis reaction of aluminium (Al) causes the release of H⁺, lowering pH and potentially causing the formation of toxic aluminium species. It is therefore critical that application rates do not exceed the buffering capacity of the treated system. In addition, eutrophic systems often experience photosynthetic driven alkaline pH shifts, resulting in Al solubilisation and the formation of the toxic Al species (Al(OH)₄⁻). The Bay of Plenty Regional Council has initiated several alum dosing programmes in the Rotorua lakes district in an effort to reduce lake trophic levels. Currently, continuous alum dosing is undertaken on the Puarenga and Utuhina Streams discharging to Lake Rotorua and the Waitangi Soda Stream discharging to Lake Rotoehu. In addition, seven discrete alum applications have been conducted on Lake Okaro since 2003. This has resulted in the total applications of 444.2, 55.2, and 2.8 tonnes of Al to lakes Rotorua, Rotoehu and Okaro respectively. Current estimated continuous lake water dose rates for Lake Rotoehu 6.72 µg Al l-1 and Lake Rotorua 2.62 µg Al l⁻¹ are low by international standards and the estimated maximum dose applied to Lake Okaro of 0.22 mg Al l⁻¹ lake water was also conservative. A literature review was undertaken to provide guidance on a number of concerns associated with alum dosing of the Rotorua lakes. This includes; (1) the likely concentration thresholds for acute toxicological effects from Al dosing and whether current dosing programmes are likely to exceed them; (2) the fate of flocculated Al in lake sediments and whether they pose an ecological hazard; (3) the risk of the buffering capacity of water in the Rotorua lakes being exceeded leading to release of toxic Al species; and (4) the risk to biota of burial by Alfloc and its potential to disrupt lake processes. The toxicity of Al is closely associated with pH, and acute toxic effects are likely to be a combination of physiological responses to both acidic pH and Al. Fish appear to be the most susceptible group to Al toxicity with respiratory disruption initially occurring at pH 6.0 due to gill irritation by colloidal Al. This is followed by increasing levels of osmoregulatory disruption peaking at pH 3.0 where Al³⁺ is the predominant species. Toxic effects may also manifest under alkaline conditions (pH >8.5), and although the precise mechanism is unknown it is theorised that the gills may be the primary site of action. As well as pH, susceptibility to Al toxicity is dependent on a number of factors including species, life stage, and even whether the organism has had previously exposure to Al. In addition, a number of chemical components have an ameliorating influence; foremost amongst these are dissolved organic matter (DOM), silica and calcium concentrations.