Aluminium in AMD streams on the Stockton Plateau: is dilution a management solution?

Abstract

Acid mine drainage (AMD), has had detrimental effects on water quality and ecology in the waterways of the Stockton Plateau, site of New Zealand’s largest open-cast coal mining operation. This has been partly attributed to elevated Al concentrations. Chemical analysis, geochemical modelling and macroinvertebrate sampling, was undertaken in T35, T31, Herbert and Cypress Stream waters, and the Mt Frederick quarry lake water in order to ascertain Al concentration and speciation, and ecological impacts. The effectiveness of dilution as a long term, environmental management strategy for these low volume, AMD waterways, was investigated with experimental dilutions and geochemical modelling. Sulphate concentrations provide a reliable indicator of AMD impact, and in the waterways studied, ranged from 2.18mg/L in un-impacted waters, to 217 mg/L in the most affected samples. In those streams without AMD impacts, pH ranged from ≈4.0- 6.0 and dissolved aluminium concentrations from 0.10 - 0.27 mg/L. In AMD impacted waters, pH=3.4 - 6.6 and dissolved Al=0.37 - 27 mg/L, although this is reduced to 0.03-0.08 mg/L in high pH samples (>5.6) where Al-hydroxide precipitation has removed dissolved Al. Al speciation and toxicity is highly dependent on pH as well as the presence of complexing ligands. The free ion Al3+, is considered to be the most toxic Al species and was predicted to dominate in samples with pH =3.8- 4.8. At pH<3.9, toxicity is mitigated by formation of the AlSO4+ complex, in high SO4 samples. At pH>4.8 fulvate and/or hydroxide complexes were predicted to dominate speciation and toxicity to decline, although at high pH, Al-hydroxide precipitates may have a detrimental ecological effect. The variable strengths of Al-complexes explained the different dissolved Al concentrations obtained from the two analytical methods used; the ICP-mass spectrometry method involved an aggressive acid preservation step (pH≤2), and provided a measure of ‘total dissolved Al’, while the on-site Aluminon spectrometry method did not require sample preservation and did not appear to break down strong Al complexes during the analytical procedure, measuring free-ion and weak complexes only. This results in the Aluminon method providing a better measure of bio-available Al concentrations. Macroinvertebrate taxa and EPT richness provide a good indication of AMD impact. Both of these ecological variables identified a dissolved Al threshold, above which taxa richness declined rapidly. The ICP-MS dissolved Al threshold was 0.6mg/L, however the toxic component will vary according to water chemistry. The Al3+ threshold was 0.42 mg/L and is consistent with an Aluminon method dissolved Al threshold of 0.5mg/L. Dilution with circum-neutral pH waters, such as those of the Mt Frederick quarry lake, to remediate small volume, AMD impacted drainages such as Herbert Stream is feasible, but presents significant challenges. The degree of dilution required depends on the environmental targets selected, with dilution to 85% or 95% quarry water, required to achieve the Al3+ and ICP-MS dissolved Al thresholds respectively. Dilutions above 85% quarry water are likely to precipitate Al-hydroxide. Significant changes in Al speciation and hence toxicity resulting from minor changes in dilution, present significant challenges for managing variable flow regimes.