Stream communities in acid mine drainages : investigating longitudinal recovery and remediation possibilities.

Abstract

The mechanisms of acid mine drainage were investigated in stream communities on the West Coast of the South Island of New Zealand, with specific regard to the Denniston/Stockton areas. To do this, a field survey of AMD impacted streams indicated that some natural recovery occurred without anthropogenic management, particularly in Charming Creek and Darcy Stream, and to a lesser extent in the Waimangaroa River. This recovery is associated with improvements in water quality due to the dilution of AMD by non-impacted tributaries. Rapid Creek is a small, contaminated stream that was used as a model to assess the mechanisms of AMD toxicity. Iron precipitate on substrate altered water chemistry and proved toxic to Deleatidium mayflies. Substrate with iron precipitate harbours more organic biofilm, but in-situ experimentation did not detect a difference in the invertebrate community associated with either iron-precipitate coated substrate, or substrate without iron. Experimentation over 96-hours indicated that low pH (<4) was the primary cause of toxicity, and that dissolved metals were secondarily toxic. Techniques to rectify these causes of toxicity were investigated. Dilution with distilled water, and the commercial cation-exchange products Bauxsol™ and Zeolite were used to treat AMD water, and the survival of Deleatidium subsequently exposed to this water was compared. Bauxsol™ and Zeolite improved mayfly survival after 96 hours by 50%. Dilution, up to 8-fold did not improve mayfly survival. Extrapolation indicated that dilution up to 64-fold would be required to mitigate the effects of acidic AMD entering Rapid Creek, even though the natural levels of pH are generally <5. The combined results of this study suggest that active remediation of AMD impacted streams would facilitate an improvement in the condition of the ecosystems, provided specific regard is given to the particular circumstances of each stream.