ENVIRONMENTAL IMPACT OF ACID MINE DRAINAGE ON THE AVOCA RIVER: Acute Toxicity Experiments in the Field Using Macro-invertebrates

Abstract

Field toxicity experiments were conducted in the Avoca River (Ireland) during August and September 1994 in order to evaluate the toxicity of acid mine drainage (AMD) to a number of selected macro-invertebrate species. The main objective of the experiment was to design and evaluate a field toxicity method for rivers receiving AMD. An experimental flow-through chamber was designed and tested in the field It was found to produce reliable and reproducible results. Three test macro-invertebrates were evaluated Gammarus duebeni, Ephemerella ignita and Baetis rhodani. Only the former species proved both robust and sensitive enough for toxicity assessment work using the chambers. Three replicates were used at each test site comprising of twenty individuals per replicate (five per chamber). Large specimens of a small size range were used to eliminate size/age effects. Mortality at the control site was acceptable at <10%. The difficulty encountered in using the more sensitive insects may be overcome by collecting animals in early spring. However, their smaller size may make them more suitable for laboratory-based experiments. A significant toxicity effect was only recorded within the mixing zone of the river, with the completely mixed water at Avoca Bridge showing no significant toxicity. From the water data, the AMD had only a moderate impact on the river during the experimental period. Using the mean AMD index this varied from 92.4 over the experimental period at the upstream control site to 90.3 and 93.5 below A voca bridge and above the IFl plant respectively. Water quality in the mixing zone is very variable depending on flow characteristics, but with the area clearly severely impacted. In the mixing zone the AMD index varied from 71.3 to 95.6 over the same experimental period. While the field toxicity tests indicated that only the mixing zone was impacted, routine biological surveillance has shown the impact to be on a more extensive scale with the entire river below the mines (12km) severely damaged. These results suggest that factors other than water toxicity are responsible for the elimination of species in the lower Avoca, although the water quality may result in long term chronic toxicity.