WATER QUALITY OF THE SURFACE WATERS AT AVOCA MINES AND THE AVOCA RIVER: Interim Report May to October, 1994.

Abstract

During this interim period of six months from May to October, 1994, 42 separate surface water sites were monitored. Surface waters were classified into nine discrete source categories. These are 1. Springs and seepage from spoil; 2. Surface runoff; 3. Leachate streams; 4. River mixing zone; 5. Contaminated river; 6 .Uncontaminated surface runoff; 7. Contaminated streams containing leachate; 8. Uncontaminated river; and 9. The lake in Cronebane Pit. Each source category has a distinctive water chemistry, although there is some overlap between categories for some parameters. The mine derived waters were all significantly affected by acid mine generation on the site. While the river was severely impacted by the discharge of diluted AMD via leachate streams (adits), contaminated streams, interflow and groundwater. The results show that the Avoca mine is an active acid mine producing site causing severe surface water pollution. While categories 1 and 2 waters can be classed as raw AMD (mean AMDI 10.0 and 24.2 respectively), the water in the leachate streams (category 3) are significantly diluted and buffered by groundwater infiltration. This category comprises of three separate leachate streams and each one is significantly different. The Shallow Adit is least diluted and so has the highest metal concentrations. While of the two that discharge into the river, the Deep Adit is the more polluting. The mean AMDI for these adits are 10.6 for the Shallow Adit, 26.6 for the Deep Adit, and 32.6 for the Ballymurtagh Adit. Compared to other similar mine sites, the leachate emanating from the Avoca mining area is highly polluting. The river is severely impacted by the AMD (category 5) downstream of the mines, although in terms of water chemistry the river recovers fairly quickly. Two factors are influential in this recovery. First the effect of the Aughrim River which has a similar flow to the Avoca River and which is more buffered due to a small area of limestone within its sub-catchment. Secondly, the effect of the fertilizer factory (IFI) downstream of the confluence of the two rivers which discharges a large volume of strong ammonia wastewater. The upstream uncontaminated river site shows a largely clean river with trace amounts of Zn, Fe and Cu (mean AMDI 97.0). However from the data this site may be subject to occasional AMD contamination. The absence of any surface water input would suggest that groundwater discharge is affecting the river at this site, even though this is not reflected by the biological investigations. A site further upstream should be selected for future work. In terms of identifying and quantifying AMD, and AMD impacted surface waters, both pH and AMDI are powerful at discriminating between source categories. However, the AMDI is also able to quantify AMD contamination in terms of the strength of the AMD and the actual impact and recovery within the river.