Mass and Water Balance Evaluations to Support the Selection of Remedial Action Alternatives at the Avoca Mine Site, County Wicklow, Ireland

The Avoca Mine in County Wicklow, Ireland is a volcanogenic massive sulfide deposit which has been mined for copper intermittently over the past 300 years using both underground and open pit methods. As a result of the mining activities, large areas of the site are covered with mine wastes, and the Avoca River has been impacted by Acid Rock Drainage (ARD) containing metals concentrations which have made the stream habitat unsuitable for native trout and salmon.

CDM performed a detailed water and mass balance evaluation consisting of analyzing the water chemistry and measuring water levels for 17 groundwater wells, and measuring flows and water chemistry at five locations along the Avoca River, five tributaries, and seven flowing mine adits. In addition, a tracer test was performed in the Avoca to evaluate potential underflow.

The results indicated that the Avoca River is part of a dynamic system that reacts rapidly to precipitation events. In general, the Avoca River is gaining, but some reaches can become losing during high flow. Groundwater seepage flows are generally low, but may be significant during the rainy season. Overland flow is believed to be an important component during storm events within the areas where waste rock is present next to the river. The river sediments temporarily store copper via attenuation, which is released over time.

Based on the results, the remediation measures under consideration include; removal of tailings and capping of waste rock near the river and in recharge areas, treatment of the adit discharges, capture and treatment of impacted groundwater and removal of impacted sediments in the reaches just down-gradient of the adit discharge points.