Acid mine drainage composition and the implications for its impact on lotic systems

Intensive sampling of acid mine drainage from adits draining the abandoned copper and sulphur mines at Avoca, south-east Ireland, has confirmed a seasonal variation in the Zn:Cu ratio observed previously. The variation in the Zn:Cu ratio leads to extreme variations in the toxicity of the drainage, and linked with increased adit flows and surface runoff during wetter months, often results in higher river toxicity, even during high river discharge rates. This seasonal variation in toxicity has important ramifications for the control of AMD to surface waters. Sampling over a three-year period shows that the rate of discharge from the adits follows a seasonal cycle being high in the spring, declining through summer to reach lowest flows in autumn and rising again in the winter. During the intensive sampling study over a 13-month period, the discharge rate in the Deep Adit varied from 8.5 to 37.3 l/s compared to 6.3 to 35.2 l/s in the Ballymurtagh Adit. This represents a total discharge rate for the two adits of between 1265 and 6270 m3/d. Significant weights of cations were discharged from the two adits, ranging from 169–1279 kg/d for Fe, 69–459 kg/d for Zn, 1.5–34.7 kg/d for Cu, and 0.167–0.875 kg/d for Cd, with the Deep Adit contributing on average 40.4, 70.4, 66.7 and 80.9% of each metal respectively over the sampling period of 13 months. The impact of AMD on the river biota is severe with a major decrease in the number of taxa and faunal abundance. The species deficit at all sites below the mines is in excess of 87%. Dipterans dominate impacted sites with Chironomidae abundant. Fish are eliminated except for juvenile eels close to the estuary.