Mobility and bioavailability of arsenic, lead, copper, and zinc at the Avoca Mine Site, County Wicklow, Ireland

Abstract

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 spoils, and the Avoca River has been impacted by ARD containing metals concentrations which have made the stream habitat unsuitable for native trout and salmon. An evaluation of the bioavailability of lead and arsenic within the spoils material was performed using a bioassay technique developed by the University of Colorado, ICP, and electron microprobe (EMP) analyses, while the leach ability of zinc and copper within the spoils was evaluated using ICP, and EMP techniques. While the spoils contained on average 900 mg/kg arsenic and 15,000 mg/kg lead, only 0-28% of the lead (average of 6%) and 0-8% of the arsenic (average of 0.5%) was bioavailable. EMP results showed the lead and arsenic to be present mainly within plumbojarosite and other hydroxysulfate phases, which are stable under the low pH conditions within the spoils pore water. The copper and zinc concentrations within the spoils averaged 1750 mg/kg and 420 mg/kg, respectively. EMP results showed that copper and zinc was present in the spoils mainly as primary sulfide minerals. Copper and zinc concentrations were low within the secondary minerals such as plumbojarosite and other hydroxysulfate, reflecting their higher mobility in the system and explaining why copper and zinc are present in much higher concentrations in the ARD and the Avoca River than arsenic and lead.