Pre-mining acid rock drainage in the Talvivaara Ni–Cu–Zn–Co deposit (Finland): Natural peat layers as a natural analog to constructed wetlands

• High Cu, Fe, Ni, Zn, S, and ash contents in peat were gauged at black schist area.
• Bedrock and soil composition affect the characteristics of peat formations.
• Besides groundwater chemistry, acid rock drainage influenced the peat chemistry.
• Peat naturally attenuates groundwater and surface waters in pre-mining conditions.
• The capacity of peat to attenuate AMD can be decreased before mining activities begin.

Altogether 62 peat, 411 bedrock, and 1163 glacial till samples were selected in pre-mining conditions around the present Talvivaara Ni–Cu–Zn–Co mine in Finland. The samples were collected from different lithological areas representing mineralized (> 0.07% Ni) and non-mineralized bedrock. The influence of bedrock and till composition, and subsequent groundwater and surface water quality, on the metal and sulfur concentrations in the peat deposits was evaluated. The peat samples at the non-mineralized mica schist study site M1 showed moderate increase in element concentrations (Cu, Ni, S) and decreasing electric conductivity (EC) as a function of depth, whereas at the mineralized black schist study sites M2 and M3 the heavy metal (Fe, Ni, Zn) and sulfur concentrations and EC exhibited a significant increase towards the Carex-dominated bottom layers. Besides anomalous metal (up to 12.0 wt.% Fe, 758 mg/kg Ni, 5270 mg/kg Zn) and S (13.6 wt.%) concentrations, the site M2 presented anomalously high ash content (median 35%) and low pH (3.4), which were attributed to the acid rock drainage derived from an adjacent mineralized black schist outcrop. Samples from site M2 were subjected to sequential extractions to study elemental distribution. The studied peatland is a natural analog to constructed wetlands in passive treatment of acid mine drainage waters, though the retention capacity is lower due to low pH and decreased buffering capacity.