SENSITIVITY OF MINE SPOIL HEAPS FROM AN ABANDONED LEAD MINE IN MID WALES (UK) TO CHANGES IN pH OF NATURAL WATER SYSTEMS IN THE CONTEXT OF CLIMATE CHANGE

A case study of cerussite-dominated mine tailings (5-7 wt% Pb) from the former lead-zinc Grogwynion Mine, Mid Wales (UK), is presented with the objective of providing an improved understanding of the chemical speciation that controls element release from the waste in the short and long term. An experimental and modelling approach was used to examine the effects of pH changes in fresh waters interacting with the waste heaps and of the projected increase over time of atmospheric PCO2 on Pb mobility. Results of pore water analysis and pH-dependent leaching experiments indicated that cerussite controlled Pb solubility in the pore waters and leachates, maintaining concentrations < 10 mg L at pH > ~6 . As pH decreased upon acid addition the cerussite mineral phase dissolved. The precipitation of anglesite in the acid domain of the experiment had a negligible effect in reducing the concentration of dissolved Pb deriving from cerussite dissolution, as a result of the limiting concentrations of SO4 in solution. The output from the simulation of the projected PCO2 increase on the solubility of cerrussite in equilibrium with freshwater suggested a limited increase in Pb solubility. Other factors such as changes in temperature and the intensity of microbial activity and related CO2 production may also influence the solubility changes of such carbonate systems. Clearly, an understanding of the integrated effects of projected climate changes for a given environment is required to quantify the potential impacts of climate change scenarios on mine contaminated environments.