The role of sulfide minerals in the genesis of groundwater with elevated geogenic arsenic in bedrock aquifers from western Quebec, Canada

The primary arsenic-bearing minerals and their relation to the mobilization of arsenic in groundwater are investigated in a mineralized area of the Canadian Shield (Province of Quebec, Canada). Bedrock core and groundwater samples were collected from three mining exploration boreholes completed in the fractured bedrock aquifer. Chemical analyses of groundwater reveal that the boreholes contain high arsenic concentrations ranging from 0.11 to 0.46 mg/l. Mineralogical analyses of core samples indicate that arsenic is mainly present as arsenopyrite (FeAsS) and gersdorffite (NiAsS), with arsenic and sulfur contents ranging from a few mg/kg to more than 3.1 and 1.3 wt%, respectively. Core leaching experiments show that the release of arsenic in oxidizing and weak alkaline water largely depends on the mineralogy of arsenic. Gersdorffite-rich samples release larger amount of arsenic than arsenopyrite-rich samples, which is mainly attributed to the high reactivity of gersdorffite compared to arsenopyrite. However, the high arsenic concentrations in groundwater are not associated with the occurrence and extent of gersdorffite-rich zones in the bedrock. These results are interpreted as evidence that the local weathering of sulfide minerals is not the main mechanism of arsenic mobilization in borehole waters. The groundwater composition strongly suggests that the mobilization of arsenic is related to the reductive dissolution of Fe and Mn oxyhydroxides in the downgradient part of the aquifer. Thus, it is proposed that the high arsenic concentrations in groundwater are mainly driven by (1) the initial release of arsenic through sulfide oxidation under oxidizing conditions (recharge areas), (2) the sequestration by sorption onto FeMn oxyhydroxides and clay minerals, (3) the transport of arsenic in dissolved and particulate forms in groundwater through fractures, and (4) the (re) mobilization by dissolution of Fe-Mn oxyhydroxides under reducing conditions (downgradient areas). This study highlights the potential role of sulfide stability and the importance of redox conditions in the mobilization of arsenic in groundwater from areas of sulfide mineralization. More detailed information is needed regarding the geochemical behavior of arsenic-bearing sulfides in bedrock aquifers under natural conditions and its implication for geogenic arsenic in groundwater.