Sources of geogenic arsenic in well water associated with granitic bedrock from Nova Scotia, Canada

Abstract

Arsenic toxicity in drinking water is a global issue, with chronic exposure causing cancer and other health concerns. Groundwater from geochemically similar granites from mainland Nova Scotia, Canada, can have high and low levels of arsenic. The origin of this variation is uncertain, but different mineral hosts for arsenic could explain the disparity. The lability of arsenic from different minerals was assessed using laser ablation inductively coupled plasma mass spectrometry combined with calculations based upon well water data. Pyrite has the highest arsenic concentration (mean: 2300 μg/g, n = 9), is unstable in the groundwater system, and can release arsenic during oxidation. However, oxidation products replacing pyrite can adsorb arsenic, modifying the amount released. Cordierite has low arsenic concentrations (mean: 7.3 μg/g, n = 5) but is abundant and relatively soluble. Thus, cordierite could be a previously unrecognized source of arsenic in metapelitic rocks from metamorphic terranes. Pyrite from one of the granites studied was not oxidized, which in addition to the absence of cordierite in these same granites could account for the lower arsenic levels observed in associated well water. The results of this study can be used to identify potential geogenic sources of arsenic in other granitic terranes and reduce the risk of exposure through drinking water.