Arsenic in Fractured Bedrock Aquifers: The Influence of Prograde Metamorphism on Arsenic Mineralization in the Bedrock

The occurrence of natural arsenic in sedimentary aquifers around the world is well documented. However, arsenic concentrations in bedrock aquifers are of increasing concern in many parts of the world, including the United States. A groundwater study in Augusta, Maine reported approximately 30% of domestic well waters with arsenic concentrations greater than 10 µg/L (n=796). Wells tap groundwater in meta-sedimentary bedrock that exhibits varying degrees of metamorphism; increasing (prograde) metamorphism, and contact metamorphism associated with granitic intrusions.

This study was initiated to examine the concentration and mineralogy of arsenic within metamorphic rocks, specifically rocks affected by prograde metamorphism. Fourty-eight rock samples were collected along a transect from low grade slates/phyllites to high grade schists/gneisses. Seventy-five percent of the low grade rocks had detectable arsenic (mean As 70 mg/kg) compared to As detected in only 7% of the high grade rocks. Arsenic concentrations correlate with pyrite presence, thus detailed mineralogical studies (SEM-EDX, XRD, XRF, thin sections) have been employed to determine the association between arsenic and pyrite (FeS2) which converts to pyrrhotite (FeS) with increasing metamorphism.

It is hypothesized that during metamorphism, arsenic is removed from higher grade metamorphic rocks and retained in low grade metamorphic rocks containing arsenian pyrite, which may remain a potential source of groundwater arsenic today. Our objective is to map the variation in the As concentration in the aquifer in relation to groundwater flow patterns through the various grades of metamorphism in the bedrock.