A COMPARATIVE ANALYSIS OF ARSENIC OCCURRENCE IN ADJACENT BEDROCK UNITS, MAINE

An understanding of arsenic occurrence in rocks of the Waterville Formation in Maine have revealed interesting trends potentially associated with increasingly metamorphosed bedrock units in the Greater Augusta area. Previous data indicate whole rock arsenic concentrations in the metamorphosed pelitic Waterville Formation decrease with increasing metamorphism, As averaging 48 mg kg^-1 (n= 17) in low grade rocks, decreasing to 35 mg kg^-1 (n= 10) in medium grade rocks, and 10 mg kg^-1 (n= 13) in high grade rocks. In contrast, new data for the adjacent metamorphosed calcareous sandstone and dolostone Vassalboro Formation shows increasing whole rock arsenic concentrations with increasing metamorphism, As averaging 20 mg kg^-1 (n=6) in low grade rocks, 25 mg kg^-1 (n=13) in medium grade rocks, and 37 mg kg^-1 (n=13) in high grade rocks. Previous data indicate an association between arsenic and pyrite, biotite, and garnet in the Waterville Formation rocks but less is known about the arsenic-mineral associations in the Vassalboro Formation, which is characterized by the appearance of different index minerals in the metamorphic sequence (e.g., ankerite, amphibole, zoisite, diopside).

Caution must be applied when interpreting arsenic trends with increasing metamorphism particularly since geologic heterogeneity ensues and preliminary datasets are relatively small. Ideally, knowledge of the arsenic-mineral associations can help to explain arsenic distribution in groundwater and this geochemical data will be a useful complement to the large groundwater database available for the Greater Augusta region. Current research also includes a study on the leachability of arsenic from host rocks of both formations in addition to analysis of temporal variation (over 24 hours) in arsenic concentration in groundwater extracted from the Vassalboro Formation. Much remains unanswered with respect to the processes controlling arsenic release from, and distribution within, crystalline aquifers. The ultimate goal is to enhance our knowledge of the mineral controls on arsenic occurrence so that processes contributing to its release to groundwater are better understood and can be used for improving predictions of populations at risk of arsenic exposure via consumption of groundwater from crystalline bedrock environments.