ARSENIC AND BIOTITE IN A FRACTURED CRYSTALLINE AQUIFER WITH ELEVATED GROUNDWATER ARSENIC, MAINE USA

The association between As, sulfides, and oxide minerals is well documented. Less is known about the association between As and silicate minerals. Biotite has been proposed as a possible As host mineral, but it is difficult to isolate a representative sample of biotite from unconsolidated aquifer sands, such as those in the As-rich sedimentary aquifers of southeast Asia. Therefore, the potential for biotite, and perhaps other silicate minerals, to influence As concentrations in groundwater, remains uncertain.

In this study, 46 rock samples were collected from the prograde metamorphic Waterville Formation of central Maine. Biotite and other target minerals were identified in thin sections using optical microscopy, then individual mineral grains were analyzed by laser-ablation HR-ICP-MS. To our knowledge, this is the first groundwater study to directly analyze As concentrations in biotite minerals, without the uncertainty associated with separating biotite from an unconsolidated matrix. Arsenic concentrations in biotite varied with metamorphic grade. No As was detected in the poorly formed biotites of the low grade rocks (upper greenschist facies). Arsenic in high grade sillimanite-muscovite rocks ranged from 1 to 570 mg kg^-1 (µ=174 mg kg^-1; n=13). Another silicate mineral, garnet, also contains notable As concentrations (µ=78 mg kg^-1; n=9). The garnets are zoned, with some zones reporting higher As concentrations. In addition, preliminary data suggests that graphite may contain appreciable amounts of As; up to 798 mg kg^-1 in low grade rocks.

While the exact nature of the As association between these silicate minerals and graphite is unclear, the data presented herein increases our awareness of minerals other than sulfides and oxides to potentially host As. This work has helped identify geologic units in this area, and potentially other crystalline aquifers, that may pose an increased risk of elevated As concentrations in groundwater. In addition, it may provide insights for sedimentary aquifers containing these minerals. Further work is underway and includes molecular scale analyses using synchrotron radiation; and biogeochemical weathering studies of these As host minerals.