FRACTURE SURFACE CHEMISTRY IN AN ARSENIC-RICH BEDROCK AQUIFER

Fracture surfaces in four bedrock cores from a small watershed in Bayside, Maine show considerable variability even within a single borehole. The chemistry of the ground water is controlled by the processes occurring at the surfaces of these fractures. The bedrock is primarily a sulfidic, interbedded pelite and metasandstone that has been intruded by small gabbroic to granitic plutons and crosscut by many quartz and tourmaline veins. In areas where veins are present, the rock commonly fractures along the veins which allows the vein chemistry to strongly influence the chemistry of the ground water.

Examination of the chemistry of the ground water in this watershed reveals that the high-As (> 100 ug/l) and low-As data sets have significant differences in chemistry that may be due to differences in bedrock mineralogy. For example, the high-As data shows a strong positive Ca/Mg, Ca/pH, Mg/pH correlations (suggestive of carbonate dissolution) that are not seen in the low-As data. Also, the high-As data has a As(III):Fe(II) slope 17 times that in the low-As data. The good positive correlation of As and Fe in the high-As data and the excellent As(III)/Fe(II) correlation in both data sets indicates that reductive dissolution of Fe-oxyhydroxides may control As concentrations.

In order to determine what processes are occurring at these fractures and how secondary minerals are controlling the ground-water chemistry, XRD, electron microprobe analyses and leach tests are being conducted. Preliminary microprobe mounts made of cross sections of the fractures from one of the rock cores show no arsenic-bearing minerals on the fracture surfaces. The fractures are exposed to separate leach tests of NaOH and oxalate. The NaOH raises the pH and releases easily desorbed material and the oxalate releases easily dissolved constituents, allowing us to determine if the arsenic is primarily adsorbed or co-precipitated on fracture surfaces. Preliminary leaching results from one of the cores show a correlation of As with S in both leachates, but none between As and Fe. This is in conflict with the ground-water chemistry that shows a correlation between As and Fe. Examination and testing of fracture surfaces at other locations within the watershed will be presented and may clarify this discrepancy between the ground-water chemistry and the leach test analysis.