ARSENIC DISTRIBUTION AND GEOCHEMICAL RELATIONSHIPS IN A FRACTURED-BEDROCK AQUIFER

The spatial distribution of aqueous arsenic concentrations measured in a fractured, sulfidic schist aquifer (Bayside, Maine) reveals only one well with arsenic concentrations greater than 10 ppb in the upland, recharge areas. A tight spatial cluster of domestic wells yielding ground water with elevated (> 100 ppb) arsenic concentrations occurs in the discharge area of our study watershed. The strong clustering of high arsenic concentrations in plan view indicates probable fracture-related processes. Unlike many other areas, the ground water does not show a correlation between arsenic and pH. Nor is there a direct correlation between arsenic and Eh, but all the samples with arsenic concentrations > 25 ppb do have low Eh (< 50 mV). Sequential leach tests were conducted on six fracture surfaces from a bedrock core collected within the arsenic cluster. The fractures were first leached with NaOH to release easily-extractable, adsorbed material, and then leached with an oxalate buffer to dissolve precipitated material. There is no correlation between As and Fe in either the NaOH or the oxalate leaching that would be expected if As is associated with Fe (III) oxyhydroxides. There is, however a correlation between As and S and between As and Zn, indicating that arsenic on these fracture surfaces may be associated with poorly-crystalline As and Zn sulfides or oxides. A correlation between As and Zn is not seen in the water samples, indicating that arsenic concentrations are likely controlled by different reactions occurring in individual fracture systems. Visual inspection of the fractures reveals different weathering effects. Most of the fractures are obviously weathered, some have sulfides growing on the surface, and most have a thin black or white coating, or a green crust. The fractured rock with the highest extractable As, Zn, and S, but low extractable Fe is cut by late-stage tourmaline veins that may have mobilized As. The chemistry of ground water in fractured-bedrock systems may not be easily explainable by a single process because of the variability of the reactions within individual fractures.