The geochemistry of a mercury-contaminated bedrock aquifer in the vicinity of the Murray Brook mine

Abstract

The Murray Brook mine is located in North central New Brunswick and has been mined between 1989 and 1992 for gold from a gossan cap overlying a massive sulfide deposit. The cyanide heap-leaching method which was used to recover the gold caused the release of other heavy metals that were present in the ore, including mercury. Previous workers monitoring the area found elevated mercury concentrations in the groundwater below the tailings. Work performed during the summer of 1997 showed that elevated mercury concentrations in the groundwater and surface waters still persist in the vicinity of the mine. Total cyanide concentrations in the ground and surface waters were shown to be below analytical detection limits (5 μg/L). This suggested that concentrations of mercury in the ground and surface waters may be influenced by other naturally occurring ligands. A fine, white precipitate was discovered on the stream bed of Gossan Creek and was analyzed by means of X-ray diffraction and transmission electron microscopy (TEM) in an effort to determine its morphology and composition. It was detem1ined that the material was an amorphous substance consisting mainly of aluminum and silica with traces of iron, copper and sulfur. A second precipitate found to exist in the groundwater was also analyzed using the TEM. The precipitate was found to have a mineral composition and structure resembling that of either allophane, Al203(Si02)2 or boehmite, AlO(OH), but is probably not limited to these two minerals. Equilibrium geochemical complexation modeling suggested that in the absence of cyanide, pH and chloride would be the dominant factors responsible for the increased mobility of mercury. At present, the effects of cyanide on mercury mobility have not been ruled out. Cyanide was not detected in the samples that were taken but may still persist at concentration levels that are adequate to mobilize the mercury.