ANALYSIS OF TRACE METALS IN STREAM SEDIMENTS NEAR AN ABANDONED COPPER MINE, BRISTOL, CONNECTICUT

The Bristol Copper mine operated from the early 1800s to the 1950s along the base of Mine Mountain near tributary streams to the Farmington River. The mineralization occurs along the western border fault of the Hartford basin and consists of primarily barite, chalcocite, bornite, chalcopyrite, galena, and pyrite. In order to assess the potential for trace metal contamination due to runoff from this abandoned mine, fifty-three stream sediment samples (<64 micron fraction) were collected from twenty-three sites. Sites were sampled along Unionville Brook and Copper Mine Brook downstream from the abandoned mine in an area with sedimentary bedrock and east of the border fault. Sites were also sampled in Copper Mine Brook and unnamed tributaries upstream from the mine in metamorphic bedrock. A weak acid digestion was used to prepare samples for ICP-OES analysis to determine the concentration of major trace metal elements. A mercury analyzer was used to determine mercury concentrations on freeze-dried samples.

The results show a strong correlation to the geology of the area. Streams draining the mine area and associated sedimentary rocks have particularly high concentrations of certain metals. For example, most copper values are above 40 ppm and locally reach 284 ppm. Barium levels typically range from 100 to over 400 ppm, and lead locally exceeded 75 ppm; while arsenic levels typically are 1.0 to 1.3 ppm. However, certain metals such vanadium, boron, chromium, mercury and zinc showed no distinct distribution pattern and were found to be distributed everywhere. Interestingly, cobalt, beryllium, titanium, magnesium, cadmium, and thallium are highest in streams that drain the metamorphic rocks to the west. Most of the metal concentrations exceeded the Threshold Effect Level, while in some samples levels of zinc, copper, and chromium surpassed the Probable Effect Level. One of the sites has a mercury concentration of 1.0 ppm, which is considered highly toxic.