COMPLEXITIES OF ARSENIC CONTAMINATION IN FRACTURED BEDROCK WELLS OF CONNECTICUT

Arsenic in groundwater has recently become a health concern for many residents of Connecticut as more wells test positive for this tasteless and odorless, cancer causing metalloid. The source of arsenic remains unknown owing to the complex nature of the fractured bedrock aquifer and unknown background water quality conditions that allow naturally occurring arsenic to leach from the source. An analysis of arsenic in domestic wells was completed for Lebanon, Connecticut in the summer of 2014. Results of this analysis indicated that wells with significantly high arsenic concentrations occurred on the eastern side of town intersecting the Brimfield Schist, the Hebron Gneiss, and the Scotland Schist at the surface. Wells also exhibited a relatively high pH, low iron concentrations, and low manganese concentrations. In cooperation with the Connecticut Department of Energy and Environmental Protection and the Department of Public Health, domestic wells within the high arsenic concentration area were re-sampled and analyzed for arsenic, as well as several other constituents, in the summer of 2015 to better understand the groundwater quality conditions attributing to this phenomenon. As expected, many of the wells within the area tested positive for arsenic and pH values continued to be alkaline. Preliminary results suggest complexation given the following results: arsenic was present under oxic and reducing conditions; arsenic was present at a range of pH values; although a negative correlation exists between arsenic and dissolved oxygen, the relationship was not strong and arsenic was present at all levels of dissolved oxygen; and, although many of the wells did not test positive for uranium, wells that had uranium also had arsenic. Alternatively, observed arsenic concentrations and other groundwater quality parameters may be a result of well bore mixing. Correlations between arsenic concentrations and age and depths of wells were both positive yet the weak correlations suggest water from different fractures under non-equilibrium conditions may alter the water chemistry. Continuation of this study will include the identification of water bearing fractures high in arsenic for this area of the State and a three-dimensional analysis of fractures to predict additional locations where arsenic may be present.