ARSENIC IN BEDROCK AND BEDROCK GROUND WATER IN NORTH CENTRAL MASSACHUSETTS

Two public water supply wells, located within the Merrimack Belt, North Central Massachusetts, have arsenic concentrations at levels of 20 to 30ppb. New monitoring wells were installed to gain additional data about the origin and pathways of arsenic within this aquifer. Arsenic in filtered water samples taken at five-foot intervals show an interesting up-and-down-and-up pattern with depth. Arsenic levels first increase up to a maximum of 189ppb with a concurrent decrease in oxidation-reduction potential (ORP) to -145 mV. With further depth, arsenic decreases sharply while ORP increases. In two monitoring wells that reached the bedrock, water is once again reducing and arsenic levels are slightly in excess of 100ppb. There is also a pattern of increasing arsenic in bedrock waters after the installation of the wells. During this time arsenic increased from 44ppb to 139ppb which suggests that the bedrock is the source of arsenic contamination for bedrock waters.

This study presents results of microscopic and microprobe analyses on bedrock samples recovered from two monitoring wells at the study site. Whole rock analysis of the bedrock samples yields 25 and 11ppm of arsenic respectively. Electron microprobe study shows a progressive oxidation of sulfide minerals and that weathering of arsenic-bearing sulfides may be the possible source of arsenic. However, arsenic in pyrites is highly variable ranging from below detection at approximately 400ppm to 2646ppm, considered to low to explain observed arsenic levels in bedrock waters. A detailed scanning of two polished sections with the most abundant sulfides revealed a presence of another sulfide mineral not previously recognized in these samples. The newly identified sulfide contains up to 49wt% arsenic, 17 to 24wt% Co, 5 to 8wt% Ni, and 5 to 8wt% Fe. The atomic ratio of (Co,Fe,Ni) to As and S is consistent with the stoichiometric formula of cobaltite. We suggests that cobaltite may be an important source of arsenic in the bedrock.