NATURALLY OCCURRING ARSENIC IN OVERBURDEN IN CENTRAL MASSACHUSETTS

Presence of elevated levels of arsenic in a zone that traverses N-S across Central Massachusetts had been periodically noted and reported, however, without a specific reference to neither the origin of arsenic nor the arsenic sources. Suspected sources included past applications of lead arsenate in orchards as a control for codling moth, industrial applications in metal and leather processing plants, and/or release from natural sources. An accumulated set of data in the archives of state environmental agencies provides a confirmation of the widespread reports of arsenic levels that are well above the regulatory “background” levels (17 ppm) in overburden. We report arsenic data that (1) were compiled from selected sites listed with the Massachusetts Bureau of Hazardous Waste within this region; and (2) data obtained by this study on samples of overburden obtained from drilled profiles at 18 randomly selected sites. The compiled data are for sites within a corridor along the NNE-SSW trending tract that passes through the geographic center of the state. The drilled sites cover a 10 by 10 miles subsection within this tract. Both data sets have similar arsenic frequency distribution curves (histograms) with identifiable two frequency subsets: 20 to 50 ppm and 50 to 800 ppm. Samples in the first category are approximately 10 to 20 times more populous than samples of the second category. Comparison with distribution curves for lead proves that there is no connection between lead and arsenic therefore suggesting that lead arsenate is not the arsenic source. Microprobe analysis of sulfides from bedrock samples confirms the presence of pyrites (FeS2) and cobaltites (CoAsS) in the underlying bedrock formations. Pyrites contain negligible arsenic, however, arsenic levels in cobaltites range from 30 to 50 % by weight. Elevated arsenic occurrences within the overburden of the Central Massachusetts is best explain by the derivation of overburden from local bedrock formations and by the occasional incorporation of sulfides into the overburden which may account for the observed arsenic “hot” spots. Combination of both modes of arsenic sources best explains the arsenic frequency distribution curves. The Central Massachusetts “arsenic province” is part of a larger lithotectonic zone as previously reported by Ayotte and others (1999 – WRIR 99-4162).