MIGRATION PATHWAYS OF ARSENIC IN HIGH ARSENIC ZONES IN CENTRAL MASSACHUSETTS

The presence of elevated levels of arsenic in a zone that traverses N-S across Central Massachusetts had been periodically noted and reported. 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 (30 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 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. Both data sets have similar arsenic frequency distribution curves (histograms) with identifiable two frequency subsets: 20 to 50 ppm and 50 to 800 ppm. A detailed analysis of arsenic distribution reveals a pattern of arsenic migration from the hillside sites to the sites located in valleys and flat lying areas. Within a small geographic area near the City of Worcester we analyzed and compiled a database of 283 arsenic concentration data in soils colleceted from near the ground surface up to a depth of 30 ft bgs. Soil samples from 126 hillside sites have an average arsenic content of 31 ppm with the range of 4 to 281 ppm. By contrast, 157 samples of soils taken from the sites in the valleys or flat lying areas have an average of 71 ppm and a range of 7 to 770 ppm. From our dataset a pattern of arsenic migration can be observed where a substantial proportion of arsenic had been transported from the hillsides toward the valleys. We suggest that groundwater recharged at higher elevations dissolves small amounts of arsenic which is then transported to the flat lying aquifers where the dissolved arsenic is allowed to precipitate on the aquifer material surfaces prior to its discharge to the surface drainage system. It is likely that a changing redox facilitates the dissolution and precipitation: slightly reduced water due to a decaying organic litter can mobilize minor arsenic which is then reprecipitated when the groundwater enters a zone of atmospheric influence near its discharge point.