ARSENIC SOURCES AND PATHWAYS IN THE OVERBURDEN OF CENTRAL MASSACHUSETTS
Rudolph Hon 1, Kevin Doherty 2, Thomas Davidson 1, William C. Brandon 3, Carol L. Stein 4, and David F. McTigue 4.
1 Department of Geology & Geophysics, Boston College, Chestnut Hill, MA 02467,
2 Knoll Environmental, Inc., 69 Wexford Street, Needham, MA 02494,
3 Office of Site Remediation and Restoration, USEPA Region I: New England Region, 1 Congress St, Boston, MA 02114,
4 Gannett Fleming, Inc, 15 Willard Road, New Ipswich, NH 03071

Elevated levels of arsenic in unconsolidated layers within a zone that traverses N-S across Central Massachusetts had been at times noted, although without a specific reference to the source(s) of arsenic. Suspected sources included past applications of lead arsenate in orchards as a control for coddling moth, industrial applications in metal and leather processing facilities, and/or 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 data on arsenic that (1) were compiled for selected sites listed with the Massachusetts Bureau of Hazardous Waste within this region; and (2) data analyzed by this study on samples of overburden obtained from drilled profiles at randomly selected sites in Central Massachusetts. The compiled data include 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 two frequency subsets: 20 to 50 ppm and 50 to 800 ppm. Comparison with distribution curves for lead shows no correlation between lead and arsenic suggesting that lead arsenate is not the likely source for samples with elevated arsenic. Microprobe analysis of sulfides from bedrock cores confirms a presence of two different sulfide phases: pyrites (FeS2) and cobaltites (CoAsS) in the underlying formations. Pyrites contain negligible amounts of arsenic, however, arsenic levels in cobaltites range from 30 to 50 % of As by weight. Elevated arsenic values in the overburden of Central Massachusetts are best explained by natural origin where the bulk of the overburden layer is derived from the local bedrock formations. An occasional presence of cobaltite clusters within the overburden is the likely explanation for the observed arsenic "hot" spots. Arsenic pathways have been recently documented in a water-supply aquifer located within the same geological zone. During installation of the monitoring wells, soil and groundwater samples were collected along vertical profiles between the top of the overburden aquifer and bedrock at three locations. Reducing conditions (ORP ­50 to ­200 mV) were encountered in the upper ~45 ft of the aquifer. Throughout this interval, total arsenic and iron yielded a strong correlation ranging up to a maximum of 189 µg/L and 21,900 µg/L, respectively. Below this redox boundary at ~45 ft bgs, dissolved arsenic and iron levels dropped below detection limits, and ORP increased correspondingly (0 to 100 mV). Soil analyses showed significant correlations between solid-phase iron and arsenic, aluminum, cobalt, copper, manganese, nickel, and zinc. These data and results obtained using PHREEQC to model arsenic adsorption are consistent with the reductive dissolution of iron oxides in the upper part of the aquifer and release of sorbed arsenic.