T192 Arsenic: From Nature to Human I

Wednesday, 31 October 2007
8:00 AM - 12:00 PM

POSSIBLE SOURCES OF ARSENIC AND ITS TRANSPORT MECHANISM IN CENTRAL MASSACHUSETTS

Paper number: T192-4
Program: Technical Sessions
Time: 8:35 AM – 8:50 AM

Authors

Abstract

A northeasterly trending transect across Central Massachusetts, U.S.A., exhibit significantly elevated arsenic levels in its glacial overburden cover. Arsenic data are clustered in the range of 20 to 50 mg/kg although higher values are common occasionally exceeding 500 mg/kg, Ground waters percolating through the overburden (and bedrock as well) show elevated arsenic whenever a reducing environment is encountered, typical values are in the 20 to 500 ppb range or even higher. The uptake mechanism of arsenic by ground water is likely by reductive dissolutions of ferric hydroxide and by a simultaneous release of adsorbed arsenicals.

The primary source of arsenic, prior to its sequestration onto ferric hydroxide surfaces in the overburden, is inferred from the multi-elemental correlations of As with Ni, Co, Cu, Zn, Pb, and Mn. Arsenic in the overburden positively correlates with Ni, Mn, and Fe, (Co, Cu, Zn, and Pb were not analyzed). Pond bottom sediments sampled from the area where reducing groundwater discharges into a surface water reservoir are enriched in arsenic (80 to 150 mg/kg) and other metal elements. Correlations with Fe suggest reverse arsenic sequestration by ferric hydroxides at the GW-SW redox interface. Arsenic in the pond bottom sediments positively correlates with Ni, Co, Cu, Zn, and Pb. Zn and Pb are in 100's mg/kg, Cu, Ni, Co are in the 20 to 100 mg/kg range. Higher Zn and Pb values indicate sulfide mineralization which is also supported by field and electron microscope observations. Co, Ni, and As were detected in the bedrock samples as cobaltite-gersdorffite sulfide grains. Whole rock samples of underlying sedimentary and low grade metasedimentary bedrock taken from drill cores have average concentration of Zn and Pb 78 and 51 mg/kg respectively. Concentrations of Ni (73 mg/kg), Co (26 mg/kg), Cu (30 mg/kg) compare with average As of 18 mg/kg. The primary source of arsenic in the Central Massachusetts transect appears to be the underlying bedrock, partly mineralized during the low grade regional metamorphism by hydrothermal exhalative processes.