ARSENIC AND PATHWAYS OF SELECTED TRACE ELEMENTS IN CENTRAL MASSACHUSETTS

Arsenic levels in excess of order of magnitude over average arsenic concentrations in uncontaminated soils (3 to 6 ppm) have been observed in Central Massachusetts. Corresponding ground waters have been noted for elevated arsenic as well (up to 1000's ppb), and very high arsenic had been observed at points of ground water discharge into the surface water systems. The role of anthropogenic land use factors and the status of natural arsenic conditions are often confusing in regards to the arsenic sources and pathways of the contaminants. In this presentation we provide data on arsenic and its proxy correlations with Ni, Co, and Zn, as well as Fe. An earlier study of 169 soil samples analyzed for As, Ni, and Fe (from a small area of approximately 3 x 3 miles) reveals Ni – As, and Ni – Fe correlations (R^2 = 0.666 and 0.795, respectively) suggesting a similar source despite different speciations of Ni and As in soils. The likely source mineral is cobaltite (CoAsS) identified by a microprobe analysis in several fresh bedrock samples from a drill core. Co (19 to 24 wt%) is partially substituted by Ni (5.5 to 6.6 wt%) while arsenic (43 to 44wt%) is near stoichiometric. No significant amounts of Co or As were detected in pyrites. Average Ni content in pyrites is 0.36 wt%. Studies of arsenic contaminated ground water within the same region have shown that arsenic in ground water is due to redox potential change from the oxidizing to reducing conditions. Arsenic absorbed on hydrous ferric oxide (HFO) coatings in soils is liberated by a reductive dissolution of the HFO substrates. A reverse process is observed when arsenic contaminated GW is discharged into an oxidizing surface water environment. Pond bottom sediments at contaminated GW discharge points have elevated arsenic that also correlates with Ni, Co, and Zn. Correlations of As with Fe, Ni, Co, and Zn yield statistically significant correlation coefficients for pond bottom sediments in areas not affected by anthropogenic past activities (R^2 = 0.849, 0.873, 0.909, 0.881, respectively). Past tannery disposals interfere with these patterns by superimposition of different trace metal patterns. Landfill leachates create highly reducing environments leading to sulfide precipitations in the affected soils and drop in the levels of Ni, Co, and Zn in the discharging ground water.