GEOCHEMICAL STUDY OF ARSENIC SPECIATION IN OVERBURDEN SOILS, CENTRAL MASSACHUSETTS

Central Massachusetts is a region with an elevated range of natural arsenic in the overburden (10’s to 100’s ppm), largely overlying bedrock sequences of the Merrimack Belt. Elevated arsenic poses environmental concerns due to the potential for arsenic mobilization by ground water, which may exceed 10 ppb, the EPA limit for drinking water supply. A definition of the forms of As in these materials is essential toward an unambiguous input for the development of geochemical models for the fate and transport of As in the aquifers of this region.

A sequential extraction procedure after Tessier et al. (1979) was used, with an objective of identifying the chemical/mineralogical nature of arsenic within the overburden materials. Samples of the overburden material were subjected to solutions designed to sequentially leach a specific fraction, progressing from exchangeable metals, to those bound to carbonates, then to those bound as reducible complexes, followed by metals bound to organic matter, and finally an aqua regia extract of the residual fraction. Leachates were analyzed for concentrations of As, Fe, and Mn, among other metals. Twenty selected samples of the overburden material within the elevated arsenic zone were collected at several different localities from depths of 0-3 ft. (7 samples), 3-6 ft. (7 samples), or 6-10 ft. (6 samples). All samples were oven dried and passed through a 2 mm sieve. Two grams from this fraction were treated in sequence by different solutions and each solution analyzed separately.

From the total of six extraction fractions, only 3 fractions detected significant amounts of As, Fe, and Mn: the fraction containing reducible compounds, the fraction containing organic material, and the aqua regia extract. Not considering the aqua regia extract, over 90% of the bulk leachable As and over 80% of leachable Fe and Mn were released in the reducing step and smaller amounts were measured in the organic fraction. From the results we can conclude that the bulk of As is bound in the Fe-Mn complexes known to exist as coatings on pebbles and grains in the overburden. This data also supports conclusions from previous studies which suggest that reducing conditions in groundwater are responsible for the mobilization of As from the overburden material.