ANALYSIS OF ARSENIC SPECIATION IN ULTRAMAFIC ROCKS BY SEQUENTIAL CHEMICAL EXTRACTION: IMPLICATIONS FOR TACONIAN FLUID SOURCE AND MODERN AQUIFER CONTAMINATION

Arsenic-enriched ultramafic rocks from northern Vermont are being studied using a sequential extraction process to determine arsenic speciation. Attention was brought to high arsenic levels in these rocks when groundwater with elevated arsenic (above the EPA MCL of 10 ppb) was discovered in bedrock wells in north-central Vermont. Previous work suggests that the arsenic source is metasomatized ultramafic rocks, specifically serpentinites and talc-carbonates found within Rowe-Hawley Belt, a tectonic assemblage of thrust slices in the suture zone of the Ordovician Taconian Orogeny. Arsenic levels in these rocks are highly elevated, with concentrations up to 450 ppm in serpentinites and up to 1104 ppm in talc-carbonates. Arsenides, sulfides and other minerals frequently associated with arsenic have not been detected in these samples, indicating that other minerals host the arsenic. XRD analysis of a wide range of ultramafic specimens from northern Vermont indicates that samples are either dominated by (1) antigorite with small amounts of carbonate and chromite-magnetite, or (2) talc and/or magnesite. It is predicted that arsenic may be hosted in tetrahedral sites of antigorite or talc as As(V) substituted for Si, in magnesite as arsenate anions substituted for carbonate anions, or adsorbed onto mineral surfaces. A 3-step sequential extraction is being applied to determine arsenic speciation, using 1 M ammonium nitrate (pH = 7) to extract exchangeable ions, then 0.11 M acetic acid (pH = 3) to dissolve carbonates, and lastly aqua regia to dissolve antigorite and talc. At each extraction step, ICP-MS, XRD and FTIR will be used to determine geochemistry, mineral content, and bond configurations, respectively, in remaining pulps and extracted solutions. Preliminary FTIR and XRD data indicate disorder in antigorite crystallographic structures that may be the result of arsenic substitution in tetrahedral sites. It is expected that the sequential extraction process will elucidate the meaning of these initial observations. Implications of these findings will have relevance to both to the modern-day fate and transport of arsenic within the bedrock aquifer as well as to assessing the source of arsenic-enriching fluids responsible for Paleozoic metasomatism.