EXAMINING THE POTENTIAL EFFECT OF METAMORPHISM ON ARSENIC CONCENTRATION IN METAPELITE BEDROCK AQUIFERS: A CASE STUDY OF THE TACONIC SEQUENCE

The purpose of this study is to examine the role of metamorphism on arsenic concentrations in slates and phyllites, rock types that are common in bedrock aquifers of New England and adjacent SE Canada. Elevated levels of naturally-occurring As have been reported in bedrock wells that produce from chlorite grade slates and phyllites in the northern Appalachians, notably in the Central Maine Belt (Maine and NH) as well as in the Taconic region (VT and NY). In some areas within these belts, > 20% of bedrock wells contain > 10 ppb As; however, previous work suggests that As is depleted in higher-grade biotite and garnet zone rocks and that wells which produce from these higher-grade rocks are similarly depleted in arsenic.

This study is designed to sample across a low-grade metamorphic gradient and control as many other variables as possible, specifically by sampling rocks of similar depositional setting and age. Samples are predominantly from the Austin Glen (n=8) and Pawlet (n=19) formations, units which are correlative late Ordovician flysch containing greywacke and pyrite-bearing black shale, slate and phyllite. The Austin Glen (E NY State) is un- to weakly-metamorphosed whereas the Pawlet in Vermont has been metamorphosed to chlorite-grade slate and phyllite. Some samples from the upper Poultney Formation, where it is stratigraphically in contact with Pawlet, will be used in the final analysis.

REE data (n=10) obtained to date indicate that Austin Glen shales and Pawlet and Poultney slates and phyllites are geochemically very similar; data also show that Pawlet and Poultney phyllites (highest-grade rocks in the study) contain 23 % less As than do unmetamorphosed Austin Glen shales, and certain fluid-mobile elements (Pb, Ni, Zn) are also depleted in the higher-grade rocks. While more data is forthcoming, results to date are consistent with data from other sequences, including the Connecticut Valley-Gaspe sequence, where biotite and garnet zone phyllites and schists contain 85 % less As and Pb than unmetamorphosed shale equivalents. An additional 20 analyses of variably metamorphosed Austin Glen and Pawlet rocks will be carried out and supplemented with SEM and XRD data.