THE EFFECT OF METAMORPHISM ON ARSENIC CONCENTRATION IN METAPELITE BEDROCK AQUIFERS: A CASE STUDY OF THE CONNECTICUT VALLEY/GASPE SEQUENCE (NE VERMONT AND SE QUEBEC) (Invited Presentation)

The purpose of this study is to examine As concentrations and mineralogy in shales, slates, phyllites and schists of the Siluro-Devonian Connecticut Valley-Gaspe (CVG) sequence across a strong along-strike metamorphic gradient in NE Vermont and SE Quebec. In the northern Appalachians, elevated levels of naturally-occurring As have been reported in bedrock wells that produce from chlorite grade slates and phyllites, notably in the Central Maine Belt of Maine and NH as well as in the Taconic region of Vermont and in the CVG of SE Quebec. Within these areas, often > 20% of wells contain > 10 ppb As; however, the incidence of elevated arsenic in bedrock wells producing from phyllites and schists (biotite, garnet and staurolite grade) of the CVG of Vermont is very low – only 2-3 % of bedrock wells producing from these rocks contain > 10 ppb As. The apparent decrease in As content of bedrock wells with increasing metamorphic grade indicates that metamorphism may play a crucial role in controlling As concentration of metapelites as well as As content of groundwater hosted by these rock types.

To test the hypothesis that As is driven out of metapelites with increasing metamorphic grade, this study is designed to control as many factors as possible, particularly protolith depositional setting and age. Samples have been obtained from a suite of variably-metamorphosed metapelites from the CVG, including unmetamorphosed to low-grade shales and slates of the Compton, Famine and Ayers Cliff formations of SE Quebec as well as stratigraphically correlative medium to high-grade phyllites and schists of the Gile Mountain and Waits River formations of NE Vermont. Preliminary data indicate the following: (1) phyllites and schists from NE Vermont (> biotite zone) contain average whole rock As of 2.1 ppm (31/32 values are < 6 ppm, including many pyrite-rich samples); (2) a literature search of As concentration in Paleozoic shales (N = 281, including USGS standard SDo-1) indicates that average As concentration in unmetamorphosed shales is 53 ppm (range = 8 to 823 ppm); (3) analysis of 30 low-grade shales and slates (< chlorite zone) from Quebec is in progress. Recrystallization of pyrite during prograde metamorphism or desorption of As from sediments during metamorphism are two possible explanations for the apparent decrease in As with metamorphic grade.