GEOCHEMICAL ANALYSIS OF GROUNDWATER QUALITY IN THE FRACTURED BEDROCK AQUIFER OF THE TOWN OF CRAFTSBURY, NE VERMONT

The bedrock geology of the Town of Craftsbury is divided by a major lithotectonic contact, the Richardson Memorial Contact (RMC), which separates metamorphic rocks of the Pre-Silurian (PS) Green Mountain Belt (GMB) to the west from those of the Silurian-Devonian (SD) Connecticut Valley Belt (CVB) to the east. The RMC is a Silurian unconformity that is locally coincident with a Devonian fault. The GMB is composed of phyllites and phyllitic quartzites of the Moretown and Cram Hill fms; the CVB consists of siliceous marbles, phyllites and isolated granites. The Ordovician Taconian Orogeny only deformed and metamorphosed PS rocks whereas the Devonian Acadian Orogeny affected rocks on both sides of the RMC.

Both bedrock and surficial geologic maps of Craftsbury were completed in 2010 and are a framework for evaluating groundwater quality and quantity. There are 328 accurately located water wells in Craftsbury and 96% were completed in bedrock. The focus of this study is to test the groundwater from each major bedrock formation for a suite of metals, non-metals, anions, gross alpha (screening test for radioactivity), temperature, pH, conductivity, DO, and ORP. This water chemistry will be compared with the whole rock formation chemistry to see if geochemical “fingerprints” can be recognized and also with other correlative along-strike areas containing the RMC.

A recent groundwater study in the Montpelier and Barre West quadrangles (VT) found that some wells completed in SD rocks had elevated Gross Alpha and radium levels, whereas wells completed in PS rocks west of the RMC did not. Gamma-ray surveys by Walsh and Satkoski (2005) demonstrated that SD rocks are more radiogenic than PS rocks, and that phyllite beds in the Waits River Fm were elevated in radionuclides relative to quartzite and marble beds.

Preliminary geochemical analysis of the major bedrock formations by ICP-AES shows that the Moretown Fm contains ~twice as much average Zr as the SD rocks. The SD rocks contain significantly more Ca than the Moretown Fm and less Ba or V.