PHYSICAL AND CHEMICAL HYDROGEOLOGY OF A WELL COMPLETED IN THE FRACTURED ROCK AQUIFER (FRA) OF MORRISTOWN, VERMONT, USA

The Morristown Corners public water supply serves a small rural subdivision and consists of three wells completed in 1971 (Well 7), 2009 (Well 8), and 2024 (Well 9). The third well was drilled to supplement the insufficient yields of the first two wells, which were already being treated for elevated As and Mn. Over-pumping of the older wells also liberated Fe and caused low pH (3.8), likely from pyrite oxidation in the FRA. The purpose of this study is to physically (geophysical logging and a pumping test) and chemically (groundwater chemistry) characterize the hydrogeology of the newest well and entire well field.

The well field straddles an east-dipping Ordovician thrust fault that juxtaposed rusty-weathering, black, sulfidic phyllites of the Ottauquechee Fm (east, hanging wall) with green phyllites of the Jay Peak Fm (west, footwall), both of ~Cambrian age. Regional planar structures include a composite S_1-2 foliation deformed by upright asymmetric F₃ folds with an axial-planar crenulation cleavage (S₃). Shallowly dipping unloading fractures with variable strike are the latest planar structure.

Geophysical logs (borehole camera, fluid temperature and conductivity, gamma, caliper, and acoustic televiewer) show that Well 9 penetrated the thrust fault at 80 m depth. Heat Pulse Flowmeter surveys identified stacked vertical flow cells with groundwater entering the well from fractures at 120 m and 65 m, and exiting at 95 m. A 72-hour pumping test demonstrated strong and very weak hydraulic connections between Wells 9 and 7 and between Wells 8 and 9, respectively.

Pre-, syn-, and post-pumping groundwater geochemical samples from all wells showed: 1) increased sulfate, calcium, magnesium, and uranium in Well 9, with decreased Fe, suggesting pyrite dissolution along flow paths and Fe oxide precipitation, and 2) increased uranium but decreased calcium, arsenic, and Fe in Well 8, reflecting redox shifts and arsenic adsorption or co-precipitation with Fe oxides.

These findings highlight groundwater-rock interactions, redox dynamics, and structural controls on hydraulic connectivity. Uranium mobilization suggests oxidizing conditions in previously reducing fractures. Structurally partitioned groundwater geochemical zones defined by a 3D network of foliations and fractures control flow and recharge pathways.