6 Applied Geological Investigations at State Geological Surveys in the Northeastern Section of GSA (Posters)

This technical session will highlight the high-level, applied work done by state geological surveys to address diverse environmental issues including geologic hazards, groundwater and surface water, and economic mineral deposits in the Northeast Section region.

Sunday, 22 March 2026
8:00 AM - 12:00 PM
CCC, Ballroom C

Hydrogeology of a Fractured Carbonate Aquifer Being Utilized for Open-Loop Geothermal Energy, Middlebury Vermont

Paper number: 6-1
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

The heterogeneity of fractured rock aquifers adds complexity to aquifer properties across short distances, which is an issue when trying to predict well yield, flow patterns and water chemistry. A geothermal well field at Ilsley Public Library in Middlebury consists of 4 wells in Ordovician carbonates on the overturned east limb of the asymmetric Middlebury Synclinorium in the hanging wall of the Champlain Thrust fault. Some lithologic contacts are stratigraphic; others are reactivated bedding-parallel thrusts and may have younger rock slivers along them. One such thrust is mapped in the field area and projects beneath the library and should intersect the wells.

This project seeks to understand the carbonate aquifer hosting an open-loop low-temperature geothermal system currently in development. Open-loop geothermal systems draw groundwater (~10 oC) from at least one production well, deliver it to a heat pump to exchange thermal energy, then inject water into at least one injection well, emphasizing the need for a reliable groundwater source and sink. The Ilsley system includes 2 production wells 50-60 m downhill of 2 injection wells. These wells were studied by (1) logging with a downhole Mt. Sopris instrument (T, resistivity, natural gamma, caliper, acoustic borehole imager [ABI], heat-pulse flowmeter), (2) continuous downhole color video recorded with a GeoVISION borehole camera, (3) stable isotopes (dD, d18O), (4) groundwater recharge ages (CFCs, 3H), and (5) chemical analysis of major and trace elements in well water. All indicate a high degree of heterogeneity, notably that one of the production wells was hydrologically isolated from the other three when pump tested; and, the other production well has a stratified heterogeneous composition of low-TDS young (20 yr) groundwater atop high TDS old (68 yr) groundwater, separated by a sharp transition at 44+3 m depth. Based on the borehole camera survey alone, no abrupt lithological or structural changes were observed that would support the presence of a thrust fault. Structural data derived from caliper and ABI logs was used to calculate the attitudes of planar structures e.g. beds, foliations, and fractures. These data will be integrated with all other logging data and chemical parameters to construct a 3D fence diagram that shows the “plumbing” of the well field.

Geological Society of America Abstracts with Programs. Vol. 58, No. 2

doi: 10.1130/abs/2026NE-411272

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