When municipal governments invest public taxpayer capital into civic buildings, the design bar is extraordinarily high: systems must deliver multi-decade reliability, drastic operational cost reductions, and zero environmental hazard to municipal drinking water supplies.
In Groton, Massachusetts, the Town Building Committee embarked on an ambitious municipal modernization plan covering Town Hall and the Public Library. The goal was to achieve LEED green building certification, reduce long-term town operating budgets, and protect Groton's pristine Nashoba Valley drinking water resources. GDNE was selected to engineer and construct the vertical ground-source borefield.
Geology of the Nashoba Terrane
The subsurface geology beneath Groton consists of ancient metamorphic crystalline rock—predominantly Nashoba Formation gneiss with granitic intrusions. This competent bedrock provides high thermal conductivity (approximately 1.5 to 1.8 BTU/hr-ft-°F), making it an outstanding medium for vertical heat dissipation and extraction.
Environmental Safeties: Aquifer Isolation
Because the borefield was located within proximity to municipal groundwater recharge zones, GDNE engineered an environmentally failsafe loopfield design:
- Closed-Loop Isolation: Zero groundwater is withdrawn or returned. The ground loop is a permanently sealed closed loop.
- Impervious Bentonite Grout Seals: Each 450-foot borehole was pressure-grouted from the bedrock base to the topsoil with high-solids bentonite grout. This creates an impermeable geological plug that permanently isolates shallow surface runoff from deep bedrock fractures.
- Food-Grade Propylene Glycol Transfer Fluid: Rather than conventional synthetic refrigerants, the ground loop circulates a biodegradable, non-toxic food-grade USP propylene glycol solution with organic corrosion inhibitors.
Municipal Performance Deliverables
- Facilities Served: Groton Town Hall & Public Library complex
- Annual Operating Savings: > 55% reduction vs. baseline oil/propane HVAC
- LEED Contribution: Critical Energy & Atmosphere (EA) optimization credits
- Equipment Lifespan: 50+ year warrantied HDPE subsurface infrastructure
Preserving Civic Aesthetics in Historic Groton
Like many historic New England towns, Groton takes immense pride in its colonial architecture and Town Common aesthetics. Conventional split-system heat pumps or rooftop chiller units would have created unwanted mechanical noise and visual clutter on historic civic facades.
By placing the entire thermal generation field underground beneath parking and manicured lawn space, the Town achieved state-of-the-art climate control with zero visible footprint. Quiet indoor air handlers and water-to-water heat pumps maintain whisper-quiet reading rooms in the library and comfortable administrative offices in Town Hall.
Read more about our municipal and institutional engineering solutions in our project case studies portfolio.
