EVALUATION OF ARTIFICIAL RECHARGE AND GROUNDWATER FLOW IN A SMALL NEW ENGLAND AQUIFER

Artificial recharge is receiving increased attention as a means to manage groundwater and surface water resources in the Northeast US, where small stratified-drift aquifers are common. The water supply system serving the University of New Hampshire and Town of Durham has implemented an artificial recharge system that recharges the Spruce Hole Aquifer with surface water from the nearby Lamprey River. The system has been in operation since 2016 and the existing hydrogeological, geophysical, atmospheric, and system monitoring datasets, along with new geochemical data, provide the foundation for the development of a high-resolution groundwater model to better understand the impacts of system usage and environmental conditions on groundwater flow and transport in the aquifer. Simulation design focuses on evaluating surface water storage efficiency, sustainable groundwater yield, and drought resiliency across a range of observed and potential conditions.

The model uses a suite of open-source resources including python packages, Modflow-setup, and flopy, that discretize grid-independent source data files, execute MODFLOW6 groundwater flow and transport simulations, and provide post-processing options of results. A python package is developed to process existing datasets, generate model source data files, execute the automated parameter estimation, and analyze model output.

Model calibration with PEST quantifies parameter correlations and uncertainties using existing datasets collected during preliminary hydrogeological investigations, studies conducted to permit the production well and artificial recharge system, and annual aquifer monitoring from 2012-2023. Data from ongoing on-site monitoring of surface and groundwater stations, including water levels, streamflow measurements, and water quality samples analyzed for stable isotopes and trace metals, are also used to help calibrate the model, characterize the fate and transport of artificial recharge water in the aquifer, and estimate groundwater discharge into the adjacent Chesley Brook.

Results demonstrate the influence of aquifer properties, artificial recharge system usage, and environmental conditions on storage efficiency and sustainable yield and provide a case study for applying artificial recharge in other settings.