The City of Guelph, Ontario, Canada relies on 21 groundwater wells to supply water from an underlying fractured dolostone aquifer to ~135,000 residents. Following the implementation of the Clean Water Act (2006), Ontario municipalities are required to evaluate threats to source water. The approach requires developing wellhead protection areas based on groundwater travel times, which can be complex to estimate in urban environments with variability in infrastructure, land use, and recharge. The variety and age of potential contamination source zones, following decades of urban and industrial activities, provides impetus to assess aquifer vulnerability by utilizing the best-available characterization and monitoring techniques.
This study involves a municipal supply well and surrounding aquifer system, where trace concentrations of trichloroethylene (TCE) have been detected since the mid-1990s. In collaboration with the City of Guelph and their consultants, a 3-D sentry monitoring network was designed surrounding the municipal supply well to monitor variable hydraulic and contaminant conditions for early assessment of trends in water quality at multiple depths and directions within the capture zone. Four boreholes were drilled radially around the supply well (3 km2 area, ~70 m deep), intersecting flow paths between the well and suspected sources. Of the four boreholes, one ‘golden spike’ location was selected as a method calibration borehole where a suite of high-resolution geophysical and hydrophysical datasets were collected, including VOC analysis of rock core samples, following the discrete fracture network–matrix field approach (Parker et al. 2012).
Robust golden spike data, including continuous core logging and transmissivity profiles, informed the placement of 7-10 depth-discrete ports in each multilevel system (MLS), forming a 3-D monitoring network. Ports targeted zones with active groundwater flow, rock core VOC detections, and aquitard layers, while minimizing the cross-connection of distinct hydrogeologic units. MLS design incorporated variable tube diameters to accommodate ports with different functions, and a dedicated fiber-optic cable for monitoring flux variability in the back-filled system.
MLS hydraulic head profiles show the presence of strong downward vertical gradients, induced by pumping, surrounding the supply well. This indicates the presence of an aquitard unit separating two zones of relatively high hydraulic conductivity and, most importantly, two aquifer units with distinct recharge and possible contaminant input areas and residence times.
At municipal supply wells impacted by aged chlorinated solvent plumes, sentry monitoring is a valuable approach to understand 3-D spatial and temporal variability in flow and contaminant distributions, allowing for timely and cost-effective decision-making for supplying safe water to the community. The improved understanding of subsurface heterogeneity and flow conditions around the supply well will be used to update the groundwater flow model, evaluate future risks to water resources and improve strategies for long-term urban aquifer management.