Geochemical and isotopic evidence for pumping-induced impacts to bedrock groundwater quality in the City of Guelph, Canada

Abstract

Groundwater can be a key water resource in urban environments, providing a source of freshwater for community needs. The City of Guelph relies on groundwater for a population of over 130,000 people. Thus, groundwater quality is a critical component of long-term water management. This study assesses the effect of urban, industrial and agriculture activities on groundwater quality using a suite of multidisciplinary methods including geological, hydrological, geochemical, and isotopic tools. Multi-level systems (with depth-discrete ports), conventional well clusters, and municipal production wells were used to monitor hydraulic head and collect groundwater samples. Geochemical and isotopic measurements included redox parameters, major anions and cations, VOC, tritium and δ34S and δ18O in sulfate. Hydraulic head data show vertical gradient profiles characteristic of a multi-layered flow system within a shallow and deep Silurian bedrock aquifer and an intermediate aquitard varying in occurrence. Head loss disappears near production wells, showing enhanced vertical hydraulic connectivity between shallow and deep aquifers, attributed to pumping and long open intervals. Hydrochemical data show no impact of nitrate and high chloride is observed in the shallow and deep aquifer, attributed to seasonal road salt and the underlying aquitard unit, respectively. The aquitard unit also control the high sulfate in the deep aquifer which is supported by the isotope data on sulfate. Facilitated transport of shallow groundwater into the deeper aquifer is supported by the tritium data, showing the presence of recent groundwater throughout the whole depth of the two aquifers in some areas, likely due to the varying aquitard presence and enhanced vertical flow due to proximity from pumping municipal wells and active rock quarry. The results of this research suggest long term implications for groundwater resource management of sedimentary bedrock aquifers, where there is increasing groundwater demand due to population growth and potential for continual water quality degradation.