Abstract

Groundwater accounts for approximately 98% of available freshwater, with >2 billion people relying on it as a primary drinking water source. Notwithstanding its importance, specific groundwater quality parameters - namely microbial concentrations and non-Escherichia coli coliforms (NEC) - remain understudied. The current study sought to address this gap by modelling three distinct Contamination Indices (CI) corresponding to E. coli concentration, NEC concentration, and the NEC:E. coli concentration ratio. CIs were developed for south Ontario (115,693 km2) using ∼1 million samples from ∼290,000 wells collected between 2010 and 2021. To permit modelling, CIs were linked to 50 subregion-specific variables which impact groundwater quality (e.g., well depth, aquifer type, mean daily precipitation volumes); Generalized Additive Models (GAM) were subsequently developed and associated non-linear partial effects were calculated. Findings suggest NEC concentrations may appropriately indicate a source's long-term potential for generalized contamination, as the NEC model exhibited high deviance explained (91.9%) due to significant associations (p < 0.05) with factors influencing and/or representing groundwater recharge. A daily summer rainfall “tipping point” was identified, with volumes >3 mm being associated with NEC concentration reductions (p < 0.0001), potentially due to subsoil saturation and/or aquifer contamination dilution. Regions with predominantly deep wells in bedrock aquifers were associated (p < 0.0001) with low NEC:E. coli ratios, i.e., localized contamination mechanisms (e.g., contaminant bypass or short-circuiting) likely dominate in these regions. The presumption that deeper aquifers/wells are “safer” may thus be due for reconsideration. The importance of understanding and inferring contamination mechanisms cannot be overstated, as it serves as a foundation for evidence-based source protection and testing recommendations.

Keywords

Contamination mechanisms, E. coli, Groundwater contamination modelling, Total coliforms

Publication Date

2026-01-01

Publication Title

Water Research X

Volume

30

Deposit Date

2026-01-21

Funding

The authors would like to acknowledge the on-going efforts and dedication of the provincial Public Health Ontario water testing laboratories, whose continued service and contributions enable this important work to continue on behalf of Ontario's rural residents and in support of safe drinking water for all. This work was financially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) Collaborative Research and Training Experience (CREATE) program for Leaders in Water and Watershed Sustainability (the LEADERS CREATE), the Canadian Foundation for Infectious Diseases (CFID), and the Ontario Graduate Scholarship (OGS). This work was financially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) Collaborative Research and Training Experience (CREATE) program for Leaders in Water and Watershed Sustainability (the LEADERS CREATE) , the Canadian Foundation for Infectious Diseases (CFID) , and the Ontario Graduate Scholarship (OGS) .

Creative Commons License

Creative Commons Attribution 4.0 International License
This work is licensed under a Creative Commons Attribution 4.0 International License.

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