Seasonal Source Water Temperature as a Predictive Indicator for Legionella Outbreak Risk in Ontario Healthcare Facilities: A Correlation Analysis Using PWQMN Baseline Data and Provincial Surveillance Records 2019–2025

Ontario's 2024–25 fiscal year produced the largest Legionella outbreak in Middlesex-London Health Unit history — 107 confirmed cases — while simultaneously recording a 10-fold increase in provincial drinking water exceedances, 578 trihalomethane (THM) exceedances across Ontario drinking water systems compared to near-zero in all prior years, and 192 low chlorine residual adverse water quality incidents (AWQIs).This paper presents a spatiotemporal ecological analysis examining whether source water organic loading in Ontario's Grand River and Thames River agricultural watersheds creates a predictable cascade of disinfection stress — quantified through THM exceedance patterns — that elevates population-level Legionella risk. Using five years of Ontario Drinking Water System AWQI records (2020–2025, n=18,114 incidents), 60 years of Ontario Provincial Water Quality Monitoring Network (PWQMN) source water chemistry data, and Public Health Ontario legionellosis surveillance data (2019–2025, mean 354 cases annually), we document three convergent findings. First, monthly low chlorine AWQIs in 2024–25 peaked in May through September (range: 163–288 per month), preceding and coinciding with Ontario's documented June through August Legionella outbreak season with a 4–8 week temporal lag consistent with the 2–6 week predictive window established in Paper 2 of this series. Second, health units drawing from Grand River agricultural watershed sources averaged 1,800+ AWQIs in 2024–25 — 2.6 times higher than health units drawing from Canadian Shield and Lake Superior sources. Third, THM exceedances in 2024–25 were elevated across all 12 months, consistent with a sustained province-wide organic-chlorine demand crisis rather than a seasonal or episodic event.We propose that THM exceedances, when concurrent with low chlorine residual AWQIs, serve as sentinel biomarkers of the Disinfection Gap: the condition in which source water organic demand exceeds treatment plant chlorination capacity, depleting free chlorine residuals before they can suppress Legionella growth in downstream cooling tower systems. This is Paper 4 of the Presignal Source Water Intelligence Series. Related work — Paper 1 Grand River: DOI 10.5281/zenodo.19653415 Related work — Paper 2 Legionella Seasonal: DOI 10.5281/zenodo.19695022 Related work — Paper 3 Multi-Parameter Framework: DOI 10.5281/zenodo.19935260