Contrasting environmental drivers of Shiga toxin-producing Escherichia coli (STEC) O157 and O26 serogroups in surface water and private groundwater supplies of a karst agricultural catchment in western Ireland

Study Region: This study was conducted in the karstic Black River sub-catchment in western Ireland, where well-draining soils and rapid groundwater recharge promote strong surface-groundwater connectivity and increase the vulnerability of groundwater supplies to microbial contamination. Study Focus: This year-long study assessed the prevalence and environmental drivers of Shiga toxin-producing Escherichia coli (STEC) O157 and O26 in surface waters and groundwater wells from May 2023 to May 2024. Nineteen sites (5 river sites, 14 groundwater wells) were sampled fortnightly. Detection of O157- and O26-associated genes was performed using enrichment-free low-volume filtration with quantitative PCR. Logistic regression identified climatic and anthropogenic predictors of STEC presence. New Hydrological Insights for the Region: O157 was detected in 58% of samples (44% surface water, 63% groundwater), while O26 occurred in 27% (29% surface water, 26% groundwater). O157 presence in groundwater was positively associated with cattle density and rainfall, and negatively associated with well depth, suggesting mobilisation and rapid transport following precipitation. In contrast, O26 presence was positively associated with domestic wastewater treatment system density and negatively associated with rainfall, indicating different contamination pathways. In surface waters, O157 was associated with increased cattle density and low discharge, while O26 was associated with low rainfall. These findings reveal distinct drivers and transport pathways for the two dominant STEC serogroups and provide new insight into the dynamics of clinically significant STEC contamination in Irish surface and groundwater systems.