Sources of generic Escherichia coli and factors impacting guideline exceedances for food safety in an irrigation reservoir outlet and two canals.

Nearly half of all cases of foodborne illness are associated with plant-based foods such as leafy greens and raw flour. An important potential source of pathogen contamination along the food-production continuum is irrigation water, which has led to the implementation of increasingly stringent agricultural irrigation water quality requirements. To better understand factors impacting irrigation water quality, we investigated sources of generic Escherichia coli and how they varied temporally among different sampling sites. Precipitation, Campylobacter species distribution, and physicochemical water quality parameters were also investigated to substantiate microbial source tracking findings. Biweekly sampling was conducted at a reservoir outlet and two downstream canals in southern Alberta, Canada, throughout two irrigation seasons, the latter of which was notable for drought conditions. Overall, 50% of canal samples exceeded Alberta's irrigation guideline for E. coli (100 E. coli per 100 ml), whereas all reservoir samples were below guideline limits. Collectively, E. coli source apportionment, Campylobacter species distribution, and physicochemical water quality data suggest runoff from surrounding agricultural land was a contributing factor to E. coli guideline exceedances in Year 1 only. In Year 2, the majority of exceedances occurred later in the season when there was little precipitation and were largely attributed to cosmopolitan E. coli from wild birds and cattle. Similarities in E. coli host-source and Campylobacter species distributions between the reservoir and canals when the guideline was exceeded suggest the reservoir could be a primary source of E. coli during drought. Increased bacterial concentrations in canals were likely due to environmental conditions that promoted bacterial survival and in-situ proliferation. Our findings support previous accounts that many E. coli isolates possess enhanced survival capabilities, which has implications to bacterial water quality assessments and risk mitigation, particularly under drought conditions.