Spatiotemporal Variability in Microbial Quality of Western US Agricultural Water Supplies: A Multistate Study

• Published in: Journal of environmental quality Vol. 47; no. 5; pp. 939 - 948
• Main Authors: Partyka, Melissa L., Bond, Ronald F., Chase, Jennifer A., Atwill, Edward R.
• Format: Journal Article
• Language: English
• Published: United States The American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Inc 01-09-2018
• ISSN: 0047-2425; 1537-2537
• DOI: 10.2134/jeq2017.12.0501

In 2011, the US Congress passed the Food Safety Modernization Act, which tasks the US Food and Drug Administration to establish microbiological standards for agricultural water. However, little data are available for the microbiological quality of surface water irrigation supplies. During the 2015 irrigation season, we conducted a baseline study on the microbial water quality of large irrigation districts in California (n = 2) and Washington (n = 4). Monthly samples (n = 517) were analyzed for bacterial indicators (fecal coliforms, enterococci, and Escherichia coli) and pathogens (Salmonella spp., E. coli O157, and non‐O157 Shiga toxin‐producing E. coli [STEC]). Although there was a high degree of variability (μ ± SD = 59.13 ± 106.0), only 11% of samples (56/517) exceeded 126 colony‐forming units (CFU) 100 mL−1, and only six samples exceeded 410 CFU 100 mL−1. Two volumes of water were collected for pathogen analysis (1 L and 10 L); prevalence of Salmonella in 10‐L samples (68/149) was nearly double of that found in 1‐L samples (132/517). We found STEC during ∼9% of sampling events (58/517); serotypes O26 and O45 were the most common at 31 and 26%, respectively. Pathogens were not associated with exceedance of the regulatory threshold, yet the odds of detecting Salmonella increased approximately threefold (odds ration [O.R.] = 3.14, p < 0.0001) for every log increase in turbidity. Microbiological outcomes were highly district‐specific, suggesting drivers of water quality vary across spatiotemporal scales. The true risk of contamination of produce from irrigation water supplies remains unknown, along with the optimal monitoring strategy to improve food safety.
Core Ideas
Western US irrigation water supplies rarely exceed FDA agricultural water standards.
Indicator bacteria are not predictive of pathogen prevalence or concentration.
Prevalence of pathogens in irrigation water is higher than previously reported.
Odds of detecting Salmonella improved >270% with an increase in sample volume.
More research is needed to provide science‐based guidance to the produce industry.