Science-Based Monitoring for Produce Safety: Comparing Indicators and Pathogens in Water, Soil, and Crops

Abstract

Using treated wastewater (TWW) for crop irrigation represents an important opportunity for ensuring adequate food production in light of growing freshwater scarcity worldwide. However, the environmentally sustainable approach of using TWW for irrigation can lead to contamination of produce with fecal pathogens that may remain in treated water. The overall goal of this research was to evaluate the correlation between the presence of fecal indicator bacteria (FIB) and that of a suite of human pathogens in TWW, the irrigated soil, and crops. Field experiments were conducted to compare secondary and tertiary TWW with dechlorinated tap water for irrigation of tomatoes, a typical commercial crop, in Israel, a semi-arid country. Human pathogens including bacteria (Salmonella), protozoa (Cryptosporidiumand Giardia), and viruses (Adenovirus [AV Types A, B, C & 40/41] and Enterovirus [EV71 subtypes]) were monitored in two field trials using a combination of microscopic, cultivation-based, and molecular (qPCR) techniques. Results from the field trials indicate that microbial contamination on the surface of tomatoes did not appear to be associated with the source of irrigated waters; FIB contamination was not statistically different on tomatoes irrigated with TWW as compared to tomatoes irrigated with potable water. In fact, Indicator bacteria testing did not predict the presence of pathogens in any of the matrices tested. High concentrations of FIB were detected in water and on tomato surfaces from all irrigation treatment schemes, while pathogen contamination on tomato surfaces (Cryptosporidiumand Salmonella) was only detected on crops irrigated with TWW. These results suggest that regular monitoring for pathogens should take place to accurately detect presence of harmful microorganisms that could threaten consumer safety. A notable result from our study is that the large numbers of FIB in the water did not appear to lead to FIB accumulation in the soil. With the exception of two samples, E. coli that was present at 10³ to 10⁴ cells/100 mL in the water, was not detected in the soil. Other bacterial targets associated with the enteric environment (e. g., Proteusspp.) as well as protozoal pathogens were detected in the TWW, but not in the soil. These findings suggest that significant microbial transfer to the soil from TWW did not occur in this study. The pattern of FIB contamination on the surfaces of tomatoes was the same for all treatment types, and showed a temporal effect with more contamination detected as the duration of the field trial increased. An important observation revealed that water quality dramatically deteriorated between the time of its release from the wastewater treatment plant and the time it was utilized for irrigation, highlighting the importance of performing water quality testing throughout the growing season at the cultivation site.