1a. Objectives (from AD-416)
Objective 1: Elucidate and quantify mechanisms and factors of pathogen and indicator bacteria fate and transport from animal sources to irrigation waters. Evaluate the effects of soil and vegetation properties on parameters of pathogen fate and transport with surface runoff to irrigation water sources. Assess contribution of bottom sediments as potential pathogen reservoirs in agricultural landscapes. Research the fate and transport of pathogen and indicator bacteria in irrigation water delivery systems. Objective 2: Develop models and computer-based tools to recommend and implement site-specific diagnostics, monitoring, and prediction of the fate and transport of pathogen and indicator bacteria that affect the microbiological condition of irrigation water. Develop bacteria fate and transport components for USDA-ARS hydrologic models to simulate the effect of bottom sediments, periphyton, and bank soils on microbiological quality of surface waters intended for irrigation. Develop a farm-scale irrigation system model that will use site specific environmental and management data to provide input data for quantitative microbial risk assessment of irrigation waters.
1b. Approach (from AD-416)
An integrated approach including laboratory research, field research on irrigation systems, and mathematical modeling will be used. Experiments and monitoring will be carried out to (a) evaluate the effects of soil and vegetation properties on pathogen fate and transport with surface runoff to irrigation water sources, (b) understand and quantify pathogen and indicator bacteria fate in potential pathogen reservoirs associated with irrigation systems, such as bottom sediments in surface waters, and biofilms in irrigation equipment, and (b) microbial exchange between these reservoirs and flowing or stagnant waters. Mechanistic models will be developed to allow for (a) analyzing possible changes in pathogen and indicator bacteria concentrations along hydrologic pathways from animal sources to fields, and (b) improving resource allocation to monitor pathogen and indicator bacteria occurrence along the pathways.
3. Progress Report
The first comprehensive review has been compiled to address the issue of the microbiological water quality and its effect on produce contamination by water-borne pathogens. Topics covered included: temporal and spatial variability, and regional differences, in pathogen and indicator organism concentrations in water; direct and circumstantial evidence for contaminated water as a source of food-borne pathogens; fate and transport of pathogens and indicator organisms in irrigation systems, and the role of environmental microbial reservoirs;and current standards for irrigation water quality, and risk assessment. Pilot field experiments were carried out to test the hypothesis that microbial quality of irrigation water can be substantially altered by the association of E. coli with biofilms in pipe-based irrigation water delivery systems. The aluminum pipes in the sprinkler irrigation system were outfitted with coupons that were extracted before each of 2-hour long irrigations carried out with weekly intervals. Water from the creek water and sprinklers, residual water from the previous irrigation, and biofilms on the coupons were analyzed for E. coli. High E. coli concentrations in the water remaining in irrigation pipes between irrigation events were indicative of E. coli growth. Substantial differences were found between E. coli concentrations in creek water at the pump’s uptake and the water from the sprinklers, leading us to believe E. coli were released from the pipe’s inner surfaces to flowing water, previously captured from the flowing water. The population of bacteria associated with the biofilm on pipe walls was estimated to be larger than that in water in pipes in the first three irrigation events and comparable to one in the fourth event. Laboratory experiments were carried out to screen and rank methods of E. coli extraction efficiency from biofilms on typical irrigation pipe materials. Biofilms were grown on aluminum and PVC coupons for a week. Extraction methods included sonication, beads application, and variety of shaking procedures. No significant differences have been found between methods so far. The database accumulated in EMFSL from 2004 through 2011 has been reanalyzed to evaluate the loss of microorganisms to on-site infiltration during their release from land applied manure and manure slurries. The predictive models of the overland manure-borne microorganism fate and transport currently assume that all microorganisms eventually can be released to runoff. Our re-analyzis showed that this assumption is far from accurate. From 30% to 80% of microorganisms stay put as they enter soil with infiltrating water at the manure application site. To improve the characterization of streambed sediments as reservoirs of microorganisms, we initiated the research of the response of the “smart tracer” resazurin to the microbial respiration. We observed the high sensitivity of the fluorimetric determination of resazurin transformation rates. We also estimated background levels of fluorescence in stream conditions at the wavelengths of resazurin and its product determination.
1. Pilot experiments were conducted to evaluate the ability of biofilms in irrigation pipelines to serve as reservoirs of pathogen and indicator microorganisms. The population of bacteria associated with the biofilm on pipe walls was estimated to be larger than that in water in pipes in three of four irrigation events. Biofilm-associated E. coli altered microbial quality of irrigation water. This work is the first experiment on the impact of biofilms on microbial quality of irrigation waters. Flushing of the irrigation system may be a useful management practice to decrease the risk of microbial contamination of produce. Because microbial water quality can be substantially modified while water is transported in an irrigation system, it becomes imperative to monitor water quality at fields, rather than just at the intake. Disinfecting irrigation pipelines has to be considered as the viable management practice to improve food safety of irrigated produce.