Development of a model to predict the impact of sediments on microbial irrigation water quality

Microbial contamination of surface water is a complex dynamic and influenced by many interacting factors (e.g. runoff into streams from rainfall). Sediments at the bottom of waterways have been show to harbor 10 to 10,000 more fecal bacteria than surface waters, and when re-suspended may cause large increases in the over laying water resulting in increased risks of exposure to disease causing microorganisms. However, their significance in manmade canals is unknown. The goal of this project is to determine under what conditions bacteria and viruses could re-suspended from sediments found in manmade irrigation canals and to identify potential “hotspots” of accumulation of microbes in the canals. This information can then be used to design sampling strategies and irrigation events to minimize exposure of pathogens to crops.

Our recent work on the occurrence of Salmonella and Escherichia coli in irrigation canals in Southern Arizona have shown that these bacteria occur in concentrations ten times greater in the sediment than the overlaying water. In addition, other pathogens, such as Listeria monocytogenes and human pathogenic enteric viruses occur in aquatic sediments. We believe that this reservoir can be a significant source of foodborne pathogens. For the first time, we propose to develop an integrated model that could form the basis of managing this threat to food crops and microbial water quality in surface waters. The overall goal is to develop a baseline of data to determine how sediment, pathogens, and flow properties affects the quality of irrigation water. This will be accomplished through both laboratory experimental studies and field investigations. In the laboratory, a series of experiments will be conducted in a flume to identify flow and sediment conditions which would result in the resuspension of sediment-bound bacteria/viruses in irrigation channels, quantifying the impact of re-suspension of different levels of bacteria/viruses in overlaying water, and suggesting possible guidelines for the occurrence of indicator bacteria and pathogens in the sediments of irrigation channels. Previous work has been limited to only two sediment types and one enteric bacterium and bacterial virus. Because of the wide variety of the structural/surface chemistry properties of bacteria and viruses, varying types of viruses and bacteria need to be studied to develop a more practical model of their behavior in sediments. Increasing evidence suggests that waters used in the irrigation of produce are a significant source of the pathogens responsible for many illnesses, such as diarrhea, hepatitis, listeriosis, etc. Arizona ranks third nationally in the production of fresh produce and second in the production of lettuce, broccoli, spinach, and melons. Farmers in Arizona and California account for approximately 98% of the commercial domestic output of lettuce. All of California and Arizona’s produce is grown by irrigated agriculture. Thus, the microbial quality of irrigation waters is a key issue.

1. Identify factors which would result in the re-suspension of sediment-bound bacteria/viruses in irrigation channels (i.e., rainfall events, wind), specifically E. coli, L. monocytogenes, MS2 virus and phiX174 virus (also used as a surrogate for enteric viral pathogens). 2. Quantify the impact of resuspension of different levels of these bacteria and viruses on the quality of the overlaying water. 3. Suggest guidelines for growers/producers to minimize the occurrence of pathogenic bacteria and viruses in the irrigation water.

1. Virus resuspension from sediment was found to vary with the type of virus. Thus, foodborne viruses, such as norovirus and hepatitis A virus, may have different potentials for contaminating the overlaying water. 2. Models were developed to predict the resuspension of viruses, E. coli and L. monocytogenes based on microorganism type, sediment type, flow rate, velocity, and shear stress. These models can be used to predict the impact of these microorganisms in sediment in the overlaying water under varied operational conditions during irrigation events. 3. Data collected in field studies was analyzed using machine learning and it was found that E. coli in the sediments influences the quality of the irrigation water in canal systems if the standard is 126 E. coli /100 ml. 4. No L. monocytogenes was detected in the sediments or water of irrigation canals in Arizona, and do not appear to be a source in this region. 5. The highest concentrations of E. coli in sediments were found in those of splitter boxes (diversion boxes), and these could potentially be reservoirs of E. coli in irrigation systems. These concentrations were 10 to >1,000 times greater than those found in canal sediments. Splitter boxes should be studied to determine their impact on irrigation water quality. 6. Sediments collected repeatedly from some locations always had high concentrations of E. coli. These may be point sources of E. coli contamination of the overlaying water. Studies should be conducted to determine why these locations always have high levels of E. coli in the sediment. E. coli in the sediment could be an indicator of long-term E. coli concentrations in the water.