"Artificial channels are a lot different, we believe because operators control the flow of water," said Gerba, a University of Arizona microbiology professor. "How are these canals different from riverine systems, and could they be managed differently? Do we find more organisms in the sediment than in the surface waters? Are they a risk and do they create problems? We're still sorting that out."
Even if sampling finds pathogens in canal sediment, how do environmental events - such as canal flows - affect population dynamics and pathogen resuspension in surface water?
"It's a big puzzle, and we're trying to find one aspect that plays a role in this or to rule it out," Gerba said.
The goal is to establish baseline data and ultimately, an integrated model to help growers determine potential risks tied to canal irrigation water.
Joining him as co-principal investigator is Jennifer Duan, Ph.D.
, who is a professor at the University of Arizona Civil Engineering and Engineering Mechanics Department and holds a joint appointment in the Hydrology and Atmospheric Sciences Department.
Gerba said irrigated agriculture in the Southwest creates unique ecological challenges because canals or ditches serve nearly all fields. Decades ago, the desert Southwest was home to cotton and other field crops that weren't directly consumed by humans, so pathogens potentially found in irrigation water or sediment were not top of mind. But as the Southwest has increasingly become the center of winter produce production, potential risks posed by microorganisms are gaining attention.
Little research has been conducted on canal sediments, and of that research, it is generally limited to two sediment types, one enteric bacterium, and one virus, Gerba said. His project will take a broader approach and involves several sediment types, including sand, silt and clay; two bacterial pathogens and two enteric viruses. It also will examine how nutrient loads, and particularly nitrogen levels, may affect pathogen populations and/or resuspension.
Part of the project involves laboratory experiments with a hydraulic flume that compare the effects of flow rate, velocity and sediment type on the resuspension of sediment-bound pathogens in overlaying water. This is where Duan's expertise in hydraulics and sediment transport comes into play. The results will be incorporated into a model, which may be able to predict the risk of potential pathogen reservoirs developing under specific conditions.
In the desert Southwest, produce planting begins in late summer, with harvest beginning before Thanksgiving. The season usually runs through May, depending on the weather. Although the researchers collect field measurements from canals and ditches year-round, they increase sampling during the production season. Among the measurements they take are flow velocity, flow depth, water temperature, suspended sediment concentration, bacterial counts in water, and bacterial counts in the bed sediment. Results from both laboratory experiments and canal sampling will be incorporated into a predictive tool. Gerba said he envisions growers entering a number of environmental variables, such as weather, water sampling results, and canal flow, and the model would provide a type of risk assessment. But identifying a pathogen risk, should there be one, is just one part of the project.
"It makes no sense to me to find a problem without finding a solution," he said. "If it's an issue, then we'll figure out how to manage it."
As such, Gerba said he hopes research results also could be incorporated into guidelines producers could use to minimize the occurrence of pathogenic bacteria and viruses in irrigation water.