“Taking some samples periodically is just not going to catch unexpected situations,” Dr. LaBorde says. “You can’t ask people to do more unless there are some assurances that what they are actually doing is going to have a public benefit. This E. coli sampling, for instance, sometimes it works, sometimes it predicts, sometimes it doesn’t. But all the studies that have come out say it’s a very poor predictor of human pathogens in water.”
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Explore This IssueApril/May 2019
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Phyllis Posy, vice president, strategic services and regulatory affairs, for UV-water-treatment firm Atlantium Technologies, agrees. “Taking a sample is just that—an indication of that time and place,” she says. “How many folks are going to sample right after the rain stops, or when the temperature is coldest or warmest? We don’t catch the extremes and that may be when the contamination is measurable.”
Compounding the problem, says Dr. LaBorde, is the likelihood that sampling will miss contaminated silt in a reservoir or ditch. In the case of the Yuma E. coli outbreak, he recalls, “Some people say it might have been silt at the bottom of a ditch that was stirred up. Bacteria can attach to silt, so you may not have picked them up in the water itself. But if that silt or sediment is upset by mixing and flowing, then it can get into the irrigation system and onto the produce.”
Ozeran and Latack also note the difference between surface-water quality and the presence of pathogens in water sediment. While changes to weather can change the constitution of surface water, “sediment that collects at the bottom of a reservoir, canal, or stream tends to have higher concentrations of bacteria than the water itself. With a rainstorm causing hillside runoff or flooding, sediment and the bacteria living there can become suspended in the water. Turbid water after a storm therefore may have a higher concentration of bacteria than it had before the storm.”
While Ozeran and Latack encourage sampling as frequently as possible, and recording general site conditions while sampling (such as whether a flock of birds has recently been in the reservoir) in order to be informed about conditions affecting water quality, they acknowledge that available technology doesn’t allow constant monitoring, making constant measures for the moment impossible.
United Fresh’s Dr. McEntire puts it even more bluntly: “The bottom line is that there is no good testing protocol.”
Drinking Water Is Not Irrigation Water
Dr. LaBorde says a foundational problem with discussions about water safety is that standards applied to drinking water are not applied to irrigation water, with good reason. Initial approaches to E. coli were based on incidences of sudden contamination such as a baby’s diaper in a small lake, which is difficult to apply to irrigation.
“Transferring [an approach grounded in public health epidemiology] over to water that gets on a plant, that’s different to your exposure when you’re swimming and your mouth is open and you’re gulping water,” Dr. LaBorde adds. He notes that research on this subject is ongoing, with researchers trying to improve the means of quickly identifying fecal contamination. “But it’s very frustrating for everybody because the systems just aren’t that good right now, and while there is a lot of research to improve that, it takes time. I don’t think there is a good answer right now. The thinking has been, ‘Well this is good enough for now.’ We’ve got to do something.”
Meanwhile, says Posy, it’s unwise to attempt to apply EPA drinking water standards to irrigation due to volume alone. She stresses that EPA has zero responsibility to oversee agricultural water, since its domain is limited to drinking water.