Larissa Winder

Cocooned between rolling peaks on either side, the city of Gonzales sits like an emerald jewel. The produce grown here outnumbers the people 1,000 to 1 and you are more likely to catch sight of an idling tractor far off in the fields than gridlock traffic. It is green as far as the eye can see and the aroma of sun-warmed tomatoes and fresh-cut broccoli fills the air. 

Our technicians walk up and down these fields, their feet clad in hard-rubber boots. Their expert gazes run over the sprawling greenery, taking in every detail. Usually, this particular stretch of land is serviced by one or two people, but today they are working in a much larger team because they have an important job to do. Today is 21 days before these growers plan to harvest. 

Testing and Metrics

In California, the FDA does not require that growers test their water. Growers are, instead, governed by individual entities, such as the Leafy Green Association (LGMA). The LGMA specifically was formed in 2007 in response to a spinach E. coli outbreak in 2006 wherein 200 people became ill. An LGMA certification is extremely important to growers who have or wish to place their produce on the shelves of large grocery chains. They use that leverage to set standards, or metrics, to ensure safe, healthy produce makes it to consumers. (Ward,2020) 

These metrics are in a constant state of change as technology and the science around water-borne pathogens evolve. Of the 430 checkpoints of an LGMA audit, 100 pertain to irrigation. A quarter of those checkpoints specifically pertain to treated water. Your water treatment provider often offer guidance to ensure that each of these metrics are met. (Ward, 2020)

Many of these checkpoints are audited on this day when growers are tasked with treating all water being sourced from open-air bodies, such as reservoirs. The purpose of this treatment is to ensure that the water being delivered to the soon-to-be-harvested produce via overhead sprinkler systems does not exceed standard LGMA water requirement. The LGMA requires that a minimum of 3, 100 ml samples taken 2 of 3 must be non-detect for generic E.coli and 1 must not indicate a microbial load of greater than 10 MPN generic E.coli. All 3 must be 99 MPN or less for Total Coliforms or an adequate log reduction of Total Coliform as compared to an untreated source water sample. (Ward, 2020)

The Treatment

The treatment can vary between growers and crops. For this particular grower, we are treating tomato plants being irrigated by overhead sprinklers, fed by a reservoir. As such, they have opted to utilize calcium hypochlorite, an oxidizer. Other options include sodium hypochlorite, potassium hypochlorite, and PAA. All of these chemicals work to oxidize any and every cause of turbidity, including non-invasive matter. The treatment is pushed through the system within minutes of reaching the irrigation sprinklers. It is now time to test the water that is finally reaching the field. 

Our resident food safety expert, Garrett Dana, has already inspected the system and has noticed some modifications that the grower has made. In the back of his head, he files the observation away in case he needs it. Now, he crouches low to the ground and watches the irrigation sprinklers rise in the last block of fields on the docket today. With a 10 mL glass vial in his hand, he approaches carefully until he is within arm’s reach. He places the vial in the pressurized stream, which sends a spray of water towards him. When the vial is properly filled, he moves away with ease.

Garrett’s colleague is on hand with a plain white packet containing a powdered chemical, dihydropyrimidine dehydrogenase (DPD). They join each other at the tailgate of the truck looking out at the produce grown in Salians Valley, Garrett empties the packet into the vial. Then, he shakes it. As the water agitates, it begins to change color. Slowly but surely, the once totally clear liquid is a rosy pink. 

“That’s not what we’re looking for,” Garrett says, sliding the vial into a pocket-sized device he had produced from the truck’s cab. 

It is a colorimeter. With it, they should be able to analyze the exact shade the DBP has turned the water. This shade corresponds with a range of residual chemicals that can be detected in the already-treated water. 

Garrett is right; the water should be a much darker pink. The low residuals indicate that the mix of oxidizers being run through the system is not sufficient. The entire block of crops is about to fail. That is unacceptable. 


The team immediately transitions into a new mindset: remediation. Garrett hops on the phone and lets his contact at the main pump know that the readings came back at a much lower number than expected. If they cannot find the cause of the discrepancy and remedy it within 24 hours, this grower’s LGMA certification may be on the line. One by one, he runs through the list of common reasons for such a low residual return. 

First, he instructs his colleague at the water source to visually assess it. If there are obvious signs of debris or an algae bloom, they may not have pushed enough chemistry to properly treat the water. This would mean that the oxidizers are not sufficient enough to kill bacteria and still may be present in the final sample. 

Second, Garrett scans the wide-open fields for the telltale halo of water on the horizon. This could indicate that there has been a breach in one of the underground pipes. If the system is losing water, it is also losing chemistry. Again, a cause for low residuals. 

Finally, he sends a technician to investigate a hunch. Within minutes, he gets the call. The technician explains that he agrees with Garrett’s theory that the growers had modified their system to accommodate a spacing issue. In doing so, they had placed the flow meter, the device that analyzes distribution uniformity (DU), too close to the paddle mechanism that allows for accurate measurement. The three-foot clearance that is standard has been reduced by over a foot. Due to this modification, the water that had previously flowed without hindrance was back flowing into the paddle after hitting the container’s siding. The meter was reading a much lower flow rate than it was delivering in reality. Therefore, we had pushed a lower rate of treatment than needed. 

Garrett instructs the technician to recalibrate. He gets the notification that the chemistry has been readjusted and we wait. Finally, he approaches the sprinkler once again, this time with a new vial and an unopened packet of DPD. He is already giving the water a vigorous shake by the time he makes it back to the truck. All eyes are on the tiny container as it began to change. 

“Now, that’s it,” he says holding up the container of dark pink liquid. 

The colorimeter confirms his evaluation. The numbers are sitting comfortably within an acceptable range. He nods at his colleague who takes one more sample to confirm. These tomatoes will be sitting on grocery shelves very soon. From the moment of the initial test to the moment of success, only ten minutes have passed. 

Communication is Key

Garrett’s last step is to give the field manager of the produce grown in Salians Valley a call. They go through each block, one by one, and discuss the preliminary test results. Garrett makes him aware of the remediation and explains the need to take the system modification into account the next time treatment is administered. They agree on a plan and he documents it carefully. With him, communication is the foundation for a better experience for both the growers and his technicians.

The team packs up their samples and heads to an off-site lab where they will be further analyzed. Today, one more step has been completed in this grower’s season. With an LGMA certification, they are confident that their produce will nourish the end-consumer, not make them sick. At the end of the day, that shared goal is what brings us to your fields and keeps us there through many seasons more. 

Sections of this article have been revised for accuracy on (1/6/21). For up-to-date information on LGMA metrics and more, visit[LW1]



A. Ward, personal communication, December 21, 2020

Horsfall, Scott. LGMA Audit Data Released on New Water Requirements. California LGMA, 30 September 2020, Accessed 1 October 2020. 

California LGMA. (2020) Commodity Specific Food Safety Guidelines: For the Production and Harvest of Lettuce and Leafy Greens.

Don't forget to share this post!