A simple method to reducing biological contamination in irrigation water
There are some surprisingly simple mitigation strategies that can make contaminated surface water use still very economical in your fruit and vegetable acreage.
High generic E. coli counts can happen in surface water bodies all over the state. Even a simple rain can cause significant spikes that would put surface water into the danger zone. Sometimes a grower may have to irrigate a produce crop just after sending in a sample of irrigation water and not yet have the results. If that sample came back with excessive generic E. coli counts, what is a grower to do?
Wait. Yes, wait. Research suggests that for every day following an irrigation event, the generic E. coli levels drop by at least 0.5 log. Put another way, if you wait two days after irrigating the crop, whatever the E. coli level was at the time of application drops by nearly a decimal place. For example, if I irrigate with surface water that has 10,000 cfu/100 ml of E. coli and wait one day, I will have dropped my contamination load to just over 3,000 cfu/100 ml. The U.S. Food and Drug Administration (FDA) is proposing that if growers factor in the daily loss of generic E. coli to the irrigation water quality, they can effectively be in compliance of the law, if that adjusted number falls within the threshold.
When using this as a mitigation technique, it’s important to keep track of the irrigation schedule in addition to your harvest dates. In the event of an audit, both will be needed to verify compliance.
It is important to remember that the Food Safety Modernization Act (FSMA) rule requirements may be markedly different than third party audit requirements. If you are required to have a third party audit in addition to meeting FSMA, check with your auditing body to see if having a pre-harvest interval to mitigate a high bacterial count in irrigation water meets their standard.