Water Testing Basics
Many food safety programs recommend testing the water used for irrigation and agricultural chemical or commercial fertilizer application. If pathogens are in the water, the water may contaminate the produce.
How often to test your water will depend on a number of factors. If you grow a crop that can be consumed raw (e.g. lettuce) and the water you use to irrigate varies over time (e.g. river or pond fed by runoff), test your water at least a couple of times a year so that you have a better understanding of your water risk. Seasonal environmental changes or the presence of effluents or run-off can greatly affect the quality of a water source. In a study posted in Water Research (2009), Dr. Graham Wilkes and researchers noticed seasonal patterns where pathogens and parasites were most frequently detected during the fall months in Eastern Ontario's South Nation River basin. Cryptosporidium was detected in over 70% of the samples and the bacteria Salmonella and Campylobacter were detected in 15% and 34% of samples respectively.
Rainfall and run-off can do more than just add pathogens to a water source. These events can disturb the natural "settling" process of pathogens that may already exist in the water. The Wilkes' study saw an increased detection of pathogens days after high accumulated rainfall and spring/fall discharge. The researchers believed that the disrupted sediment and vegetative matter in the water courses caused the spike in pathogen detection. So be aware that bacterial levels can increase significantly in water sources even a few days after heavy rainfall - especially if you need to overhead irrigate a crop that can be consumed raw with this water.
While testing for a specific pathogen is possible, it is less expensive to test for an organism that can indicate the potential for pathogens in the water. Currently, generic E. coli may be the most appropriate indicator for predictive efforts. The higher the numbers of E. coli present in the water, the greater the likelihood that Dr. Wilkes also found pathogens present.
The current recommendation is that E.coli should be at or below 100 bacteria per 100 ml water. This value is similar to Ontario's Recreational Water Standards. If the water is considered "safe" to swim in, it is also a low risk for contamination of food crops.
When testing your irrigation water, collect the water sample at the point where it contacts produce, such as at emitters or when it leaves the guns or risers, or when it "rains" down on the crop. Refrigerate the water sample immediately and have it transported under refrigerated conditions to a water testing laboratory within 24 hours. For more information, see the OMAFRA factsheet Improving On-Farm Food Safety through Good Irrigation Practices, Order No. 10-037.
Wilkes, G.A et al., (2009). "Seasonal Relationships among Indicator
Bacteria, Pathogenic Bacteria, Cryptosporidium Oocysts, Giardia Cysts,
and Hydrological Indices for Surface Waters within an Agricultural Landscape."
Water Research, 43(8), pp. 2209-2223.
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