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Are you using surface water to irrigate, or well water to wash veggies? Remember to test all your farm's water

 Annalisa Hultberg, Extension Educator, food safety

If you are using surface water sources like ponds, creeks, rivers or water that has been collected from roofs for irrigation of your produce, remember that these sources of water are open to the environment and can be contaminated with human pathogens like Salmonella and toxigenic E. coli that can sicken people if it is on your produce. Even well water, though much more protected than surface water, can become contaminated if your well is compromised.  Read on for tips to reduce the potential for water to contaminate your fresh produce this season.

What are the risks with using surface water for irrigating fruits and vegetables?

While surface water like water pumped from a pond, stream, creek or river might seem like a cheaper alternative to ground water, this water must be used with caution. Water used for irrigation can be a source of contamination for our fruits and vegetables if the water contains human pathogens, such as Salmonella or E. coli from human or animal feces. 

It should be noted that we are talking about using surface water for irrigation and pre-harvest, not postharvest use like washing vegetables. You cannot use surface water for any postharvest uses like washing vegetable.

What are the risks? Below is a test result from the Zumbro river. The detectable E. coli concentrations were more than the limits for the test, or more than 2,419 units of E. coli in 100 ml of water. That means that this water contained such large amounts of E. coli that it was even more than was detectable by the lab's tests, and if we wanted a true number we would have to gather another sample and they would have to retest using different methods.  Such high numbers means that there is a large amount feces in the water. This water would be very risky to use for irrigation and would likely to cause human illness if consumed.

Where did the E. coli come from?  Think about everything that goes into a river. Runoff from animal grazing fields, leaking septic and sewer systems, waste and garbage from humans in and near the water and waste from marine mammals. While not all surface water sources are the same level of risk, all are more open to contamination than ground water.

As a rule, surface waters are considered the highest risk to produce safety since their quality can be highly variable and they are susceptible to contamination from animals and other sources of fecal contamination. This water is much more likely to contain pathogenic E. coli and Salmonella than well water sources.

Microbiological risks to surface water might include:

  • Livestock 
  • Birds
  • Humans recreating
  • Runoff from nearby fields, livestock and roadways
  • Dead animals
  • Dumping and trash

How to reduce risks when using surface water for irrigation?

Avoid direct contact with produce

Take steps to reduce contact of the water with the edible portions of the crop, such as by using drip tape under plastic. This has the added advantage of using much less water than overhead, and also reduces plant disease and mildew that can come with continually wetting plant foliage. Water applied with overhead irrigation is more likely to touch the edible portion. 

Apply to produce that won't be eaten raw

Apply to produce that are not likely to be eaten raw. For example, potatoes, winter squash and sweet corn. If there is contamination in the water, the cooking will likely serve as a "kill step" to reduce potential contamination.

Wait after application before harvest

Allow a "die off" period between application of the surface water and harvest of the produce. Bacterial die-off levels in water are highly variable, and the number of days required for adequate die-off varies greatly depending on the initial level of contamination of the water, the crops, the UV and wind conditions, and other field conditions. For this reason, relying on die-off alone to reduce bacterial levels in water is not a good strategy. 

The FSMA rule allows 4 days from water application to harvest to use water that does not meet initial standards, meaning it has high contamination levels. This assumes a 0.5 log die-off per day.  


Test the water for bacterial contamination to understand the baseline bacterial load. Know that this level will change rapidly, however, as surface water is open to the environment. It is recommended that surface water be tested 5 times per season. For more info on testing click here.

What to test for?

There are different tests you can ask the laboratory to run, depending on your use and requirements. For most farms, it is best to test for the presence of generic E. coli since this is the best indicator of the presence of fecal contamination in your water.

When you call the lab tell them you have agricultural water, if it is surface or well water, and that you want a quantitative generic E. coli test. 

Where to find a lab near you

Laboratories that test water quality may be private or public, such as county-operated. A complete list of certified labs is available from the Minnesota Department of Health here.  The tests average about $25 - $40 per sample for generic E. coli.

How to collect a sample 

1. Call ahead to the lab you will use for the testing. Tell the lab that you want your water tested for quantified total generic E. coli, and that it is agricultural water that will be used for irrigation or postharvest use with fresh produce.

2. The lab will send sample collection containers in the mail or you pick them up.

3. Carefully read and follow the directions included with the sample containers.

4. Wash your hands before collecting the sample! (This is very important!) You do not want a false positive. 

5. Collect water from as far out into the source that is possible if from surface water. If you are collecting from a well, let the hydrant run for a few minutes before collecting the sample.

6. Carefully open the sample container, ensuring that you do not touch or otherwise contaminate any interior portion of the container.

7. Follow instructions for filling, and do not allow the container to overflow.

8. Keep the sample cool, such as in a cooler on ice, and return it to the lab within the specified time frame; usually 6-30 hours.

Water being collected for testing from a farm pond. The collection bottle was attached with electrical tape to a painters extension pole. 

A sample container like this will be given to you by a testing lab. Make sure to use this container to gather the water, not another bottle that you might have on hand. 

How often should I test? 

Surface water should be tested frequently, at least 3 to 5 times per season. Consider testing at planting, during peak irrigation, and near harvest. 

Municipal water does not need to be tested, but a water bill proving that water comes from a regulated (or managed) municipal source might be needed for GAP audits. 

Ground water like wells should be tested once per year.

Interpreting the results

For most ground water, generic E. coli should test at less than 1, or not detectable in a 100 mL sample.

If your test indicated the presence of generic E. coli, generally this means that the well or distribution system is compromised in some way, and that the water is contaminated.

This water does not meet requirements for potability, and should not be used for drinking, handwashing, washing produce, or other postharvest uses until the problem is addressed and the water is retested.

The FSMA Produce Safety Rule and most GAP audits allow farms to use water with small amounts (e.g., less than 410 CFU/100 mL) of generic E. coli present for production uses such as irrigation, but this should be done with caution since the water has measurable fecal contamination.

Depending on the results, water source, and how the water is used, actions such as well disinfection may be needed. After determining the cause of contamination and correcting it, well disinfection can
be done by hiring a licensed well contractor. You can also do it yourself using chlorine bleach. Be sure to follow guidelines from the Minnesota Department of Health. 
treatment to ensure it has returned to safe levels.

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