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NC State Extension

Water Testing

Documentation determining the quality of irrigation and wash water is required for the GAP certification (USDA GAPs Audit). As described here, most of our farmers use drip and microsprinkler irrigation and their wash water source is well water. As a farmer, you might be wondering, “How do I go about testing my water? What are the steps? Who do I call?” An email from one of our farmers who had contacted a lab for water testing shows there’s more to just water testing than a single sample from a spigot:

Microbiological testing of irrigation and wash water is used to track safety, not for daily monitoring activities. These water testing records become very important in the event of a microbiological outbreak investigation. It is important to document the frequency and results of each water test for comparison purposes. Changes in results may help identify problems such as contamination. For wells and municipal water sources, if generic E. coli are below detection limits for five consecutive samples, the sampling frequency should be annually. Surface water should be tested three times during the season at minimum (beginning of the season, the peak, and during harvest). If funds allow, during the season irrigation water should be tested on a monthly basis.

Before sampling

  1. Contact the selected laboratory (NC Labs for Generic E. coli Testing, NC Labs for E. coli and Coliform Testing with Prices) prior to collecting the sample (sample delivery (accept samples) times, collecting instructions, pricing per sample, testing method, etc)
  2. Collect samples from cold water tap in sterile containers, usually provided by the testing laboratory.
  3. Do not open container until just before taking the water sample. Never set the cap down or leave it off longer than it takes to collect the sample. Hold the cap so that it is facing down to avoid having debris settle in it.
  4. Do not rinse sample bottles prior to taking samples. This may introduce residual chlorine into the bottle and kill bacteria (not for surface water).
  5. Use a new pair of rubber gloves for each sample.
  6. At no time should the sampler’s fingers come in contact with the inside of the sample container.
  7.  Using a marker record the date, sample location, and sample time on an unopened container.
  8. It is undesirable to take samples too near to the bank or too far from the point of drain off, or at depth above or below the point of drain off.
  9. Take a sample at or near the pump connected to the surface water source by inserting a Tee in the line on the outlet side.
  10. If more than one sample is to be tested, all samples should be collected within 18 hours.
  11. The time between collection and start of analysis shall not exceed 24 hours.
  12. Always take extra bottles and sample request forms.
  13. Select your method for testing: the Colilert® method (24 + 2 hrs, Generic E. coli and coliforms) is recommended with quantitative results (not presence/absence).


If funds are low, use a single sample at the point of use (end point) to account for the entire irrigation system. If funds are available or you plan on participating in the cost share program, I recommend a single sample from the water source (wellhead, surface water, etc) and a single sample at the point of use (end point) for irrigation and wash water. Your results will be representative of the water quality throughout your system. This way you will be able to identify if your water is becoming contaminated through your system, either irrigation lines or at the wash station. If you do find some level of contamination, you can isolate it either to the water source (i.e. crack well casing, inflow from above due to faulty well seal, contaminated runoff, wildlife contamination, etc.) or to the above-ground (i.e. irrigation or wash station) system.

For irrigation water samples

  1. Run the irrigation system for the amount of time to flush the ‘hold up’ volume of the system plus an additional 5-10 minutes.
  2. Collect samples from the sprinkler/drip system (not the intake area).

For wash water samples

  1. When collecting samples from the distribution system tap make sure to remove any attachments, such as aerators (Kitchen faucets with swivel arms are not recommended locations for sampling.)
  2. Open the tap fully and allow the system to run for at least 10 minutes (or the time to flush out the ‘hold up’ volume) before the sample is taken.
  3. Slowly fill the container to the line as indicated. (Do not let the container overflow.)
  4. Tightly cap the container.


  1. The sample should be delivered to the laboratory as soon as possible, and no longer than 24 hours after its collection.
  2. Samples should be placed in a cooler with ice or gel packs during transportation.
  3. Check with specific lab for any additional protocols.

Turnaround time and results

Using the Colilert test for Generic E. coli/coliforms results will be available after 24 + 2 hours. Laboratory results can be delivered via fax, email, or mail. All results should be kept in the food safety manual.

Based on risk assessment research, the recommended generic E. coli testing is as follows:
For water not coming in direct contact with the edible portion of a plant:

  • Acceptance Criteria: Less than or equal to 126 MPN/100mL (geometric mean of 5 samples)
  • Acceptance Criteria: Less than or equal to 576 MPN/100mL (for any single sample)

For water coming in direct contact with the edible portion of a plant:

  • Acceptance Criteria: Less than or equal to 126 MPN/100mL (rolling geometric mean n=5)
  • Acceptance Criteria: Less than or equal to 235 MPN/100mL (for any single sample)

These standards were set for by the EPA as the Bacterial Water Quality Standards for Recreational Waters (EPA).

When results indicate high counts. A more aggressive sampling program (i.e., sampling once per week instead of once per month) should be instituted if an explanation for the contamination is not readily apparent. Do not use water from that water system, in a manner that directly contacts edible portions of the crop, until the water can meet the outlined acceptance criteria for this use.

We are entering the stage of the project where farmers are compiling their food safety manual. I have sent out via email several templates that are available on the internet for our farmers to choose from (Plan 1, Plan 2, Plan 3). An important section of the food safety manual that requires testing from a laboratory is water testing.


Dr. Chip Simmons (website)

Diane Ducharme (website)

For questions about water testing or the content of this page, please contact Dr. Chip Simmons at

Article first appeared (with minor changes) as Opening Markets post, July 25, 2011.