FAQ: Irrigation Water Quality

Question: My well water test came back high for coliform bacteria but negative for E.coli, is the water okay to use for irrigation?

Answer: It depends on the bacteria level. Look at your test results for the concentration number.

Your irrigation water should be tested specifically for fecal coliforms and generic E. coli, and the test used should not be a simple positive/negative but should determine the number of E. coli present. Typically Iowa farmers will use the University of Iowa Hygienic lab for water testing.

While the presence of generic E. coli does not correlate directly with the likelihood of pathogens being present, it does suggest that the water has been exposed to fecal matter that may contain pathogens. We test for indicators as it is a more cost effective activity than testing for all possible pathogens. Thresholds exist for water quality for different uses: zero E. coli are allowed for wash water and drinking; higher levels are permissible for irrigation water as that water is often impacted by UV rays and drying.

For irrigation water coming in direct contact with the edible portion of a plant, if the average is below 126 MPN/100 mL and highest single sample is below 235 MPN/100 mL then your water is acceptable for agricultural use (EPA, 1986). For water not coming in direct contact with the edible portion of a plant, if the average is below 126 MPN/100 mL and highest single sample is below 576 MPN/100 mL then your water is acceptable for agricultural use. If either number exceeds those tolerances, then you need to take remedial action.

NOTE: Most testing labs will return results as MPN/100 mL but some will report in CFU/100 mL. These measures are equivalent, so regardless of the measurement units, you are looking for the same thresholds (126/235 or 126/576).

Don’t forget to keep your results in your food safety manual.

Remember it is important to choose the highest quality source possible for agricultural irrigation. Water can be contaminated by sediment, agricultural runoff, chemicals, or any of the major microbial contaminants, such as bacteria, viruses, or parasitic organisms. The water tests mentioned above only test for fecal bacteria. Irrigation methods that reduce water contact with produce such as drip are recommended over overhead irrigation.

Source: GAPs for Small Diversified Farms, North Carolina State University and Carolina Farm Stewardship Association.

Am I Certified?

Question: If I attend an Iowa Level 1 or Level 2 GAP class, is my farm GAP certified?

No. While attending a class is a great way to learn about food safety, it does not make your farm certified. Three things need to happen in order for a farm to become “GAP-certified.”

  1. The farms need to implement an on-farm food safety program with a manual containing policies, procedures (SOPs) and logs.
  2. The farmer needs to contact a certifying auditing agency like USDA AMS, Primus, NSF or other certifying body to come to the farm and audit the farm’s food safety program. Because Iowa does not have a state GAP inspection program, auditors usually come from out-of-state and audits will cost $600-$2000 depending on the audit used.
  3. They need to pass the audit (for example, a score 80% or higher on a USDA GAP audit).

While it is highly recommended that farmers attend a Level 1 and Level 2 GAP class to learn about food safety principles and to receive assistance crafting a farm safety manual, attending class is not a requirement to become GAP certified. In fact, there are several templates available that can guide farmers through the process.

ISU Extension specialists are also available to assist with questions and to perform mock audits.

See also:

Will I be GAP certified if I come to your class?

Food Safety Decision Tools

The National Good Agricultural Practices (GAPs) website has added farm food safety “decision trees” to help farmers identify risks and implement food-safe practices. Nine decision trees are available, including worker hygiene, soil amendments, wildlife management, and postharvest handling. Each decision tree includes samples of recordkeeping logs and standard operating procedures, as well as template farm food safety plans.

The purpose of the Decision Trees is to:
1.    Help you identify risks and practices that reduce risks;
2.    Prioritize the implementation of practices to use limited resources wisely;
3.    Familiarize you with the terms and methods necessary to understand and follow requirements and expectations for food safety from buyers, farm markets, schools, and federal regulations.

Begin with “How to Use the Decision Trees” and reviewing the Checklist to identify which Decision Tree you should complete first.
When you are ready to begin writing your farm food safety plan be sure to check out the the How to Write an SOP document and Recordkeeping Sheets. There is a Glossary in case any terms are unfamiliar.

Managing GAP and Organic Certifications

Are you an organic grower managing NOP certification as well as GAPs audits? The team at New Mexico State University has developed an informative website to aid in the dynamic management of organic system plans and Good Agricultural Practices. No need to duplicate the paperwork effort – there are lots of helpful hints to share!


For additional information, contact Nancy Flores (naflores@nmsu.edu).

Will I be GAP certified if I come to your class?

This is a common question.

No.  You will not be GAP certified if you attend a workshop offered by ISU Extension and Outreach.  ISU Extension cannot GAP certify farms.

The GAP certification process is similar to the organic certification process.  You need to have a farm safety plan and documentation in place before a GAP auditor can visit your farm.  GAP auditors may be from the USDA or another third-party company; you decide which certifying agency depending on your buyers’ requests.  The auditor will visit your farm, review your farm safety plan and check your documentation to make sure you are doing what you said you were going to do in your plan.  GAP audits range in cost from $500-$1200.

While you can certify your farm without attending a class, the information provided by ISU Extension will make it easier for you to identify food safety risks on your farm.  Level 1 GAP discusses basic food microbiology.  Level 2 GAP gets you started on your food safety plan.  Level 3 GAP gets you out on a farm to look for farm safety risks in person.

One more item to remember.  One of the Good Agricultural Practices is regular training for yourself and your employees.  An attendance certificate is an important piece of documentation to include in your food safety binder.  Be sure to ask for an attendance certificate for any trainings you attend!


Aquaponics and Food Safety

I’ve received a few questions about aquaponics and food safety.  Specifically can I get GAP certification while growing my produce with aquaponics?

Aquaponics is a food production system that uses nutrient-rich water from fish culture to irrigate and fertilize plants. After the plants have absorbed the nutrients, the water is recirculated to the fish rearing tanks. This combination of aquaculture and hydroponics recycles both water and nutrients, resulting in an efficient use of resources. However, when food plants are grown in the presence of fish culture effluent, food safety considerations become very important.

Here are some documents to help with developing food safety plans from the University of Hawaii.

On Farm Food Safety: Aquaponics

Microbial Water Quality Related to Food Safety with Aquaponics


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