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.