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NC State Extension

Quality of Irrigation Water

Sitting in a shady area next to the barn, a farmer asked me ‘how can my pond water become contaminated?’ Irrigation water can be a source of contamination depending on how clean it is when it’s delivered to the crop. Surface water may contain pathogens and human parasites and there are several routes these pathogens can get into the water such as wild animals and runoff. Just a few of the microorganisms transmitted through water in the U.S. are E. coli, Salmonella spp., Shigella spp., Cryptosporidium, Giardia, Toxoplasma, norovirus, and Hepatitis A virus (check out this CDC page on water contamination). Well water is less likely to be contaminated (although it can be). It is recommended that uncovered wells and surface waters be tested monthly during production and at the beginning of production for closed systems such as wells. Irrigation methods also contribute to contamination if the water comes in direct contact with the edible portion of the plant such as overhead (spray) in contrast to drip irrigation.

Testing of all agricultural water for the presence of generic E. coli is recommended. This is a special group of coliform bacteria that reside in the intestinal tract of warm-blooded animals. The notorious pathogen E. coli O157:H7 is a member of this group. High counts of fecal coliform indicate the water MAY have been contaminated with E. coli O157:H7 and/or other human pathogens. The presence of high levels of generic E. coli in the water indicates that it may be unsafe to use to irrigate crops or wash fresh produce. It is an indication that the source of contamination needs to be identified, or that an alternative source of irrigation water should be used. Testing prices for generic E. coli in NC can range from $15-80 (NC Labs for Generic E. coli Testing, NC Labs for E. coli and Coliform Testing with Prices).

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). They are referenced in the Commodity Specific Food Safety Guidelines for the Production And Harvest of Lettuce
and Leafy Greens, 2010
), developed by the California Leafy Green Products Handler Marketing Agreement (LGMA). The LGMA was formed by California farmers to protect public health by reducing potential sources of contamination in California-grown leafy greens.

Over the past three weeks, the farms I visited use a variety of sources to irrigate their crops during production. Rain: Springhouse Farm, Vilas (5 acres) natural and collected in rain barrels and well water (with water can when necessary) Creek: Zydeco Moon Farm, Grassy Creek (5 acres) Pond: Vollmer Farms, Bunn (25 acres), Combination of pond and well water: Black River Organic Farm, Ivanhoe (16 acres), Harland’s Creek Farm, Pittsboro (5 acres) and Maple Springs Gardens, Cedar Grove (5 acres) Well water only: Perrywinkle farm ,Chapel Hill (5 acres), Rocking S Farm, Piney Creek (8 acres) Berry Patch Farm, West Jefferson (2 acres), Contrarian Farms, Westfield (3.5 acres)

How does water factor into the GAP certification process? Under the Water Quality Risks section, 1-3 A water quality assessment has been performed to determine the quality of water used for irrigation purposes on the crop(s) being applied. 1-4 A water quality assessment has been performed to determine the quality of water used for chemical application or fertigation method. Total of 15 points and a document is required to show conformance to the question. Under the Field Harvesting and Transportation section, 2-15 Water applied to harvested product is microbially safe. Total of 15 points and record is required to be kept showing an action was taken.

It is important to reduce the risks associated with water that comes in contact with fruits and vegetables and the first step is determining the quality of water. Water quality testing will determine if the water is safe to apply to crops in the form of irrigation and application of pesticides and fertilizers.

Article first appeared (with minor changes) in Opening Markets blog, June 9, 2011.