Purpose: The study evaluated generic E. coli in open surface waters used for irrigation and overhead cooling of apples in Washington.
Methods: Variation in water quality was examined based on region, source, holding and delivery of water. Multiple sites from each location (ex. 2 canal, 3 pond) were evaluated for consistency. Samples were tested for E. coli using the FDA-BAM Most Probable Number method (2013, 2014) and IDEXX Colilert®-18, Quanti-Tray®/2000 system (2012); and for presence of Salmonella and E. coli O157:H7 (2013).
Results: For 319 samples evaluated in 2013 (24 locations) using the MPN method, 16 samples (5%) exceeded the originally proposed FDA standard for any single sample (235 MPN/100 ml generic E. coli). One site of 82 (1%) exceeded the originally proposed rolling geometric mean standard (126 MPN/100ml). IDEXX Colilert®-18 system results did not always align with FDA-BAM MPN values; biological variation likely contributed to this observation. For 523 samples evaluated using 2013 and 2014 MPN data (14 locations), 1 location (7%) exceeded the currently proposed FDA standard for statistical threshold value (410 MPN/100 ml generic E. coli). None of the locations exceeded the proposed geometric mean standard (126 MPN/100 ml). Fewer sources of water exceeded the supplemental proposed standards (1) compared to the originally proposed standards (16). E. coli O157:H7 and Salmonella were isolated from sites associated with a river (1), canal (1), waterbox (1) and ponds (3); however, these samples were not associated with locations that exceeded the currently proposed regulatory standards.
Significance: Evaluation of generic E. coli at multiple points within an open surface water delivery system can assist growers in evaluating risk associated with fecal contamination but may not align with risk of pathogen contamination.