Purpose: The study evaluated fecal coliform and generic E. coli levels in open surface waters used for irrigation and overhead cooling of apples in Washington.
Methods: Sampling locations (24) in three regions of central Washington were identified and selected sampling sites examined variation in water quality from the water source, holding and final water delivery method. Multiple samples were collected from sampling sites to evaluate consistency between bacterial levels at multiple points within a site. Water samples were collected monthly for at least three months (June – August) with 98 total sampling sites. Water samples were tested for fecal coliforms and generic E. coli using the Most Probable Number method and IDEXX Colilert®-18, Quanti-Tray®/2000 system.
Results: For 293 samples evaluated using the MPN method, approximately 15 samples (5.1%) exceeded the currently proposed FDA standard for any single sample (235 MPN/100ml generic E. coli). The southern region had the highest number of samples exceeding the proposed single sample standard (10) followed by central (4) and northern (1) regions. Sites exceeding the single sample standard were evenly distributed over time (5 each month), and were collected from canals/laterals (7), ponds (6), river (1) and a water box (1). Only one of 98 sampling sites (1%) exceeded the proposed rolling geometric mean standard (126 MPN/100ml). Results using IDEXX Colilert®-18 system did not always align with MPN values; however, biological variation likely contributed to this observation.
Significance: Understanding trends in generic E. coli levels associated with individual, open surface water bodies and water used for direct produce contact can assist growers, scientists and regulators in determining optimal risk management strategies.