Purpose: The purpose of this study is to determine the relationship between fecal indicator bacteria and pathogens in irrigation water supplies and whether large volume sampling for pathogens is more informative for microbial water quality.
Methods: Seven sites were sampled monthly at overpasses along canal networks in the Southwest US. Using ultrafiltration (20 l) samples were analyzed for indicator E. coli and fecal coliforms, Salmonella and E. coli O157:H7. Environmental and water quality parameters were collected at each site. Sampling for 2013 was conducted from June to Novemberl; sampling for 2014 will be performed from January to April.
Results: A total of 76 water samples were collected over 6 sampling events. To date, Salmonella was found in 29 screened samples while no samples tested positive for E. coli O157:H7. The presence of Salmonella was not associated statistically with an increase in indicator bacteria (E. coli and fecal coliforms). Average indicator E. coli counts (CFU/100 ml) were well below the standard of 235 CFU/100 ml (μ = 46.1, σ = 82.4) and did not vary significantly (P > 0.05) across the sampling period. Fecal coliforms varied across time and space (event and site).
Significance: The water quality standard adopted by the produce industry and slated for inclusion in the Food Safety Modernization Act for the safe application of irrigation water is 235 CFU/100 ml for E. coli. Our data indicate that the presence of pathogens in irrigation water supplies is not predicted by an excess in the current water quality standard in a major southwestern desert produce production region.