Genetic evidence for the offsite transport of E. coli associated with land application of Class B biosolids on agricultural fields Original Research Article

The land-application of Class B biosolids is tightly regulated to allow for natural attenuation of co-applied pathogens. Since many agricultural fields that receive biosolids are artificially drained through subsurface tiles, it is possible that under scenarios of excessive drainage associated with heavy rainfall events, co-applied pathogens might be carried offsite to contaminate nearby surface waters. To address this concern, we used genetic as well as traditional methods to investigate the impact of rainfall on the offsite drainage of Escherichia coli from agricultural fields during biosolids application. Water samples from field drain tiles and a reference field (no biosolids applied) were collected pre-, during and post-biosolids application, while samples of applied biosolids were collected on site during application. The samples were analyzed for E. coli-density and community- and isolate‐fingerprinting to assess the genetic link between E. coli in drainage water and those co-applied with biosolids. In contrast to E. coli densities present in the reference field drainage, our results revealed that post-application drainage water collected from biosolids treated fields contained significantly higher E. coli densities following heavy rainfall events, as compared to light rainfall events. Also, in contrast to the reference field, heavy rainfall correlated significantly with increased similarity of E. coli community fingerprints occurring in biosolids to those draining from treated field. Fingerprinting of individual E. coli revealed a high similarity (> 94%) between some isolates collected from biosolids and post-application drainage water. Using a combination of enumeration and genetic typing methods, we show that heavy rainfall following biosolids application to agricultural fields induced the offsite transport of biosolids-associated E. coli, potentially compromising the quality of water draining through the watershed.