Assessment of composite sampling for determining the levels of fecal indicator and antibiotic resistant bacteria in irrigation water

The microbial quality of irrigation water is a major worldwide health concern. Assessments of irrigation water quality are conducted via the measurement of the fecal indicator bacteria Escherichia coli. More recently, the levels of antibiotic resistant bacteria in irrigation water have been highlighted as an emerging concern as they may be spread from surface waters to crops and soils. Composite sampling has been recommended in cases when larger sample sets cannot be collected across a waterbody. However there have been no reports which have compared the results of composite sampling with the spatial means or medians of fecal and antibiotic resistant bacteria within irrigation waters. The objectives of this work were to evaluate the representativeness of composite samples for estimating the levels of fecal and antibiotic bacteria in several irrigation ponds. In August and September of 2022, water samples were collected from dense sampling grids within five irrigation ponds in Maryland and Georgia, USA. Concentrations of generic, tetracycline, and cefotaxime-resistant E. coli and total coliforms were enumerated in all samples. Three composite samples were created for each pond: a composite of the interior, bank, and full sample sets. In general, we found the distributions of generic and antibiotic resistant bacteria concentrations did not significantly differ between bank and interior samples. Concentrations of antibiotic resistant bacteria ranged substantially across all the waterbodies. On average, the composite samples fell between the 60^th and 70^th percentile of the concentration distributions. In only a 9 and 14.5 % of cases (n = 90) did the composite sample value significantly differ (p < 0.05) from the mean or median, respectively, of the entire sample sets. Results of this work indicate that composite sampling may accurately be used to estimate the spatial mean or median of generic and antibiotic resistant bacteria concentrations in irrigation waters. These results will be used to improve the estimation of fecal contamination of irrigation waters as well as the presence of antibiotic resistant bacteria.