Temporal stability of E. coli concentrations in waters of two irrigation ponds in Maryland

Authors: Pachepsky, Yakov; Kierzewski, Rachel; STOCKER, MATTHEW - Orise Fellow; MULBRY, WALTER - Hood College; SELLNER, KEVIN - Hood College; LEE, HOONSOO - US Department Of Agriculture (USDA); Kim, Moon

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/7/2017
Publication Date: 11/17/2017
Citation: Pachepsky, Y.A., Kierzewski, R.A., Stocker, M., Mulbry, W., Sellner, K., Lee, H., Kim, M.S. 2017. Temporal stability of E. coli concentrations in waters of two irrigation ponds in Maryland. Applied and Environmental Microbiology. 84:e01876-17. https://doi.org/10.1128/AEM.01876-17.
DOI: https://doi.org/10.1128/AEM.01876-17

Interpretive Summary: Microbial quality of irrigation water has become an essential food safety issue. Design of microbial water quality monitoring is complicated by the substantial spatial and temporal variability of the Escherichia coli concentration in freshwater sources. We hypothesized that there is a spatial pattern in E. coli concentrations in surface water sources such that in irrigation ponds there are areas where the concentrations are persistently mostly larger than average concentration across the pond, and other areas where the concentrations are mostly persistently smaller than the average concentration across the pond. We tested this hypothesis by doing multiple surveys in the same locations at two irrigation ponds in Maryland. It appeared that persistent in time areas with relatively high and relatively low concentrations indeed exist. The highest concentrations were found at inlets and outlets of ponds. Results of this work will help in developing guidance on monitoring of microbial water quality of irrigation water sources.

Technical Abstract: Fecal contamination of water sources is an important water quality issue for agricultural irrigation ponds. Escherichia coli is a common microbial indicator used to evaluate recreational and irrigation water quality. We hypothesized that there may exist temporally stable spatial patterns of E. coli concentrations across ponds meaning that there are areas that always have higher than average concentrations of E.coli and areas that always contain lower than average E.coli concentrations. To test this hypothesis, we sampled two irrigation ponds in Maryland biweekly during the summer of 2016. Sampling location grids were established with 34 locations across one pond and 22 locations across another pond. Environmental covariates- temperature, turbidity, conductivity, pH, DO, chlorophyll-a and nutrient concentrations - were measured in conjunction with E. coli concentrations. The temporal stability was assessed for each location using mean relative differences between measurements in a location and averaged measurements across ponds. Spearman correlation coefficients were computed to study the correlation between location ranks for each type of measurements. Temporally stable spatial patterns in E.coli concentrations were well expressed for both ponds. Temporal stability was found for most of the environmental covariates. In the pond interior, larger relative mean differences in algae corresponded to smaller mean relative differences in E.coli concentrations with a rank correlation coefficient of '= 0.819. Turbidity and ammonia concentrations were two other environmental covariates with the largest positive correlations between their location ranking and E. coli location ranking. Ten-fold differences were found between geometric mean E. coli concentrations in locations that were consistently high or consistently low. The existence of temporally stable patterns in E. coli concentrations can affect results of microbial water quality evaluation in ponds and should be accounted for in microbial water quality monitoring design.