Depth-dependent concentrations of E. coli in agricultural irrigation ponds

Authors: STOCKER, MATTHEW - ORISE FELLOW; SMITH, JACLYN - ORISE FELLOW; Pachepsky, Yakov

Submitted to: Water
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/19/2022
Publication Date: 7/21/2022
Citation: Stocker, M.D., Smith, J.E., Pachepsky, Y.A. 2022. Depth-dependent concentrations of E. coli in agricultural irrigation ponds. Water. 14(14):2276. https://doi.org/10.3390/w14142276.
DOI: https://doi.org/10.3390/w14142276

Interpretive Summary: Current rules and regulations for microbiological testing of irrigation water sources ignore possible changes in E. coli concentrations with depth in irrigation ponds. Failing to account for the vertical variation of E. coli concentrations can lead to underestimating the risk of using a waterbody for irrigation. The objectives of this work were to evaluate variations in E. coli concentrations by sampling irrigation ponds at multiple depths and at different times of the day. While monitoring three ponds over three growing seasons, we found that in two of the three ponds, the E. coli concentrations substantially differed by water depth in one of three ponds. In addition, the time of the day was the significant factor in two of eleven cases when the largest concentrations were found in the mornings. This work will be useful for improving sampling strategies that aim to characterize the fecal contamination of irrigation pond waters effectively.

Technical Abstract: The microbial quality of irrigation waters is assessed by measuring the concentrations of Escherichia coli (E. coli). In most cases, monitoring and assessments have been performed by sampling the water at the surface, thus ignoring potential gradients in E. coli concentrations formed with depth. The objective of this work was to evaluate the dependence of E. coli concentrations on the water sampling depth at different times of the day. Three irrigation ponds were sampled in Maryland eleben times during the growing seasons of 2019, 2020, and 2021. Water samples were collected at the surface (0 cm) and then in 50 cm depth intervals at 9:00, 12:00, and 15:00. The YSI sonde was used to characterize physicochemical water quality variables such as dissolved oxygen, temperature, and pH. Measured E. coli concentrations were generally low (< 50 cells 100 mL-1) and varied by a pond, depth, and time of day. Analysis-of-variance testing revealed that E. coli concentrations did not significantly differ by sampling depth in two of the three ponds (p > 0.05). However, the sampling time of day was significant at 2 of 11 of the observation dates across ponds and in those cases the average concentrations across the pond were in the order of 9:00 > 12:00 > 15:00. This work shows that E. coli concentrations measured in irrigation ponds may be substantially different depending on the sampling depth and time of day and that these factors should be accounted for in the monitoring design.