Intra-daily variation of E. coli concentrations in agricultural irrigation ponds

Authors: STOCKER, MATTHEW - Orise Fellow; SMITH, JACLYN - Orise Fellow; HILL, ROBERT - University Of Maryland; Pachepsky, Yakov

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/26/2022
Publication Date: 4/13/2022
Citation: Stocker, M., Smith, J., Hill, R., Pachepsky, Y.A. 2022. Intra-daily variation of E. coli concentrations in agricultural irrigation ponds. Journal of Environmental Quality. https://doi.org/10.1002/jeq2.20352.
DOI: https://doi.org/10.1002/jeq2.20352

Interpretive Summary: When contaminated waters are used for irrigation, there is a potential to transfer dangerous microbes to crops and soils. Consumption of contaminated produce has resulted in numerous multi-state foodborne outbreaks. The microbial quality of irrigation water is assessed by measuring E. coli concentrations and comparing them to regulatory standards. Very little work has been done to evaluate the effect of the time of the day on E. coli concentrations in irrigation water sources. The objective of this work was to determine if the time of day is a significant factor for microbial water quality determinations. Water in three Mayland irrigation ponds was monitored for coli concentration in 2019, 2020, and 2021. Samples were taken across the ponds at 9:00 am, 12:00 pm, and 3:00 pm. The sampling time of day the significant factor in two of the three ponds. E. coli die-off rates were several times different between ponds. Results of this work will be of use for irrigation water managers and users in that they show the need to sample irrigation water sources during morning hours to obtain conservative estimates of the microbial water quality.

Technical Abstract: The microbial water quality is determined by comparing observed E. coli concentrations to the regulatory thresholds. Measured concentrations can be expected to change throughout the course of a day in response to diurnal variation in environmental conditions such as solar radiation and temperature. Therefore, the time of day at which samples are taken is an important factor within microbial water quality measurements. Little, however, is known about the diurnal variations of E. coli concentrations in surface sources of irrigation water. The objectives of this work were to evaluate the diurnal dynamics of E. coli in three irrigation ponds in Maryland over several years and to determine the water quality parameters to which E. coli populations are most sensitive. Water sampling was conducted across the ponds at 9:00, 12:00, and 15:00 on a total of 17 dates in the summers of 2019 to 2021. A One-way analysis of variance revealed significant diurnal variability in E. coli concentrations in Ponds 1 and 2 whereas no significant effects were observed in Pond 3. E. coli die-off rates (k) calculated between sampling time points in the same day were significantly higher in Pond 2 than both Pond 1 and Pond 3 and these rates ranged from 0.005 to 0.799 hr-1 across ponds. Concentrations of dissolved oxygen, pH, and turbidity exerted the most control over E. coli populations. Results of this work demonstrate that sampling in the early morning hours provides the most conservative assessment of the microbial quality of irrigation waters.