Depth-Dependent Concentrations of E. coli in Agricultural Irrigation Ponds

Microbial water quality surveys of irrigation sources are conducted by measuring the concentrations of generic E. coli. The objective of this study 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 eleven times during the growing seasons of 2019–2021. Water was collected in replicates at the surface (0 cm) and then in 50 cm depth intervals at 9:00, 12:00, and 15:00. Ponds 1 and 2 were sampled to 150 cm, whereas Pond 3 was only sampled to the 50 cm depth due to it having a shallower average depth. An analysis of variance test revealed that E. coli concentrations significantly differed by depth in only one pond (p > 0.05) but on multiple dates. Additionally, the sampling time of day was significant at only two of eleven of the observation dates across ponds; in those cases, the average concentrations across the pond increased in the order of 9:00 > 12:00 > 15:00. This study shows that E. coli concentrations measured in irrigation ponds may substantially differ depending on the sampling depth and time of day, and that these factors should be accounted for in the monitoring design.