Temporal Stability of Microbial Water Quality in Two Irrigation Ponds in Maryland.

Microbial quality of irrigation waters attracts substantial attention and has recently become regulated. The microbial quality of irrigation water depends on the spatial distribution of source water with respect to the intake location and water quality changes during transportation to fields. The objective of this work was to determine whether a temporal stability pattern is present in spatio-temporal distribution of the microbial water quality indicator E. coli in irrigation ponds. The temporal stability means that there are parts of the pond where the concentration of E. coli is mostly lower than the average in the pond and parts where this concentration is mostly higher. The field research was done on two irrigation ponds in Maryland. Grids of 19 and 34 sampling locations were designed for the first and the second pond, respectively. The grab sampling was done from the bank and from the interior of the pond from kayak biweekly. Mean relative differences were computed for each location. Overall, concentrations of E. coli were significantly higher near banks as compared with pond interior water. The temporal stability of E. coli concentrations was detected at both ponds for pond interiors. The high observed variability of E. coli concentrations results in the need of using multiple samples. Knowledge of the presence of temporal stability in E. coli concentrations can help to decrease the number of samples by removing the spatial trend manifested by temporal stability.