Elucidating spatial patterns of E. coli in two irrigation ponds with empirical orthogonal functions

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

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/26/2022
Publication Date: 3/29/2022
Citation: Stocker, M., Pachepsky, Y.A., Hill, R.L., Kim, M.S. 2022. Elucidating spatial patterns of E. coli in two irrigation ponds with empirical orthogonal functions. Journal of Hydrology. 609:127770. https://doi.org/10.1016/j.jhydrol.2022.127770.
DOI: https://doi.org/10.1016/j.jhydrol.2022.127770

Interpretive Summary: The microbial quality of water used for irrigation is assessed by measuring the levels of E. coli which is a fecal indicator organism. Current regulations of E. coli monitoring in irrigation ponds and reservoirs do not consider the spatial variability of E. coli in such water bodies used for irrigation, such as ponds located on farms. Failing to account for areas that tend to have consistently higher or lower E. coli may result in under-or overestimation of the microbial water quality conditions and associated risks. This work's objective was to find patterns of the spatial variability of E. coli concentrations in two working farm ponds in Maryland using the pattern recognition method called the Empirical Orthogonal Function (EOF) analysis. The EOF analysis showed clear spatial patterns of E. coli levels within both ponds, which corresponded to areas with consistently higher and consistently lower concentrations. These patterns reflected human activities in the ponds and in surrounding lands. We expect this work to be helpful for a wide range of water quality professionals involved in developing monitoring designs and sampling strategies to assess the microbial quality of water used for irrigation.

Technical Abstract: The microbial quality of irrigation water is assessed through the measurement of E. coli concentrations. E. coli concentrations have been shown to vary highly both in time and in space. Water sampling for E. coli design could benefit from discovering spatial patterns in E. coli concentrations that are stable in time. This work's objective was to research temporal variability of E. coli concentrations in two working irrigation ponds using an Empirical Orthogonal Function (EOF) analysis. E. coli, pH, specific conductance, dissolved oxygen, and temperature were measured biweekly at 23 and 34 locations at ponds P1 and P2, respectively, from 2016 to 2018. The first three E. coli EOFs accounted for > 80 % of the total variance each year at both ponds. In a given year, EOF1 explained from 34 to 54% and from 45% to 60 % of the total variance in ponds P1 and P2, respectively. Examination of EOF1 revealed distinct zones of relatively high and low E. coli concentrations related to known microorganism sources and transport conditions. The spatial patterns of E. showed a significant negative correlation with spatial patterns of temperature, pH, and DO coli in both ponds. Overall, the EOF analysis provided ample information about the temporally stable spatial patterns of E. coli in the ponds by examining the deviations from the spatial averages across the ponds. The EOF analysis holds the promise to be a valuable tool for effective microbial water quality monitoring design.