Temporal stability of E. coli concentration patterns in two irrigation ponds in Maryland

Authors: Pachepsky, Yakov; KIERKZEWSKY, RACHEL - University Of Maryland; STOCKER, MATTHEW - Orise Fellow; Mulbry, Walter; Millner, Patricia; Shelton, Daniel

Submitted to: European Geophysical Society Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 2/21/2017
Publication Date: 4/23/2017
Citation: Pachepsky, Y.A., Kierkzewsky, R., Stocker, M., Mulbry Iii, W.W., Millner, P.D., Shelton, D.R. 2017. Temporal stability of E. coli concentration patterns in two irrigation ponds in Maryland. European Geophysical Society Meeting. Vienna, Austria on April 23-27, 2017.

Interpretive Summary:

Technical Abstract: There are about nine millions ponds in USA, and many of them serve as an important agricultural surface water source. E. coli concentrations are commonly used as indicator organisms to evaluate microbial water quality for irrigation and recreation. Our hypothesis was that there exists a temporally stable pattern of E. coli concentrations across the pond, i.e. there exist some zones where E. coli concentrations are mostly or always lower than the average concentration across a pond for any sampling event, and other zones where E. coli concentrations are mostly or always higher than the average concentration across a pond for any sampling event. Two irrigation ponds in Maryland were sampled biweekly in summer of 2016. Samples were taken in 22 locations across one pond and at 34 locations across another pond on grids. Environmental covariates – temperature, pH, DO, turbidity, nutrient concentrations, and chlorophyll a concentrations were measured in samples along with generic E. coli concentrations. Methods of temporal stability assessment included estimating Vachaud’s mean relative differences, computing the Spearman correlation coefficient, and deriving empirical orthogonal functions. All three methods indicated the presence of temporally stable patterns in E. coli concentrations and environmental covariates. Pond shape and the distance from the shore seemed to serve as controls. Temporal stability was found for the most of environmental covariates. Smaller mean relative differences of E. coli concentrations corresponded to larger mean relative differences of chlorophyll a. Presence of temporal stability in E. coli concentrations across ponds means that (a) the selection of sampling locations may strongly affect the microbial quality assessment, (b) the microbial quality of water at the pumping intake for irrigation can be affected by the intake location, and (c) one or more locations can be found where E. coli concentrations remain close to the median concentrations across the pond, and can be used to characterize the overall microbial water quality.