Effect of the time scale on the uncertainty of geometric mean concentrations of fecal indicators in creek water under baseflow conditions

Authors: JEON, DONGJIN - Gwangju Institute Of Science And Technology; Pachepsky, Yakov; HARRIGER, DANA - Wilson College; PICARD, RACHAEL - Wilson College; Coppock, Cary

Submitted to: Environmental Research Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/11/2019
Publication Date: 11/10/2019
Citation: Jeon, D., Pachepsky, Y.A., Harriger, D., Picard, R., Coppock, C.R. 2019. Effect of the time scale on the uncertainty of geometric mean concentrations of fecal indicators in creek water under baseflow conditions. Environmental Research Letters. 10:1-5. https://doi.org/10.1038/s41598-020-58603-5.
DOI: https://doi.org/10.1038/s41598-020-58603-5

Interpretive Summary: Concentrations of E. coli are used to evaluate the suitability of surface waters for recreation, irrigation, and other health-related purposes. Rather than use a single concentration measurement, several measurements over week, month, or season are combined in a single value of the geometric mean concentration. This value has as uncertainty caused be variation of environmental condition during the observation period. Knowing this uncertainty is needed to assess the risk of making wrong decisions about the microbial water quality. It is not known if the uncertainty of the geometric means changes with the length of the observation period. We researched this question with the 3-year E. coli and enterococci monitoring data from the Conococheague creek in Pennsylvania, and found that the uncertainty in geometric mean concentration statistically significantly increases with the length of the observation period. This finding will be of interest for researches and consultants who evaluate microbial water quality for health-related use.

Technical Abstract: Geometric mean concentrations of fecal indicator bacteria E. coli and enterococci are commonly used to evaluate the microbial quality of irrigation, recreation, and other types of waters, as well in the watershed -scale microbial water quality modeling. It is not known how the uncertainty of those concentrations depends on the on the time over which they are computed. We analyzed data collected during the three-year study in daily and weekly sampling campaigns under baseflow conditions at the Conococheague creek in Pennsylvania. Standard deviations of logarithms of geometric mean concentrations were computed over weeks, months, and seasons. The increase in standard deviations from weekly to seasonal time scale was on average about 0.1 and 0.2 for log (E. coli) and log(enterococci), respectively, and in most cases was statistically significant. This may need to be accounted in evaluating uncertainty of measurements for modeling purposes and in risk assessment of microbial water quality.