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Escherichia coli Concentrations in Urban Watersheds Exhibit Diurnal Sag: Implications for Water‐Quality Monitoring and Assessment
Author(s) -
Desai Anuradha M.,
Rifai Hanadi S.
Publication year - 2013
Publication title -
jawra journal of the american water resources association
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.957
H-Index - 105
eISSN - 1752-1688
pISSN - 1093-474X
DOI - 10.1111/jawr.12039
Subject(s) - water quality , environmental science , escherichia coli , watershed , morning , daytime , indicator bacteria , hydrology (agriculture) , sampling (signal processing) , diurnal temperature variation , zoology , fecal coliform , environmental chemistry , chemistry , atmospheric sciences , biology , ecology , botany , physics , biochemistry , geotechnical engineering , optics , machine learning , detector , computer science , gene , engineering
The variability of indicator bacteria over a fine resolution time scale on the order of minutes has yet to be fully understood. In this study, we collected more than 700 Escherichia coli samples at a 10‐ and 30‐min resolution in an urban watershed in Houston. A Bacteria Diurnal Sag ( BDS ) marked with daytime exponential decay followed by an exponential nighttime regeneration was observed. This pattern was observed during all sampled events but varied depending on other variables. The concentrations during a 24‐h period varied 1 to 5 orders of magnitude and the fecal load was at least 10 times lower than what would be obtained using a single morning E. coli measurement, the typical sampling scheme in most monitoring programs. Decay rates, ranging from 3.67 to 24.7/day, decreased E. coli concentrations to below the water‐quality standards from 14:00 to 18:00 h and were strongly influenced by water temperatures and solar radiation intensities. Rapid regeneration occurred on the order of 9.41 to 64.1/day allowing E. coli concentrations to return to their pre‐decay levels. The data indicated that four to six samples taken between 06:00 and 18:00 h may be sufficient to define the BDS depending on stream conditions, and that a threshold concentration of approximately 100 MPN /dl (most probable number in a deciliter) existed for the studied urban watershed. These findings have significant implications for water‐quality monitoring, regulation, and compliance.