Escherichia coli Concentration as a Function of Stream Order and Watershed Size

This research examines the relationship of Escherichia coli concentrations to stream order and watershed size and considers the implications on water quality standards. To assess geospatial effects, E. coli data were obtained from 743 monitoring stations in the Central Great Plains, Cross Timbers, and South Central Plains ecoregions of Texas and Oklahoma. Median and geometric mean E. coli concentrations were analyzed for correlation with stream order and watershed size at each site. Comparison of the three ecoregions revealed E. coli concentrations were highest in the westernmost Central Great Plains and lowest in the easternmost South Central Plains. Similarly, the strength of E. coli's correlation with stream order and watershed area decreased with ecoregion moving west to east. Thus, incorporating ecoregion approaches when defining stream water quality standards is supported. Analysis showed no significant relationship of stream order or watershed size to E. coli concentrations in least-impacted watersheds (i.e., watersheds with minimal wastewater discharge and urbanization). Conversely, analysis of data from all sites showed a weak negative relationship between E. coli concentration and stream order and watershed size, with E. coli concentration generally decreasing with increasing stream order and watershed size. However, variability in smaller watersheds and lower-order streams supports continued use of site-specific studies to determine appropriate standards. Three-parameter exponential models provided an approach to estimate E. coli concentrations using Shreve stream order and watershed area and identify outlier streams potentially affected by anthropogenic activities where further investigation or remediation may be warranted.