Indicator Bacteria Concentrations of Two Northwest Arkansas Streams in Relation to Flow and Season

Concentrations of indicator bacteria such as fecal coliform (FC) and fecal streptococcus (FS) are often usedto assess the suitability of waters for their intended use(s) and to allocate resources for water quality improvementmeasures. There is evidence, however, that concentrations of FC and FS can be influenced by variables such as seasonand flow rate during sampling, which could lead to biased results. The objective of this study was to assess the impacts ofseason and flow rate on concentrations of FC and FS. Fecal indicator bacteria concentrations were measured forapproximately three years at five sites on two Northwest Arkansas streams. Flow data were collected at two of the fivesites. Land use in the basins draining the streams was primarily pasture (57-90%) and forest (6-40%). Significantseasonal influences on FC and FS concentrations were detected for all sampling sites, with the highest concentrationsoccurring in summer. On the two sites with flow data, flow rate generally had a significant effect on FC and FSconcentrations during all seasons, with FC and FS concentrations increasing with flow rate. Ratios of FC and FS, whichhave been used in the past to differentiate between animal and human sources of fecal pollution, did not appear toreliably indicate the major sources of fecal indicator bacteria. The findings of this study suggest a potential for fixedsampling intervals to contribute to biased results. The issue of biased results might be avoided by sampling during timesof year and flow conditions that support the intended use(s) of the waters.