Dynamics of indicator bacteria populations in sediment and river water near a combined sewer outfall

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Abstract
Sediments collected throughout the summer of 1991 from a river bed around a combined sewer outfall were found to have geometric mean faecal coliform and faecal streptococci densities ranging between 102‐104 g‐1 and 101‐102 g‐1 respectively. During the study period, faecal coliform densities in water samples from the same river reach were several logs lower than in the sediment, but still exceeded state guidelines for primary contact in six of nine sample weeks. Bacteria densities in sediment were weakly correlated with particle size and organic content (as represented by loss‐on‐ignition) during steady‐state baseline flows. Correlations were stronger at the outfall site in the two weeks following an overflow event. Multiple regression analyses indicated that organic content was the dominant factor in explaining variability in bacterial densities in post‐overflow sediments. Using a simple combination of sampled physical parameters (flow velocity, river discharge and bed sediment size distribution) and well‐established hydrologic relations (Hjulstrom diagram, flow duration curve), it is expected that bacteria‐laden sediments around the outfall could be resuspended, on average, thirteen days per year. Resuspension of contaminated sediments, therefore, could adversely impact water quality even in the absence of overflow events.