Inter-Event and Intra-Event Variations of Indicator Bacteria Concentrations in the Storm Sewer System of the City of Östersund, Sweden

Abstract

An episode of microbiological contamination of the drinking water supply of the City of Östersund, Sweden (63°10′45″N; 14°38′09″E) prompted a study of fecal pollution in four storm drainage catchments discharging in the vicinity of the water treatment plant intake, with the overall aim of determining the presence and variation of standard fecal indicator bacteria strains and total suspended solids (TSS) in stormwater from urban catchments with specific land uses and sizes varying from 5 to 40 ha. Four bacteria strains used as indicators of fecal pollution in Sweden were studied: total coliforms, enterococci, Escherichia coli (E. coli) and Clostridium perfringens (C. perfringens). In dry weather, indicator bacteria concentrations in storm sewers conveying baseflow did not exceed 100 colony forming units (CFU)/100 mL, but during wet weather, total coliform and enterococci concentrations increased 102 to 103 times, compared to those in baseflow, and considerably less in the case of E. coli and C. perfringens. Bacteria concentrations differed significantly among the sampling sites and in the majority of events observed in the four catchments; higher bacteria concentrations were observed during the early phases of runoff. Only in one catchment, positive correlations were observed between TSS and total coliforms, E. coli, and enterococci, suggesting similar sources; in the remaining catchments, no such correlations were observed. The collected indicator bacteria data represent a useful addition to the available data on indicator bacteria in stormwater in cold-climate regions.