Annual variation and reliability of fecal indicators in a polluted river

Abstract

Fecal indicators were enumerated over a period of one year in the river Vantaanjoki discharging to the Gulf of Finland. The river was divided into three zones, one affected by nonpoint loading, another receiving treated domestic sewage, and the lower reach with no substantial further loading. The reliability of routine enumeration methods for thermotolerant coliforms and fecal streptococci was tested by isolating strains for confirmation. Fecal indicators showed wide spatial and temporal variation: the concentrations and numbers were low in summer, higher in winter under the ice cover, and highest during the periods of high discharge in spring and autumn. Of the 931 thermotolerant coliforms isolated, 74% were identified as Escherichia coli by the API 20 E identification system. A total of 3441 strains were tested for gas and indole production at 44.5°C. Of these, 73% were gas positive and 89% indole positive. Of the 3567 fecal streptococci, 67% were confirmed as catalase negative and esculin positive on bile esculin azide agar at 44.5°C. The proportion of confirmed fecal indicators varied both in different reaches of the river and seasonally. The enumeration of fecal streptococci was in some cases unreliable. Correlations between different fecal indicators and between fecal indicators and chemical water quality variables were calculated within seasons. Correlation coefficients varied with season but showed good overall compatibility.