Monitoring of fecal indicators in rivers on the basis of random sampling and percentiles

Abstract

Thermotolerant coliform bacteria and fecal streptococci were monitored in polluted rivers using random sampling and a distribution-free method of estimating percentiles. The objective was to investigate the wide temporal and spatial variation observed earlier in some of these rivers and to monitor for compliance with the standard of acceptable swimming water (1000 thermotolerant coliform bacteria per 100mL). In the summer of 1986 eleven river reaches were visited in random order 3 to 10 times, randomly sampling 2 to 13 sites per reach and visit. Thermotolerant coliform bacteria and fecal streptococci were determined by the membrane filtration technique from 341 and 232 samples, respectively. The results were evaluated on the basis of statistical parameters, percentiles and analysis of variance. Bacterial concentrations varied widely in every reach due to fluctuations of loading and/or hydrological phenomena. During dry periods the bacterial concentrations in rivers were lower than during wet periods. The standard of acceptable swimming water was temporarily exceeded during wet periods but never during dry periods. The use of random sampling and calculation of percentiles was found to provide a good overall picture of the variability of water quality in the rivers. It is recommended to evaluate the bacterial concentrations separately during wet and dry periods.