Densities of indicator bacteria in urban and rural runoff

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Virginia Tech

This study was designed to investigate the indicator bacterial densities in runoff from urban and rural sources and to compare these densities with the densities in both raw and secondary-treated, unchlorinated sewage effluents. Analysis was performed on a total of 42 samples, 22 of which were taken at the urban and the rural sites and correlated with storm flow. The remaining 20 samples were taken, 5 at each of the 4 sites, in order to establish respective baseline densities for total coliform, fecal coliform, and fecal streptococcus. Results indicated that mean values for total coliforms and fecal coliforms during dry flow were within Virginia State standards for urban runoff, but exceeded those two standards 100 percent and 91.9 percent of the time, respectively, during storm flow conditions.

Mean values for total coliforms and fecal coliforms during dry flow at the rural site exceeded Virginia State standards 100 percent and 75 percent of the time, respectively. During storm flow the standards for both of these indicators were exceeded 100 percent of the time.

It was also determined that the increase in densities typically lagged behind flow increases and maintained this lag while returning to baseline values.

Finally, the FC/FS ratio as originally proposed by Geldreich was statistically supported as being valid for use in the identification of fecal pollution sources. Individual ratios were less than 0.7 at the urban watershed for 80% of the dry flow samples and 64.6 percent of the storm flow samples. For the rural site, FC/FS ratios were less than 0.7 for 25 percent of the dry flow samples and 81.8 percent of the wet flow samples. Samples of raw and secondary treated sewage exhibited ratios above 4.0, 40 percent, and 25 percent of the time respectively.