Monitoring Fecal Coliform in Stormwater Discharge as Part of the Ambient Monitoring Program Required in the NPDES Permit

Monitoring fecal coliform bacteria in stormwater from sites located in the City of Chattanooga, as part of the National Pollutant Discharge Elimination System, occurred over two years at 100 sites located on streams and ditches in the City. Sampling for the first year of this project was initiated in November 1997, and concluded in late September 1998. The second year samples were collected beginning in January 1999, and concluded in August 1999, Three types of samples were collected: preliminary, to refine laboratory techniques; "wet" season, collected during the late winter and early spring, representing a period of the year having higher rainfall; and "dry" season, collected during summer months, representing a period of the year having little rainfall. Water samples were collected in sterile containers, stored on ice, and returned to the laboratory within 4 hours of collection. At the laboratory, standard laboratory procedures were used to dilute and filter aliquots of the water samples onto sterile 0.45 micron filters, which were incubated on M-FC medium at 44.5 +/- 0.2 C for 24 hours. Fecal coliform contamination of Chattanooga streams was fairly widespread. During "wet" season sampling approximately 90% of the sites tested had fecal coliform counts greater than 4 cfu/100 ml. During the "dry" seasons nearly 82% of the sites had fecal coliform counts > 4 cfu/100 ml. Levels of fecal conforms in samples collected in the "dry" season were greater than during the "wet" season, with 60% of the sites tested having fecal coliform counts > 200 cfu/100ml, while "wet" season samples with fecal coliforms >200 cfu/100ml accounted for only 24% of samples collected. Further analyses of 1998 "wet" season data indicated that mean values of fecal coliforms during high flow were 316 cfu/100ml (n=33), while low flow conditions resulted in reduced numbers of fecal coliforms, with mean values of 195 cfu/100ml (n=46) observed. Additionally, levels of fecal coliform contamination were different in different areas of Chattanooga: North Chattanooga was found to have the highest coliform levels (mean value of 482 cfu/100 ml), the Chattanooga Creek area was found to have intermediate streamwater fecal coliform levels (mean values of 373 cfu/100 ml), and outlying areas of Chattanooga had the lowest stormwater fecal coliform counts (mean values of 255 cfu/100 ml). Finally, the presence of wetlands in City streams resulted in reduced levels of fecal conforms in the stream water downstream of the wetlands (reductions of nearly 35% for four wetland systems studied). Removal of concrete channelization, allowing riparian vegetation to regrow in the area, also resulted in reduced fecal conforms in stormwater flowing through the stream channel (resulting in nearly 50% reduction in these bacterial numbers downstream).