Tracking and Investigating Microbial Sources in Gainesville's Urban Creeks
Abstract:Gainesville's urban creeks have a history of high concentrations of indicator bacteria that consistently exceed regulatory standards. Potential sources include leaking sanitary sewer pipes, faulty septic systems, illicit interconnections between subsurface pipes, direct human disposal, pets, and wildlife. The state has determined that TMDLs are required to reduce the level of fecal coliforms in Gainesville's urban creeks. This study consisted of an intensive three-phased creek sampling program that incorporated conventional bacterial counts and innovative microbial source tracking (MST) techniques:
•Phase 1 - Synoptic Sampling. Synoptic sampling was conducted in the three urban watersheds to determine "hot spots" with high bacteria levels. Initially, a total of 43 sites were selected, and water samples were collected and analyzed for fecal coliform, total coliforms, and Enterococcus species using conventional methods. Site selection was conducted after an analysis of land use and potential contributors.
•Phase 2 – Detailed Microbial Source Testing Pilot. Pilot MST program was implemented at three high priority sites; and
•Phase 3 - Focused Sampling Program. Final MST program was conducted for 10 hot spots. MST methods used were ribotyping analysis, and DNA marker tests for human Enterococcus , Bacteroides, human enteric virus, and Polyomavirus. The laboratory results were interpreted in the context of land use patterns in the watersheds. As the study progressed and "hot spots" were identified, other sampling sites were added to bracket the suspected source area and thereby better define the range and extent of bacterial contamination. Results from Phase indicated that virtually all of the sample sites exceeded one or more of the traditional regulatory guidance values for total coliform, fecal coliform, and Enterococcus during each sampling event. Ribotyping and human DNA marker tests were conducted during Phases 2 and 3. A total of 41 samples representing 23 sampling sites were analyzed by ribotyping E. coli isolates. Each isolate was placed in one of the following categories: human, dog, bird, wildlife, indeterminate human or animal, indeterminate animal, or unknown. Of the total 31 sampling sites analyzed for the presence of DNA markers, 27 (87%) showed evidence of human impact for one or more sampling interval. This investigation demonstrated that the traditional bacterial indicators show elevated levels of coliforms and Enterococci in nearly every sample, regardless of season, flow condition, landscape position, adjacent land use, or proximity to different land uses. The ubiquitous nature Copyright ©2007 Water Environment Federation. All Rights Reserved of the high counts indicated that high bacteria levels are not reliable stand-alone predictors of human fecal contamination. Results of ribotyping analyses show that multiple non-human sources as well as human sources contribute to the problem. In many areas of Gainesville, levels of total coliforms, fecal coliforms and Enterococcus would still exceed regulatory standards even if all human sources were eliminated. Persistence and regrowth of indicator bacteria in Gainesville's creeks is likely contributing to the high bacterial counts. Further, MST techniques, used in a toolbox approach, were valuable in determining bacteria source host species, in estimating the relative contribution of various host species, and identifying stream reaches that have been impacted by human fecal contamination. Characteristics of the various DNA markers and the patterns of detection were also useful in determining whether or not the bacteria resulted from a recent event or from residual contamination, and whether the source was likely to be from a collection system or from a discrete source such as a septic tank. Control methods designed to reduce fecal bacteria are effective only to the extent that they target the specific sources of those bacteria by expanding our ability to identify these sources, the evolving MST technology will aid in the development of management strategies for protecting Gainesville's surface water resources. The City of Gainesville has TMDL reduction goals for several of its urban creeks. These results are being used to address the required reductions for TMDL compliance.
Document Type: Research Article
Publication date: 2007-10-01
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