Stormwater drains - illegal sewer connection

Aim

To identify the source of high levels of microbial indicators detected in a series of open, concrete-bottomed stormwater drains, which flowed through an urban area. The appearance of the water was clear but the microbial indicators were suggesting faecal pollution.

The sampling sites B1, B3 and B4 were located along the same stormwater drain, with B1 and B3 being approximately 900 metres downstream of B4. Site B2 was located on a drain that flowed into the main outlet (Figure 1).

Figure 1: Stormwater drain sampling sites

Approach taken

Samples were initially analysed for E. coli, with very high levels of faecal contamination in samples B1, B3 and B4 (6,000 – 18,000 MPN/100 ml) and lower levels in sample B2 (300 MPN/100 ml). These initial results were further evaluated by analysis of faecal sterols, fluorescent whitening agents (FWAs) and molecular markers to determine potential sources of the high microbial indicator levels.

Results

The faecal source indicators all showed very strong evidence of human faecal pollution at site B4. The faecal sterol levels for B1 and B3 were lower in comparison to B4, but still noteworthy, and the ratios supported human pollution as the source of faecal indicators. Site B2, in contrast, contained levels of faecal sterols that were close to the detection limit and therefore it was not applicable to use ratio analysis for this site. These results were supported by FWA analysis with site B4 containing 20-fold higher levels compared with sites B1 and B3. However, all three sites contained levels of FWAs indicative of human faecal pollution. The human Bacteroidetes PCR marker was positive for all three sites B1, B3 and B4. In contrast, site B2 situated on a drain that flowed into the main outlet contained no significant human faecal source indicators.

Conclusion

The results from all faecal indicators suggested that the drain on which site 2 was located was not the source of the high numbers of bacterial indicators. Site B4, being approximately 900 metres upstream of Sites B3 and B1, was identified as being closest to the source of faecal contamination. With the scientific evidence provided, the local council was able to narrow the investigation for the human faecal source to localities above site B4.

This case study provided a powerful example of combining a basket of determinants to trace the potential source of faecal contamination in an aquatic environment.