Identification of suitable indicators for tracing human faecal contamination and sewage in urban catchments
Date of Issue2014
School of Civil and Environmental Engineering
Singapore-MIT Alliance Programme
Faecal contamination, particularly from human sources, to surface waters poses a threat to public health especially when the surface water is used for water supply and/or for recreational purposes. Faecal indicator bacteria (FIB) have been used widely as surrogates for faecal contamination but were originally developed in temperate climates. FIB are known to experience growth and decay in the environment and therefore their use has been questioned, especially in tropical environments. This has motivated the present study in Singapore, a tropical and highly urbanised country, in which identification of human-related sources is important. This study aimed to suggest sampling protocols, identify suitable tracers as a function of land use and identify evidence of leaking sewers as sources of human faecal and sewage contamination in a highly urbanised catchment. Preliminary data analysis shows that sampling location influences surface water quality and surface water quality parameters have diurnal variations. Therefore, in this study, dry-weather samples were collected hourly over 12 and/or 24 hr at 13 upstream areas with specific land uses: 6 high-density residential, 5 low-density residential, 1 commercial and 1 industrial. A suite of potential tracers which includes FIB, chemical parameters and HF183 is analysed. Hourly sampling over 24 hr conducted at eleven residential sites revealed diurnal variation of FIB which closely follows the pattern of sewer flow. Daytime FIB concentrations were significantly higher than nighttime FIB concentrations. Field tracer tests conducted also provide qualitative evidence linking sewage exfiltration and transport to surface drains via preferential flow paths. FIB concentrations in samples collected hourly from 04:00 – 07:00 were significantly lower than in those collected from 12:00 – 15:00. Particularly, sampling at 12:00 and 14:00 results in significantly higher FIB concentrations while sampling at 05:00 and 04:00 or at 05:00 and 06:00 results in significantly lower FIB concentrations than sampling at other times throughout the day. Therefore, samples for water quality monitoring can be collected at these suggested sampling times. Correlation analysis shows that land use is an important factor in drain water quality. There is stronger correlation among water-quality parameters when only samples with high normalised FIB concentrations are considered. Acetaminophen, orthophosphate, cholesterol, cholestanol, coprostanol, sucralose and saccharin are potential tracers for high-density residential areas. Chloride, cholestanol, coprostanol and caffeine are potential tracers for low-density residential areas. In general, higher HF concentrations are accompanied by higher concentrations of FIB, faecal stanols, pharmaceuticals and sucralose. Correlation analysis and analysis of variance of FIB, chemical and HF data shows that high-density residential is more contaminated than low-density residential. Overall, the diurnal pattern of FIB concentrations, agreement among tracers and more contaminated high-density than low-density residential support the hypothesis that leaking sewers are sources of human faecal and sewage contamination in urban storm drains in Singapore.
DRNTU::Engineering::Civil engineering::Water resources