Water quality studies in Kranji Catchment, Singapore : use of organic tracer and PEDs for identifying potential sewage sources

Abstract

The objective of this study was to identify organic compounds that could serve as indicators of potential human fecal contamination sources to the Kranji Reservoir in Singapore that could be used as confirmation indicators along with bacteria indicators. The compounds chosen as potential tracers were nonylphenol and nonylphenol ethoxyltes (NPEOs), as these are indicative of sewage and have been found at measurable concentrations in rivers and streams elsewhere in Asia. Polyethylene devices (PEDs) were used as passive samplers to measure NPEOs in stormwater drains in Kranji catchment. Fieldwork was conducted in the month of January 2010. PEDs were deployed in different land use areas in order to obtain an overview of concentration ranges across the Kranji catchment and thus provide an indication of where and why this family of compounds could be found. Storm drains vary in design and since experience with PEDs in storm drains was limited, different types of drains were selected for deployment. The PED deployment design changed depending on the type of drain and the available tools and local conditions that were found. Subsequent chemical analysis of the PEDs was done at MIT. PED deployment times in Kranji were not long enough for mass transfer equilibrium between the water and the PED to be reached, and thus a model for kinetic sampling was used to estimate concentrations in the water. The results of fieldwork done for this study showed that survival of the PEDs was highly dependent on type of drains and flow intensity. The results of the laboratory analysis showed that NPEOs were present in non-residential areas of the catchment. The kinetic sampling model produced results that, although highly uncertain, give an indication of potentially risky concentrations in the water. These analysis lead to the recommendation that PEDs should be deployed pre-loaded with tracers to better model equilibrium in stormwater drains. Deployment should be considered at or near sites where human DNA factors and/or E. coli have been found. More information is needed on household detergent ingredients in Singapore to further evaluate using NPEOs as sewage indicators. Future studies should be done to confirm estimated NPEOs concentrations in Kranji water to verify if there is a risk to the ecosystem's health.