A comparative microbiological assessment of river basin sites to elucidate fecal impact and the corresponding risks

Abstract

The study aims to assess and compare the concentration of microbial contaminants, their sources and distribution in surface water and sediment, and to determine the impact of seasonal variations and corresponding risks of faecal contamination using conventional and molecular methods. Historical data analysis was conducted using E. coli values from the eThekwini Water and Sanitation (EWS) department for 66 months (2009-2014). E. coli and Enterococci were analysed in surface water and sediment samples using the mFC/ spread plate and Colilert-18 (IDEXX) methods. The impact of seasonal variations was assessed using E. coli and Enterococci data collected during rainfall and no rainfall events, using an auto-sampler and sediment trap in parallel. Conventional standard membrane filtration methods using mFC agar, Slanetz & Bartley/ Bile Esculin and Brilliance E. coli selective agar were compared to the enzymatic Colilert-18 and Enterolert (IDEXX) test methods along the Isipingo and Palmiet Rivers. In addition, comparison of the analytical performance of droplet digital PCR (ddPCR) and qPCR for the detection of Salmonella targeting ttr gene in river sediment samples collected from the four sites of the Palmiet River in Durban, South Africa was done. In order to assess the public health risk associated with exposure of men, women and children to microbial pathogens in polluted surface water during recreational activities, the QMRA tool was employed in relation to the risk exposure to pathogenic E. coli, Campylobacter, Salmonella and Shigella. Also, the risk associated with crop irrigation (on farmers) as well as the consumption of crops irrigated with surface water from the Isipingo river was determined.

Analysis of the historical data gave a baseline of the two rivers of interest, thus helps understand the current situation of the rivers enabling researchers to pick up potential gaps. In this study after the analysis of the historical data it was evident that at the Palmiet river, microbial analysis must be conducted around the QRI settlements which is a major pollution source.

Also, from this study it was found that sampling points situated close to wastewater treatment plants, pump stations or informal settlements were of major concern, thus were considered for the study. It was found that sediment exhibited higher microbial concentrations than surface water, which was observed in both rivers. Also, rainfall had a significant impact on microbial variability. Higher microbial concentrations (indicator organisms) were observed in surface water after a heavy rainfall as appose to when there was no rainfall. This was due to contamination that is washed off into the river and sediment resuspension. Methodology comparison revealed that Colilert-18 and Brilliance E. coli were more selective compared to mFC agar. Brilliance E. coli /Coliform agar was comparable with Colilert-18 IDEXX, which was also observed with Slanetz & Bartley and Enterolert IDEXX. However, when mFC agar was compared with Colilert-18 IDEXX, significant difference was observed. In comparison of two Molecular methods, ddPCR were found to be fully amenable for the quantification of Salmonella and offer robust, accurate, high-throughput, affordable and more sensitive quantitation than qPCR in complex environmental samples like sediments. Quantitative Microbial Risk Assessment (QMRA) relating to recreational and occupational exposure showed that children were at the highest risk of getting infected. Also, it was observed that the probability of infection upon exposure to surface water from the Isipingo and Palmiet rivers was significantly high, hence exceeded the WHO guidelines values. Risk assessment on crops revealed that pathogenic bacteria may pose a risk to the consumer, however, a 9-log reduction may be achieved according to the WHO multi-barrier approach which involves proper washing and proper cooking of the crop before ingestion.

Overall the sampling points that had the highest pollution level and constantly exceeded the WHO and DWAF guidelines at the Isipingo river were the points situated and named “Next to the WWTP”, and “Downstream of QRI” at the Palmiet River.