Impacts of natural and anthropogenic factors on microbiological water quality indicators along an urban riverine tropical wetland

Impacts of natural and anthropogenic factors on microbiological water quality indicators along an urban riverine tropical wetland  

Flavia Byekwaso^1,3,6, Guenter Langergraber², Gabriele Weigelhofer^1,3, Rose Kaggwa⁴, Frank Kansiime⁵,  Thomas Hein^1,3

¹ University of Natural Resources and Life Sciences, Vienna (BOKU), Department of Ecosystem Management, Climate and Biodiversity, Institute of Hydrobiology and Aquatic Ecosystem Management, Gregor-Mendel-Strasse 33, 1180 Vienna, Austria

² University of Natural Resources and Life Sciences, Vienna (BOKU), Department of Landscape, Water and Infrastructure, Institute of Sanitary Engineering and Water Pollution Control, Muthgasse 18, 1190 Vienna, Austria

³ WasserCluster Lunz, Dr. Kupelwieser-Promenade 5, 3293 Lunz am See, Austria

⁴National Water and Sewerage Corporation, P.O. BOX 7053, Kampala, Uganda

⁵ Makerere University, Department of Environmental Management, P.O. BOX 7062, Kampala, Uganda

⁶ Ministry of Water and Environment, Climate Change Department, P.O BOX 20026, Kampala, Uganda

ABSTRACT

 

Water quality monitoring is essential for understanding seasonal variations in microbiological indicators and their implications for public health. Waterborne bacteria and pathogens are a significant cause of human diseases, especially in developing countries.  The study aimed to understand the factors that cause seasonal changes in the concentrations of microbiological water quality indicators along a riverine tropical wetland. In total, 144 water samples were collected for 12 months at six sites along Lubigi wetland in Kampala, Uganda, receiving varying stormwater and wastewater inputs from urban water infrastructure during the dry and wet seasons. Water samples were analysed using specific microbiological assay tests for Escherichia coli, faecal coliforms, heterotrophic plate counts, Enterococcus and Salmonella species. Generally, the highest concentrations of microbial contamination were detected during the dry season at all sites. There was a decreasing trend in microbial contamination for all the selected five microbiological indicators with increasing distances from the sources of stormwater and wastewater inflows in the upstream reaches towards the downstream areas of Lubigi wetland. Nitrogen compounds, Escherichia coli, faecal coliforms, Enterococcus and Salmonella species originated from stormwater, whereas wastewater discharges delivered primarily phosphorus compounds, organic matter and heterotroph plate counts. E. coli and heterotrophic plate counts were positively correlated with water temperature and salinity. E. coli, faecal coliforms and heterotroph plate counts were positively associated with Biological Oxygen Demand (BOD5). Escherichia coli, faecal coliforms and Enterococcus species were positively correlated with NH₄-N. Escherichia coli, faecal coliforms, and heterotrophs dominated with high concentrations during the dry seasons, while Enterococcus and Salmonella species were more prevalent in the wet season. Escherichia coli, faecal coliforms and Salmonella showed insignificant logarithmic reductions during both seasons, showing when the carrying capacities of Nsooba main channel and Lubigi sewage treatment plant systems were exceeded. Enterococcus species showed no reduction in both seasons, which implied continuous high in-stream contamination. Heterotrophs showed significantly higher logarithmic reduction during the wet season than in the dry season. This suggested a concentration reduction during the wet season and loading/increase in the dry seasons. Our research findings may be used by the public health sector to understand relationships between the occurrence of surface water quality microbiological indicators and the prevalence of diseases through strategic seasonal monitoring and evaluation in Kampala and the region.