Geochemical tracers reveal urban-derived contamination of groundwater and springs in Gulu city, Uganda

Groundwater is consumed by over 2 billion people worldwide, though it is susceptible to microbial and chemical contamination (Alley et al., 2002; Parker et al., 2010). In East Africa, as in many places, displacement from rural to urban areas increases stress on local infrastructure including water and sanitation (UNCDF, 2018). In this study, we utilise wastewater tracers to examine urban-derived inputs to groundwater under rapidly developing urban areas in Gulu District, Northern Uganda. Bulk and fluorescent dissolved organic matter (DOM), microorganisms (total coliforms and E.coli) and inorganic tracers of anthropogenic waste (NO3–, SO42–, Cl/Br) were characterised from boreholes (from 3–76 m depth; n = 113), protected springs (n = 20) and surface water from handpump drainage pools (n = 2; Richards et al., 2023). Our results indicate that NO3– was elevated in water sources in the Gulu city urban area and the Cl/Br ratio was elevated in springs, compared to concentrations in the more rural Aswa County area (p < .05; both). Interestingly, human and animal waste indicators E.coli and Tryp:FA ratios (Baker, 2002) displayed no significant difference between the rural and urban settings (p > .05). Some construction and maintenance-related aspects of the boreholes, as spot assessed by sanitary risk observations, did not apparently correspond to the indicators of microbial contamination. Rather, results suggest that DOM prevalence is primarily depth controlled in Gulu District. We considered the distribution of organic, inorganic and microbial analytes with regards to the potential source and fate of contaminants. As the population of many urban areas increase, this study offers valuable insights useful for water management planning. Acknowledgements A Dame Kathleen Ollerenshaw Fellowship is acknowledged for LAR and for GJLW’s PhD studentship. A UKRI-GCRF-Newton-ODA 2022-2023 Award via UoM to LAR et al. supported this project. We thank field support from Nancy Aromo and Monica Adokorach (Gulu University).  References Alley, W.M., Healy, R.W., LaBaugh, J.W., Reilly, T.E., 2002. Flow and storage in groundwater systems. Science 296, 1985–1990. Baker, A., 2002. Fluorescence properties of some farm wastes: Implications for water quality monitoring. Water Res. 36, 189–195. https://doi.org/10.1016/S0043-1354(01)00210-X Parker, A.H., Youlten, R., Dillon, M., Nussbaumer, T., Carter, R.C., Tyrrel, S.F., Webster, J., 2010. An assessment of microbiological water quality of six water source categories in north-east Uganda. J. Water Health 8, 550–560. https://doi.org/10.2166/wh.2010.128 Richards, L.A., Wilson, G.J.L., Wu, R., Muloogi, D., Hamisi, R., Denwood, T., Nuwategeka, E., Bhattacharya, P., Huck, J., Polya, David A., 2023. Water Quality in East Africa: Bringing Together Traditional Monitoring, Community Science and Artificial Intelligence Approaches. Presented at the AGU Annual Meeting 2023, San Francisco. UNCDF, 2018. Local Assessment for Equitable Growth in Gulu and Mbale Municipalities, Uganda. UN Capital Development Fund, New York.