Using hydrochemical tracers to assess impacts of unsewered urban catchments on hydrochemistry and nutrients in groundwater
Department of Water Science and Engineering, UNESCO-IHE Institute for Water Education, PO Box 3015, Delft, Netherlands; Makerere University, Department of Civil and Environmental Engineering, PO Box 7062, Kampala, Uganda; Section of Water Resources, Delft University of Technology, PO Box 5048, Delft, Netherlands; Department of Civil Engineering, Central University College, PO Box DS 2310, Dansoman-Accra, Ghana
We applied graphical methods and multivariate statistics to understand impacts of an unsewered slum catchment on nutrients and hydrochemistry of groundwater in Kampala, Uganda. Data were collected from 56 springs (groundwater), 22 surface water sites and 13 rain samples. Groundwater was acidic and dominated by Na, Cl and NO3. These ions were strongly correlated, indicating pollution originating from wastewater infiltration from on-site sanitation systems. Results also showed that rain, which was acidic, impacted on groundwater chemistry. Using Q-mode hierarchical cluster analysis, we identified three distinct water quality groups. The first group had springs dominated by Ca-Cl-NO3, low values of electrical conductivity (EC), pH and cations, and relatively high NO3 values. These springs were shown to have originated from the acidic rains because their chemistry closely corresponded to ion concentrations that would occur from rainfall recharge, which was around 3.3 times concentrated by evaporation. The second group had springs dominated by Na-K-Cl-NO3 and Ca-Cl-NO3, low pH but with higher values of EC, NO3 and cations. We interpreted these as groundwater affected by both acid rain and infiltration of wastewater from urban areas. The third group had the highest EC values (average of 688μS/cm), low pH and very high concentrations of NO3 (average of 2.15mmol/l) and cations. Since these springs were all located in slum areas, we interpreted them as groundwater affected by infiltration of wastewater from poorly sanitized slums areas. Surface water was slightly reducing and eutrophic because of wastewater effluents, but the contribution of groundwater to nutrients in surface water was minimal because o-PO4 was absent, whereas NO3 was lost by denitification. Our findings suggest that groundwater chemistry in the catchment is strongly influenced by anthropogenic inputs derived from nitrogen-containing rains and domestic wastewater. © 2013 John Wiley & Sons, Ltd.
Acid rain; Calcium; Catchments; Cluster analysis; Effluents; Groundwater; Groundwater geochemistry; Hierarchical systems; Housing; Infiltration; Multivariant analysis; Nutrients; pH; Positive ions; Precipitation (meteorology); Rain; River pollution; Runoff; Sanitation; Surface waters; Water quality; Electrical conductivity; Groundwater chemistry; Hierarchical cluster analysis; Hydrochemical tracers; Multivariate statistics; On-site sanitation systems; Slums; Uganda; Hydrochemistry