| dc.description.abstract | The absence of proper sanitation in urban slum areas in African mega cities is a multi-faceted problem where a socio-economic as well as a technical approach is required to come to a sustainable solution. Consequences of the lack of sanitation in slums can be the deterioration of groundwater and surface water quality by an increased input of nutrients such as nitrogen and phosphate species and an input of bacteria/viruses. Additionally the deterioration of surface water quality can lead to eutrophication. This research focused on the presence of nutrients in groundwater and the processes governing their fate. A monitoring network of 26 wells was installed in the Bwaise III slum area in the capital city of Uganda, Kampala. Water levels were registered weekly and slug tests were performed to determine the horizontal hydraulic conductivity. Parameters measured in the field on water samples were EC, pH, temperature, alkalinity, orthophosphate, nitrate and ammonium. In the laboratory, samples were analyzed for all cations and anions. Soil samples were analyzed for grain size and geo-available metals. Additionally, a phreeqc model was set up in order to validate the groundwater flow velocity and the possibilities for cation exchange occurring in the soil.
A NE-SW hydraulic gradient of approximately 3 m/km was derived from the groundwater level measurements with a hydraulic conductivity of 3.43 m/day. The groundwater flowing through the region ended up in Nsooba channel. Chemical analyses showed Bwaise acted as an effective nitrate reducing system where, in the southwestern parts, iron was also being reduced. Ammonium was well established in the region with values up to 1 mmol/l. Orthophosphate presence was 2-10 µmol/l in groundwater which was markedly less than the calculated input of 26.3 µmol/l. The geo-available metal content of the soil combined with the coarse grain sizes encountered showed that sorption to metal oxides is an unlikely mechanism for the marked decrease in orthophosphate. Mineral saturation indices revealed that hydroxyapatite could regulate orthophosphate concentrations in the nitrate reducing part of Bwaise and vivianite in the iron reducing part of Bwaise. The phreeqc model showed that different values for the assumed porosity and cation exchange capacity can produce the same pattern seen in the chloride and sodium values in the field.
Contrary to conclusions raised by other researches, our study showed that under the right circumstances, pit latrines as a means of managing waste in slum areas can be a favourable option from a nutrient management point of view especially in relationship to disposal of waste in drains that discharge in surface water. | |
