Surface water – groundwater chemical quality relationships in Msimbazi river basin, Dar es Salaam city

The study focused on the evaluation of groundwater recharge rates in the Kizinga catchment in Dar es Salaam region. The study area is drained by river Kizinga having its upper reaches in Pugu and Kisarawe hills with a total surface area of 191 kn2. Determination of groundwater flow direction, using hydraulic heads has shown that groundwater flow is controlled by natural topography and its recharge takes place at the Pugu and Kisarawe hills by infiltration of local precipitation. Groundwater in the study area has been found to occur in sands, gravels and limestone’s. The mean value of transmissivity and hydraulic conductivity obtained using the pump test data records, were 1.14×10˙3m2/s and 9.021×10˙5m/s respectively. Water balance, hydraulic and chloride profile methods were used in the determination of groundwater recharge rates resulting in 81.3 mm/year , 31.6 mm/year and 45.4 mm/year, respectively. A mean recharge rate of 52.8 mm/year was estimated being equivalent to 10.1×10 6m3/year indicating that 4.7% of the long term means annual precipitation of 1124 mm ends groundwater recharge. Borehole yields whose mean specific capacity is 2m2/h was 3.4×10 6m3/year by the year 1997. Two years later the total yield tripled to 9.1×10 6m2/year which is an increase of about 179% of groundwater production. Data collected indicate that, the natural groundwater recharge rates was by the end of 1999 nearly equal to that of groundwater abstraction rates in the study area. It is therefore suggested that alternative sources of freshwater supply be provided to meet the increasing demand and avoid depletion of groundwater resources.