Groundwater exploitation and hydraulic parameter estimation for a Quaternary aquifer in Dar-es-Salaam Tanzania

The fact that groundwater exploitation has largely increased since 1997 in the Dar-es-Salaam aquifer, calls for a directed attention towards possible problems of aquifer overexploitation that may arise in the near future. Hydraulic parameters are important for developing local and regional water plans as well as developing numerical groundwater flow models to predict the future availability of the water resource. The determination of aquifer parameters through pumping tests has become a standard step in the evaluation of groundwater resource potential. The pumping tests in the study area were conducted in August 2004 and August 2005, where 39 boreholes were tested out of 400 visited. In the study area there are over 1300 recorded boreholes drilled by Drilling and Dam Construction Agency (DDCA) by the year 2005. Total groundwater exploitation in the study area was estimated at 8.59 × 106 m3/year, based on yield data collected during the 2004–2005 field campaigns. The pumping tests included single-well tests and tests with measurements on the pumping well and at least one observation well. The tests were conducted for 6 h and 30 min. The pump was shut down after 6 h of pumping and the remaining 30 min were used for recovery measurements. The pumping test analysis methods used include: Neuman type curve matching and Walton type curve matching, checked by specific well capacity assessment and Thiem–Dupuit/Thiem’s method. The curve-matching results from the aquifer tests show the following parameters: an average transmissivity and hydraulic conductivity of 34 m2/d and 1.58 m/d, respectively for the unconfined aquifer; the semi-confined aquifer has an average value of 63 m2/d and 2.14 m/d for transmissivity and hydraulic conductivity, respectively. For the case of the storativity, the unconfined aquifer has an average elastic early-time storativity of 0.01, while the lower aquifer has an average storativity of 3 × 10−4. Specific well capacity method and Thiem–Dupuit/Thiem’s method confirm results for transmissivity and hydraulic conductivity of the semi-confined aquifer, while values for the unconfined aquifer are somewhat larger (by a factor of 2–3). The hydraulic parameters calculated appear to reasonably agree with the geological formation of the aquifers, as deduced from borehole descriptions.