Yohana Enock Mtoni

Saltwater intrusion and nitrate pollution in the coastal aquifer of Dar es Salaam, Tanzania

Dar es Salaam Quaternary coastal aquifer is a major source of water supply in Dar es Salaam City used for domestic, agricultural, and industrial uses. However, groundwater overdraft and contamination are the major problems affecting the aquifer system. This study aims to define the principal hydrogeochemical processes controlling groundwater quality in the coastal strip of Dar es Salaam and to investigate whether the threats of seawater intrusion and pollution are influencing groundwater quality. Major cations and anions analysed in 134 groundwater samples reveal that groundwater is mainly affected by four factors: dissolution of calcite and dolomite, weathering of silicate minerals, seawater intrusion due to aquifer overexploitation, and nitrate pollution mainly caused by the use of pit latrines and septic tanks. High enrichment of Na+ and Cl− near the coast gives an indication of seawater intrusion into the aquifer as also supported from the Na–Cl signature on the Piper diagram. The boreholes close to the coast have much higher Na/Cl molar ratios than the boreholes located further inland. The dissolution of calcite and dolomite in recharge areas results in Ca–HCO3 and Ca–Mg–HCO3 groundwater types. Further along flow paths, Ca2+ and Na+ ion exchange causes groundwater evolution to Na–HCO3 type. From the PHREEQC simulation model, it appears that groundwater is undersaturated to slightly oversaturated with respect to the calcite and dolomite minerals. The results of this study provide important information required for the protection of the aquifer system.

Enhancing Protection of Dar es Salaam Quaternary Aquifer: Groundwater Recharge Assessment

Water balance for Dar es Salaam Quaternary coastal aquifer (DQCA) was calculated according to the method of Thornthwaite and Mather. Monthly potential evapotranspiration estimates were calculated from 39 years (1971-2009) of routine meteorological data using Penman-Monteith, Thornthwaite, Hargreaves and Hamon methods. Results were compared in order to show possible differences that could be attributed to the methods. Determination of groundwater recharge rates gave a mean value of 184 mm year-1 which is equivalent to 71.39 x 106 m3 year-1 indicating that 16.5% of the long term average annual precipitation of 1114 mm ends up as groundwater recharge. Groundwater abstraction from DQCA was estimated to be 69.3 x 106 m3 year-1. These results are alarming in two aspects: high abstraction rate and the increasing trend of borehole drilling. Groundwater abstraction which is nearly equal to the amount of the groundwater recharge is far greater than the least conservative sustainable yield calculated at 70% (equivalent to 49.97 x 106 m3 year-1).