Abstract:
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 control-
ling groundwater quality in the coastal strip of Dar es
Salaam and to investigate whether the threats of seawater
intrusion and pollution are influencing groundwater qual-
ity. 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 over-
exploitation, 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–HCO 3 and
Ca–Mg–HCO 3 groundwater types. Further along flow
paths, Ca 2? and Na ? ion exchange causes groundwater
evolution to Na–HCO 3 type. From the PHREEQC simu-
lation 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.
