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Main Article Content
General Hydrogeochemical Characterization of Groundwater in Coastal Aquifers of Kilwa Kisiwani Oceanic Island, South East Tanzania
Abstract
The main objective of this study was to determine the hydrogeochemical
processes controlling the groundwater chemistry on the Island and assess
its suitability for human consumption and irrigation. The methodology
used in the study include hydrogeological assessment of the study area
and laboratory analysis for general physico-chemical parameters of
groundwater. Other parameters such as Soluble sodium percentage
(SSP) or % Na+
, Sodium Adsorption Ratio (SAR), Magnesium adsorption
ratio (MAR), Residual sodium carbonate (RSC), hydrochemical facies
classification, and Water Quality Index (WQI) were calculated using
empirical formulae. The effects of salinization of groundwater were
classified using the chloride-Bicarbonate (Cl/HCO3) and the Na+
/Clratios. The major geological units of the study area are sandstone, clays,
alluvium, beach sands and laterite soils of Cretaceous to Quaternary age.
A total of 21 groundwater samples were collected and analyzed for
physical parameters and major ions. Analysis results for pH shows a
mean value of 7.47 while the mean value for TDS is 1618.47mg/L.
Groundwater classification based on the piper trilinear diagram shows
that the dominant hydrochemical facies are Ca-Cl (42%), Ca-Mg-Cl
(33%) and Ca-HCO3 (23%). The cation exchange processes indicates
that Ca2+ replaced the Na+
and this is an indication of freshening
geochemical processes in the aquifer matrix. The findings also show that
mean value for total hardness (TH) is 92.44 mg/L, soluble sodium
percentage (SSP) or Na% is 15.29%, Sodium adsorption ratio (SAR) is
1.08, Electrical conductivity (EC) is 2447.1 μS/cm, Residual sodium
carbonate (RSC) is -11.29 meq/L, and Magnesium adsorption ratio
(MAR) is 56.89%. Water quality index (WQI) results show that the
groundwater is of very poor quality since the WQI value is 298.8 which
is in the range of (200 - 300), this range is designated as very poor water
quality for human consumption.
