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 identify the hydrogeochemical processes controlling the groundwater chemistry on the Island. 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 slightly neutral conditions while TDS value is slightly higher than the permissible limit. 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 show that mean values for total hardness (TH),  soluble sodium percentage (SSP) or Na%,  Sodium adsorption ratio (SAR),  Electrical conductivity (EC),  Residual sodium carbonate (RSC),  Magnesium adsorption ratio (MAR) and Kelly’s ratio (KR) are within permissible values. The effects of salinization of groundwater were classified using the chloride-Bicarbonate (Cl/HCO₃) and the Na⁺/Cl^- ratios.  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.