Preliminary Investigation into the source of salinity in water samples from a coastal aquifer. Case Study Ningo-Prampram area, southern Ghana

Abstract

To contribute towards understanding the state of coastal groundwater resources in Ghana, this study employs field sampling, laboratory analysis, statistical and geochemical analysis to reveal the possible sources of mineralization/salinization in an unconfined gneissic aquifer in the eastern part of the Accra,  Ghana. To do this, samples from 16 groundwater sources 11 shallow hand dug wells and 5 deep boreholes were sampled and analyzed for physical and chemical parameters.  Ionic ratios involving chloride and other ratios  (Ca²⁺/SO₄^2- ,  Ca²⁺/Mg²⁺,   Ca/(HCO₃+SO₄)), scatter plots: Ca/(HCO₃+SO₄)  and Na⁺/Cl^- were employed  to understand possible contribution of the sea to groundwater salinization. Geochemical tools were employed to  understand various mechanisms responsible for  salinization  and hydrochemical water types.  Results indicated  that groundwater  in the study areais saline with average EC values > 1mS/cm and water hardness ranging from moderately hard water to very hard water. The dominant ions (Na, Ca, Cl and HCO₃) are implicated as the major determinants of mineralisation. The aquifer is influenced by seawater and indicated by: ionic ratios > 1 for  for  Ca²⁺/SO₄^2- ,  Ca²⁺/Mg²⁺  and  Cl/HCO₃  and low ratios (<1) for SO₄^2-/Cl^-,  K⁺/Cl^-, and  Mg²⁺/Cl^-  for samples of  from both HDW and BH, Chadda’s diagram  resulting in  Na -Cl water type  for all samples and the dominance of  Na and Cl in  all  water types (Na-Ca-Cl-HCO₃ , Na-Ca-Cl , Na-Cl  and Na-Cl-HCO₃)  obtained from Piper’s plots.  Base ion exchange processes influence the groundwater chemistry of almost all the samples (93.8%). Seawater intrusion in deep groundwater sources inferred from high mineralization of deep groundwater (depth > 20 m) and high average Seawater mixing Index (SMI) of 1.2 in boreholes. Silicate weathering, evaporite dissolution and wastewater infiltration are additional processes contributing to salinization of the aquifer