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Of recent, the rapid deterioration of groundwater quality in Port Harcourt, Southern Nigeria due to unregulated exploitation resulting from increasing growth in the oil and gas production activities has become a major concern. The predisposition of groundwater to pollution and the realization of its serious health and economic consequences demand knowledge of the ambient groundwater quality and of the processes leading to an improved understanding of the groundwater in the area. Groundwater samples were collected from eighteen (18) representative boreholes spread over the Port Harcourt City. This was done to assess and determine the geochemical processes occurring within the aquifer systems using groundwater chemistry and ionic ratios. Properties such as electrical conductivity, pH and major ion concentrations, such as Ca, Mg, Na, K, Cl, HCO3, and SO4, of groundwater were taken into consideration. Concentrations of these cations and ions in the groundwater systems of the area vary spatially and temporally. Abundance of these ions are in the following order: Ca > Mg >Na > K = HCO3 > Cl > SO4 > NO3. Ca - Mg - HCO3 and Ca- Mg- SO4- Cl are the dominant hydrochemical facies of the study area. Results show that ion-exchange processes, carbonate and silicate weathering are responsible mechanisms for the groundwater chemistry of the area. Hydrochemical indices (Mg/Ca, Cl/HCO3 and Cation Exchange Values (CEV) generally indicates low- salt inland waters, with minimal marine influence. The hydrochemical evidence reveals the importance of recent management decisions (reduced exploitation/controlled pumping) in determining the evolution and distribution of groundwater salinity within the aquiferous zones. This framework, as the study observes, will lead to improved understanding of the hydrochemical characteristics of the aquifer systems of the area.