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Water chemistry in the shale bedrock of the Cretaceous-Tertiary of the Cross River and Niger Delta hydrological basins has been investigated using major ions. To carry out a characterization of the water bearing units, 30 and 16 representatives surface and groundwater samples were collected. The evolution of the water is characterized by enhanced content of sodium, calcium and sulphate as a result of leaching of shale rock. The spatial changes in groundwater quality of the area shows an anomalous concentrations of ions in the central parts, while lower values characterize the eastern part of the basin covering Ogoja, Ikom and Odukpani areas. The values of total dissolved solids (TDS) and ions increases down gradient in the direction of groundwater flow. The dissolution of halite and gypsum explains part of the contained Na ? , Ca 2? , Cl -and SO 4 2-, but other processes such as ion exchange, silicate weathering and pyrite oxidation also contribute to water composition. The assessment with contamination indicators such as TDS, hardness, chloride, nitrate and sulphate indicates that the water in area is suitable for human consumption in some locations. Modelling using MINTEQA2 program shows that the water from all the shale water bearing units are under saturated with respect to gypsum.

The present work is a review on the quality characteristics of groundwater in Nigeria. The aim was to collate, synthesize and analyse hydrochemical data from available literature in order to evaluate the physical and chemical character, quality and determine the sources of ions. The review process covered four major groundwater sedimentary basins viz Benin, Benue, Niger Delta, Sokoto and the Crystalline Basement Complexes. The issue of saline groundwater in parts of Benue and Niger Delta Basins were given attention. In general, data show that on the average, TDS for the groundwater from the different basins was < 250 mg/l compared to the saline groundwater with average TDS as high as 15700 mg/l. The high salinity was attributed to dissolution of salts and seawater intrusion. Nitrate concentration on the average is high especially, for Sokoto Basin indicating anthropogenic pollution. Descriptive and multivariate statistics together with cross plots indicate that the major geologic controls on the groundwater chemistry are chloride dissolution, weathering (silicate and carbonate) and ion exchange. With respect to agricultural and irrigation purposes, the groundwater are excellent besides the water from the saline groundwater from Benue Basin and the coastal aquifers.

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