Abstract
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+, Ca2+, Cl− and SO42−, 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.
Highlights
The Cross River Basin (Nigeria) is often faced with difficulties in supply of water for drinking, domestic and irrigation use
Fifty percent and 70 % of all the groundwater and surface water samples, respectively, indicate that the water of the study area is acidic (Tables 1, 2). This may be attributed to the oxidation of sulphide minerals and sulphur contained in the shale, while the neutrality of the pH may be due to the buffering effect of limestone associated with the Abakaliki Shale
The minimum value of turbidity in groundwater was obtained at location GW 12 and the maximum value of 56.78 NTU was obtained at location GW16 (Abakaliki Shale)
Summary
The Cross River Basin (Nigeria) is often faced with difficulties in supply of water for drinking, domestic and irrigation use. This is partly attributed to the fact that the area is underlain by low permeability shale bedrock. Most rural people have no fresh drinking water. This scarcity of water seriously threatens the survival of the local population and contributes to poverty and water borne disease. The water supply in most of these rural communities is almost exclusively through shallow wells and surface water. The use of water for irrigation makes water a critical resource. Little is known about natural phenomena that govern the chemical composition of water and anthropogenic factors that may affect them
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