Integrated geophysical, geochemical and hydrogeological investigation of shallow groundwater resources in parts of the Ikom-Mamfe Embayment and the adjoining areas in Cross River State, Nigeria

摘要

Information generated from geophysical, geochemical and hydrogeological data has been used in assessing the groundwater resource potential, quality and usability and in mapping flow directions within the shallow subsurface of the Mamfe Embayment, Cross River State, Nigeria. The electrical resistivity technique in which the Schlumberger's vertical electrical sounding field procedure has been adopted was the geophysical method employed; lithology logs from drilling records, discharge rates, static water level information were the hydrogeological information utilized, while the geochemical techniques involve analyses of water samples. Apparent resistances were measured using different resistivity meters including OYO McOhm (model 2115), ABEM ter-rameter (SAS300B and SAS1000 models) and IGIS (SSP-ATS-MRP model) with maximum current electrode separation reaching 1 km in some communities. Geological information was used as control in the modelling and interpretation of all geophysical data. The physico-chemical parameters of the water samples from the different water sources in the area were determined using different analytical techniques and in some cases, by in situ direct measurement of some parameters. Measured values of electrical conductivity, static water level, available aquifer discharge information and calculated SAR and %Na were integrated into the geophysical and hydro-geological results. The shallow subsurface of the area is segmented into four hydrogeological provinces [crystalline basement province (CBP), Cross River Plain Province (CRPP), Nkporo-Afikpo Shales Province (NASP) and alluvial/buried river province (ABRP) with localized groundwater flow patterns]. Results indicated that the alluvial (discharge rate of 3.83 L/s), fractured sandstone (discharge rate of 2.43 L/s) and basement (discharge rate of 1.80 17s) aquifers are more yielding than the aquifers in areas covered with deformed shales (discharge rate of 0.62 L/s) and siltstone aquifers (discharge rate of 0.97 L/s). The aquifer horizons are inhomogeneous and anisotropic with topography and lithology exerting significant influence on groundwater flow direction. However, there appears to be some high yielding aquifers at depths greater than 100 m in the CRPP areas although researches on their distribution are still ongoing. Precipitation is the major source of recharge and the water is enriched with Na~+, K~+, Ca~(2+), Mg~(2+), HCO_3~-, Cl~-, SO_4~(2-) and NO_3~- throughout the year. Graphical analyses of hydrochem-ical data using Piper and Stiff diagrams show that Ca-(Mg)-CO_3-HCO_3 is the dominant water facies. Results from EC, SAR and %Na show that the water is fresh and belongs to the good-to-excellent class and is, therefore, suitable for domestic, agricultural and industrial use.