Hydrochemical and Isotopic Characterization of Groundwater in the Buem, Voltaian and Togo Formations of the Volta Region, Ghana

Abstract

Variation in groundwater chemistry and isotopic composition has been observed in the northern part o f the Volta Region. The geology o f the study area consists o f quartz schist, quartzitic sandstone, shales, mudstones and siitstone. The extent to which the water chemistry and isotopic composition has been affected by geology is however not known. This study therefore seeks to determine the general chemical character of the groundwater in relation to the different lithologies and determine the source(s) and origin o f recharge to the aquifer systems. This study combines geology with the physico-chemical and isotopic data o f groundwater samples systematically obtained from boreholes in the area. The distribution o f physico-chemical parameters define three hydrochemical zones controlled mainly by mineralogy and degree o f weathering in the various lithologies: The quartz schist and quartzitic sandstone aquifer is dominate by slightly acidic (mean pH is 5.9) and relatively low TDS; the shale aquifer by near neutral (mean pH is 6 .8 ) and medium TDS; and mudstones and siitstone aquifer by neutral (mean pH is 7.2) and relatively high TDS. However, apart from the slightly acidic water occurring in some parts o f the south, which falls below the accepted lower limit o f 6.5 (WHO, 1993), the chemical composition o f the groundwater in the area is generally of acceptable chemical quality. The quartz schists, quartzitic sandstones and shales aquifers are characterized by Calcium+Magnesium-Bicarbonate, Calcium-Bicarbonate and Magnesium Bicarbonate water types. These water types are controlled mainly by dissolution of plagioclase feldspar, ssricites and minor carbonate materials. Sodium-Bicarbonate facies on the other hand is found in the mudstone and siltstone aquifers and influenced mainly by cation exchange and/or dissolution o f albite-rich plagioclase. The relationship between deuterium and oxygen-18 shows two classes o f water. The relatively enriched samples are confined to the south while the depleted samples are in the north indicating, probably, continental or latitude effect. However, all the samples lie along the global meteoric water line suggesting that groundwater recharge in the area is by rainfall with no evaporation prior to infiltration. This may indicate rapid flow towards the potentiometric surface.