Hydrochemical characteristics of groundwater in sedimentary, metamorphic and volcanic aquifers in Ndian Division, South West Region, Cameroon.

Abstract

Wells and springs are the dominant potable water sources in the area of study used by the population. Studies of the characteristics of aquifer types and the hydrochemistry of groundwater in sedimentary, metamorphic, and volcanic aquifers in the southern part of Ndian Division indicated the presence of the following three types of aquifers-sedimentary (alluvial), metamorphic (weathered/fractured gneisses) and volcanic (jointed basalts and pyroclastic materials). Fieldwork involved collecting details on rock characteristics such as the mineralogy, grain-size and texture. Physical parameters (pH, EC and temperature) were measured in-situ. Cations were determined by AAS and anions by ion chromatography. The pH values for sedimentary aquifers (5.50-8.00), metamorphic (6.10-7.40) and volcanic (5.80-7.60), reflected slightly acidic, neutral and basic water sources. The mean concentration of total dissolved solids (TDS) in the groundwater increased from metamorphic (8.84 mg/l), to volcanic (17.08 mg/l), and to sedimentary (40.48 mg/l) aquifers, representing the degree of mineralisation in these rock types. Alkalinity increased from metamorphic (14.53 mg/l), volcanic (19.18 mg/l), to sedimentary (57.64 mg/l). Based on the electrical conductivity, all the water sources were suitable  for drinking and for irrigation. Relating the major groundwater chemical facies to their geologic occurrence showed that the water in sedimentary rocks was characterised by Na-HCO₃ (90%) and Ca-(HCO₃)₂ (10%), in the metamorphics by Na-HCO₃ (100%) and in the volcanic aquifers by Ca/Mg-(HCO₃)₂ (70%) and Na-HCO₃ (30%) water types. Based on WHO (2004) norms for potable water, 37 water sources were suitable for drinking and one water source (open well at Bekora) unsuitable due to excess nitrate and bromide concentrations of 77.28 mg/l and 0.08 mg/l respectively. Relatively based on EC and TDS, the most mineralised water was found in the sediments, the volcanics intermediate and the least mineralised water was in the metamorphic formations. The results constitute useful baseline information for future provision and management plans of water resources in the area.

Keywords: Ndian Division, rock types, hydrochemistry, physical properties, groundwater, unconfinedaquifers.