Modelling and Hydrochemical Assessment of The Groundwater in The Gezira Area -Sudan

Abstract

The main purpose of the current research was to study the chemical quality of the groundwater in the Gezira area to achieve better management of its water resources. The study was carried out by utilizing the data obtained from recent and historical analyses of more than 1300 of groundwater samples collected from various wells, which were selected to cover the whole area of study. The analyzed data was filtered using the imbalance ratio equation to determine the error in the chemical analysis of each sample. Only a 5% error was considered acceptable. The analyzed data was classified into two distinct classes pertaining to the Nubian and Gezira aquifers. The area of study, which is greater than 26000 km2, encompasses the two Niles and is approximately bounded by latitudes 13=616;-15.=494;=616; North and longitudes 32=616;-34.5=616; East. Using GIS techniques, estimation models for the chemical constituents of the Gezira groundwater have been conveniently devised. The developed models can be used as primary predictors for estimating the expected values of the chemical constituents of the groundwater pertaining to new locations in the map. The GIS tools were utilized efficiently and a comprehensive map of the Gezira groundwater was produced. The map is composed of all the collected data and the established models of the chemical parameters (Integrated model). The developed integrated map was extended to include also the mapping of groundwater clusters, ionic ratios and saturation indices. The essential chemical parameters of the groundwater were analyzed using conventional statistical methods. The values of the mean concentrations of the total dissolved solids, Sodium, Sulphate, Silica and Chloride ions were higher in the groundwater of the Gezira formation than in the Nubian aquifer. The groundwater of both the Nubian and Gezira aquifers was found to be slightly or moderately alkaline and most of the analyzed chemical parameters were found to lie within the permissible ranges stated by the WHO and Sudanese standards except in the Gezira aquifer where very confined areas were noted to have high levels of salinity. The sources of salinity in the Gezira area were found to be related to the high concentrations of the ions of Ca, Mg, Na, HCO3, and Cl in the Gezira aquifer and Na, HCO3, Mg, SO4 and Cl in the Nubian aquifer. The classification of the Gezira area groundwater was carried out using cluster analysis and the Piper, Durouv and Schoeller diagrams. The main water type and the dominant anion in both the Nubian and Gezira aquifers were found to be (Na-Ca-HCO3) and HCO3, respectively. Factor analysis was employed to study the factors that control the mineralization of the groundwater. The factors for the Gezira aquifer were: (SO4, CaCO3, TDS, Cl, and EC), (alkalinity HCO3), and (Ca, Na, and Mg). The factors for the Nubian aquifer were: (EC, TDS, CaCO3, and SO4), (alkalinity), and (Mg and Ca). Composition diagrams and ionic ratios were used to examine the cation exchange processes, and the hydrogeochemical processes that occur in the groundwater. The dissolution of sulphide minerals, halite and magnesium carbonate minerals has been noted to take place in the Gezira aquifer. The oxidation of sulphide minerals and dissolution of carbonate and dolomite have been identified in the Nubian aquifer. High values of bicarbonate, calcium, and magnesium were obtained close to the Blue Nile and high values of bicarbonate, sodium and potassium were also obtained close to the White Nile, indicating the contribution of the two rivers to the recharge of the groundwater in the Gezira area. The evaporation process in the Gezira and Nubian aquifers is minor as indicated by the low correlation between Na and Cl and as shown from their composition diagrams. The negative ion exchange, where Ca and Mg are replaced by Na in the water is the dominant process in both aquifers. The groundwater in both aquifers has been found undersaturated in terms of anhydrite, CO2, and Gypsum, and oversaturated in terms of Aragonite, Calcite and Dolomite. It is worth mentioning here that the hydrochemical analysis of the groundwater in the Gezira area has yielded similar facies for both the Nubian and Gezira aquifers. This is an indication of interconnection of the two aquifers