Abstract:
Boreholes and wells are the most reliable sources of groundwater in Lokoja and its environs. Some aspects of hydrochemical properties of the groundwater resource have been assessed. However, stable isotopes characterisation to delineate recharge sources necessary for effective management of the groundwater system has not been undertaken. This work was designed to undertake an evaluation of hydrochemical and stable isotopes characteristics of groundwater in Lokoja, north central Nigeria in order to assess its quality and recharge sources.
Field geological mapping of Lokoja was carried out with rock samples collected and cut into slides for petrographic study. Physicochemical parameters were measured in the field using Hanna HI9861-3 digital meter. A total of 129 groundwater samples were collected during dry (65) and wet (64) seasons for hydrochemical analyses using inductively plasma emission spectrometry for cations and ion chromatography and titrimetry for anions. Thirty-nine samples were also collected and subjected to stable isotopes analyses using mass spectrometry. Data evaluation involving statistical correlation, estimation of Water Quality Index (WQI), Sodium Absorption Ratio (SAR) and hydrochemical plots for groundwater were carried out.
Migmatite-biotite gneiss, granite, sandstone and alluvium were the major rock units in the study area. Migmatite-biotite gneiss and granite were mostly composed of quartz, microcline, plagioclase and biotite. The water pH ranged from 5.5-10.6 and 5.4-9.1, while total hardness ranged from 4.11-472 and 1.2-260 mg/L in dry and wet seasons respectively. Electrical conductivity of 30-860 and 70-1,730 µS/cm, and total dissolved solids of 31-1,040 and 55-1,273 mg/L suggested low to moderately mineralised groundwater system. The concentrations of Ca2+, Mg2+, Na+ and K+ (mg/L) for dry and wet seasons were 1.12-103.52 and 0.1-79; 0.32-52.02 and 0.2-57.1; 1.61-122.27 and 3.4-157.7; and 0.55-107.17 and 1.5-72.2, respectively. The bicarbonate (HCO3-) was the dominant anion with values ranging from 5.16-128 and 10.3-398.7 mg/L, indicating recharge from CO2 charged precipitation. The nitrate concentration ranged from 2.0-72.5 and 0.03-59.4 mg/L during dry and wet seasons. The concentrations of As, Cr, Fe, Mn, Pb, Cu and Zn were <0.05 mg/L, indicating low background concentrations and hence no contamination with respect to trace elements. The analyses of stable isotopes revealed deuterium (δD) ranged from -23.9 to 11.7‰ and δ18O -3.9 to -1.2 ‰, while deuterium excess (d-excess) of -6.3 and 16.2 ‰, indicated precipitation as the main recharge source with possible effects of kinetic evaporation. Positive correlations (r>0.5) exist between TDS and all major ions except HCO3-, indicating geogenic solute input to the overall mineralisation of the groundwater. Estimated WQI showed that 56.9 % of the samples fell within excellent class, while 35.4 and 7.7 % belonged to good and poor water classes, respectively. The estimated SAR revealed that 12.3, 70.8 and 16.9 % of the samples have low, medium and high salinity risk for irrigation. Hydrochemical characterisation revealed three dominant water types, which were Ca2+-(Mg2+)-HCO3-, Ca2+ -(Mg2+)-Cl—SO42- and Na+-(K+)-Cl--SO42-, generally controlled by precipitation, weathering and dissolution of bedrocks.
The groundwater system in Lokoja and its environs has low to moderate mineralisation and limited migratory history. Recent precipitation was confirmed as the source of groundwater recharge.
