ASSESSMENT OF GROUNDWATERQUALITY INTARSAND-BEARING FORMATION OF EASTERN DAHOMEY BASIN, SOUTHWESTERN NIGERIA

Abstract

The tarsand-bearing communities of Ondo State depend on groundwater in which bitumen is frequently found filling pores and crevices of host rocks. Several works have been done with the view to assessing water quality in Eastern Dahomey without emphasizing on the depth of the groundwater. Hence the study evaluated groundwater quality in selected boreholes and hand-dug wells with respect to depth. Groundwater samples were collected and analysed for physicochemical characteristics and heavy metal concentration with respect to depth of the groundwater. The parameters of interest that were analysed in accordance with standard methods include E/C, TDS, pH, total hardness, temperature, Cl-, SO42-, NO3-, K+, Fe2+, Mn, Cd, Cr and Pb. Spatial autocorrelation analysis was conducted on the parameters using the Moran’s I test in GeoDa®1.8.10 software environment. The spatial pattern of parameters with positive correlation were determined using Inverse Distance Weighted (IDW) algorithm. The potability of the water was also analysed using the Water Quality Index. The calculated Water Quality Index and depth of the groundwater were subjected to Correlation. The result shows that the concentration of the heavy metals (Fe2+, Mn, Cd and Cr) had value above the reference level with depth exceeded 60m.The calculated Water Quality Index based on Standard Water Quality Classification Schemefor all the sampled groundwater shows that 27.27% of the groundwater sampled were excellent water and they were all hand-dug wells with depth ranged between 3m – 8m. For the borehole alone, 50% were good water with depth ranged from 35m to 50m while 50% of the sampled boreholes were very poor to unfit for drinking with depth between 60m and 100m which shows that water quality in the study area decreases as the depth increases to 100m. The regression analysis carried out shows that there is a strong positive relationship betweenWater Quality Index and depth of the groundwater. It shows that the value of Water Quality Index increases as the depth increases to 100m. The study has revealed that water quality varies with depth in the study area. Hence water quality in bitumen formation varies with depth.