ASSESSMENT OF GROUNDWATER VARIABILITY OVER DIFFERENT GEOLOGIC FORMATIONS ACROSS KEBBI STATE, NIGERIA.

Saadu, U.W.* and Ali, B.M.

This study attempts to assess the variability of groundwater composition over three different geologic units in Kebbi State. Nine (9) boreholes were purposively selected and studied. From each bore hole the lithology and water samples were analyzed. Boreholes from Southern Kebbi State (undifferentiated basement complex) were found to be composed of schist and granodiorite with some patches of clay formations. In contrast to this, the lithology of boreholes in Northern and Central Kebbi State is largely composed of sands, inter-bedded by clay formations. In general, lithology was found to differ from northern to southern parts of the study area. In Gwandu Formation, sodium was found to have the highest mean concentration (1081±2 mg/l). Bicarbonates (655l±2 mg/l) was found to be the outstanding parameter under Illo Formation. In undifferentiated basement complex, chloride is the major chemical parameter (498.3±2 mg/l). Sodium, chloride, zinc, magnesium and phosphates were found to be in excess of WHO and national standard for drinking water quality (NSDWQ) reference guidelines across the three geologic units. Whereas mean salinity, TDS, potassium, calcium, copper and iron were found to be within WHO and NSDWQ reference guidelines. The study concludes that the observed variability in physico-chemical composition of groundwater in the study area could be derived from the underlying rock formations, even though, other environmental factors such as climate, and land-use pattern, affect groundwater composition. This could only be explained by detailed studies that compare the effects of geology, climate, and land-use on groundwater quality over a wide range of spatial and temporal scales.

Key words: Geological Formation, Groundwater Composition, Kebbi State, Lithologic Log.