Hydro-chemical studies and assessment of trace elements and bacterial contamination of shallow groundwater of Oyo area, southwestern Nigeria

Groundwater from various shallow hand-dug wells is the only reliable source of water supply in the ancient town of Oyo in southwestern Nigeria. The present work studied the water quality in twenty-five wells across this area by measuring the ancillary parameters, and analyzing the major and trace elements and taking the inventory of the wells as well as assessing the total coliform and Escherichia coliform in sampled water. The field measurement showed that the groundwater is slightly acidic with an average pH of 6.6 and fresh from the total dissolved solids values between 70 and 630 mg/L. From the average concentrations of major ions, the dominance order of the cations constituents in groundwater is in the order- Mg 2+ > Ca 2+ > K + > Na + and HCO - > CI - > NO 3 - > SO 4 2- for anions. The ranges of trace elements concentrations were; Iron II- 0.01 – 0.06; Chromium VI: 0 - 0.34; manganese 0 – 1.2 in mg/L, while Copper was between 0.38 – 32.7; Cadmium 0.03 – 1.87; lead 0.02 – 3.57; Zinc 0.01 – 7.79; and Arsenic 0.05 – 7.35 in µg/L. The total coliform count ranges from 6 – 1860 cfu/100 mL while Escherichia coliform (E. coli) units ranges from nil in three wells representing 12% of the wells, 1 – 50 units (28%), 51 – 100 (20%), 101 – 500 (20%) and > 500 units (20%). The groundwater is grossly polluted with nitrate and coliform bacteria while four wells were found to be contaminated with manganese and chromium.