Hydrogeology and Water Quality Assessment of the Middle Aquiferous Horizon of Onitsha and Environs in Anambra Basin, Eastern Nigeria

Onitsha is a commercial, ecclesiastical and administrative city located on 6º49′30″E and 6º8′30″N in South-Eastern Nigeria with a high population density. The study area is close to a confluence and occupies a landmass of 300 km2 and geologically belongs to the Ameki group in Anambra basin. The lithology of ten (10) wells labeled W5, W7, W9, W12, W16, W20, W21, W22, W23, and W24 were obtained from which a geologic fenced diagram was generated; four aquiferous horizons were detected which range from shallow (15-24m), upper (30-32m), middle (<90m) and the deep (>90m). The study revealed that the flow direction of the groundwater is multidirectional which was influenced by the piesometric heights; the study also revealed that there is a depression (sinkhole) at the middle aquifer occasioned by the massive extraction of groundwater from the aquifer; this is as result of the population density of the study area with several functional boreholes taping its water from the middle aquifer on an hourly basis.
Biogeochemical analyses were carried out on the groundwater from these wells and the result obtained when compared to WHO standard shows that the water was severely polluted particularly with coli form count, for example the an average of 14.7 total coli form count was recorded against 0 recommended by WHO. The cations and the anions were analized using acceptable method such as American Public Health Association, Allem et al; 1974 and Adeleke and Abegunde 2011, while the heavy metals were analized with AAS model PYE Unicam SP 2900. The pH value of the groundwater was found to range between (6.5–7.05); other parameters measured include total hardness (TH), total dissolve solid (TDS), Electrical Conductivity (EC) and alkalinity has the following range of values (64–120 mg/l), (10–40 mg/l), (71–109 s/cm) and (56–220 mg/l) respectively. The cations and anions were obtained as follows; Ca2+ (1.46–8.62 mg/l), Mg2+ (0.72–2.57 mg/l), Cl- (153.8–449.7 mg/l), PO42- (7.5-61.5 mg/l) and NO3- (6-21.6 mg/l). The heavy metals includes; Fe (0.27-0.65 mg/l), Zn (0.04-1.16 mg/l), Mn (0.03-0.52 mg/l) and Ba (0.02-0.26 mg/l). The Chemistry of the water was traced to anthropogenic activities due to high chloride and heavy metals measured; this is influenced by the countless number of septic tanks, open dumpsite, battery manufacturing companies, and effluent from pharmaceuticals and textiles industries amongst others present in the area. The authors recommend that drillers and borehole contractors should therefore drill deep into the deeper aquifer that is capped with clay/shale body for potable water while the water from the middle aquiferous horizon should be well treated before consumption by applying Adsorption and Osmosis filter method etc.