Assessment of groundwater quality in shallow coastal aquifers of Okrika Island, eastern Niger delta, Nigeria

Abstract

This paper investigates the aquifer system and groundwater quality of Okrika Island using lithological logs, static water level (SWL) measurements and physico-chemical analyses of sampled waters from ten boreholes. The borehole logs identify three (3) major lithologies within the Benin Formation. These include clayey sand, clay and sand. The clays are silty in places, while the sands are fine to coarse and poorly sorted. In some boreholes, clays occur as topsoil and could be up to 9 m thick. In the other boreholes, the topsoils are either sand or clayey sand. Two main sand aquifer units were delineated. The static water levels are relatively shallow (o.5 7.9 m) making the aquifers in the area vulnerable to pollution from solid wastes and leachates. The pH values vary from 6.6 7.7 with an average of 7.1 which indicate slightly acidic to slightly basic groundwater. The concentrations of chloride are between 10 and 300 mg/L with a mean of 92.28 mg/L. Total alkalinity values are between 81 and 86 mg/L. Iron
concentrations range from 0.01 0.1 mg/L with an average of 0.04 mg/L. Sulphate concentration ranges from 0.02 20 mg/L. These values are low when compared with the WHO (2006) standard for potable water. The calcium  hardness obtained falls between 10 and 62 mg/L. Magnesium hardness is low in the water and ranges in concentration between 6 and 17 mg/L while total hardness values of the groundwater samples in the area range from 2.5 100 mg/L, indicating that the water is soft to moderately hard. The bicarbonate concentration measured at Isaka II is 81mg/L while the total dissolved solids (TDS) values fall between 10 300 mg/L. Except at few locations where elevated chloride concentrations were recorded, the groundwater is generally potable.

Keywords: Water Quality, Hydrogeology, Saline Water Intrusion, Okrika Island, Niger Delta.