OCCURENCE AND FATE OF ARSENIC IN THE HYDROGEOLOGICAL SYSTEMS OF NIGERIA

The hydrogeological system in Nigeria is based on the geology of the entire country which comprises of basement complex /crystalline rocks and sedimentary formation. The recognized existing hydrogeological systems in Nigeria include Sokoto sedimentary area, Chad sedimentary area, Upper Niger sedimentary area, Benue sedimentary area, Ogun/Osun sedimentary area, Lower Niger sedimentary area, Cross River sedimentary area and the Basement complex rock area all of which cover a total area of 923,800 km2. Due to diversification of water collection facilities like spring water, dug wells and rain harvest in the southern hydrogeological system the northern part of the country recorded the highest number of borehole wells.

The occurrence and fate of arsenic in some of the hydrogeological systems in Nigeria has not been known with certainty due to paucity of data. However, it is believed to occur naturally in trace form within the aquifers. The available data on the occurrence of total arsenic in aquifers located within the southern and southwestern part of the hydrogeological systems revealed a concentration range of 0.03 to 0.25 ppm in the Basement complex and a range of 0.06 to 0.40 ppm in the sedimentary terrain. The sources of arsenic in these regions have been found to be both geogenic and anthropogenic. The limestone deposits and the associated shale units around Ewekoro and Shagamu which played host to most of the aquifers in this hydrogeological regime contains arsenic concentration in the range of 3.6 to 30.2 ppm while the arsenic content of the underground water in this regime ranged from 0.04 to 0.16 ppm. The thick sand body of the coastal plain sediments in the hydrogeological regime of the southern Nigeria enhances the leaching of the arsenic content of the industrial waste and effluents into the aquifers.

More work is anticipated on the arsenic content of the entire hydrogeological system of Nigeria in other to generate a comprehensive data on it's occurrence, concentration, speciation and persistence which will help in determining the quality of the aquifer in term of the arsenic safe range and impacts on health of the consumers.