Occurrence and distribution of fluoride in groundwater of chad formation aquifers in Borno state, Nigeria

Abstract

Borno State is largely underlain by the Chad Formation, which is the youngest stratigraphic sequence in the Chad Basin and most prolific in terms of groundwater resources. The Formation consists of the three prominent water bearing zones known as the Upper, Middle and Lower zone aquifers. The Middle and the Lower zone aquifers are confined while the Upper zone aquifer is unconfined, semi- confined and confined in some places. A total of 175 water samples were collected from boreholes and hand dug wells tapping these aquifers. The samples were collected using plastic bottles and at each sampling point pH, temperature and electrical conductivity were measured using portable pH/EC meter. In the laboratory, samples were analysed for fluoride concentration using Spectrophotometer with detection limit of 0.01mg/l. The Upper zone aquifer has fluoride concentration within the WHO acceptable limit of less than 1.5mg/l, except for a sample in Magumeri which has concentration of 1.8mg/l. The Middle and Lower zone aquifers have relatively higher concentrations, with the highest of 4.6mg/l in Gubio and 9.00mg/l in New Marte respectively. The relatively higher concentrations (above the WHO limit of 1.5mg/l) in these aquifers might be because of leaching from the confining clay layers, enhanced by long residence time and high temperature. The alkaline nature of water in these aquifers and their moderate electrical conductivity favour dissolution of fluoride. In the study area, cases of mild to severe dental fluorosis have been observed in areas with high fluoride concentration – above the WHO limit. In New Marte for instance, where the highest fluoride concentrations was recorded, cases of skeletal fluorosis have been noticed