The Distribution of Fluoride Ions in the Groundwatersofthe Baringo Bogoria Lake Basin

Abstract

The ground waters of the area extending from the southern shores of Lake Bogoria to the northern shores of Lake Baringo, within the floor of the Rift Valley Kenya have been examined in relation to fluoride concentrations. The area is defined by co-ordinates 35° 45' - 36° 20' E and 1° 00' - 0° 15' N, it covers approximately 6000 knr'. Fluoride concentrations as high as 50 ppm had been reported in this area, resulting in abandonment of some boreholes. Most members of the community suffer various effects of skeletal and dental fluorosis. One hundred and twenty sampling sites were visited with one hundred ofthe boreholes being sampled, while surface sources included samples from; Lakes Baringo and Bogoria, the main rivers Molo and Perkerra and nine springs, seven of which are Lake Bogoria springs. The ion selective electrode technique was used to analyze for fluoride concentration. Other chemical constituencies analyzed included Mg, Ca, and HC03 for correlation purposes. PCW ATEQ was used to analyze the chemical data to fmd out the main minerals comprised in these rocks that maybe responsible for the fluoride ions. These minerals include biotite, phlogopite, fluorspar, fluoroapaptite, etc mainly found in weathered volcanic and sedimentary rocks in this area. Recharge of the groundwater bodies is mostly by lateral ground water flow from the Tugen hills where rainfall is higher than the lowland plains. This was established from the piezometric map drawn and the three dimension view of the piezometric levels. Maps showing the distribution of the fluoride ions in the ground water were also drawn using ArcView 3.2 and Surfer. The results showed that the ions were mainly concentrated within the central part of the study area, which lies on through of the rift valley. The concentrations were mainly controlled by geology. A correlation of the fluoride concentration to that of Mg2+,Ca2+and HC03- showed fluoride had negative linear relationship with Mg2+,and Ca2+ ions, however it has a general positive linear relationship with HC03- The pH of most waters sampled ranged from 7-10 on the pH scale. This means that waters with high concentrations of Mg2+,Ca2+will generally have low concentrations of fluoride ions. The results of statistical analysis also indicate that Na+ has the highest mean for cat ions and HC03- has the highest mean for anions and that these ions have a very high positive correlation with fluoride ions. With the water in this area having concentrations of fluoride that exceed the set standard of 1.5ppm, the prevalence of skeletal and dental fluorosis was evident. Due the low social economic standards of the community Bonechar defluoridation method was adopted. The efficacy of this technique rated at ninety percent. The findings in study will facilitate future planning and development of groundwater resource in this area by the various stake holders.