Concentration levels and patterns of fluoride in groundwater resources from Kendu Bay area, Kenya

Abstract

Water samples from twenty eight boreholes and twenty six wells were sampled in Kendu Bay area which lies between longitudes 34° 35' and 34° 40' E and latitudes 0° 21' and 0° 27' S. The sampling was done in two seasons: dry season in the month of January 2006 and rainy season in the month of April in the same year. Deficiency or excess of fluoride in drinking water may be hazardous causing dental caries or dental fluorosis respectively. The samples were therefore analysed for fluoride and pH in the laboratory with the aim of determining the concentrations of fluoride in the study area, comparing these concentrations in the rift zone with those outside the rift, hence establish the factors controlling fluoride concentrations in the study area. Ion selective electrode method was used for the fluoride and pH determination in the laboratory. Temperature determinations were done in the field on site of each sampling point and an average of three temperature readings were taken on fresh sample of each sample using a -] O°C-100°C range mercury thermometer. Using computer. techniques the overall distribution of fluoride levels, temperature and pH values in the study area in different seasons were plotted. The distribution patterns have been explained in terms of geology. These patterns do not change but the concentration changes. The field and results .obtained from the study area were subjected to statistical analysis. The regression curves and values of moment correlation coefficients indicate that there is significant relationship between fluoride and pH in both seasons. Temperature has slight effect on fluoride concentration while there is no relationship between fluoride concentrations and depth. Component factor analysis results show that pH is the

parameter that has the highest significant control of fluoride in the study area. Q-Q plot results and the descriptive statistics reveal that fluoride concentrations in the dry season are higher than rainy season, also fluoride concentrations in the lowland are higher than in upland, hence a general pattern in the decrease in the fluoride level from dry season to rainy season. This same pattern applies to temperature and pH. Topographical corrections were done and the elevations obtained in metres were used to determine the groundwater level above sea level. This was used to draw groundwater contours. The groundwater flow within the area of study has been determined by the use of groundwater contours. The flow direction of groundwater is roughly SE-NW-N direction and is the same direction as the surface water. The recharge and discharge areas have also been established from the groundwater contour map. In relation to groundwater flow direction the concentration levels of fluoride are higher in the discharge areas and lower in the recharge areas. Fluoride concentration levels increase with the direction of flow of groundwater. Quality aspect of the groundwater resources in relation to domestic use has been discussed. From this it is established that the waters are not suitable for domestic use.