Fluoride in small quantities promotes dental health, and is essential for growth of bones and is an essential element for the human body. So fluoride is an essential element for the human body yet in excess may cause dental fluorosis. Many health problems may arise either through deficiency or excess of fluoride. Drinking water is the main source of fluoride required by humans. Thus understanding the geochemistry and behavior of fluoride in surface water and groundwater is vital for human health. This is significant for Sri Lanka as the majority of the population does not have modern pipe-bone water systems especially in the rural areas. They depend entirely on dug and deep hand pump wells, rivers and channels for their domestic water requirements. A problem of growing concern is the excessive concentrations of fluoride found in many dug wells and water supply bore holes in the low plains of the dry zone, notably in the Anuradhapura and Polonnaruwa districts. The fluoride concentration in many of these wells exceeds 1.5 mg/l and poses a health hazard to consumers. In these areas, wide prevalence of dental fluorosis is found specially among children of school going age. The slow rate of groundwater movement and high evapo-transpiration in the low plains of the dry zone tend to increase the fluoride concentration.
Within the areas of high fluoride bearing groundwater, it is still possible to find certain areas specially close to irrigation tanks, with considerably low fluoride concentrations with regard to drinking water standards. Surface water containing low fluoride concentrations from the irrigation tanks could be a major governing factor for this situation. The possible effects of surface water infiltration on groundwater fluoride concentrations was studied in Polonnaruwa district, in a small village situated below the major irrigation tank; Parakrama Samudraya. The behavioral patterns of the groundwater table and groundwater fluoride content revealed that the concentration in groundwater of the area is controlled by the infiltration of the surface water from the tank and the irrigation channel. It was found that the fluoride in groundwater is low in the vicinity of the surface water bodies especially down-slope. Fluoride concentration increases in the direction of groundwater flow and away from the surface water bodies within the area of influence of the surface water. On account of the widely spread surface water irrigation network in Polonnaruwa district, it could be expected that the low fluoride areas close to irrigation tanks and channels are a result of infiltration and dilution of fluoride content by the surface water.