Most cases of drinking-water resource degradation are in direct association with the contamination of water as a result of anthropogenic activities, e.g. pesticides and fertilisers from agriculture, tailings from mining operations, effluents from industrial processes, chemical spills, etc. While contaminants of anthropogenic origin will likely continue to be a major cause of the impairment of drinking-water resources, naturally-occurring drinking-water hazards, although less commonly reported, do exist — and they play a substantial part in the threat to public health and livelihoods of millions around the world. One such hazard is fluoride. In small amounts, fluoride is beneficial for oral health. However, prolonged exposure to high doses can lead to irreversible dental and crippling skeletal fluorosis. An estimated 200 million people from nearly every continent in the world are exposed to high concentrations of naturally-occurring fluoride in water that exceeds the World Health Organisation's guideline value. The case of high-fluoride drinking-water in the provinces of Chiang Mai and Lamphun shall be the focus of this talk. The key environmental processes which control the transport of fluoride from source to sink will first be presented. Then, the risks of vulnerable populations to fluorosis and the implications of the occurrence of these high-fluoride waters to the local water resource governance will be discussed.