There are four main geographical regions in the country: the north, the central plains, the northeast, and the south. Thailand has a total of 25 river basins in the country and Thailand's annual rainfall is around 1,700 mm. Despite the annual southwest monsoon, Thailand is subject to drought, particularly the northeastern region.[37] As of 2002, Thailand had available less water available per person than any other country in Asia, and nearly one-third of its water was "unsuitable for human consumption."[38] Non-potable water was a result of increasing untreated domestic sewage, industrial waste water, and solid hazardous wastes.[38]
A critical environmental problem that Thailand faces is water pollution.[1] Like air pollution, water pollution is most serious in the populous central region, with high levels of industrial and domestic waste water. The depletion of the water table around Bangkok has led to land subsidence which has exacerbated flooding.
Coastal waters also face challenges. The Gulf of Thailand is polluted by domestic waste water, and further by waste from industry and tourism. High pollution levels are found at the mouths of the Chao Phraya, Tha Chin, Pak Panang, Pattani, and Ranong rivers. Coastal water quality in most areas, however, is within established limits.
Water pollution has become obvious in many areas. In 1997, hundreds of thousands of fish and other aquatic life in the Nam Phong River died as a result of industrial pollution.[39] Large amounts of arsenic were found in the groundwater in Nakhon Si Thammarat Province, a result of mining in the area.[40] Pollution affects the marine environment. Red tides, caused by excessive algae growth and a result of pollution, oil spills, and invasive species are some of the factors that are affecting Thailand's marine biodiversity.[3]
Another major source of pollution are the heavy metals that have seeped into the rivers of Thailand. In the Chao Phraya estuary, mercury levels have far exceeded normal standards, and high concentrations of heavy metals on the river bed poses a serious threat to ecosystems