66% of water, 15% from surface water sources, and 13% is mined from underground. Non-potable water is a result of untreated domestic sewage, industrial waste water, and solid hazardous wastes. This is a critical environmental problem for Thailand. According to the Pollution Control Department, the agricultural sector is the largest polluter as the nation's farms discharged up to 39 million m3 of wastewater per day in 2016. The industrial sector ranked second, discharging 17.8 million m3 per day. The residential sector ranked third with 9.6 million m3 per day. Wastewater treatment processes in the residential sector were only 18% effective, while only 52% of wastewater was treated. Surface waters In 2003, Thailand's Pollution Control Department (PCD) monitored the quality of 49 rivers and four lakes in Thailand. Findings revealed that 68% of water bodies surveyed were suitable for agriculture and general consumption. Only less than 40% of Thailand's surface waters were in poor or very poor quality. According to the survey of major rivers and lakes by PCD, no surface water was categorized as "very good" quality (clean water suitable for aquatic animals and human consumption after normal treatment). Surface water quality varies widely in the different regions in Thailand. Surface water monitored in the northern, central, and southern regions appear to have poor quality, while water in the eastern region was fair. Compared to other regions, the rivers and lakes monitored in the northeastern region had good quality surface water. In terms of dissolved oxygen (DO), surface water in the northern region ranks the best, approximately 6 mg/L, followed by the northeastern region with DO concentrations of around 4 mg/L. The central, eastern, and central regions rank the lowest, about 2 mg/L. The highest concentration of total coliform bacteria (TCB), among surface waters monitored, was found in the central region with concentrations of TCB higher than 4,000MPN (most probable number)/100mL. Coastal waters In 2003, PCD set up 240 monitoring stations in Thailand's 23 coastal provinces and on significant islands. In 2003, monitoring results showed that coastal water of 68 percent of the stations were in "very good" and "good" quality. Thirty percent of the stations were in "fair" condition and only three percent were in "poor" quality. Compared with past data, coastal water quality was shown to have deteriorated, specifically in the areas into which four main rivers flow. The chief indicators of pollution were DO and TCB. Water