WATER QUALITY OF A LARGE TRIBUTARY OF THE MEKONG RIVER IN THAILAND - SPATIOTEMPORAL VARIATIONS AND POTENTIAL CAUSES

Surface water quality degradation has been an environmental problem which raises great concerns across the world. The Mun River, one of the largest tributaries of the Mekong River and an important agricultural area in Thailand, is investigated to determine its status, identify spatiotemporal variations, and distinguish the causes of the variations. Water quality data over the last two decades (1997-2017) from 21 water quality monitoring sites distributed along ~ 800 km of the river was used. Eight parameters: dissolved oxygen (DO), biochemical oxygen demand (BOD), total coliform bacteria (TCB), fecal coliform bacteria (FCB), total phosphorus (TP), nitrate-nitrogen (NO₃-N), ammonia nitrogen (NH₃-N), and suspended sediment (SS), were used as representative indices to assess the water quality of the river. A water quality index (WQI) method is applied to assess the general water quality. Marked spatiotemporal fluctuations were observed in both individual parameters and the WQI, though the water quality in general, is good. Spatial variation in water quality was detected from the upper to the lower reaches, and the upper reaches generally having lower WQI values than those in the lower reaches. Seasonal variation of water quality was also observed, and WQI values in August (flood season) were generally among the lowest, compared to other seasons. TP and NH₃-N are the main parameters which impact water quality in the upper and lower reaches of the Mun River, respectively. Excessive phosphorus, the primary cause of water quality degradation and eutrophication in the upper reaches, comes mainly from erosion and leaching from agricultural soil, while nitrogen (NH₃-N), the primary parameter for water quality degradation in the lower reaches, potentially comes from raw sewage discharging into the river from towns or cities.