Assessment of Eutrophication in Middle-Lower Chao Phraya River (2020–2022)

Abstract

This study investigated eutrophication in the Chao Phraya River from 2020 to 2022, focusing on ten monitoring stations between Ayutthaya and Samut Prakan. Distinct seasonal dynamics associated with the monsoon were observed. During the wet season (May–November), agricultural and urban runoff elevated BOD, suspended solids, and nutrient concentrations. In contrast, during the dry season (December–April), reduced freshwater discharge intensified salinity intrusion. Hypoxic conditions, with dissolved oxygen (DO) frequently dropping below 2 mg·L⁻¹ near the river mouth. Nutrient enrichment–particularly dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP)–was strongly associated with phytoplankton blooms, especially near the salinity front where chlorophyll-a concentrations peaked. Nutrient levels increased downstream, driven by untreated discharges from urban, agricultural, and industrial sources. Wastewater nitrification increased nitrate levels, while nitrate overall comprised 41.36% of the total DIN. Peaks in phosphate and total dissolved phosphorus (TDP) followed agricultural runoff, notably in November 2020. Principal component analysis (PCA) revealed clear spatial distinction between middle and lower river stations, with the lower section characterized by elevated nitrogen, phosphorus, and chlorophyll-a concentrations. The AARL-PC Score (a trophic state index) supported these findings, showing consistently high trophic scores in downstream stations indicative of eutrophic conditions, particularly during June and July 2020. Overall, the results indicate a worsening ecological condition compared to historical records and highlight the urgent need for integrated pollution control and sustainable river basin management to protect this tropical urban river.