Effects of Monsoonal-driven Flood Dynamics on Plastic Transport in the Mekong River Floodplain around Phnom Penh, Cambodia

Abstract:

The goal of this research was to assess the loads of plastic moving through the Mekong river floodplain around Cambodia’s capital Phnom Penh (PP), driven by increased flows and urban runoff during the monsoon season. We conducted a field campaign in August and September 2019 with a total of eight sampling events at the four river reaches flowing in or out of the city: the Mekong upstream, downstream, Tonle sap, and Bassac. Our method was developed and well-tested in Tampa, Florida: semi-stationary, multi-point cross-sectional sampling with a 500 μm mesh neuston net. In adaptation to the larger system, we captured plastic concentrations at five to seven surface points along the river span and 4 points down the center water column, with a maximum sampling depth of 12 m. An Acoustic Doppler Current Profiler (ADCP) was used to create a detailed flow and discharge profile of the same cross-sections. Samples were processed in the laboratory and a subsample of plastics analyzed with a microRaman spectrometer. We found over 40.000 particles in our 80 samples. Count/mass concentrations and loads from the four sites tell a story on how plastic is released, mixed, and redistributed through this urban river river intersection within a major Southeast Asian city. Hundreds of tons of plastic are carried from PP downstream the Mekong. The Tonle Sap river, connecting to the world’s largest freshwater fisheries, carried similar and even higher quantities of plastic. The majority of plastic is floating at the surface part a share of it moves deep down the water column. This research links data on plastic loads and concentrations with hydrologic measurements and river hydrodynamics at an extraordinary study site. This work provides an important insight into plastic pollution in Cambodia and its possible impact on Cambodian society and ecosystems, as well as the impact of Southeast Asia on global plastic input into the ocean.