GROUNDWATER AND SURFACE WATER QUALITY OF BOENG CHEUNG EK WASTEWATER TREATMENT WETLAND, PHNOM PENH, CAMBODIA

Boeng Cheung Ek wetland is a large, peri-urban, free water surface, wastewater treatment wetland that services approximately 90% of combined storm water and untreated municipal wastewater from the city of Phnom Penh, Cambodia. Flooded area in the wetland basin cycles seasonally between 20 km² and 13 km² during the wet season and dry season, respectively. Untreated wastewater input to the wetland is an average of 2.04 m³/s during the dry season with a maximum of 12 m³/s during storm events. The objective of this study is to characterize the spatiotemporal variability of contaminants in wetland surface water and determine whether infiltration to groundwater has occurred. Significant communities adjacent to the wetland still rely on household wells as their primary drinking water source. Nitrate, ammonia, phosphate, potassium, chloride, E. coli, heavy metals, H and O isotopes, and physicochemical parameters were determined for three sampling seasons: July 2017, August 2017, and January 2018. Of the 10 household and community wells sampled, two wells located 180 m apart had high levels of chloride indicating contamination by wetland surface water. The difference in concentration between inflow at two wastewater pumping stations and outflow from the wetland via two outlet streams reflected reductions in the range of 40-88% (ammonia) and 43-87% (phosphate). Additionally, the study presents the potential of sUAS and SfM photogrammetry for inexpensively producing high resolution digital elevation models. A 1.2 km² DEM with an average ground sampling distance of only 6.67 cm was compared to eight RTK-GPS ground control points yielding a mean vertical error of 4.96 m and standard deviation of 0.67 m. Data from this emerging technique has applications including referencing well elevation for groundwater modeling, measuring surface water level and stream gradient, and serving as an input for flood modeling and risk studies. At this point, 22% of the wetland has been filled in for development. While there are plans to fill in nearly the entire wetland, the construction of a wastewater treatment plant has yet to begin. We hope this research will help inform the sustainable management of wastewater in the city.