Impacts of Urban Wastewater on Water Quality of the Lake at Rach Gia Bay in the Mekong Delta, Vietnam

Aims: The Mekong Delta of Vietnam is facing impacts from climate change and sea level rise. Two extreme weather conditions – drought and flood – have occurred much more often recently. Faced with the challenges of drought, the government of Vietnam has an idea of constructing a super sea dyke (SSD)/barrage at Rach Gia Bay – Kien Giang province to create a fresh water lake that will provide water to water scarce regions in the Long Xuyen Quadrangle (LXQ) and especially, Ca Mau Peninsula (CMP). Three options for the SSD have been proposed: Option I: short route, 30 km, creating a small lake with a surface area of 357 km² and a volume of 609 million m3; Option II: short route track, 31.8 km, creating a lake with a surface area of 425 km² and a volume of 795 million m3; and Option III: long route, 47.5 km, forming a large lake with a surface area of 823 km², and a value of 2.58 billion m3 in volume. Furthermore, each option includes at least a sluice gate and a navigation lock. The objective of this study was to assess the impacts of wastewater discharges on the water quality in the proposed barrage.

Place of Study: Rach Gia City, Kien Giang province, Vietnam.

Methodology: Based on primary and secondary data for wastewater concentration estimates and the MIKE 21 FM model, water pollutant (TSS, BOD5, COD, total nitrogen, total phosphorous) fate in the barrage under Option II (the consensus-based optimal option) was assessed.

Results: The results show that only considering the untreated wastewater from Rach Gia City would make the lake become a wastewater repository, especially for organic pollutants, as it will receive a total wastewater volume of 28,432 m3/day from domestic sources and 16,711 m3/day from industrial sources with a total load of TSS, BOD and COD being up to 25,482; 12,281; 21,074 kg/day, respectively. The MIKE 21 Flow Model FM was used to simulate and evaluate water quality of the lake considering different methods/modes of discharging treated wastewater into the lake. The results show that even though wastewater might be treated to meet the water quality standards of Vietnam (Class B), the lake still could become contaminated locally; different locations of treated wastewater discharge (e.g. 1 point or 5 points near the bank or even at the middle of the lake) do not help to ensure uniform water quality in all areas of the lake.

Conclusion: An optimal option proposed is to control water quality not only for Rach Gia City, but also for other rivers and canals which are flowing to the lake to improve the quality of water supply for different purposes.