An Assessment of Pollution Load Capacity of Son La Hydropower Reservoir in the Northwest Mountains of Vietnam Do Xuan Duc (1) (*), Luu Duc Hai (1), Do Huu Tuan (1) (1) VNU University of Sciences, Vietnam National University, Hanoi, Vietnam * Correspondence: dxduc.ces@gmail.com Abstract: Son La hydropower reservoir is the largest reservoir in the Northwest region of Vietnam on the Da River, its water surface area, length, capacity and average water level are 225 km2, 120 km, 9,26 billion m3, and 215m respectively The basin which provides water for Son La hydropower reservoir has an area of 11,075 km2, including 16 districts and cities and over 100 communes and wards in provinces of Son La, Dien Bien, Lai Chau, and Yen Bai Water resources within the basin play a decisive role in the water environment composition, biological structure of the wetland ecosystem in Son La hydropower reservoir Economic, social and humanitarian activities in the above basin area have direct and indirect impacts on the reservoir water environment This study calculates the pollution load capacity of Son La hydropower reservoir with nine parameters of pollutants from the basin including (DO, BOD5, COD, TSS, Fe, NH4+, NO2-, NO3-, PO43-), basing on the water levels and reservoir capacity when operating Son La hydropower factory Keywords: pollution load; hydropower reservoir; Son La; Northwest Mountains of Vietnam Introduction The concept of environmental capacity was first used by American scientists when determining the appropriate grazing density in the Kaibad plateau in the early 20th century (Edwards et al 1974) According to GESAMP, environmental capacity (also known as "absorptive capacity) or" assimilative capacity "is defined as" environmental properties "field and its capabilities in regulating a certain activity without causing unacceptable environmental impacts” (GESAMP 1986) According to calculations it takes 21.7ha of mangroves to absorb phosphorus from the wastewater of 1ha of intensive shrimp farming and 7.2ha of mangroves for nitrogen waste of 1ha of shrimp farming Intensive, for semiintensive shrimp farming, the area is 2.4ha (for nitrogen) and 2.8ha (for phosphorus), respectively (Robertson et al.1994) P-limited wetlands also have the ability to assimilate low P loads without significant changes in ecosystem structure and function (Richardson et al.1999) Results indicated that 3863.75 tons of COD, 410.24 tons of NH3-N and 51.63 tons of P was discharged into Taiping Lake in 2011 While Taiping Lake still has large pollutant carrying capacity according to the current water quality management target (Li et al.2014) Photpho that can be assimilated in hydroelectric dams (David et al 2015) In reservoirs, hydrology was an important and additional factor modulating phytoplankton in these tropical reservoirs, directly removing phytoplankton populations and their potential zooplankton grazers by washout, and also affecting nutrient availability (Rangel et al.2012) The reservoir has the ability to load short-term pollution with nutrients from the input source for contaminated rural areas (Ryding et al.1980) The filtration effect of flowing water according to reservoir structure can reduce pollutant concentration by about 64% for suspended solids and 79% for lead (Legret et al.1996) Agricultural land use is directly related to basin pollution, these pollutants are observed to have a negative impact on water quality in freshwater reservoirs in terms of eutrophication and toxicity The pollution load related to pesticides and fertilizers is calculated from the unit load based on the area used (Tanik et al.1999) The loading reservoir is polluted with nitrogen (N) and phosphorus (P) from the seasonal basin, thereby identifying the major polluting sources for the reservoir (Shen et al 2014) Wetlands are used to treat contaminated water NH4+, BOD5, COD, PO43obtained from the Buriganga River in Bangladesh (Saeed et al.2016) This study calculates the pollutant loading capacity of the Son La hydropower reservoir in northwest of Vietnam with nine categories of pollutants from the basin, including (DO, BOD5, COD, TSS, Fe, NH4+, NO2-, NO3-, PO43-), basing on water level and reservoir capacity when operating Son La hydropower plant Estimating nitrogen load with multiple pollution sources using SWAT model: A case study in Cau River basin in northern Vietnam, analyzing pollution load depending on rainfall (Tran et al.2017) The activities of basin residents change the nitrogen cycle and photpho cycle in Nui Coc Lake (Vietnam), the study demonstrates about 66% of total annual nitrogen and 79% of total annual phosphorus removal in the reservoir (Le et al.2014) The formation of dams and reservoirs in the Red River basin of Vietnam, including the Da River system (Hoa Binh hydroelectric dam), has been shown to have capacity for metals (Dang et al.2010) Calculated the economic value of forest hydrological services for Hoa Binh Hydroelectric Plant in Vietnam, indicate that the economic value of forest hydrological services for electricity production ranges from 26.3 million USD to 85.5 million USD per year; and that the longevity of the hydroelectric plant can be prolonged by about 35-80 years, depending on the state of forest cover in the watershed (Nguyen et al.2013) In this study, we would like to assess how, after the formation of the Son La hydropower reservoir on the Da River (Northwest of Vietnam), is this reservoir capable of loading pollutants from the basin? Pollution load according to operating water level is calculated and discussed Methodology 2.1 Study area Son La hydropower reservoir is a reservoir on the Da River, this is the largest reservoir in the Northwest region of Vietnam The water surface area is 225 km2, the length is 120km, connecting provinces of Northwest Vietnam, namely Son La, Dien Bien and Lai Chau Capacity of 9,26 billion m3, the average water level is 215m The lake is located in the basin of Son La hydroelectric lake, with magnetic coordinates 21015’15’’ to 22045’10’’ North latitude, to 102050’10’’ from 104035’15’’ East longitude 2.2.Data collection and analysis Monitoring data of Son La hydropower reservoir inlet and outlet, this source of data is the basis for analyzing the pollutant loading capacity of Son La hydroelectricity reservoir Table Quality of inlet and outlet water of Son La hydropower reservoir, 2017 List Parameter Unit Input Output National technical regulation on surface Dissolved Total suspended Laichau Sonla water quality hydroelectric hydroelectric 08:2015/MONRE, Column dams dam A2 mg/L 5.2 5.4 ≥5 mg/L 25 15 30 mg/L 4.2 4.0 mg/L