An Assessment of Pollution Load Capacity of Son La Hydropower Reservoir in the Northwest Mountains of Vietnam45247

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 Scienc

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

1 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 semi-intensive 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, PO43-

obtained 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

2 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 3 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 1 Quality of inlet and outlet water of Son La hydropower reservoir, 2017

regulation on surface

Laichau hydroelectric dams

Sonla hydroelectric dam

water quality 08:2015/MONRE, Column

A2

oxygen(DO)

solids(TSS)

oxygen demand

BOD5 (20oC)

demand COD

(calculated by N)

(calculated by N) (

(calculated by N)

(calculated by P)

Source: Sonla hydroelectric company, 2017

Data: the correlation between flood elevation, water level and reservoir capacity during operation of Son La hydropower plant, 2017, to calculate pollution load capacity according to reservoir operating water level

Table 2 Water level and operating capacity of Son La hydropower reservoir in 2017

hydroelectric lake in the year

Lake water level (meters)

Lake capacity (million m3)

Noted

Apr -

Aug

capacity Jan -

Mar

capacity Sep -

Dec

capacity Source: Sonla hydroelectric company, 2017

2.3 Formula for calculating pollutant load capacity of Son La hydroelectricity reservoir

Load capacity of hydropower reservoirs is estimated through formulas, using the

circular: 76/2017/MONRE, Vietnam Ministry of Natural Resources and Environment

Mtn = (Cqc - Ci.out) x Vh x 10-3 x FS (1)

In which:

- Mtn: wastewater receiving capacity, load capacity for each pollution parameter of the lake, its unit is kg

- Cqc: limit value of surface water quality parameters according to technical standards

of surface water quality for use purpose of the lake, its unit is (mg/l), in that case of that lake

is the level A2 of National technical regulation on surface water quality 08:2015/MONRE

separate areas, the unit of calculation is mg/l

- Vh: The volume of the lake and is determined on the basis of the volume of the Son

La hydroelectric lake on the correlation between elevation level, water level and reservoir capacity during the operation of Son La hydroelectric plant

- FS: Safety factor, taken as 0.7

3 Results

3.1 Pollution loading of Son La hydroelectric lake reservoir at 175m water level

Based on the output water quality data in the water sample at the foot of the Son La hydroelectric dam (Table 1) and the lake capacity data (Table 2), calculate the possibility of pollution load for 09 parameters (DO, BOD5, COD, TSS, Fe, NH4+, NO2-, NO3-, PO43-) of the lake at 175m, volume of 2,756 (million m3) Plug the numbers into the formula:

Mtn = (Cqc - Ci.ra) x Vh x 10-3 x FS (1), have the following results

MBOD5 = (C6 - C4) x 2.756 x 0,7 = 3,858 (tons)

Figure 2.Pollution loading capacity of Son La hydropower reservoir at 175m water level

With a water level of 175m, 2,756 million m3 in volume Son La hydroelectric reservoir

is capable of withstanding 59,475 (tons) of pollutants, including: 3,086 tons (DO); 28,938 tons (TSS); 3,858 tons (BOD5); 11,575 tons (COD); 1,350 tons (Fe); 522 tons (NH4+); 1,929 tons (NO

2-); 8,006 tons (NO3-), 210 tons (PO43-)

3.2 Pollution loading of Son La hydroelectric lake reservoir at 190m water level

Based on the data of the output water quality in the water sample at the foot of the Son La hydroelectric dam (Table 1) and the volume of the lake (Table 2), it is possible to calculate the pollution load capacity for 09 parameters (DO, BOD5, COD, TSS, Fe, NH4+, NO

2-, NO3-, PO43-) of Son La hydroelectric lake at a water level of 190m, a volume of 6,504 (million

m3) Plug the number into the formula (1)

Mtn = (Cqc - Ci.ra) x Vh x 10-3 x FS (1), the following results

MDO = (C7 - C5,4) x 6.504 x 0,7 = 7,284 (tons)

MTSS = (C30 - C15) x 6.504 x 0,7 = 68,292 (tons)

MCOD = (C15 - C9) x 6.504 x 0,7 = 27,316 (tons)

MFe = (C1 - C0,3) x 6.504 x 0,7 = 3,187 (tons)

MNO3- = (C5 - C0,85) x 6.504 x 0,7 = 18,894 (tons)

MPO43- = (C0,2 - C0,091) x 6.504 x 0,7 = 496 (tons)

Figure 3 Pollution loading capacity of Son La hydroelectric reservoir at 190m water leve

With a water level of 190m and a volume of 6,504 million m3, Son La hydroelectric reservoir has a load capacity of 140,359 (tons) of pollutants, including: 7,284 tons (DO); 68,292 tons (TSS); 9,105 tons (BOD5); 27,316 tons (COD); 3,187 tons (Fe); 1,233 tons (NH4+); 4,552 tons (NO2-); 18,894 tons (NO3-); 496.2 tons (PO43-)

3.3 Pollution loading of Son La hydroelectric lake reservoir at 215m water level

Based on the data of the output water quality in the water sample at the foot of the Son La hydroelectric dam (Table 1) and the volume of the lake (Table 2), it is possible to calculate the pollution load capacity for 09 parameters (DO, BOD5, COD, TSS, Fe, NH4+, NO

2-, NO3-, PO43-) of Son La hydroelectric lake at a water level of 215m, volume of 9,260 (million

m3) Plug the number into the formula (1)

Mtn = (Cqc - Ci.ra) x Vh x 10-3 x FS (1), the following results

MDO = (C7 - C5,4) x 9.260 x 0,7 = 10,371 (tons)

MTSS = (C30 - C15) x 9.260 x 0,7 = 97,230 (tons)

MCOD = (C15 - C9) x 9.260 x 0,7 = 38,892 (tons)

MFe = (C1 - C0,3) x 9.260 x 0,7 = 4,537 (tons)

MNO3- = (C5 - C0,85) x 9.260 x 0,7 = 26,900 (tons)

MPO43- = (C0,2 - C0,091) x 9.260 x 0,7 = 706 (tons)

Figure 4.Pollution loading capacity of Son La hydroelectric reservoir at 215m water leve

With a water level of 215m, the total volume of the lake reaches 9,260 million m3, Son La hydroelectric lake has a load capacity of 199,838 (tons) of pollutants, including: 10,371 tons (DO); 97,230 tons (TSS); 12,964 tons (BOD5); 38,892 tons (COD); 4,537 tons (Fe); 1,756 tons (NH4+); 6,482 tons (NO2-); 26,900 tons (NO3-); 706 tons (PO43-)

4 Conclusions and discussion

The study identified the monitoring data of water quality of Son La hydroelectric reservoir on input and output parameters In addition, certify the water level and operating volume of the Son La hydroelectric plant by year

Calculation results demonstrate the ability to load about 399,672 tons of pollutants/year of Son La hydropower reservoir with 09 parameters including: (DO, BOD5, COD, TSS, Fe, NH4+, NO2-, NO3-, PO43-)

From January to March, water level of 190m, reaching a useful volume of 6,504 (million m3), the lake is capable of loading 140,359 (tons) of pollutants

From April to August, 175m water level, reaching a shallow volume of 2,756 (million

m3), lake capable of loading 59,475 (tons) of pollutants

From September to December, the water level is 215m, volume of 9,260 (million m3), the lake is capable of loading 199,838 (tons) of pollutants

The study results have confirmed that the bigger the water level and the volume of the Son La hydroelectric reservoir will be able to load pollutants higher than the water level and the lower volume

The results of calculating the pollutant loading capacity of the Son La hydroelectric reservoir are the basis for controlling the sources of pollutants from the seasonal basin of the year, ensuring the capacity of receiving and assimilating pollutants of the lake while maintaining water environment functions

Acknowledgments

The authors would like to thank the Son La Hydropower Company for allowing the use of the environmental monitoring data source of Son La hydroelectric lake

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