Assessment of Pollution Load into Thi Nai Lagoon, Viet Nam and Prediction to 2025

Based on the statistical data of social-economic state and development planning to 2025 of Binh Dinh province, the pollution load from development activities has been calculated by using the environmental rapid assessment method. The results showed that development activities in Binh Dinh province create about 30.7, thousand tons of COD;17.1 thousand tons of BOD; 9.1 thousand tons of Nitrogen; 6.5 thousand tons of Phosphorous and 379 thousand tons of TSS from living activities, aquaculture, industry, farming and land leaching. Until 2025, this amount will be increased 1.4 2.6 times. The main pollution sources are from living activities including residents and tourists and livestock farming. Therefore, treatment of waste from these sources is very necessary to minimize the waste amount discharged into the Thi Nai Lagoon.


I. INTRODUCTION
Thi Nai lagoon which located in Binh Dinh province is a tropical salty brackish lagoon with the area about 5,060ha, the length of over 10km and width of about 4km. The lagoon is poured by dense river network, in which Con river and Ha Thanh river are the largest ones. Con River is over 178km in length with total basin area approximately 3,067km 2 and flow velocity 58.84m 3 /s. Ha Thanh river is 58km in length with total basin area of about 580km 2 and flow velocity 13.6m 3 /s. Both rivers originate from the high mountains, inclined from east to west. In the rainy season, flood and leaching seriously happens, whereas drought occurs in the dry season.Flow velocity differencebetween two seasons is more than 1,000 times (Geography of provinces and cities in Vietnam, Volume 4). This research has used the environmental rapid assessment method, calculated pollution load based on the emission factors according to UNEP (1984) [11], San Diego-McGlone (2000) [9], Tran Van Nhan, Ngo Thi Nga (2002) [2] and the number of residents, tourists, animal, industrial production. This method has been used to evaluate the pollution load discharged into Halong Bay -Bai Tu Long [10]. Pollution load discharged into Thi Nai Lagoon has been estimatedby analysing pollutants discharged into the lagoon and waste treatment capability in the region.

* Pollution from living and tourism activities
Pollution load which generated from residents in the coastal districts has been caculated by population statisticsin the region and the pollution emission factor per capita. The components which selected for calculating pollution load was BOD, COD, SS, TN and TP. Pollution load from tourists has been calculated based on the number of tourists and residential days multiplied with the pollution emission factor. Pollution load from the districts in Binh Dinh province has been calculated by capability of pollutants discharged into the Thi Nai Lagoon. Pollution load from living sources (Qsh) is the total pollution load from residents (Qdc) and tourists (Qdl). Qsh = Qdc + Qdl(tons/year) -The formula for calculating pollution load from residents Pollution load from this source has been calculated based on the population of the districts and living pollution load per capita. Qdc = P x Qi x 10 -3 Qdc: Pollution load from residents(tons/year) P: Population of districts (person) Qi: Living pollution load of substance i (kg/person/year)

* Pollution from industry
Industrial pollution sources in the region has been calculated based on industrial production yield in related districts multiplied with the pollution emission factor of the industrial type. Industrial pollution load from other areas in the province has been calculated based on capability of bringing pollutants into the areas: Industrial pollution load has beeb calculated according to the formula: Qij = Vj x Cij x 10 -6 j =1,n Qij: Pollution load of substance i from industrial sources j (tons/ year) Vj: Annual wastewater volume from the facility j (m 3 /year) Cij: Content of substance i in wastewater from facility j (mg/l) n: The number of industrial facility in the region. This formula has been used to calculate pollution load from coal industry activities. When there no data on the wastewater composition, pollution load has been calculated based on the volume of some main industrial products of the districts and typical wastewater composition (Table 2).

* Pollution from agricultural sources -Pollution from livestock farming
The pollution from livestock farminghas been calculated by the number of livestock and pollution load unit. The number of livestock taken from Binh Dinh Statistical Yearbook. Pollution load unit perlivestockrefered from "Guidance on themaster planning of basin drainage system" -Japanese Drainage Association.
Pollution load by livestock farminghas been calculated based on total annual livestock and pollution load unit for livestock (Table 3). The amount of pesticides used in agriculture has been known based on the statistics of the Department of Agriculture and Rural Development.

* Pollution from aquaculture source
Pollution load from aquaculture source has been calculated based on aquaculture production yield in the region multiplied with the pollutant emission factor for each type of aquaculture production yield. The amount of generated pollutants depends on the form and species farming, in which shrimp and cagefish farming industry has the most significant discharge.

* Pollution from soil leaching
Pollution load from soil leaching has been calculated by land use area and pollution load unit. Pollutant load unit per area for BOD, COD, TN and TP refered from "Guidance on the master planning of basin drainage system" -Japanese Drainage Association. Pollution load due to soil leachinghas been calculated based on data on thetypes of land usearea, the average number of rainy days in the area and pollutant load unit from soil leaching of the types of land use.

* Estimating pollutant load discharged into the Thi Nai Lagoon
Pollution load estimation discharged intothe lagoon from different sources depending on following formula: ∑Qij = ∑Qij generated x Rij x (1 -rj Hij) ΣQij -Total pollution load of substance i discharged into the lagoon from the source j (04 sources) ΣQijarising -Total pollution load i generated from the source j Rij-Pollution load factorfrom the waste sourses discharged into lagoon corresponding to i and j rj -Percentage of treated wastewater from source j Hij -Treatment efficiency corresponding to i and j Pollution load factor from the waste sourses Rijdischarged into lagoon depends on the type of pollution sources, pollutants, terrain slope, rainfall, distance from the pollution source to the basin and some other mitigation process. However the survey and calculation has been quite costly and complex, we have used the pollution load factor table of Ha Long Bay -Bai Tu Long in Quang Ninh provincedone by JICA, 1998 (table 6) to estimate the amount of pollutant load from coastal sources discharged into the Thi Nai Lagoon. Because theterrain of coastal areas of Thi Nai Lagoon similar to the terrain of Quang Ninh province (Midland-hill and coastal plain areas), theaverage value of thefactorhas been selected corresponding to each substance for each source.

III. RESULTAND DISCUSSION 1. Currentpollutant load generated in the region and prediction to 2025
These sectors which generate waste in the province include tourism -resident, aquaculture, agriculture (mainly cattle and poultry farming), industry and soil leaching. Pollution load from each source has been calculated in detail below.

Pollution load from living source
The total population in Thi Nai Lagoon is 172,678 people (in 2013), which occupies for about 11% of the population in Binh Dinh province. Meanwhile the population in the North of Thi Nai lagoon is 20,319 people, in the southwest of lagoon is 147,861 people, in the southeast of lagoon is 3,698 people and in the Con Chim area is 800 people (Le Thi Vinh, 2011).
In the overall planning of social -economic development of Binh Dinh province by 2020, the rate of natural population growth aimed to reduce 0.

Pollution load from industrial source
Binh Dinh industry has been differentiated into three distinct subareas since 2000: industrial subarea in Con river downstream, coastal subarea and mountains subarea. The researched area is fully located in Con River downstream subarea where have resources of alluvial deposit, building stone and agriculture, forestry and fisheries. There are 49% of the production facilities, 70.4% industrial employment and 91.5% of the province's industrial production in this subarea. Quy Nhon is the largest industrial center with 67.7% of the province's industrial production (Geography of the provinces and cities in Vietnam, Volume 4).
In 2012, the amount of main industrial products in the area was estimated about 11.9 thousand tons of frozen seafood, 3.3 thousand tons of vegetable oil, more than 27. Expected growth rate of industrial production value of the region has been equal to national industrial growth rate, about 12-13%/year to 2020 and the period from 2021 to 2030 to reach about 11-12%/year. Corresponding to the growth rate of industrial production value in each period, the forecasted industrial waste volume generated in the research area until 2025. According to the master planning after 2020, the industrial parks area in research area will reach about 1766 ha (including Phu Tai, Long My industrial zones and industrial area in Nhon Hoi economic zone) and the total area of the industrial parks which operate in future will be 944ha.  If the livestock partition structure continuously remained at present, the number of livestock and poultry in the researched area to the year 2020 will be nearly 29 thousand of buffalo, cows and nearly 77 thousand of pigs and more than 1 million of birds. According to Binh Dinh Agriculture development plan period 2016 -2020, the growth speed of farming is 5%/year. If this speed has been preserving to 2025, the total amount of cattle and poultry will be 36.8 thousand of buffalo, cows, more than 97 thousand of pigs and about 1.4 million of poultries.    Inconsiderable  Inconsiderable  -6 NH4 0,9 1,2 33,33 7 PO4 1,5 2,0 33,33 The result in the above table shows that the pollution load from aquaculture in Thi Nai lagoon increased 29 -33% in comparison with present due to the limitation of farming area and without untrolled farming.

Pollution load from soil leaching
Con river and Ha Thanh river which originates from Thi Nai lagoon flows into the sea through Quy Nhon watergate . Con River is the largest river in the province, more than 178 km in the length with total basin area approximately 3,067 km 2 and flow velocity about 58.84 m 3 /s. The length of Ha Thanh river approximately 58km with total basin area about 580 km 2 and flow velocity about 13.6m 3 /s. Both rivers originate from the high mountains, incline from east to west. In the rainy season, flooding and leaching severely happen, whereas depletion in the dry season. The flow velocity in the flood season and dry season is different more than 1,000 times. (Geographic in provinces and cities in Vietnam, Volume 4).
There are two rainy seasons in Binh Dinh province: the abundant rainy season lasts from September to December and the little rainy season lasts from January at the August. There are 03 rainy areas here. The researched area is the little rainy area with precipitation less than 1,800mm located near the middle of the province stretching from coast to the western border which includes the majority of Quy Nhon city, TuyPhuoc, AnNhon, Phu Cat, De Gi district, narrow stripin the north of Tay Son district and half of the south of Vinh Thanh district. The average rainy day per month is highest in October and November and lowest in February and March. The average rainy day per year from79 days(Vinh Kim station inVinh Thanh district) to124 days (Quy Nhon station), the average rainy day per year about 100 days in whole province. Add up specialized land and living land into residential land, forestry land considered belong to forest and grasslands, the remaining is vacant land. The documents related to land use planning in the region that we have collected include Binh Dinh province and Tuy Phuoc district land use planning to 2020. The land use planning to 2020 of 4 other districts and Quy Nhon city have not been implemented. Therefore, except Tuy Phuoc district, the land use structure of the remaining districts and city to 2020 and 2025 will be followed to the average rate of increase and decrease of each type of land use in Binh Dinh province.
Currently, unused land occupies15.2% of the province land, in which mountain mainly occupies 12.7% , unused flat land occupied1%, rocky mountain without plant occupies 0.7%. It is expected that unused land about1. 19% of the total area of Binh Dinh province in 2020.Thus, the land use structure hardly big changed in the next period.Agricultural and unused land may be slightly decreased due to changing use purpose for specialized land and building land.The forest land is hardly increased. Basically, in 2025,the land use structure in Binh Dinh province will not be major changed in comparision with 2020.

Total pollution load in Binh Dinh province
Pollution load in whole province calculated by summarizing all discharged sources (Table 15 and  Table 16). The data in Table 15 indicate that annual pollution load in the researched area approximate 30.7 thousand tons of COD; 17.1 thousand tons of BOD; 9.1 thousand tons of N-T, 2.4 thousand tons of P-T and 289 thousand tons of TSS. Among the discharge sources from human activities, the discharge sources from livestock has been the main pollution load discharge into the Thi Nai lagoon, followed by the discharge source from industrial and living activities, the discharge source from aquaculture has taken very little waste. It is expected that in 2025, total pollution load in the researched area occupies about 76.2 thousand tons of COD, more than 37.7 thousand tons of BOD; 14.7 thousand tons of N-T (in which about 128.32 tons of NO3 + , NO2and more than 4 thousand tons of NH4 + ); more than 3.6 thousand tons of P-T (in which about 1.3 thousand tons of PO4 3-) and more than 206 thousand tons of TSS (Table 16).
The comparison between discharge sources by 2025 has showed that the pollution load from industrial has been the largest, followed by the pollution load from livestock, living and soil leaching. The pollution load from aquaculture has been the smallest in comparison with others.
Compared to the pollution load in 2012, the pollution load in 2025 will be increased 1.4 times to 2.6 times, in which the concentration of ammonium will be increased highest (2.6 times) due to the increase of waste from the industrial and livestock activities. In particular, the concentration of TSS will be decreased by 28% compared to present due to the main source of TSS has been still soil leaching. Because of land use change in 2025, cultivated soil area should reduce, the concentration of TSS from soil leaching will be decreased.

Pollution load discharged into Thi Nai lagoon
Binh Dinh province has not really pay attention to the wastewater treatment issue (the percentage of treated waste water is low: about 20% of industrial waste water, 10% of livestock waste water). Additionally, the sewerage system has not been completed (only about 10% of the households installed sewage system). Besides, the terrain sloped from west to east, so the pollution load from soil leaching into the lagoon has been high (from 51% -90% of all discharge sources, excluding BOD only about 18%). Thus, the annual pollutants in Thi Nai lagoon as follow: About 17.8 thousand tons of COD; 3.8 thousand tons of BOD; more than 6.4 thousand tons of NT (including 36 tons of NO2and NO3and more than 01 thousand tons of NH4 + ); 2,016 tons of P-T (nearly 515 tons of PO4 3-) and nearly 140 thousand tons of suspended solids, Table 17.
The comparison between the pollution load from discharge sources has showed that: Among the pollution load discharged into the Thi Nai lagoon, BOD occupies for 21.79%, COD occupies for 57.77%, N-T and P-T occupies from 68.50 to 86.19% and TSS occupies for 47.83%.
As for organic waste, pollution load from industrial sources has been major (about 36 -49%). The main products of industry which generate organic waste including frozen shrimp, rice milling, beer. Besides, there has much organic waste from livestock sources, occupies for 26% -29%, followed by the pollution load from living source (11-18%) and soil leaching (12-15%).
As for N and P compounds, pollution load from soil leaching has been major (about 34 -39%), followed by the pollution load from industrial and living source. The pollution load from aquaculture has been minor source. However, the source should be concerned because it is direct source. The comparison between the pollution load from discharge sources into Thi Nai lagoon shows that: As for organic pollution load, the industrial sources is major, followed by living, farming and soil leaching. As for nutrients, the farming source is major, followed by industrial, living and soil leaching source. As for TSS, the soil leaching source is major, followed by living source, other sources are minor.
In general, in comparison with the pollution load generation, pollution load discharged into the Thi Nai lagoon occupies about 6 -23% for organic compounds, from 40-70% for the nutrients, and about 31.2% for TSS. It is remarkable that phosphate occupies 67.12%. This shows that the necessity of improving the phosphate treatment efficiency.

IV. CONCLUSION
The annual pollution load in Binh Dinh province approximate 30.7 thousand tons of COD; 17.1 thousand tons of BOD; 9.1 thousand tons of N-T, 2.4 thousand tons of P-T and 289 thousand tons of TSS. Among the discharge sources from human activities, the discharge sources from farming has been the main pollution load discharge into the Thi Nai lagoon, followed by the discharge source from industrial and living activities, the discharge source from aquaculture has taken very little waste. It is expected that in 2025, total pollution load in the researched area occupies about 76.2 thousand tons of COD, more than 37.7 thousand tons of BOD; 14.7 thousand tons of N-T (in which about 128.32 tons of NO3 + , NO2and more than 4 thousand tons of NH4 + ); more than 3.6 thousand tons of P-T (in which about 1.3 thousand tons of PO4 3-) and more than 206 thousand tons of TSS.
The annual pollutants discharged into Thi Nai lagoon as follow: About 17.8 thousand tons of COD; 3.8 thousand tons of BOD; more than 6.4 thousand tons of NT (including 36 tons of NO2and NO3and more than 01 thousand tons of NH4 + ); 2,016 tons of P-T (nearly 515 tons of PO4 3-) and nearly 140 thousand tons of suspended solids. Prediction results in 2025 shows that the pollution load which discharged into Thi Nai lagoon about 17.8 thousand tons of COD; 2.4 thousand tons of BOD; 6.7 thousand tons of NT (including about 52.2 tons of soluble nitrogen NO2and NO3and nearly 1.7 thousand tons of NH4 + ); 2.55 thousand ton of P-T (about 867 tons of PO4 3-) and 64.5 thousand tons of suspended solid.

V. ACKNOWLEDGEMENTS
This article written on the basis of documents entitled "Assessing the environmental threshold of some typical coastal basins in Vietnam for sustainable development" coded KC.09.17/11-15 of the Institute of Marine Environment and Resources.