HEAVY METAL CONCENTRATIONS IN SURFACE WATER OF THE LOWER MEKONG RIVER BASIN (VIETNAM)

Purpose: The research is carried out to determine the concentration of heavy metals in surface water of the downstream Mekong River. Methods: The sampling method follows the Vietnamese national standard (TCVN 6663-6:2018, ISO 5667-6:2014). The heavy metal indicators analyzed using an atomic absorption spectroscopy machine. The lowest detection level of the measurement method is approximately 0.2µg (on average for all indicators measured). Results and discussion: Among the 5 parameters studied in December 2023, the average concentration has not exceeded the national standards of Vietnam (the maximum allowable values for parameters affecting human health), but it is approaching the limit (especially Zn at 454.69 µg/l compared to 500 µg/l). At many locations, the levels of Zn and Cu have exceeded the standards (at Cửa Đại, Ba Lai, Cung Hầu). Compared to our research conducted in 2013, except for Cd, the average concentrations of all other parameters are increasing. Compared to the standards of some countries such as the United States, Canada, and Russia, the levels of heavy metals in the research area are high, especially Zn and Cu which are much higher. Implications of the research: The heavy metal concentration in the study area increases towards the sea, correlating with the pH index and river-sea mixing. Human activities contribute to heavy metal accumulation in the lower Mekong region's water. Management agencies must develop policies and measures to minimize NCDs' impact and ensure sustainable development.


INTRODUCTION
The Mekong River is about 4,500 km long, with a basin area of about 810,000 km 2 , and a flow of 475 million m³/year.The river originates from high mountainous areas located on the Tibetan Plateau (China) at an altitude of nearly 5,000 m.The upstream section of the river flowing in the mountains and plateaus of Chinese territory is called the Lang Thuong River with a northwest -southeast direction, reaching the upper plateau of Laos, the new river has the official name of the Mekong River.The river flows through Laos, Myanmar, Thailand, Cambodia with many large cities along the river such as Luang Prabang, Vientiane, Pakse, Phnom Penh.In Vietnam, the Mekong River is called Cuu Long River.The Mekong River flows into two parallel branches: the Tien River and the Hau River, about 230 km long from the Vietnam -Cambodia border through many rice fields, orchards, and major Vietnamese cities such as Chau Doc, Long Xuyen, Can Tho, Cao Lanh, Sa Dec, Vinh Long,... Before emptying into the East Sea, the river creates a large delta with 9 estuaries: Tien River: 6 estuaries (Tieu, Dai, Ba Lai, Ham Luong , Co Chien, Cung Hau), Hau river: 3 gates (Dinh An, Bat Sat, Tran De, of which Bat Sat gate today is muddied) (Phung Thai Duong, 2022).
Because it flows through many countries with countless tributaries, the Mekong flows through many agricultural, industrial and residential areas, so the downstream area and river mouth are places where pollutants and heavy metals of local origin accumulate from nature and people's socio-economic activities.Every year, the amount of heavy metals carried out by the water from the upstream and accumulated in the water in the estuary is relatively high, the possibility of infecting the ecosystem is very high.Recognizing the dangers that heavy metals bring to people in the region, many research projects have been conducted such as: Tereshchenko et al. (2023), S. Costa-Böddeker et al (2017), L.V. Muoi et al (2022), Cenci R. M (2004), Bui Thi Nga (2011), Savichev O.G (2013), Phung Thai Duong (2013), Phung Thai Duong (2016).However, the current data is old and needs to be researched and updated.
The Mekong Delta has a tropical monsoon climate with high rainfall, but the dry season is long and profound, so people often use surface water to pump into rice fields and irrigate orchards.Mekong River surface water is also used in aquaculture activities as well as serving people's daily activities.This not only helps heavy metals enter the ecological chain but also directly affects health and community life.Therefore, studying the heavy metal content existing in the environment, especially river surface water here, is extremely necessary.
With the analysis of 5 indicators in the main estuary and tributaries of the Mekong River, the article will focus on analyzing and evaluating the heavy metal content in water of the study area through comparison with Vietnamese standards and other countries.experts around the world as well as research results in other estuaries in Vietnam have recently been published.In addition, we also used R software to analyze the correlation between the pH geochemical index and each heavy metal indicator and between heavy metal indicators with each other.Hopefully the research results will serve as a basis for rational orientation in production activities and people's daily activities to ensure sustainable development in the Mekong Delta, an area heavily affected by Climate Change.

STUDY AREA
The lower Mekong River region, including: 2 branches of Tien River and Hau River; gates: Tieu, Dai, Ba Lai, Ham Luong, Co Chien, Cung Hau, Dinh An, Tran De; and tributaries flowing into the above estuaries, with sample numbers arranged as shown in Figure 1, Table 1.Sampling was carried out in December 2023.

SAMPLING TOOLS AND EQUIPMENT
Model 1120-G42 (Wildco-USA).The equipment is made of transparent acrylic material, with a polyurethane lid, so it does not affect the sample quality.6 Sampling methods: for main rivers, a boat is used for sampling (by lowering a rope), for tributaries, samples are taken using a pole.When sampling on a boat, we avoid contaminating the sample with stirred-up mud or debris from the boat, we turn off the boat engine, and samples are collected from the boat's bow or at a distance from the boat's hull to prevent sample contamination.

SAMPLING METHOD
The sampling method is carried out according to the Vietnamese national standard (TCVN 6663-6:2018, ISO 5667-6:2014).At each location, after determining the coordinates on the map, go to the site and take samples about 10-15m away from the bank (for tributaries) and about 30-50m away (for main rivers) to avoid contamination from dust and sand from the path.The sampling point should have well-mixed and flowing water, meaning it should not be a whirlpool or stagnant water where the flow is not representative of the main water body.For tributaries, choose two sampling points at the upper source and near the river mouth (where it flows into the main river).
To ensure accuracy, the samples are taken from a depth of about 30cm below the surface to avoid interference from sunlight and floating organisms.At each location, three large samples are taken (each large sample consists of three small samples taken 5m apart and mixed together to form a representative sample).The large samples are spaced 20m apart around the initial sampling position.The sampling is done according to the isosceles triangle rule.The representative sample of the site is taken after blending the three large samples.The equipment is thoroughly cleaned before taking new samples.

THE PRESERVATION METHOD
The preservation method of the sample is carried out according to the national standard of Vietnam (TCVN 6663-3:2016, ISO 5667-3:2012).
The representative sample is preserved in a 0.5-liter glass bottle, avoiding exposure to light.The outside of the bottle is covered with waterproof paper, containing complete information about the sampling location.

THE METHOD FOR ANALYZING INDICATORS
The heavy metal indicators are analyzed using an atomic absorption spectroscopy machine (Model: ZEEnit 700, Manufactured by Analytik-Jena -Germany), along with an instruction manual and WinAAS Ver program that comes with the machine.The lowest detection level of the measurement method is approximately 0.2µg (on average for all indicators measured).

RESULTS
With a total of 40 samples collected, preserved, and analyzed in the end of 2023.Results of surface water analysis in the Mekong estuary show that the highest average content is Zn, followed by Cu, Pb, As and Cd respectively.From the analysis results, the following observations can be made: • Zinc concentration: average 454.69 µg/l (main river 549.62 µg/l, branch river 233.20 µg/l); the main river fluctuates from 317.65 µg/l to 1000.23 µg/l; the branch river fluctuates from 109.57µg/l to 600.38 µg/l.
• Arsenic concentration: average 0.34 µg/l (main river 0.32 µg/l, branch river 0.39 µg/l); the main river fluctuates from not detected to 0.61 µg/l; the branch river fluctuates from not detected to 0.91 µg/l.
• Cadmium concentration: average 0.27 µg/l (main river 0.34 µg/l, branch river 0.12 µg/l); the main river fluctuates from not detected to 1.27 µg/l; the branch river fluctuates from not detected to 0.21 µg/l.In general, the average concentration and the range of values fluctuate more in the main river than in the branch river.

COMPARE RESEARCH RESULTS WITH OTHER RIVERS IN VIETNAM, VIETNAMESE STANDARDS AND SOME COUNTRIES AROUND THE WORLD
Compared to the research results we conducted in 2013, it can be observed that, except for Cd which tends to decrease (from 0.37 µg/l to 0.27 µg/l), all other indicators tend to increase: Zn increases from 344.60 µg/l to 454.69 µg/l; Cu increases from 69.10 µg/l to 69.20 µg/l; Pb increases from 0.02 µg/l to 1.32 µg/l; As increases from 0.20 µg/l to 0.34 µg/l.
Therefore, the levels of Zn and Pb have increased significantly compared to 10 years ago.This indicates that the production and activities of both upstream sources and the local population contribute to the worsening heavy metal pollution downstream.This poses a threat to sustainable development in the area.
Recently, very few studies have been published on heavy metal content in river surface water in Vietnam.However, there are also two projects in Hanoi capital: Nhue River and Red River.Compared to the Red River, the average content of two indicators has exceeded many times: Zn (454.69 µg/l compared to 17.30 µg/l), Cu (69.10 µg/l compared to 34.38 µg /l).
Compared to Nhue River, the Zn content in the study area is also larger (454.69µg/l compared to 254.00 µg/l).It should be noted that compared to Hanoi, the lower Mekong River region is not the most developed industrial and urban area in Vietnam, but such content is still very high, so it is necessary to research and have strategies to Limit surface water pollution, limit the risk of entering the ecological food chain, and affecting the health of people in the area.
According to the National technical regulation on Surface water quality on regulations on maximum limit values of parameters affecting human health in Vietnam (QCVN 08: 2023/BTNMT), the average content has not exceeded the regulations, However, the Zn and Cu targets at Cua Dai, Ba Lai and Cung Hau have exceeded the limit.Compared to the standards of some countries in the world such as the United States for reference values of surface water toxicity, Canada for surface water quality to protect aquatic life, and the Russian Federation for protecting aquatic nature, the content of Heavy metals in the study area are at high levels, especially Zn and Cu, which are many times higher.

CORRELATION OF PH GEOCHEMICAL INDEX WITH EACH INDICATOR AND BETWEEN INDICATORS
Low pH is acidic, high pH is alkaline, freshwater pH with higher acidity therefore lower value.Here it is easy to see that the pH value gradually increases as it moves towards the sea.
Similar to the content of heavy metals, high pH, the accumulation of heavy metals in water also tends to increase clearly.This is easy to see through the relatively high correlation values between pH and other indicators, from 0.13 for As to 0.37 for Zn and Pb.Samples collected from tributaries (samples 29-40) have an average concentration lower than the main river, but the concentration is also relatively high.This proves that besides being brought out from upstream, the production activities of local people also contribute significantly to the accumulation and pollution of heavy metal content in the water here.
pH is an important indicator of interactions in the synthetic system "environment".The increase in pH increases the saturation level of water leading to precipitation and adsorption of minerals including heavy metals.Thus, the index of change in hydrochemical conditions in the delta is the pH, the values of which can be used for an approximate estimate of the actual forecast and trace elements in surface water.along the length of the Mekong River during the year.In addition, the correlation between indicators in the study area is very high, which is also the basis for reducing research costs, thereby helping to plan agricultural activities, manage water and ensure safety.ensure sustainable development in the delta.
The heavy metal content of most indicators tends to increase gradually towards the sea.
This proves that the further downstream, in the estuary, the stronger the interaction between the river and the sea takes place, leading to the accumulation of alluvium and accompanying heavy metals originating from the interior.

CONCLUSION
Except for Cd, the average content of the remaining indicators Zn Cd, Pb, As all increased compared to previous studies.Although on average it does not exceed Vietnamese standards, if considered individually, in many locations it has exceeded the regulations of Vietnam, the United States, Canada, the Russian Federation as well as some other regions in Vietnamese territory.
The heavy metal content in the study area is relatively high and tends to increase gradually as it moves towards the sea, from the tributary river to the main river, partly explained by the close correlation with the pH index, the impact, Mixing between rivers and seas causes more heavy metals in the water to accumulate.Besides from the upstream side, the daily life and production activities of people in the region also make an important contribution to the formation and accumulation of heavy metals in the water of the lower Mekong region.Faced with that situation, it is necessary for management agencies to develop policies and measures to minimize the impact of NCDs on human life and ensure sustainable development here.
The sample is preserved by adding 1ml of 65% concentration HNO3 solution from Merck (Germany).The sample is kept in a refrigerated container during transportation and in a specialized LKPv 1423 cabinet (Liebherr) at a temperature of around 5 0 C. Heavy Metal Concentrations in Surface Water of the Lower Mekong River Basin (Vietnam) ___________________________________________________________________________ Rev. Gest.Soc.Ambient.| Miami | v.18.n.2 | p.1-17 | e06829 | 2024.7 3.4 PREPARING THE ANALYSIS SAMPLES For the water samples, they will be filtered through a filter paper (0.4 μm) and measured at a volume of 20 ml.However, for relatively turbid water samples, we use a microwave sample digestion device before measurement (Model: MW 680 _ Manufactured: Aurora -Canada) with the instruction manual and the implementation steps provided in the TRANSFORM680 program accompanying the machine.

Figure 2
Figure 2 Correlation of pH geochemical index with each indicator and between indicators in surface water in the lower Mekong River basin