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Assessment of the current geo-environmental status in Duyen Hai, Tra Vinh province during the affor

Updated: Jan 3, 2023


TAbstract — Mangroves play important roles in: (1) providing livelihood for people; (2) protecting and preventing natural disaster; (3) reducing erosion and protecting land; (4) reducing pollution; (5) reducing the impact of climate change; (6) providing food and habitat for many animals. Tra Vinh mangroves also contribute significantly to the national mangrove system. In the economic development process, it has been destroyed heavily. In 1975, the area of Duyen Hai mangrove was over 19,000 ha. However, according to Tra Vinh Provincial FDP, the whole mangrove area was only 12,400 ha in 1980. It continued to go down to 5,924 ha in 1990 and 5,429 ha in 1992 (Tra Vinh People’s Committee, 2010). At present, there are over 8,622 ha of mangroves and to be expected to cover 9,000 ha in 2020 by ODA (VNA, 2016). Therefore, the research team conducted a fieldtrip to assess the current geoenvironment in Duyen Hai area, Tra Vinh province. The result from 15 samples show that: for surface water, the pH value from 6.39 to 8.11, all samples meets VN standard (QCVN 08 – MT:2015/BTNMT), the salinity varies from 26 to 2 ‰, the Eh value ranging from -2 mV to -91 mV, slightly redox potential; for soil samples, all the pH values of sediment are rather low, they vary from 3.7 to 7.2, the total organic carbon (TOC), total nitrogen (TN) and total phosphorus vary widely (0.5-2.41%, 0.015-0.17% and 0.04-0.07% respectively). Moreover, soil erosion was also observed in the afforestation area.





1. INTRODUCTION OF THE CHARACTERISTICS OF DUYEN HAI


The survey area is the coastal area of Duyen Hai district and Duyen Hai town, Tra Vinh province (Fig. 1). It borders on Cau Ngang and Tra Cu districts of the province in the Northwest, on Ben Tre province in the Northeast by Co Chien river, on the South China Sea in the Southeast and South with 65-kilometer coastline, on Soc Trang province by Hau River in the Southwest.


The factors of climate, temperature, light, evaporation and rainfall are quite distinguishing between the rainy season (from May to November) and the dry season (from November to April next year), including 90% of the rainfall concentrated in the rainy season. The nearer the sea, the shorter raining period. Duyen Hai district has the lowest rainfall in the province. In the dry season, the saltwater intrusion is very complicated.


It is an area with low terrain surface with frequent tidal flooding, especially in My Long Nam, which is affected by hydrological regime of Mekong River, South China Sea tide, in-field rain, irrigation and salt control works. In the flood season, the water level on the Mekong River increases rapidly, the flood flows downstream in combination with flood- and high tides. If no dyke is well-established, many areas in the province will be submerged 0.3-0.5m below the mean tide level. Changes in tidal regime, tidal flat elevation, and geochemical characteristics of the tidal flat environment have a direct influence on the distribution of mangrove species, and at the same time, affect the organic origin in the sediments of mangrove forests and the level of biodiversity in the region.


One of the socio-economic development orientations of the province is the protection, afforestation and restoration of the key coastal protection forest belt. The whole Tra Vinh province has 8,975 hectares, including up to 98% (8,770 hectares) of mangrove forest in the coastal area, mainly distributed in the coastal fringes on the salt marshes formed between river sediments and the influence of tides. Therefore, the environment in Duyen Hai is significantly influenced by mangrove forests. However, the ecosystem here is still quite rich and diverse with many species with high economic value and characteristic of the mangrove ecosystem [5]. Therefore, mangrove forests are able to maintain and develop aquatic resources, play a crucial role in carbon storage and greenhouse gas absorption.




The article aims to assess the current geo-environmental status by measuring and analyzing the basic parameters of surface water and the content of total organic carbon, total nitrogen and total phosphorus in the sediments.


2. FIELD VISIT


The field visit and sampling were carried out in September 2019.

2.1 Methodology


It is focused on surveying the distribution of ecosystems in the area such as typical plants and animals of mangrove forest; human economic activities such as crops and aquaculture.


During the field visit, 6 sites subject to the survey included the re-planting area of My Long Nam (ML1, ML2, ML3, ML4, ML5, ML6 and ML7), coastal Ba Dong (BD1), Cai Tau bridge (CT1 and CT2), exhibition house of mangrove ecosystem (NB1 and NB2), Long Toan bridge (LT1) and Lang Sat ferry terminal area (LS1 and LS2) (Fig. 1). The total point of survey, measurement and sampling was 15. At each survey point, a sediment sample of about 1 kilogram in weight was taken and placed in a PE zipper bag to measure its pH in the field. For surface water samples, parameters such as pH, Eh, salinity and temperature were measured on site.


Methods of chemical analysis of sediment samples: samples were sent to Quality Assurance and Testing Center 3 (QUATEST 3) for analysis of the following criteria:

- Total organic carbon content: under TCVN 4050:1985

- Total nitrogen content: under TCVN 4051:1985

- Total phosphorus: under TCVN 4052:1985 (UV-VIS)

2.2 Used devices


Takemura DM-15 Soil pH - Moisture Meter was used to measure soil pH, the soil moisture measurement function is not used as the soil moisture in the survey area exceeds the measurement limit of the device (up to 80%).

The Mettler-Toledo S2-Standard Kit handheld pH meter was used to measure the temperature, pH and Eh of surface water.


Bellingham+Stanley Saline 28 (38-28) electronic refract meter was used to measure the salinity of water under the content (%) of NaCl.


In addition, GPS devices and unmanned aerial vehicles (drone) were used to determine coordinates, record the entire survey area and take samples for further studies.


3. RESULTS


Based on the field visit, analysis of data measured and derived from the analysis results of total organic carbon, total nitrogen and total phosphorus, the environmental status in Duyen Hai coastal area, Tra Vinh province has the following characteristics:


3.1 Surface water environment

For surface water, the measured parameters are pH, Eh, salinity and temperature. Measurement results are presented in Table 1 and Fig. 2, 3, and 4.


TABLE 1 : MEASUREMENT RESULTS OF SURFACE WATER SAMPLE


The pH value is the basic parameter to evaluate the water environment. In Fig. 2, surface water samples were compared with QCVN 08-MT:2015/BTNMT and coastal seawater samples (BD1) were compared with QCVN 10-MT:2015/BTNMT, both of which meet the allowable limit. The pH limit for normal growth of organisms is 6.5-8.5, so water samples remain favorable for the growth of organisms.


Fig. 2: Graph of pH of surface water


The Eh value plays a crucial role in studying the existence of a substance in the natural environment. The measured Eh ranged from -2mV to -91Mv, which exists in the reducing environment. However, the reduction is more evident in coastal Ba Dong (BD1), exhibition house of mangrove ecosystem (NB1 and NB2) and Cai Tau Bridge (CT2) (Fig. 3).

Fig. 3: Graph of Eh of surface water


Salinity is an important factor for the distribution of mangrove species. The salinity of the aquatic environment was determined based on the percentage of NaCl (Fig. 4). Salinity ranges from 0.2-2.8%. However, most of the samples have the value in the range of 0.2-0.8 (11/15 samples) while other samples with the high values are mainly distributed in Cai Tau bridge (CT1 and CT2) and exhibition house of mangrove ecosystem (NB2), and coastal Ba Dong (BD1) has the highest salinity. Areas with low salinity (<1.0%) are concentrated at Lang Sat ferry terminal and mangrove forest in My Long Nam. At the time of the survey, it was the rainy season, showing the influence of flow from the Hau and Co Chien Rivers.


Fig. 4: Graph of salinity of surface water


In general, the surface water quality remains within the allowable and safe limits for the growth of organisms.


3.2 Sedimentary environment

The results of measurement and chemical analysis of sediment samples are listed in Table 2 and Fig. 5, 6, 7, 8, and 9.


For the sedimentary environment, some of the main causes potentially degrading soil quality include agricultural production; aquaculture; coastal sedimentation-erosion; changes in mineral composition; accumulated elements in the marsh environment and estuarine sediments.

TABLE 2 : ANALYSIS RESULTS OF SEDIMENT SAMPLES

The change in sediment quality is shown through pH as a basic parameter (Fig. 5). The pH value of the sediment changes insignificantly, mainly in the range of 3.7-7.4 or at 5.3 on average. Particularly, the sediment samples at the replanted mangrove forests in My Long Nam and Cai Tau bridge have low pH (pH=3-5) and are of acidic soils.


Fig. 5: Graph of pH of the sediment



Fig. 6: Re-planted forest in May 2018


In My Long Nam, it is a re-planted mangrove forest under the program “Green trees for the green planet” implemented in May 2018 (Fig. 6). In other locations, the pH value of the sediment is higher but only in the range of 6.5-7.2.


The total organic carbon (TOC) content in the sediments ranges from 0.5-2.41% or at 1.43% on average. TOC content is highest in the mangrove area (ML1) in My Long Nam and lowest in coastal Ba Dong (BD1). However, the range is neither too wide within each survey cluster (Fig. 7).

Fig. 7: Graph of total organic carbon in sediments


The assessment of sediment quality may be based on the organic content in the sediment. However, currently, Vietnam has not set out a standard on these limit values, so in the article, they are compared with those stated in the documents published by other countries. According to Hyland et al. (2000), the TOC content in the range of 0.05-3% will have no adverse influence on the abundance as well as the biomass of benthic organisms. Thus, the sediments with TOC content ranging from 0.5-2.41% in the study area are suitable, so no negative impact on local benthic organisms was found.


Total nitrogen (TN) content ranges from 0.015 to 0.17% or at 0.12% on average. No remarkable difference exists between locations, including 14/15 samples with medium TN content, except for BD1 at poor level (TN<0.08%) (Fig. 8).


Fig. 8: Graph of total nitrogen in sediments


Total phosphorus (TP) content ranges from 0.04 to 0.07% or at 0.06% on average. All samples have no significant difference and are at poor level (Fig. 9).

Fig. 9: Graph of total phosphorus in sediments

Regarding the ratio of nutrients, the C/N ratio varies in the range of 8.79-38.89 while the N/P ratio ranges from 0.83-7.20. indicating the considerable amount of the sediment containing inorganic phosphorus content (Table 2). The mangrove plants contain high cellulose and low nitrogen contents while phytoplankton tends to have rich organic nitrogen and poor cellulose, so the C/N ratio in terrigenous sediments tends to be low. Therefore, the ratio of nutrients in the Duyen Hai coastal study area shows that most of the sediments are terrigenous.

The analysis results show that the sediments in Duyen Hai coastal are acidic to weakly alkaline, the total organic carbon and total nitrogen contents are medium while total phosphorus content is poor. The study on chemical indicators contributes to determining the ecological function of coastal mangrove forests. The characteristics of the deposition of sediments and organic matters depend on the natural characteristics of the mangrove areas. The areas with frequent tidal flooding have more organic matter in sediments from elsewhere than the areas less directly affected by tides.

3.3 Risk of landslide


In My Long Nam, the shoreline is mainly composed of powder clay, containing organic humus, having weak cohesion and at risk of erosion. Fig. 9 shows that the monitoring pile rows have been partially destroyed, some mangroves planted since August 2018 with roots that have not yet penetrated deeply into soil have been washed out, and soft dikes (sandbags) have been destroyed and washed away. However, erosion is only at a mild and local level.



Fig. 10: Landslide destroyed some monitoring piles


In the future, the re-planted mangrove species with good growth are highly potential to protect the shoreline and local organisms.


4. CONCLUSION


Overall, the environmental status in Duyen Hai, Tra Vinh province shows no sign of severe degradation.


For surface water, brackish is dominant, characteristic of pH values of grade-A water limit and entirely anaerobic.


For the soil environment, the sediments in Duyen Hai coastal are acidic to weakly alkaline, the total organic carbon and total nitrogen contents are medium while total phosphorus content is poor. The quality of the surface sedimentary environment in the study area is quite good. In this regard, the organic carbon content is suitable for benthic life, and the N and P contents are not high, which have no adverse influence on aquatic life.


The mangrove area in Nam My Long suffers from regular tidal flooding and is more affected by flow, so organic matter from mangrove plants is highly potential to be washed away. However, this area is receiving the significant amount of suspended matters and phytoplankton from the estuary, shown by its C/N and N/P ratios.

The parameters presented in this article are fundamental for the surface water and sedimentary environments for the purpose of sharing information and serving as the basis for the analysis and evaluation of the subsequent results of the study topic so as to evaluate in more detail and specifically the impacts of reforestation on the environment.


REFERENCES


[1] South Vietnam Geological Mapping Division (2019). Report on “Investigation and assessment of the current environmental status of coastal areas, assessment of the sensitivity and resilience of coastal areas and estuaries to natural hazards and environmental incidents in service of socio-economic development orientation in the coast of Tra Vinh province” project

[2] Ministry of Transport (2018). Environmental impact assessment report on “Dredging and Maintenance of Navigational Channels for large tonnage ships entering the Hau River in 2018” project.

[3] National Technical Regulation on surface water quality, QCVN 08-MT:2015/BTNMT, 21-12-2015.

[4] National technical regulation on sea water quality, QCVN 10-MT:2015/BTNMT, 21-12-2015.

[5] Report on environmental status of Tra Vinh province for 5-year period (2011-2015)

[6] Hyland, J., Karakassis, I., Magni, P., Petrov, A., and Shine, J., 2000. Summary report: Results of initial planning meeting of the UNESCO.


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Dong Uyen Thanh: Born in 1978 in Soc Trang province; graduated from Environmental Geology at Ho Chi Minh City University of Technology in 2002 and having Master’s degree in Geotechnical Engineering from Ho Chi Minh City University of Technology in 2005.


From 2005 to now, lecturer in Faculty of Geotechnical Engineering & Petroleum, Ho Chi Minh City University of Technology; expertise: Mineralogy, petrology and environmental geology. Having conducted and participated in projects and articles on petrographic and mineralogical characteristics of igneous rocks, underground water quality, etc. Currently. a member of the SALINPROVE project at UNESCO-IHE Institute, the Netherlands.


Dang Thuong Huyen: Born in 1982 in Thanh Hoa. Graduated with a degree in PhD in Environmental and Urban Engineering, Kyushu University, Japan in 2010.

From 2010 to now, lecturer in Faculty of Geotechnical Engineering & Petroleum, Ho Chi Minh City University of Technology; expertise: environmental geochemistry, groundwater modeling, soil and water pollution and measures for treatment and carbon storage in mangrove forests.


Bui Trong Vinh: Born in 1977 in Hai Duong. Graduated with a degree of PhD in Coastal Engineering, Osaka University, Japan in 2009; currently, lecturer in Faculty of Geotechnical Engineering & Petroleum, Ho Chi Minh City University of Technology; expertise: environmental geology, coastal environmental protection, hazard and natural disaster prevention.


Le Thanh Phong: Born in 1975 in Long An province. Graduated from Geotechnical Engineeringat Ho Chi Minh City University of Technology in 1999 and having Master’s degree in Civil Engineering on soft ground from Ho Chi Minh City University of Technology since 2004.


From 2004 to now, lecturer in Faculty of Geotechnical Engineering & Petroleum, Ho Chi Minh City University of Technology; expertise: geological hazards and engineering geology; having participated in projects and articles on slope stabilization and riverbank erosion, etc.

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