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Water quality and the impacts of megacities on aquatic ecosystems

Water quality in South East Asian Countries and in Vietnam

It took hundreds of thousands of years for the world population to grow to 1 billion – then in just another 200 years or so, it grew sevenfold. In 2011, the global population reached 7 billion, and today, it stands at about 7.6 billion (data from 2007 was the first year in which more people lived in urban areas than in rural areas, and by 2050 about 66 % of the world population will be living in cities. This urbanization unfold in Africa and Asia (Figure 1-1), bringing huge social, economic and environmental transformations. Among all consequences of urbanization (health care, education, housing, sanitation, food, energy and water), the present work focuses on the water-related issues, and more especially on the deterioration of water quality and its impact on aquatic ecosystems.
The study is conducted in a megacity of South East Asia (Ho Chi Minh City, Vietnam), of the ASEAN region (Association of Southeast Asian Nations, Figure 1-2). This region comprises ten Southeast Asian countries that share common water related challenges and promote cooperation and facilitates economic, environmental, educational actions to fight against water pollution.
Among the most serious problems of water quality in ASEAN figure: lack of wastewater treatment system and particles released through soil leaching; domestic wastes discharged directly to river body; water pollution from small-scale industries and agricultural activities or even groundwater contaminated with coliforms (sources: Water Environment Partnership in Asia –, Alexandra et al., 2012; Stephen and Liz, 2009). The megacity of Ho Chi Minh city (Vietnam) gathers all these water-related issues and thus provides an interesting study case to evaluate the impact of megacities on the water quality and aquatic ecosystems, to anticipate scenario of evolution in the megacities of the ASEAN and to propose ways of mitigation. Some emblematic water quality issues in ASEAN are synthesises in Table 1-1.
Poor water quality; lakes and canals becoming sewage sinks; pockets of contamination and some salinity intrusion in groundwater; rapid urbanization and industrialization and transport development, expansion, and an increase in the number of oil spills contributing to the deterioration of water quality.

The megacities and their impacts on the environment in Vietnam, with a focus on HCMC

Characteristics of megacities

Csomos (2014) classified megacities based on two parameters: (i) the specific geographic regions (population distribution) and (ii) the economic performance (income per capita). The population growth is due to the positive birth/death ratio, the rural-to-urban and urban-to-suburb migration. Population growth and high population densities are driving the municipal sprawl. In ASEAN, radical spatial, demographic, social and political structural changes in urban areas took place over the last decades, associated with the economic rise of the whole region (Kraas, 2007). Megacities will get bigger in the coming years, as the population continues to grow and people continue to abandon rural areas. Some Southeast Asian cities are already some of the most populated metropolises in the world (Figure 1-1), with millions of inhabitants like Manila (Philippines, 11.9 million inhab.), Jakarta (Indonesia, 30.2 million inhab.), Bangkok (Thailand, 14.5 million inhab.), Kuala Lumpur (Malaysia, 6.6 million inhab.), Singapore (5.8 million inhab.), Yangon (Myanmar, 4.3 million inhab.) and Ho Chi Minh City (Vietnam, 8.4 million inhab.).
In Vietnam, annually, 100 000 hectares (or more) of agricultural land are recovered for construction of industrial parks, services, and transportations (Nguyen et al., 2017; Vo, 2007). In some rural areas, typically as in the Red River Delta and in the Mekong River, the population has migrated to big cities such as Hanoi and Ho Chi Minh City. This rural migration flow leads to a city expansion that witnessed very heavy overload on population, technical and social infrastructures (Decker et al., 2002). After reaching an urban proportion of the population of 10 % during the 1950’s, the speed of urbanization increased, and was around 21.5 % in the 1970’s. After the country was reunified, there was a decline in the urban proportion of the population throughout the country until 1982, when it had fallen to 18.4 %. Since then, the level of urbanization has increased gradually to overpass 35% in 2015 (Figure 1-3).
Aside from population size, megacitiesare very high consumers of materials and energy. Power generation and the energy consumption in megacities have enormous impacts on the environment that is unequally considered by policy makers. Developed and wealthy cities, such as Los Angeles, propose mitigation strategies and offer central water treatment plants and distribution facilities. They can support 100% of their population with fresh and clean water (Decker et al., 2002). This is generally not the case in ASEAN megacities.

The effects of megacities on aquatic ecosystems

Urban development patterns are closely linked to an increase in the consumption of freshwater and, therefore, in an increase in the water demand for many activities. Among them, the food consumption and different nutrition patterns in megacities are higher than in rural populations. The urban development leads an increase in the food demand and a decrease in the availability of arable lands (Phdungsilp, 2006; Hawkes et al., 2017; Popkin, 2001).
Urban expansion or municipal sprawl with the construction of urban, tourism and transportation facilities is closely linked to the decrease in arable lands, degradation of biodiversity, air and water quality (Penning de Vries et al., 2003; Zhao et al., 2006). Untreated domestic wastewaters, which are released directly into aquatic ecosystems, remain one of the most common sources of pollution. In the case of Vietnamese megacities, canals and small water bodies are generally becoming a sink for domestic and industrial wastewaters, especially in HCMC and Hanoi. According to the Viet
Nam Environment and Sustainable Development Institute, the megacities account for (only ~30 10
%) of the national population, but they generate more than 6 million tons of waste each year (nearly 50 % of the total amount of waste of the country) (Nguyen et al., 2017).

River systems in tropical area

Only 19 % of the land surface locate in tropical climate region; in which, 60 % in the South American, 31 % in Africa, 16.3 % in Asia, 8.3 % in Australia and 5.9 % in North America (Figure 1-4) (Peel et al., 2007). The most important control of the tropical climate relates to the position of the Inter Tropical Convergent Zone (ITCZ) (Syvitski et al., 2014). In tropical areas, there is a climate without strong winter since these regions receive a large amount of solar energy. The temperature is neither shift from day to night, nor from summer to winter. The tropical zones are characterized by the intense convective rainfall (Syvitski et al., 2014).
In tropical regions, such as in Vietnam, the main factor, which specifies the seasons, is rainfall (Figure 1-5). Therefore, the amount and temporal distribution of the precipitations are the important elements for disparity in climatic zones; for instance, wet (> 1800 mm), wet-dry (700-1800 mm), and dry (< 700 mm) (Latrubesse et al., 2005). All rivers in Vietnam flows through wet-dry climatic regions, which is classified as a monsoonal regime. The alternation of ocean tropical and continental atmosphere masses dominates the seasons. As warm, moisture-laden air flows from the Indian Ocean in summer, a wet season develops. In winter, a high pressure system develops over the Asian continent and becomes the source of dry air masses.

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Nitrogen cycle

There is a numerous variety of nitrogen form in the environment such as organic nitrogen, ammonium (NH4+), nitrite (NO2−), nitrate (NO3−), nitrous oxide (N2O), nitric oxide (NO) or inorganic nitrogen gas (N2). Many processes, in which microbes can transform nitrogen from one form to another, both to get energy or to accumulate nitrogen in a form needed for their growth. The diagram below shows how these processes fit together to form the nitrogen cycle (Figure 1-6).
In biogeochemistry, there are three processes of nitrogen transformation.
– First, fixation/assimilation: Nitrogen in the atmosphere is fixed by plants. However, most fixations are carried out by symbiotic bacteria known as diazotrophs. Most biological nitrogen fixation takes place by the nitrogenase enzyme, found in a wide variety of bacteria (Moir, 2011).
– Secondly, ammonification/anaerobic ammonia oxidation: When organisms (plant or animal) dye, bacteria or fungi transform organic nitrogen form back into NH4+, a process called ammonification or mineralization. This process contributes to a main proportion of nitrogen conversion into aquatic ecosystems.
– At last, nitrification/denitrification: The nitrification process converts NH4+ to NO3−, which is easy to dissolve in water and is not retained in soils (Vitousek et al., 1997). In addition, the denitrification process is the reduction of NO3− back into the inert N2 gas, completing the nitrogen cycle. This process is conducted by bacterial species such as Pseudomonas and Clostridium (Smil, 2000). Oxygen is required for nitrification but denitrification is anaerobic strict process.
In aquatic ecosystems, N enters to water column by many pathways, e.g. precipitation, soil runoff and leaching, domestic and industrial wastewater discharge. N cannot be used directly by phytoplankton under the form of N2 gas, except for cyanobacteria (Charles and Wheeler, 2012) (Figure 1-6). Other phytoplankton species use N as NH4+ or NO3- in water environment to synthesis organic matter.

Table of contents :

1. Water quality and the impacts of megacities on aquatic ecosystems
1.1. Water quality in South East Asian Countries and in Vietnam
1.2. The megacities and their impacts on the environment in Vietnam, with a focus on HCMC
1.2.1. Characteristics of megacities
1.2.2. The effects of megacities on aquatic ecosystems
2. River systems in tropical area
3. Nutrients in tropical estuaries
3.1. Nitrogen
3.1.1. Nitrogen cycle
3.1.2. Nitrogen sources
3.1.3. Nitrification-denitrification processes
3.2. Phosphorus
3.2.1. Phosphorus cycle
3.2.2. Phosphorus sources
3.2.3. The factors influencing the adsorption-desorption of phosphorus onto sediments
3.3. Silica
3.3.1. Silica cycle
3.3.2. Silica sources
3.4. Nutrients fluxes
4. Carbon
4.1. Carbon cycle
4.2. Anthropogenic influences
5. Eutrophication: causes, consequences and assessment
5.1. Causes of eutrophication
5.2. Physical factors supporting the development of eutrophication
5.3. Consequences and effects of eutrophication
5.4. Assessment of eutrophication
6. Sediments in estuary
1. Field study
1.1. The Saigon – Dongnai Rivers hydrosystem
1.2. The Ho Chi Minh City
2. Methodology
2.1. Monitoring and sampling strategy
2.1.1. Bi-monthly monitoring
2.1.2. Diurnal sampling surveys
2.1.3. Longitudinal and salinity profile surveys
2.1.4. Atmospheric deposition
2.2. Water quality and hydrological database of DONRE
3. Measurement and analytical methods
3.1. In-situ measurements of physico-chemical parameters
3.2. Filtration and preservation of samples in laboratory
3.3. Sediment preparation
3.4. Analytical methods
3.4.1. Total suspended solid (TSS)
3.4.2. Chlorophyll a and phaeopigments
3.4.3. Dissolved nutrients Nitrogen Total phosphorus (TP) and Orthophosphates (PO4 3-) Silica
3.4.4. Organic matter
3.4.5. Particulate nutrients in deposit and suspended sediment
3.4.6. Phytoplankton identification and counting
3.4.7. Physical characteristic of sediment
4. Assessment of phosphorus mobility from laboratory experiments
4.1. Experiment design
4.2. Sorption isotherm
1. Introduction
2. Materials and methods
2.1. Study area
2.2. Monitoring and sampling strategy
2.3. Laboratory analyses
2.4. Hydrological database
3. Results
3.1. Seasonal hydrological variability
3.2. Interannual variations of water quality in the Saigon River
3.2.1. Physico-chemical parameters
3.2.2. Dissolved and total nutrients
3.2.3. Organic carbon, chlorophyll-a, and phaeopigments
3.3. Longitudinal profile in the Saigon River
3.4. Nutrient ratios
4. Discussion
4.1. Level of nutrient contamination
4.2. Eutrophication status and limiting factors
4.3. Impact of HCMC on the water quality of the Saigon River
5. Conclusions
1. Introduction
2. Material and methods
2.1. Study area
2.2. Sampling campaigns
2.3. Measurement and analytical methods
2.3.1. In-situ sampling and measurements
2.3.2. Sediment preparation
2.3.3. TSS and phosphorus measurement
2.4. Experimental designs
2.5. Sorption isotherm
3. Results
3.1. Physico-chemical parameters distribution along the Saigon River
3.2. Phosphorus levels along the Saigon River
3.3. Phosphorus distribution in the salinity gradient
3.4. Phosphorus adsorption capacity onto sediment from laboratory experiments
3.5. Physical characteristics of sediments
4. Discussion
4.1. The effect of physical characteristic of sediment on the P adsorption capacity
4.2. Impact of HCMC on the P concentrations
4.3. Implication for the understanding of P dynamics within estuaries
5. Conclusions
1. Introduction
2. Material and methods
2.1. Description of the Saigon – Dongnai River basin
2.2. Database from the Vietnamese water quality survey
2.3 Additional monitoring program
2.4. Suspended sediment and nutrient budgets calculation
2.4.1. Nutrients export by rivers
2.4.2. Domestic inputs Gross nutrients inputs from urban areas Net nutrients inputs from WWTPs Net flux from urban canals to the rivers
2.4.3. Industrial inputs
2.4.4. Atmospheric depositions
2.4.5. Assumption on TSS and nutrients budgets
2.4.6. Expected nutrients inputs in 2025, 2040 and 2050
3. Results
3.1. Seasonal variation of TSS and nutrients concentrations
3.2. Seasonal and inter-annual variations of river fluxes and the effect of climatic conditions on the river fluxes
3.2.1. Seasonal variation of river fluxes
3.2.2. Inter-annual variations of river fluxes
3.3. Mean TSS and nutrient budgets in the Saigon River
3.4 Comparison between past and future nutrients inputs
4. Discussions
4.1. Budgets in contemporary Ho Chi Minh City 2012-2016
4.2. Nutrients fluxes ratios as an indicator of potential eutrophication
4.3. Future nutrients emissions from Ho Chi Minh City by 2025-2050 and recommendation for better management
5. Conclusions


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