Seminar on Environment and Development in Vietnam
Friday and Saturday, December 6-7, 1996
Common Room, University House,
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Possible Impacts Of Salinewater Intrusion Floodgates In Vietnam's Lower Mekong Delta
Water Research Foundation of Australia
Centre for Resource and Environmental Studies
The Australian National University, Canberra ACT 0200
Tel: +61 (6) 249-0660
Fax: +61 (6) 249-0757
Mike Melville and Jes Sammut
School of Geography
The University of NSW, Sydney NSW 2052
Seawater intrusion and acid production from acid sulfate soils during dry seasons, and the export of acid through drainage systems during the wet season, are major constraints to agricultural and aquatic production in Vietnam's lower Mekong Delta and in coastal floodplains in eastern Australia as well. The massive freshwater flows which inundate much of the Mekong Delta during the wet season nourish the Delta's highly productive and essential rice crops. During the dry season, the Mekong River flows are so low that sea waters intrude into the lower reaches of the River, producing brackish water conditions that are unsuitable for rice growth. At present approximately 2 Mha of land are subject to dry season salinity with saline water extending 50 km inland. Increasing diversions of upstream Mekong flows for dry season irrigation, both in Vietnam and in countries upstream, threaten to exacerbate saltwater intrusion into productive lands.
To limit saltwater intrusion into agricultural areas, salinewater intrusion floodgates have been installed or are planned for much of the lower Mekong River. In eastern Australia, many floodplains have already been drained and floodgated. Floodgate design has been based on purely engineering criteria to reduce incursions of seawater. Impacts on soil acidification, the transport and storage of acid, and the impacts of stored acid on agricultural and aquatic productivity have been ignored.
As we shall show, research in eastern Australia has shown that floodgates on estuarine, acid sulfate floodplains, promote soil acidification, lower plant production, act as large acid reservoirs, form barriers to fish migration, decrease recruitment and feeding areas, diminish tidally-driven acid neutralisation and release hundreds of tonnes of acidity into estuarine reaches. The impacts on aquatic communities are massive.
Based on Australian experience, our hypotheses are that firstly, installation of floodgates in Vietnam will results in extremely poor upstream water quality during the dry season; and secondly, the resulting water quality will cause major decreases in upstream, dry-season, irrigated crop production and fish and aquatic production.
1.1 Management of Saline Water Intrusion in Vietnam
The massive freshwater flows which inundate much of the Mekong Delta during the wet season nourish the Delta's highly productive and essential rice crops (Vo-tong Xuan, 1993; Tran Thanh Be, 1994; Vo Quang Minh, 1995). The Delta has been called Vietnam's rice-basket which emphasises its economic and social importance to Vietnam (Tran Thanh Be, 1994). During the dry season, the Mekong River flows are so low that sea waters intrude into the lower reaches of the River, producing brackish water conditions that are unsuitable for rice growth. At present approximately 2 Mha of land are subject to dry season salinity with saline water extending 50 km inland (Vo Quang Minh, 1995). Increasing diversions of upstream Mekong flows for dry season irrigation, both in Vietnam and in countries upstream, threaten to exacerbate saltwater intrusion into productive lands.
In a portion of the salt-affected area, some innovative farmers have adapted to the fluctuating freshwater-brackish water environment by evolving a rice-shrimp rotation system to maximise returns through both rice and high-value, extensive or semi-intensive shrimp production (Vo-tong Xuan, 1993). The sustainability of this system is unknown, but sedimentation during the saline phase decreases the land area available for production by up to 4% pa (Tran Thanh Be, 1994). In addition, the area suitable for the rice-shrimp system is limited and is smaller than the area impacted by dry-season salinewater intrusion. As well, the maximisation of rice production in the Mekong Delta is an imperative.
1.11 Engineering solutions
It is easy to understand why engineered structures, which limit the upstream excursion of saltwater and increase the area available for dry-season or part dry season rice production, appear attractive solutions. Indeed saline-intrusion-limiting floodgates have been installed in Vietnam with the assistance of foreign governments. Plans for protecting much larger areas are well-advanced. Floodgates have been planned, designed and in some cases installed on purely engineering grounds without recognition of the upstream and downstream impacts and the presence of reactive sulfidic (potential acid sulfate) sediments in the upstream catchment.
1.2. Sulfidic Sediments or Acid Sulfate Soils
1.21 Formation and distribution
Sediments containing sulfides, principally iron sulfides, have been formed throughout geologic time. Those of most concern are sediments which were deposited during the last 10,000 years, the Holocene period (Pons, 1973). Sea level rise, following the end of the last ice age, led to the drowning of river valleys, and the deposition of deltas and the infilling of coastal embayments and estuarine floodplains with sulfidic sediments (Thom and Chappell, 1975). There are approximately 20M ha of recent, coastal sulfidic sediments throughout the world with major deposits in the Asia- Pacific region (van Breeman, 1973). The Mekong Delta has 2.3 M ha of recent sulfidic sediments (Vo Quangh Minh, 1995), while the Australian coastal zone has about 3 M ha (White et al., 1996b). In comparison, the area of irrigated agriculture in Australia is 1.83 M ha and the total area affected by salinity in the Murray-Darling Basin is about 5 M ha.
1.22 Acid production
As long as the sulfides in the sediments remain below the water table they are innocuous. When exposed to air, such as in periods of prolonged drought, after draining for agricultural or urban development, or during excavation or dredging, they oxidise (Willett et al., 1992). Oxidation involves the conversion of sulfide minerals in the soil, chiefly pyrite, to sulfuric acid. Because of this, these sediments when oxidised are known as acid sulfate soils (Dent, 1986).
The acid produced by sulfide oxidation reacts with clay minerals in the sediments to produce acid soilwater solutions containing dissolved aluminium, iron, manganese, and other heavy metals (van Breeman, 1973; Willett et al., 1992, 1993).
1.23 Impacts of soil acidification
These soil solutions can be extremely toxic to plants. Extremely low crop production can occur for lengthy periods when this acid soil solution is within the rootzone of the crop (Dent, 1986, Moore et al. 1993). Indeed, substantial barren or scalded areas, denuded of vegetation by exposure of acid sulfate soils, have persisted in some areas for over 80 years. When this acid soil solution is displaced by rainfall into streams it can be toxic to gilled organisms, promote fish diseases (Callinan et al., 1993, 1996), change estuarine ecosystems (Sammut et al., 1995) and corrode steel and concrete engineering infrastructures (White and Melville, 1993). The generally low plant productivity associated with acid sulfate soils has been recognised for over 260 years (Pons, 1973). In Australia, the significance of acid sulfate soils was only recognised 30 years ago (Walker, 1972; Willett and Walker, 1982). It has taken over 25 years for that recognition to be translated into action.
Massive estuarine fish kills can occur following dry-season oxidation of sulfidic sediments under natural conditions (Brown et al., 1983; Hart et al., 1987) as well as under artificially drained conditions (Callinan et al., 1993, 1996; Sammut et al., 1995). In 1987, all gilled organisms in 23 kilometres of the Tweed River estuary, in northern NSW, were killed by acid drainage from the 20,000 ha sulfidic floodplain (Easton, 1989). The estuary remained effectively sterile for 18 months. The Tweed River floodplain has been extensively drained and floodgated for agricultural development and flood mitigation works (White et al., 1993; Lin et al., 1995; Wilson, 1995).
1.24 Acid sulfate soil research priorities
The expansion of agriculture in the Mekong Delta following 1975, has led to severe acid sulfate soils problems (Brinkman, 1982; Vo-tong Xuan, 1993). Indeed, acid sulfate soils have been recognised as a major impediment to crop production in Vietnam and have been a high priority area in soil research for Vietnamese agronomists and soil scientists (To Phuc Tuong, 1993). Work in Vietnam has concentrated on improving agricultural productivity (Vo-tong Xuan, 1993), although there has been recent recognition of downstream and off-site impacts (Nguyen Thanh and Wilander, 1995; Le Quang Minh, 1995). Considerable expertise has been built up in Vietnam in the chemistry, management and agronomy of acid sulfate soils. In Australia, work has concentrated on down-stream impacts of drainage from acid sulfate soils in agricultural lands and on management of acid sulfate soils.
Australia, starting from a low base in 1990, has now built up significant expertise in acid sulfate soils, particularly in coastal geomorphology, acid sulfate soil physics, chemistry and microbiology, floodplain hydrology, floodplain management and production, environmental impacts and acid sulfate soil amelioration. Australia now probably leads the world in terms of the provision to farmers and land managers of readily accessible information on acid sulfate soils, on policy development and on coordinated government response.
2 IMPACTS OF FLOODGATES ON AREAS WITH ACID SULFATE SOILS IN AUSTRALIA
In Australia, coastal floodplains have the longest record of agricultural use of any region, due to their generally favourable temperature and soil water regimes (King, 1948). Their plentiful soil water comes from excess rainfall, flooding and high water tables. In order to optimise the use of estuarine floodplain areas for animal and crop production, governments in eastern Australia have encouraged the dramatic alteration of floodplain hydrology through flood mitigation and drainage policies.
2.1 Re-engineering of coastal floodplains
Floodplain backswamps often took in excess of 100 days to drain under natural conditions. Most crops and pastures grown in coastal floodplains cannot survive waterlogging for more than 5 days (White and Melville, 1996). Because of this, flood mitigation works have been constructed in most of eastern Australia to either divert upland inflows from the floodplain, to route them through the floodplain or to drain them from the floodplain within five days. Natural, meandering drainage channels have been straightened, widened and deepened, hundreds of kilometres of side drainage canals have been constructed, often without design, levee banks raised and floodgates installed where drainage channels enter tidal streams. These floodgates also prevent saline water intrusion during dry periods.
In his pioneering study of sulfidic estuarine floodplain soils, Walker (1972) recognised the crucial links between rainfall, evapotranspiration, watertable height and drainage and the acidification of groundwater and streams through oxidation of benign sulfides in the soils. Walker cautioned that 'Artificial drainage undertaken without regard to details of stratigraphy could lead to severe acidity and salinity conditions which, in back swamp soils, could be ameliorated only over a long period.'
Both Walker's warning and his work were largely overlooked. Indeed, the role hydrology plays in the formation of acid sulfate soils and in the subsequent export of acidity into streams has been neglected world-wide until recently (Jeurissen, and van der Gun, 1986; Bronswijk and Nugroho, 1990; Hamming and van den Eelaart; 1993; To Phuc Tuong, 1993; Palko and Yli-Halla, 1993; White et al., 1993; Willett et al., 1993; Pease, 1994; Johnston, 1995; Le Quang et al., 1995; Nguyen Thanh and Wilander, 1995; Wilson, 1995; Sammut et al., 1996; White et al., 1996a,b; White and Melville, 1996).
Many of the floodplains in eastern Australia have now been drained and floodgated and adverse impacts have been documented.
2.2 Adverse Impacts of floodgates
One-way floodgates allow drainage into estuaries at low tide which makes it possible to lower water tables in sulfidic sediments by up to 1 m below their natural level and promotes oxidation of sulfidic sediments with attendant penalties on crop production. As well, floodgates also prevent the neutralisation by tidal inflows of estuarine water of acid produced naturally in the floodplain. This allows the build up of acid behind flood gates. These acid reservoirs act as barriers to fish migration, impeding feeding, recruitment and breeding (Sammut et al., 1996).
In dry times large flood gates can act as acid reservoirs, impounding acid water for up to six months. This water is released either in a slug in subsequent flood events, or as a persistent trickle during smaller rains (Sammut et al., 1996). In one 4000 ha tributary floodplain of the Richmond River, up to 700 tonnes of sulfuric acid can be impounded in the 'fresh' water behind the floodgate with about 950 tonnes of acid released into the estuarine reach in a single flood (Sammut et al., 1996). On the Tweed River, approximately 2,600 tonnes of sulfuric acid can be released annually through floodgates (Wilson, 1995). It has been estimated that drained floodplains produce between 0.1 to 0.3 tonnes of pure sulfuric acid/ha/ year (White et al., 1996a). With these rates and the amount of sulfides in the sediments it can be estimated that acid may be produced for the order of 1000 years (Sammut et al., 1996). Critical factors in this production are the climate-driven dynamics of the floodplain water balance, the operation of the drainage system and the rate of movement of water and oxidation products through the soil (White and Melville, 1996).
Acid discharges kill fish, when they cannot avoid acid water. They also cause fish diseases such as Epizootic Ulcerative Syndrome (Red Spot Disease) (Callinan et al., 1993; 1996; Sammut et al., 1995), and claimed to have major impacts on aquaculture (Simpson and Pedini, 1985), particularly oyster production, and change benthic plant communities, leading to a loss in biodiversity. Recently a Recreational Fishing Organisation has successfully sued cane producers in northern NSW for discharge of acidity through floodgates. Commercial and recreational fishers, environmental groups and aquaculturalists are demanding changes in estuarine floodplain and floodgate management. Responsible farming organisations are also seeking to adopt best practices. It is not obvious that the general community demand for improved drainage water quality is consistent with the requirement of sustained or improved productivity on coastal floodplains. Nevertheless, those remain the major goals for floodplain management in eastern Australia.
3. IMPLICATIONS FOR SALINE-INTRUSION FLOODGATES IN VIETNAM.
It is naive to attempt to extrapolate from the situation in eastern Australia to that in the Mekong Delta. Eastern Australian catchments are in general small with low flow volumes compared with the Mekong, one of the mighty river systems of the world. Rainfall and hence streamflow are highly variable in Australia from year to year and are correlated with El Niño and La Niña events. The cultivated and drained floodplains of eastern Australia are different to the raw acid sulfate soils being subjected to intensive manipulation and organic inputs, as in lowland paddy rice fields of Vietnam (Lin et al., 1995). Nonetheless, experiences in eastern Australia lead to our fundamental hypotheses that:
1. installation of floodgates in Vietnam will result in extremely poor upstream water quality during the dry season;
2. the resulting water quality will cause major decreases in upstream dry-season irrigated crop production and fish and aquatic production.
Our work in Australia suggests the following specific changes may occur after installation of saline intrusion floodgates in Vietnam's Lower Mekong Delta:
Floodgates already installed at My Phuoc in Soc Trang Province, servicing an area of 13,400 ha containing 3,400 ha of severe acid sulfate soils with the remainder being moderate acid sulfate soils, are producing acidification and stagnation of upstream water and rising watertables in the project area.
If this is so, and the above hypotheses are correct then the installation of floodgates has be significant social and economic implications for the lower Mekong Delta.
4. ECONOMIC, ENVIRONMENTAL, SOCIAL IMPACT
Both rice and fish production are extremely important to Vietnam's economy and national well-being. At present, dry season saline intrusion limits rice production. If, however, our hypothesis on the impacts of floodgates are correct, their installation could decrease rice and other crop yields as well cause declines in fish production. The consequences of both are serious.
In Australia coastal floodplains are used by agriculture predominantly for sugarcane production, dairying, grazing and tea tree production. Recent court challenges on acid discharges through floodgates threaten these industries. As well, acid discharges threaten fish production and estuarine aquaculture. The NSW fishing industry is a $20M per annum industry. The eastern inshore trawling industry in Queensland is a $100M per annum industry. Some 65% of commercial fish species spend part of their life cycle in estuaries.
The economic impacts of estuarine acidification on both the commercial and recreational fishing industries and on aquaculture have not yet been fully documented. In NSW, acid drainage from acid sulfate soils has been reported to have destroyed directly Sydney rock oysters worth $7M over the last six years. In addition, the impact of water drained from acid sulfate soil areas on diseases has been raised (Callinan et al., 1993; 1996; Sammut et al, 1995). It has been noted that outbreaks of QX disease in oysters occurred after heavy rains (Lester, 1986). Attempts to discount a link between QX outbreaks and decreases in water pH (Anderson et al., 1994) are unconvincing. It has been estimated that improved water quality management may generate at least $5M per annum in the oyster industry. The 1987 acidification of the Tweed River is estimated to have cost $3.5M in foregone production and impacts on the tourist industry. It has been estimated that decreases in acid impacted drainage could increase commercial fisheries production by $6 M per year.
In Vietnam there is a much greater reliance on fish as a source of protein than in Australia. Impediments to fish migration, breeding and recruitment caused by acidification behind floodgates in the dry season could cause major declines in this valuable food source. If our hypotheses are correct, the installation of floodgates could have major economic and social impacts in the lower Mekong Delta as they have in eastern NSW. The benefit/cost ratio of this project has the potential to easily exceed 100/1.
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