Effective management for acidic pollution in the canal network of the Mekong Delta of Vietnam: A modeling approach

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Effective management for acidic pollution in the canal network of the Mekong Delta of Vietnam: A modeling approach

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Journal of Environmental Management 140 (2014) 14e25 Contents lists available at ScienceDirect Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman Effective management for acidic pollution in the canal network of the Mekong Delta of Vietnam: A modeling approach Ngo Dang Phong a, c, *, Chu Thai Hoanh b, To Phuc Tuong a, Hector Malano d a International Rice Research Institute (IRRI), Los Baños, Philippines International Water Management Institute (IWMI), Regional Office for Southeast Asia, Lao PDR, Laos c University of Agriculture and Forestry, Ho Chi Minh City, Viet Nam d Melbourne University, Victoria, Australia b a r t i c l e i n f o a b s t r a c t Article history: Received 24 June 2013 Received in revised form 22 October 2013 Accepted November 2013 Available online 12 April 2014 Acidic pollution can cause severe environmental consequences annually in coastal areas overlain with acid sulfate soils (ASS) A water quality model was used as an analytical tool for exploring the effects of water management options and other interventions on acidic pollution and salinity in Bac Lieu, a coastal province of the Mekong Delta Fifty eight percent of the provincial area is covered by ASS, and more than three-fourths (approximately 175,000 ha) are used for brackish-water shrimp culture Simulations of acid water propagation in the canal network indicate that the combination of opening the two main sluices along the East Sea of the study area at high tide for one day every week in May and June and widening the canals that connect these sluices to the West Sea allows for adequate saline water intake and minimizes the acidic pollution in the study area On the other hand, canal dredging in the freshwater ASS area should be done properly as it can create severe acidic pollution Ó 2014 Elsevier Ltd All rights reserved Keywords: Dredging Salinity Acidity Tide Pollution Water management Sluice operation Coastal acid sulfate soil Introduction Millions of people living in tidal ecosystems of coastal zones, especially in South and Southeast Asia, are among the poorest and most food-insecure because agricultural production is hindered by seawater intrusion during the dry season Many of these coastal zones are also overlain by acid sulfate soils (ASS) Worldwide, about 13 million of coastal ASS are located in Asia, Africa and Latin America (Brinkman, 1982) ASS occupy more than 40% (about 1.6 million ha) of the Mekong River Delta of Vietnam (Minh et al., 1997) These ASS contain significant amount of pyrite material Exposure of this material by excavation, lowering of groundwater or drainage results in its oxidation and produces high acidity, thus lowering the pH of the soil and releasing highly toxic elements such as iron and aluminum (Dent, 1986; Cook et al., 2000) Significant environmental damage due to changes in land use of coastal * Corresponding author International Rice Research Institute (IRRI), Los Baños, Philippines Tel.: þ84 1285 295 400; fax: þ84 7103 734 581 E-mail address: n.phong@irri.org (N.D Phong) http://dx.doi.org/10.1016/j.jenvman.2013.11.049 0301-4797/Ó 2014 Elsevier Ltd All rights reserved floodplains with ASS has occurred in Australia (White et al., 1997; Sammut et al., 1995, 1996a, 1996b); the Netherlands (Pons, 1973); the Mekong Delta of Vietnam (Tuong et al., 1993; Minh et al., 1997b); the Pearl River Delta of China (Lin and Melville, 1994); South Kalimantan, Indonesia (Hamming and van den Eelaart, 1993) and Finland (Palko and Yli-Halla, 1993) Rainfall can leach acidic contaminants out of the soil, which in turn acidify and pollute the receiving waters (Minh et al., 1997) Acidic pollution of the water causes dramatic changes in the stream environment (Sammut et al., 1995, 1996b), including many adverse effects on plants (Dent, 1986; Xuan, 1993), fisheries, domestic water (White et al., 1997) and corrosion of engineering infrastructure (White et al., 1996) Surface runoff and sub-flow are the main routes for draining the acidity from ASS into canals (Minh et al., 2002) Macdonald et al (2004) found that runoff from ASS affected the existing sulfide-rich sediments within an estuarine lake On the other hand, Cook et al (2000) found that acidity in the drains was mainly coming from agricultural land by groundwater discharge They concluded that sub-flow is a more severe hazard than runoff for acid pollution Other studies also claimed that the source of acid loads from agricultural fields entering canal water is groundwater, leaching from drain bank edges or seepage through drain walls N.D Phong et al / Journal of Environmental Management 140 (2014) 14e25 15 Fig Soil map of Bac Lieu province, Ca Mau peninsula, Vietnam, with dense canal network in freshwater zone (F) and saline-water zones (S1, S2, S3, B1 and B2) with a low pH in the range of 3.2e4 (Blunden and Indraratna, 2000) In the Mekong Delta of Vietnam, reclamation of ASS for agriculture and aquaculture has led to widespread acidic pollution of surface water in the freshwater zone (Tuong, 1993) as well as in the saline coastal zone (Hoanh et al., 2003; Gowing et al., 2006) Tuong et al (2003) showed that inappropriate water management, land uses of ASS and acidic pollution have led to a 70% reduction in income of the farmers living in the ASS area of Ca Mau peninsula, a coastal area of the Mekong Delta of Vietnam On ASS, spoils deposited on canal embankments during construction or dredging may be oxidized and form a source of acidic pollution (Tuong et al., 1998) In this study, extensive modeling was used to explore alternative water management practices and other interventions such as canal widening to reduce acidic pollution in ASS areas Such reduction will improve water quality and provide suitable water environment for both aquaculture and agriculture in the region 1.1 The study area The coastal plain of Bac Lieu province, Ca Mau peninsula is the study area located in the south of the Mekong Delta of Vietnam (Fig 1) It is an area with a highly modified environment The three most important soil groups in the study area are alluvial soils located in the northern and eastern parts near the Bassac River, ASS mainly located in the large depression in the central and western parts, and saline soils located in the southern and western parts along the East and West seas Roughly 90% of the annual rainfall in Bac Lieu (1800 mm) is concentrated in the rainy season from May to mid-November Rice crop is dominant in the north and shrimp raising is widespread in the south, where salinity is quite common in canal water (Hoanh et al., 2003) During the dry season from mid-November to April, freshwater availability for irrigation is a major constraint to rice production The canal network comprises a main canal, the Quan Lo Phung Hiep (QLPH), which connects the study area to the Bassac River, and series of canals of different capacity (BWRMBL, 2006, Fig 1) The primary canals are perpendicular to the QLPH, at about 4e5 km apart Their typical cross-section is 30e50 m wide and 4e10 m deep The embankments of primary canals are about 10 m wide The secondary canals connect to the primary canals at km spacing, and their typical cross section is 10e15 m wide and 1.5e 2.0 m deep The embankments of secondary canals are m wide The tertiary canals are spaced at 500 m and connect to secondary canals Their typical cross section is 5e8 m wide and 1e2 m deep, with 5-m wide embankments The tide in the East Sea is semi-diurnal (two high waters and two low waters each day) with high amplitude from to m, compared with only 0.5e1 m amplitude of diurnal tide (one tidal cycle per day) in the West Sea A series of sluices along the East Sea side is operated for delivery of saline water taken from the East Sea for shrimp culture in the central part of Ca Mau peninsula or the western part of Bac Lieu province (Fig 1) These sluices are also operated harmonically with the construction of temporary dams to restrict salinity intrusion into the agricultural zone in the eastern part of Bac Lieu The only 16 N.D Phong et al / Journal of Environmental Management 140 (2014) 14e25 Fig The VRSAP-ACIDITY model, with ACIDITY in the inset Modified from Phong (2008) freshwater source is diverted from the Bassac River to Ca Mau peninsula through the QLPH canal Among these sluices, the two largest, Ho Phong (HP) and Gia Rai (GR) with  m and  7.5 m wide, respectively, are playing an important role in controlling saline-water intake The study focuses on Bac Lieu province with 58% of the area underlain with ASS, and where salinity is found on about 175,000 of brackish-water shrimp culture The province comprises of six land-use zones, F, B1þB2, S1, S2 and S3 delineated from the existing water regimes and land uses (see Fig for location) This paper answers a question raised by provincial agencies: which management practices and other interventions can be applied to reduce acidic pollution in the canal network Methodology 2.1 The model The study used an ACIDITY module (Fig 2) coupling with a hydraulic and salinity model, the VRSAP (Vietnam River System And Plains) to simulate the temporal and spatial dynamics of acidity and salinity at a regional scale in the study area Details of the ACIDITY and the VRSAP model are summarized as follows: Using the implicit finite difference scheme to solve the basic hydraulic Saint-Venant continuity and the momentum equations and the salinity advection-dispersion equation, the VRSAP model computes water level, discharge and salinity in each segment of a complex open canal network subjected to tidal fluctuations (Hoanh et al., 2001) The model requires two types of input data: (i) the configuration and dimensions of the river and canal network; and (ii) hydrological data (water level, discharge and salinity) at boundaries and initial conditions of segments, nodes and fields Water level, discharge and salinity outputted by the model were validated with observed data in 1996 in the study area (Hoanh et al., 2001) The ACIDITY module (Phong, 2008) was based on a series of field and laboratory studies in combination with statistical and GISbased analyses The module comprises of two main functions to calculate acid loads into canals, and the acid neutralization of saline affected canal water in the coastal zones These functions were not available in the VRSAP model (i) Field experiments were carried out from 1st April to 15th July 2005 at Bac Lieu province to quantify the source and the dynamic of acidic pollution in a coastal acid sulfate soil area (Phong, 2008; Phong et al., 2013) Using regression analyses of time series data, the amount of acid loads transferred from fields and canal embankments to the canal water could be quantified from environmental parameters, including cumulative rainfall, types of ASS and age of embankment deposits (Phong et al., 2013) (ii) The laboratory experiment namely “titration” (Phong, 2008) was based on the chemical reaction of seawater on sulfuric acid with the formation of carbonic acid was described by Stumm and Morgan (1996) In the experiment, the monitoring pH of a fixed volume of canal saline water sample (defined as the recipient) when it reacts with consequent added acid water drops (defined as the titrant in experiment), results in a pH curve (or titration curve) of the canal water The experiment was repeated for each combination of a given set of titrants (pH water from to 7) and recipient waters (saline water with EC of 0, 10, 20, 30 or 55 dSmÀ1) As the results of experiment, titration curves allowed the determination of pH (hence acidity) of the canal water as it mixed with the inputted acid water At each time step of the computation, the ACIDITY module calculates the acidity (or in term of pH) of canal water at each canal segment and node (junction of two or more segments) with the known salinity of canal water computed from the VRSAP and the input of simulated acidity from canal embankments or fields before being integrated into the VRSAP model in the next time step (Fig 2) The integrative VRSAP-ACIDITY model is capable of simulating the temporal and spatial variations of water pH (as an indicator of acidity), salinity and water flow in a coastal canal networks It was calibrated with the 2003-data and validated with the 2005-data of a water quality monitoring network in the study area (Phong, 2008) N.D Phong et al / Journal of Environmental Management 140 (2014) 14e25 17 Fig The proposed process for management of both salinity and acidity Modified from the APWPC of Hoanh et al (2003) This study used the VRSAP-ACIDITY model to analyze the impacts of different water management options and other resource management measures on both salinity and acidity of the canal water in the study area Some of these options and measures may have conflicting influences on water qualities, hence combined effects could not be assessed without using the model For examples, canal widening may improve the drainage, reduce pollution but it also adds new deposits to the canal embankment and increases the acid loads to the canal water, or operating the sluices to enhance acidity drainage may also reduce the salinity to a level lower than that required by brackish water shrimp culture 2.2 Acidity propagation and possible control options Under current conditions, the following highlights and interventions are influencing acidity propagation in the study area:  The dredging of canals brings disturbed acid spoils onto canal embankments and exacerbates acidity in the canal network (Tuong et al., 2003) However, the effect of dredged acid spoils along canal embankments as sources of acidity load into canal water has not been investigated in previous studies (Truong et al., 1996)  The acid neutralization capacity for reducing acidity is an important feature of seawater (Stumm and Morgan, 1996; Evangelou, 1998) In the study area, saline water from the East Sea intrudes into the coastal plain and contains high alkalinity, which implies a potential for acidity reduction (Phong, 2008) This advantage is taken into account in scenarios of sluice operation  Direction of flows in canals in the study area changes during flood and ebb tides:  The flow direction from the East Sea to the West Sea through the canal network in Ca Mau peninsula is caused by the difference between the high tide amplitude in the East Sea, ranging from to m, and the low tide amplitude, only 0.5e m in the West Sea (Fig 3a)  The dynamics of the flood and ebb tide flows for intake of saline water or for drainage of excess water through the sluices along the East Sea has been exploited in sluice operation These sluices are equipped with hinge gates, thus opening them for one-way or two-way flow directions can be easily done at slack tide when the flow has been being slowly and then changed its direction Consequently, it can be advantageous for exploring either one of three options in sluice operation, canal widening or canal dredging that affects to water flow, salinity and acidity in canals in the study area: HP and GR sluices are selected to control salinity in the study area (Hoanh et al., 2001, 2009) Adjustments in the operation schedule of these sluices can improve the water flow and saline-water intake, which could reduce acidity of water in the study area The expansion or widening of canals facing the West Sea will increase the flow of canal water from the East Sea toward the West Sea and hence will affect salinity and acidity propagation in the canal network The locations and number of dredged canals in different salinewater or freshwater zones will alter the acidity generation in those zones, then it will affect to the water quantity and quality in the study area In past years, the main concern of provincial water managers was how to bring saline water into the study area for shrimp culture without affecting agricultural production in the freshwater area (zone F in Fig 1) Alternatives for salinity management purposes 18 N.D Phong et al / Journal of Environmental Management 140 (2014) 14e25 Fig Tide variations and sluice operation schedules a Tide variations at Ganh Hao (GH) of East Sea side and at Xeo Ro (XR) of West Sea side in May and June 2003 b Operation schedule of Ho Phong (HP) and Gia Rai (GR) sluices in two example scenarios O1 and OT were examined using an existing Analytical Process to support Water Policy Changes (APWPC) suggested by Hoanh et al (2003) In this study, step is added to the existing three-step APWPC for both salinity and acidity management (see the flow diagram of modified APWPC in Fig 4):  Step 1: Land-use investigation by delineating land-use zones and determining water-quality requirements  Step 2: Applying either one or both of the following options in water-quality management: (2.a) Exploring sluice operation options and/or (2.b) Adjusting canal configuration (widening or expansion) in combination with sluice operation Options of sluice operation could be a combination of selected sluices, number of gates operated at each sluice, days of operation and control of water flow direction (one way or two ways during sluice operation)  Step 3: Checking whether simulated salinity matches with the requirement If it does not, return to step to find suitable options  Step 4: Checking whether the water with satisfactory salinity in step satisfies the acidity requirement If it does not, return to step to find suitable options In step 3, the salinity at Chu Chi, Pho Sinh, Phuoc Long and Ninh Quoi stations (locations in Fig 1) along the QLPH canal is used for checking the boundary of salinity intrusion In step 4, maps of canal water pH are generated to identify the hot-spots of water pH less than 6, assuming that rice and shrimp productions are affected when the water pH drops below this level Table 1a Scenarios of sluice operation (Group 1).a Scenario Operated sluices Baseline OB HP and GR For saline intake O1 HP and GR O2 HP and GR O4 HP and GR OE OI HP and GR HP and GR OT HP and GR For drainage OD1 HP and GR OD2 HP and GR OD3 Sluices in the freshwater zone HP and GR are closed a Sluice opening days (*) Operated as on schedule of 2003 for May Closed in June One day every week in May and June Two consecutive days every two weeks in May and June Four consecutive days every four weeks in May and June Every day in June Two directions automatically by tide one day every week in May and June One day every week on the day with highest difference in tidal amplitudes between the East and West seas One day every week at the lowest tidal water level Two consecutive days every two weeks at the lowest tidal water level One day a week at ebb tide Sluices are operated as in 2003 for other months from January to April N.D Phong et al / Journal of Environmental Management 140 (2014) 14e25 Table 1b Scenarios of canal widening combined with sluice operation (Group 2).a Scenario Canal widening W1 W2 Widening NTL and QLCC canals connected to HP and GR sluices (see Fig 9a) with the same cross section (top width ¼ 50 m, canal bottom ¼ 2.0 m below mean sea level) W1 plus widening more secondary canals connected to the West Sea (see Fig 9b) with the same cross section (top width ¼ 30 m, canal bottom ¼ 2.0 m below mean sea level) a Operated sluices are the HP and GR Sluice opening days in these scenarios are the same as in scenario OT Table 1c Scenarios of dredged canals in different zones (Group 3).a Scenario Canal dredging DF DB DS1 DS2 Dredging Dredging Dredging Dredging canals in zone F (freshwater area) in zones B1 and B2 (brackish-water area) canals in zone S1 (saline-water area) canals in zone S2 (high-saline-water area) a Zone locations are shown in Fig Sluice opening days in these scenarios are the same as in scenario OT 2.3 Scenarios for both salinity and acidity management Acidity propagation in the canal network is investigated with different options in sluice operation, canal widening or dredging under three groups of scenarios, to (Table 1a, b and c) A baseline scenario (OB) is established as a reference to compare with these scenarios In this scenario, saline water from the East Sea is taken in from January to May by sluice operation as in the 2003 records (Phong, 2008) but it is not taken in June because of acidity problems in canal water that usually occur at the beginning of the rainy season The 2003 hydrological data (water level, salinity, flow) used for model calibration (Phong, 2008) are applied in all scenarios 19 intake is the same (See Fig 3b for sluice operation schedule of O1) In scenarios OD1, OD2 and OD3, the effect of drainage during ebb tide in June is considered and no saline water is taken in from HP and GR during drainage The same salinity of intake or drainage water provides the same reduction in acidity but the flow directions in these two cases, reflecting the movements of acid water, are different In addition, three other scenarios that focus on the effect of sluice operation on acidity propagation at the beginning of the rainy season (OE, OI and OT) are analyzed In scenario OE, HP and GR sluices are opened for saline-water intake every day in June In scenario OI, HP and GR sluices are opened bi-directionally automatically by the tide for one day every week in June In scenario OT (Fig 3b), HP and GR sluices are opened for saline water intake in one day a week in May and June when the difference in tidal amplitudes between the East and West Seas is highest in that week 2.5 Group 2: canal widening combined with sluice operation Since the flow through HP and GR sluices strongly influences water acidity, enlarging the primary canals that connect these sluices to secondary canals on the West Sea side of Ca Mau peninsula can be another alternative for improving acidity conditions Among the primary canals, the Ninh Thanh Loi (NTL) and the Quan Lo-Chu Chi (QLCC) are the shortest (20e25 km) canals (Fig 9a) The increase in water flow in these canals can be considered to boost the drainage of acidity in the study area to the West Sea (Fig 3a) At present, differences in sectional canal widths from 25 m to 50 m of these NTL and QLCC canals are causing a bottleneck of acidity flow to the West Sea (BWRMBL, 2006) In addition, canal widths of the secondary canals connecting these canals to the West Sea (Fig 9b) are also not uniform, varying from 10 m to 30 m Scenarios W1 with widening of primary canals and W2 with additional widening of secondary canals are presented in Table 1b and acidity propagations in these scenarios are presented in Fig 9a and b 2.4 Group 1: operation of HP and GR sluices 2.6 Group 3: dredging canals in different zones As presented in Table 1a, in scenarios O1, O2 and O3, only one gate of the HP and GR sluices is operated on a different schedule but the number of days (4 days every 4-week interval) for saline water The location and number of dredged canals every year are important factors in generating acidity (Phong, 2008) In this Fig Simulated water pH on 30 June under baseline scenario OB Note: Ho Phong (HP) and Gia Rai (GR) are the two main sluices in sluice operation 20 N.D Phong et al / Journal of Environmental Management 140 (2014) 14e25 Fig Simulated water pH on 30 June under scenarios of saline water intake Note: Details of scenarios are presented in Table 1a scenario group, the effects of dredged canals in different zones on acidity generation and propagation in the study area are analyzed In each scenario, canal dredging is carried out in one zone only For example, in scenarios DF, DB, DS1 and DS2, canal dredging is carried out in zone F, B1þB2, S1 or S2, respectively, while canals in other zones remain the same Dredging in zone S3 is not considered because dredging in zone S2 in scenario DS2 can represent such activity in areas with high water salinity In these scenarios, the same sluice operation as in the baseline scenario OB is applied To compare the effectiveness in reducing the acidity load in canals (Eff) in different zones (F, B1þB2, S1 and S2) by saline water in canals in these scenarios, a simple Equation (1) is applied: N.D Phong et al / Journal of Environmental Management 140 (2014) 14e25 Effð%Þ ¼ TALre 100% TAL 21 (1) where  TAL [tons Hþ] is the sum of total acidity load from all canal embankments (before entering canal water) in zone from the beginning of December 2002 to the end of June 2003 The total acidity load into canal is calculated for each canal featured by the age (number of years after the last dredging) and the ASS type (severe or medium soil acidity) of the dredged spoils on the canal embankments (Phong, 2008)  TALre [tons Hþ] is the sum of total acidity load reduced by saline water in canals in acid neutralization reactions (Stumm and Morgan, 1996) during this period Although in each scenario the model was run for the period from December to June, only the simulated water pH and salinity at the nodes in the canal network on 30 June are presented and discussed in the next section in this paper because acidity on that day represents the most severe acidic pollution in each year Results and discussion 3.1 Scenario analysis 3.1.1 The baseline scenario OB The result of acidity (represented by water pH) propagation in the baseline scenario OB (Fig 5) illustrates that, when HP and GR sluices are closed in June, acidity decreases slightly in zone S3 because of the high salinity water from the East Sea whereas a large area of severe acidity (water pH 5) is found in the freshwater area (zone F) and in the western saline part of the study area (zones S1, B1 and B2) In zones S2 and S3 downstream of the QLPH canal, except two small spots of severe acidity, water quality in these zones is better with water pH ! 3.1.2 Group 1: sluice operation 3.1.2.1 Effect of opening HP and GR sluices for saline-water intake In scenario O1 of opening HP and GR sluices for one day every week in May and June (Fig 6a), canal water with pH ! remained in narrow areas in zone S3 and small parts of zones S2, B1 and B2 along the QLPH canal Compared with the baseline scenario OB, saline water in this scenario is taken in enough to reduce acidity and maintain higher water pH in these zones until the next intake of saline water in the following week In scenario O2 of opening HP and GR sluices on two consecutive days every two weeks (Fig 6b), canal water with pH ! expanded into broader areas along the QLPH canal in zones S2, S3 and parts of zones S1, B1 and B2 compared with O1 In scenario O4, which involves opening HP and GR sluices on four consecutive days every four weeks in May and June (Fig 6c), a greater amount of intake of saline water creates a broader area with canal water pH ! than in scenario O1 but smaller than in scenario O2 This expansion indicates that, when saline water is taken in on four consecutive days, surplus saline water is drained into the West Sea because the canal system in the study area cannot store all saline water As a result, in zones B1 and B2 acidic water with pH spreads out from the severe acidity spot to other parts in June The comparison indicates that, with the same number of sluice opening days (eight days in May and June), scenario O2 with two consecutive days every two weeks provides the highest reduction in canal water acidity in the study area In scenario OE involving opening HP and GR sluices every day in June (Fig 6d), canal water with pH ! dominates in almost all Fig Simulated salinity along QLPH canal on 30 June under scenarios for saline water intake (a) and drainage (b) Note: Details of scenarios are presented in Table 1a zones and eliminates most severe acidity spots except some in zone S1 and in zone F Compared with the baseline scenario OB, scenarios O1 to O4 and OE provide higher salinity in the canal system, especially along the main canal QLPH (Fig 7) Scenario O1, with opening HP and GR sluices for one day every week only, provided lower salinity than in the other scenarios Therefore, in scenario O1, the objectives of both controlling salinity intrusion and reducing acidity can be achieved, whereas, in other scenarios, the acidity reduction is better but salinity is too high 3.1.2.2 Effect of sluice operation based on tidal amplitudes Fig 6e shows that, in scenario OT with HP and GR sluices opened to allow saline water intake for one day every week in May and June when the difference between tidal amplitudes in the East and West Seas in that week is highest, canal water pH slightly increases in zones S1, S2, B1 and B2 compared with that in scenario O1 This improvement indicates that consideration of tidal variations in both East and West seas in sluice operation is a potential alternative in reduction of acidity 3.1.2.3 Effect of sluice operation without controlling flow direction In scenario OI, HP and GR sluices are opened bi-directionally automatically by the tide for one day every week in June (Fig 6b) Canal water area with pH ! becomes broader in zone S3 while canal water area with pH < still remained in zones S1, S2, B1 and B2 This situation is explained by the different flow directions during flood tide and ebb tide in the day of sluice opening, and therefore saline water does not have enough time to reach other zones as in scenarios O1, O2 and O4 This result shows that controlling flow direction by sluice operation is very important in improving canal water quality 3.1.2.4 Effect of opening sluices for drainage In scenario OD1 with drainage toward the East Sea during ebb tide for one day every 22 N.D Phong et al / Journal of Environmental Management 140 (2014) 14e25 Fig Simulated water pH on 30 June under scenarios for drainage b a Note: Details of scenarios are presented in Table 1a week and without salinity intake (Fig 8a), acidity is more severe than in scenario OB However, in scenario OD2 with drainage on two consecutive days every two weeks, canal acidity declined significantly (Fig 8b) Compared to the baseline scenario OB and scenario OD1, canal water pH improved significantly in zones B1 and B2 in scenario OD2 and the spots of acidic water pH in zones S1, S2 and S3 are narrower because the opening of sluices on two consecutive days provides sufficient time for acidic water to drain out of the study area and be replaced by freshwater from the Bassac river through the QLPH canal As a result, salinity in scenario OD2 (Fig 8b) decreased more sharply along the QLPH canal (below g LÀ1 at Ninh Quoi) than in scenario OD1 (Fig 8a) However, a slight salinity intrusion from the West Sea into the northern part of zone B1 occurs because of more drainage toward the East Sea In this scenario OD2, the areas with canal water pH around (5.7e6.3) were broader in zones B1, B2, S2 and S3 (Fig 8b) but the area with canal water pH ! in zone S3 is smaller than in scenarios O1, O2, O4 and OE (Fig 6a to d) Canal water pH ! is suitable for the healthy growth of shrimp (Brennan et al., 2000), so the sluice operation for the intake of saline water in scenarios O1 to O4 and OE is more appropriate for shrimp culture Fig Simulated water pH on 30 June under scenarios of canal widening combined with sluice operation Note: Details of scenarios are presented in Table 1b N.D Phong et al / Journal of Environmental Management 140 (2014) 14e25 23 Fig 10 Simulated water pH on 30 June under scenarios of dredged canals in different zones Note: Details of scenarios are presented in Table 1c The worst case is scenario OD3 when sluices along the freshwater zone (zone F) are operated for drainage toward the East Sea for one day every week at ebb tide in June (Fig 8c) while HP and GR sluices are closed The results show that drainage through sluices in freshwater zone F toward the East Sea attracted acid canal water with pH from zones S1, B1 and B2 into the central part of the study area Scenario OD3 also accelerates salinity intrusion from the East Sea further upstream of the QLPH canal, with its salinity around 10 g LÀ1 at Ninh Quoi (Fig 7) 3.1.3 Group 2: canal widening combined with sluice operation From the above discussion, scenario OT is the most suitable option for both salinity control and acidity reduction Therefore, sluice operation schedule in scenario OT is included in scenarios W1 (widening only canals connected to HP and GR sluices) and W2 (W1 plus widening more canals connected to the West Sea) Details of these scenarios are shown in Table 1b Compared with scenario OT, scenarios W1 and W2 brought about a broader area of canal water with pH ! along the newly widened canals toward the West Sea (Fig 9a to b) rather than just along the QLPH canal In addition, canal water pH in scenario W2 increased more significantly in zones S2, S3, B1 and B2 This improvement illustrates that sufficient and uniform cross sections of canals connected to the West Sea are important factors to improve canal flow and acidity conditions 3.1.4 Group 3: dredging canals In general, compared to scenario OB, the epicenters of acidic pollution not vary clearly when new canals are dredged in the freshwater or saline-water zones as in scenarios DF, DB, DS1 and DS2 (Fig 10a to d) Hence, the sum of total acidity load in the canal (TAL) in each zone from the beginning of December to the end of June in these scenarios is compared in Table The results show that TAL decreases in the order of zones S1>S2>B1þB2>F in spite Table Sum of total acidity load into canal (TAL) reduced by saline water (TALre) and effectiveness (Eff %) in acidity reduction under scenarios of canal dredging Scenario Sum of total acidity load (tons Hþ) in each zone Whole area DF DB DS1 DS2 Average Eff of scenarios B (B1 þ B2) F S1 S2 TAL TALre Eff TAL TALre Eff TAL TALre Eff TAL TALre Eff TAL TALre Eff 799.4 802.7 788.0 803.5 678.8 692.4 680.4 693.1 85 86 86 86 86 64.4 55.0 55.0 55.0 19.6 18.8 18.2 18.4 30 34 33 34 33 224.9 237.5 224.9 225.0 217.1 231.1 216.8 217.8 97 97 96 97 97 275.0 275.0 280.1 278.3 270.0 270.3 275.2 273.6 98 98 98 98 98 235.1 235.1 228.0 245.2 172.0.1 172.3 170.2 183.3 73 73 75 75 74 Note: Zone locations are shown in Fig Whole area is the total area of all zones (F, B1, B2, S1 and S2) Eff (effectiveness) is computed by Equation (1) 24 N.D Phong et al / Journal of Environmental Management 140 (2014) 14e25 of the salinity level in canal water and acidity generation (locations and number of dredged canals) in dredged zones This order illustrates other factors such as flow regime could strongly affect to the spatial distribution of acidity in the zone (Phong, 2008) Van Breemen (1973) also concluded that pollution could exist in places far away from its source because it was transported by the water flow Nevertheless, the Eff in zones S1 or S2 is nearly double or triple (74%e98%) that in zone F (only 33%) in all scenarios This difference indicates that dredging in the freshwater zone (scenario DF) provides a lower reduction in acidic pollution than dredging in other saline-water zones (scenarios DB, DS1 and DS2) The results agree with findings by Evangelou (1998) and Stumm and Morgan (1996) that saline water provides higher acid neutralization capacity than freshwater 3.2 Lessons learnt from scenario analysis The analysis of saline-water intake scenarios O1, O2, O4 and OE and drainage scenarios OD1, OD2 and OD3 shows that acidity propagation as well as salinity intrusion are very sensitive to the operation of sluices on the East Sea side of the Ca Mau peninsula In drainage scenarios OD1, OD2 and OD3, the area of canal water pH around (5.7e6.3) is broader but the area with canal water pH > is smaller than in scenarios O1 to O4 and OE of saline-water intake Furthermore, for more than 60% of shrimp cultivation area that needs saline water, scenarios O1 to O4 and OE are more appropriate for both saline-water intake and acidity reduction Among these scenarios, scenario O1 of opening HP and GR sluices for one day every week in May and June can be considered as the most suitable in this group As an extension of scenario O1, scenario OT with sluices opening in the proper tidal periods when the difference between tide amplitudes in the East Sea and the West Sea is highest shows a more significant improvement of canal water acidity Hence, tide amplitudes should be considered in sluice operation to provide an additional advantage in improving water quality A significant reduction in water acidity can be achieved when combining the operation of the HP and GR sluice gates with the widening of the canals that connect these sluices to the West Sea as shown in scenario W2 Shrimp farms in the study area need saline water with pH around 7; hence, this combination strategy is the most suitable option for shrimp cultivation in Bac Lieu province However, it may cause acidic pollution in the areas along the West Sea The trade-off of that scenario should be considered carefully in water management Acidic runoff and seepage flows from the canal embankment deposits and fields connected to canals are the major sources of acidity load into the canal network in the study area The analysis of scenarios in the third scenario group shows that acidic pollution are severe when canals are dredged in either freshwater (scenario DF) or saline-water zones (scenarios DB, DS1 and DS2), and that acidity can spread far from the dredged canals by water flow The model results also show a lower effectiveness of acid neutralization in the freshwater zone than in the saline-water zone Hence, the risk of acidic pollution could be extremely high if canals on ASS are dredged in the freshwater zone dredging canals, or a combination of these interventions on both salinity and acidity of canal water Sluice operation, which is an important intervention in delivering brackish water for shrimp cultivation in the study area and controlling salinity intrusion upstream of the QLPH canal, is sensitive to acidity propagation in the canal network Among the considered scenarios of opening HP and GR sluices, the model results indicate that the opening HP and GR sluices on two consecutive days in two weeks in May and June provides the highest reduction of acidity concentration in the canal system, especially along the main canal, QLPH However, the objective of both managing salinity intrusion to support brackish-water shrimp culture and reducing acidity can be best achieved by opening HP and GR sluices for one day every week at the time of the highest difference between tide amplitudes in the East Sea and the West Sea The model is a useful tool to design the canal network and to determine canal cross sections for altering the water salinity and acidity in canals as required control The result of model shows that widening the selected primary canals to 50 m and the selected secondary canals to 30 m e in combination with proper sluice operation analyzed above e helps controlling the water flow and providing positive effects on reducing acidic pollution in the canal network However, a trade-off analysis is required since these interventions may cause acidic pollution in the surrounding areas The model also offers a methodology to analyze the effects of frequency of canal dredging/excavation at different locations, as well as the properties of the embankment deposits (e.g age of the embankment deposits, types of ASS making up the deposits, etc.) on acidic pollution in the canal network The results of scenario analysis point out that acidic pollution could be more severe when canal dredging/excavation is carried out in freshwater ASS zone than in saline water ASS zone The reduction of acidity in the canal water will decrease the release of highly toxic elements such as iron, aluminum and sulfuric acid from the ASS, and limit the adverse effects of acidity on flora and fauna It will also improve the suitability of canal water for domestic use and reduces corrosion of infrastructure However, it would take many years to improve significantly the ASS properties for reducing acidity leached out from these soils by applying only water management interventions Therefore in the recent years local people have been applying lime to neutralize the acidity in their shrimp and rice fields This practice has helped to accelerating the improvement of ASS properties and reducing acidity release Acknowledgments This study was carried out in the framework of the Project CP10, “Managing Water and Land Resources for Sustainable Livelihoods at the Interface between Fresh and Saline Water Environments in Vietnam and Bangladesh”, under the Challenge Program on Water and Food (http://waterandfood.org/2011/10/21/coastal-resourcemanagement-for-improving-livelihoods/) The manuscript was prepared under the ACIAR-funded project “Climate Change Affecting Land Use in the Mekong Delta: Adaptation of Rice-based Cropping Systems” We also thank Mr Bill Hardy of IRRI for editing this paper Conclusions References The paper shows the advantages of considering land use and water salinity control before analyzing acidity generation and propagation for water quality management in a coastal area overlain with ASS The “step-by-step” analysis provides a simple method for assessing the effects of sluice operation or widening/ Blunden, B., Indraratna, B., 2000 Evaluation of surface and groundwater management strategies for drained sulfidic soil using numerical models Aust J Soil Res 38, 569e590 Brennan, D.C., Clayton, H., Tran, T.B., 2000 Economic characteristics of rice shrimp farms in the Mekong Delta, Vietnam J Aquac Econ Manag (3e4), 127e139 Brinkman, R., 1982 Social and economic aspects of the reclamation of acid sulfate soil areas Publ Int Inst Land Reclam Improv Wagening 31, 21e36 N.D Phong et al / Journal of Environmental Management 140 (2014) 14e25 BWRMBL, 2006 Topography and Information of Dredging Canals in Bac Lieu Bureau of Water Resource Management Department of Agriculture and Rural Development (DARD) of Bac Lieu Province, Vietnam (in Vietnamese) Cook, F.J., Hicks, W., Gardner, E.A., Carlin, G.D., Froggatt, D.W., 2000 Export of acidity in drainage water from acid sulphate soils Mar Pollut Bull 41 (7e12), 319e326 Dent, D., 1986 Acid Sulfate Soils: a Baseline for Research and Development ILRI, Wageningen, The Netherlands Evangelou, V.P., 1998 Environmental Soil and Water Chemistry: Principles and Applications Wiley, New York Gowing, J.W., Tuong, T.P., Hoanh, C.T., 2006 Land and water management in coastal zones: dealing with agricultureeaquacultureefishery conflicts In: Hoanh, C.T., Tuong, T.P., Gowing, J.W., Hardy, B (Eds.), Environment and Livelihoods in Tropical Coastal Zones CAB International Hamming, A.F.J., van den Eelaart, A.L.J., 1993 Soil permeability, interflow and actual acidity in acid sulfate soils, South Kalimantan, Indonesia ILRI Pub No 53 In: Dent, D.L., van Mensvoort, M.E.F (Eds.), Selected Papers of the Ho Chi Minh City Symposium on Acid Sulphate Soils, Mar 1992 International Institute for Land Reclamation and Improvement, Wageningen, pp 155e160 Hoanh, C.T., Tuong, T.P., Kam, S.P., Phong, N.D., Ngoc, N.V., Lehmann, E., 10e13 December 2001 Using GIS-linked hydraulic model for managing water quality conflict for shrimp and rice production in the Mekong River Delta, Vietnam In: Ghassemi, F., Post, D., Sivapalan, M., Vertessy, R (Eds.), Proceedings of MODSIM 2001, Natural Systems (Part 1), vol International Congress on Modelling and Simulation, Canberra, Australia, pp 221e226 Hoanh, C.T., Tuong, T.P., Gallop, K.M., Gowing, J.W., Kam, S.P., Khiem, N.T., Phong, N.D., 2003 Livelihood impacts of water policy changes: evidence from a coastal area of the Mekong River Delta Water Policy (5), 475e488 Hoanh, C.T., Phong, N.D., Gowing, J.W., Tuong, T.P., Ngoc, N.V., Hien, N.X., 2009 Hydraulic and water quality modeling: a tool for managing land use conflicts in inland coastal zones Water Policy 11, 106e120 Lin, C., Melville, M.D., 1994 Acid sulphate soil-landscape relations in the Pearl River Delta, Southern China Catena 22, 105e120 Macdonald, B.C.T., Smith, J., Keene, A.F., Tunks, M., Kinsela, A., White, I., 2004 Impacts of runoff from sulfuric soils on sediment chemistry in an estuarine lake Sci Total Environ 329, 115e130 Minh, L.Q., Tuong, T.P., van Mensvoort, M.E.F., Bouma, J., 1997 Contamination of surface water as affected by land use in acid sulfate soils in the Mekong River Delta, Vietnam Agric Ecosyst Environ 61 (1), 19e27 Minh, L.Q., Tuong, T.P., van Mensvoort, M.E.F., Bouma, J., 2002 Aluminumcontaminant transport by surface runoff and bypass flow from an acid sulphate soil Agric Water Manag 56 (3), 179e191 Palko, J., Yli-Halla, M., 1993 Assessment and management of acidity released upon drainage of acid sulfate soils in Finland ILRI Pub No 53 In: Dent, D.L., van Mensvoort, M.E.F (Eds.), Selected Papers of the Ho Chi Minh City Symposium on Acid Sulphate Soils, Mar 1992 International Institute for Land Reclamation and Improvement, Wageningen, pp 411e418 Phong, N.D., 2008 Modelling of Canal Water Acidity Due to Acid Sulphate Soils: a Case Study of the Camau Peninsula, Mekong Delta, Vietnam PhD thesis Faculty of Engineering, Civil and Environmental Engineering and Faculty of Science, Earth Sciences, The University of Melbourne, Australia The thesis online is at: http://repository.unimelb.edu.au/10187/3525 25 Phong, N.D., Tuong, T.P., Phu, N.D., Nang, N.D., Hoanh, C.T., 2013 Quantifying source and dynamics of acidic pollution in a coastal acid sulphate soil area Water Air Soil Pollut 224, 1765 http://dx.doi.org/10.1007/s11270-013-1765-0 Pons, L.J., 1973 Outline of the genesis, characteristics, classification and improvement of acid sulphate soils ILRI Pub No 18 In: Dost, H (Ed.), Proceedings of the International Symposium on Acid Sulphate Soils, 13e29 Aug 1972, Wageningen, vol International Institute for Land Reclamation and Improvement, Wageningen, pp 3e27 Sammut, J., Mellville, M.D., Callinan, R.B., Fraser, G.C., 1995 Estuarine acidification: impacts on aquatic biota of draining acid sulfate soils Aust Geogr Stud 33, 89e 100 Sammut, J., Callinan, R.B., Fraser, G.C., 1996a An overview of the ecological impacts of acid sulfate soils in Australia In: Smith, Robert J., Associates, ASSMAC (Eds.), Proceedings of the 2nd National Conference of Acid Sulfate Soils Australia, pp 140e145 Sammut, J., White, I., Melville, M.D., 1996b Acidification of an estuarine tributary in eastern Australia due to drainage of acid sulphate soils Mar Freshw Res 47, 669e684 Stumm, W., Morgan, J.J., 1996 Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters Wiley, New York Tuong, T.P., 1993 An overview of water management of acid sulphate soils In: Dent, D.L., van Mensvoort, M.E.F (Eds.), Selected Papers of the Ho Chi Minh City Symposium on Acid Sulphate Soils, Ho Chi Minh City, Viet Nam, March 1992 International Institute for Land Reclamation and Improvement, Wageningen,, pp 265e279 Publication No 53 Tuong, T.P., Du, L.V., Luan, N.N., 1993 Effect of land preparation on leaching of an acid sulphate soil at Cu Chi, Vietnam In: Selected Papers of the Ho Chi Minh City Symposium on Acid Sulphate Soils, Ho Chi Minh City, Viet Nam, March 1992 International Institute for Land Reclamation and Improvement, Wageningen Publication No 53 Tuong, T.P., Minh, L.Q., Ni, D.V., van Mensvoort, M.E.F., 1998 Reducing acid pollution from reclaimed acid sulphate soils: experiences from the Mekong Delta, Vietnam In: Pereira, L.S., Gowing, J.W (Eds.), Water and the Environment: Innovative Issues in Irrigation and Drainage E&FN Spon, Routledge, London, pp 75e83 Tuong, T.P., Kam, S.P., Hoanh, C.T., Dung, L.C., Khiem, N.T., Barr, J., Ben, D.C., 2003 Impact of salinity protection on environment, farmers’ resource-use strategies and livelihood in a coastal area Paddy Water Environ 1, 65e73 Truong, T.V., Dac, N.T., Phien, H.T., 1996 Simulation of acid water movement in canals J Hydrol 180 (1), 361e371 Van Breemen, N., 1973 Soil forming processes in acid sulphate soils In: Dost, H (Ed.), Acid Sulphate Soils, Proc 1st Int Symp Acid Sulphate Soils, vol 18 ILRI, Wageningen, The Netherlands, pp 66e130 White, I., Melville, M.D., Sammut, J., Wilson, B.P., Bowman, G.M., 1996 Downstream impacts from acid sulfate soils In: Hunter, H.M., Eyles, A.G., Rayment, G.E (Eds.), Downstream Effects of Land Use Department of Natural Resources, Queensland, pp 165e172 White, I., Melville, M.D., Wilson, B.P., Sammut, J., 1997 Reducing acid discharge from coastal wetlands in eastern Australia Wetl Ecol Manag 5, 55e72 Xuan, V.T., 1993 Recent advances in integrated landuse on acid sulphate soils In: Dent, D.L., van Mensvoort, M.E.F (Eds.), Selected Papers of the Ho Chi Minh City Symposium on Acid Sulphate Soils International Institute for Land Reclamation and Improvement, Wageningen, pp 129e136 ILRI Publ 53 [...]... suitable option for shrimp cultivation in Bac Lieu province However, it may cause acidic pollution in the areas along the West Sea The trade-off of that scenario should be considered carefully in water management Acidic runoff and seepage flows from the canal embankment deposits and fields connected to canals are the major sources of acidity load into the canal network in the study area The analysis of. .. peninsula In drainage scenarios OD1, OD2 and OD3, the area of canal water pH around 6 (5.7e6.3) is broader but the area with canal water pH > 6 is smaller than in scenarios O1 to O4 and OE of saline-water intake Furthermore, for more than 60% of shrimp cultivation area that needs saline water, scenarios O1 to O4 and OE are more appropriate for both saline-water intake and acidity reduction Among these... in the Mekong Delta: Adaptation of Rice-based Cropping Systems” We also thank Mr Bill Hardy of IRRI for editing this paper 4 Conclusions References The paper shows the advantages of considering land use and water salinity control before analyzing acidity generation and propagation for water quality management in a coastal area overlain with ASS The “step-by-step” analysis provides a simple method for. .. primary canals to 50 m and the selected secondary canals to 30 m e in combination with proper sluice operation analyzed above e helps controlling the water flow and providing positive effects on reducing acidic pollution in the canal network However, a trade-off analysis is required since these interventions may cause acidic pollution in the surrounding areas The model also offers a methodology to analyze... (1998) and Stumm and Morgan (1996) that saline water provides higher acid neutralization capacity than freshwater 3.2 Lessons learnt from scenario analysis The analysis of saline-water intake scenarios O1, O2, O4 and OE and drainage scenarios OD1, OD2 and OD3 shows that acidity propagation as well as salinity intrusion are very sensitive to the operation of sluices on the East Sea side of the Ca Mau peninsula... analyze the effects of frequency of canal dredging/excavation at different locations, as well as the properties of the embankment deposits (e.g age of the embankment deposits, types of ASS making up the deposits, etc.) on acidic pollution in the canal network The results of scenario analysis point out that acidic pollution could be more severe when canal dredging/excavation is carried out in freshwater ASS... Social and economic aspects of the reclamation of acid sulfate soil areas Publ Int Inst Land Reclam Improv Wagening 31, 21e36 N.D Phong et al / Journal of Environmental Management 140 (2014) 14e25 BWRMBL, 2006 Topography and Information of Dredging Canals in Bac Lieu Bureau of Water Resource Management Department of Agriculture and Rural Development (DARD) of Bac Lieu Province, Vietnam (in Vietnamese)... scenarios in the third scenario group shows that acidic pollution are severe when canals are dredged in either freshwater (scenario DF) or saline-water zones (scenarios DB, DS1 and DS2), and that acidity can spread far from the dredged canals by water flow The model results also show a lower effectiveness of acid neutralization in the freshwater zone than in the saline-water zone Hence, the risk of acidic. .. acidic pollution could be extremely high if canals on ASS are dredged in the freshwater zone dredging canals, or a combination of these interventions on both salinity and acidity of canal water Sluice operation, which is an important intervention in delivering brackish water for shrimp cultivation in the study area and controlling salinity intrusion upstream of the QLPH canal, is sensitive to acidity... considered in sluice operation to provide an additional advantage in improving water quality A significant reduction in water acidity can be achieved when combining the operation of the HP and GR sluice gates with the widening of the canals that connect these sluices to the West Sea as shown in scenario W2 Shrimp farms in the study area need saline water with pH around 7; hence, this combination strategy is the ... is capable of simulating the temporal and spatial variations of water pH (as an indicator of acidity), salinity and water flow in a coastal canal networks It was calibrated with the 2003-data and... management, land uses of ASS and acidic pollution have led to a 70% reduction in income of the farmers living in the ASS area of Ca Mau peninsula, a coastal area of the Mekong Delta of Vietnam On ASS,... Dredging canals in zone F (freshwater area) in zones B1 and B2 (brackish-water area) canals in zone S1 (saline-water area) canals in zone S2 (high-saline-water area) a Zone locations are shown in

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  • Effective management for acidic pollution in the canal network of the Mekong Delta of Vietnam: A modeling approach

    • 1 Introduction

      • 1.1 The study area

    • 2 Methodology

      • 2.1 The model

      • 2.2 Acidity propagation and possible control options

      • 2.3 Scenarios for both salinity and acidity management

      • 2.4 Group 1: operation of HP and GR sluices

      • 2.5 Group 2: canal widening combined with sluice operation

      • 2.6 Group 3: dredging canals in different zones

    • 3 Results and discussion

      • 3.1 Scenario analysis

        • 3.1.1 The baseline scenario OB

        • 3.1.2 Group 1: sluice operation

          • 3.1.2.1 Effect of opening HP and GR sluices for saline-water intake

          • 3.1.2.2 Effect of sluice operation based on tidal amplitudes

          • 3.1.2.3 Effect of sluice operation without controlling flow direction

          • 3.1.2.4 Effect of opening sluices for drainage

        • 3.1.3 Group 2: canal widening combined with sluice operation

        • 3.1.4 Group 3: dredging canals

      • 3.2 Lessons learnt from scenario analysis

    • 4 Conclusions

    • Acknowledgments

    • References

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