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

Effective management for acidic pollution in the canal networkof the Mekong Delta of Vietnam: A modeling approach Ngo Dang Phonga,c,*, Chu Thai Hoanhb, To Phuc Tuonga, Hector Malanod a I

Trang 1

Effective management for acidic pollution in the canal network

of the Mekong Delta of Vietnam: A modeling approach

Ngo Dang Phonga,c,*, Chu Thai Hoanhb, To Phuc Tuonga, Hector Malanod

a International Rice Research Institute (IRRI), Los Baños, Philippines

b 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

a r t i c l e i n f o

Article history:

Received 24 June 2013

Received in revised form

22 October 2013

Accepted 3 November 2013

Available online 12 April 2014

Keywords:

Dredging

Salinity

Acidity

Tide

Pollution

Water management

Sluice operation

Coastal acid sulfate soil

a b s t r a c t

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

1 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 ha 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

Expo-sure 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

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 agriculturalfields entering canal water is groundwater, leaching from drain bank edges or seepage through drain walls

* 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).

Contents lists available atScienceDirect Journal of Environmental Management

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j e n v m a n

http://dx.doi.org/10.1016/j.jenvman.2013.11.049

Journal of Environmental Management 140 (2014) 14e25

Trang 2

with a low pH in the range of 3.2e4 (Blunden and Indraratna,

2000)

In the Mekong Delta of Vietnam, reclamation of ASS for

agricul-ture and aquaculagricul-ture has led to widespread acidic pollution of

sur-face 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

in-come of the farmers living in the ASS area of Ca Mau peninsula, a

coastal area of the Mekong Delta of Vietnam On ASS, spoils

depos-ited 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

alterna-tive 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 1 km spacing, and their typical cross section is 10e15 m wide and 1.5e 2.0 m deep The embankments of secondary canals are 7 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 3 to 4 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

Fig 1 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).

N.D Phong et al / Journal of Environmental Management 140 (2014) 14e25 15

Trang 3

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 3 8 m and 3 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 ha of brackish-water shrimp culture The province

com-prises of six land-use zones, F, B1þB2, S1, S2 and S3 delineated from

the existing water regimes and land uses (seeFig 1for 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

2 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 implicitﬁnite 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 tidalﬂuctuations (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 andﬁelds 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 ofﬁeld

and laboratory studies in combination with statistical and

GIS-based 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 cu-mulative 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 moni-toring 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 experi-ment), 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 3 to 7) and recipient waters (saline water with EC of 0, 10, 20, 30 or 55 dSm1) 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 orﬁelds 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 waterﬂow 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)

Fig 2 The VRSAP-ACIDITY model, with ACIDITY in the inset.

Modiﬁed from Phong (2008)

Trang 4

This study used the VRSAP-ACIDITY model to analyze the

im-pacts 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

exam-ples, canal widening may improve the drainage, reduce pollution

but it also adds new deposits to the canal embankment and

in-creases 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

in-terventions 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 3 to 4 m, and the low tide amplitude, only 0.5e

1 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 oper-ation These sluices are equipped with hinge gates, thus opening them for one-way or two-wayflow directions can be easily done at slack tide when theflow 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 waterﬂow, salinity and acidity in canals in the study area:

1 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 waterﬂow and saline-water intake, which could reduce acidity of water in the study area

2 The expansion or widening of canals facing the West Sea will increase theﬂow of canal water from the East Sea toward the West Sea and hence will affect salinity and acidity propagation

in the canal network

3 The locations and number of dredged canals in different saline-water or freshsaline-water 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

Fig 3 The proposed process for management of both salinity and acidity.

Modiﬁed from the APWPC of Hoanh et al (2003)

Trang 5

were examined using an existing Analytical Process to support

Water Policy Changes (APWPC) suggested byHoanh et al (2003)

In this study, step 4 is added to the existing three-step APWPC

for both salinity and acidity management (see theﬂow diagram of

modiﬁed APWPC inFig 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 waterﬂow 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 2 to ﬁnd suitable

options

Step 4: Checking whether the water with satisfactory salinity in

step 3 satisﬁes the acidity requirement If it does not, return to

step 2 toﬁnd suitable options

In step 3, the salinity at Chu Chi, Pho Sinh, Phuoc Long and Ninh

Quoi stations (locations inFig 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

Fig 4 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.

Table 1a Scenarios of sluice operation (Group 1) a

Scenario Operated sluices Sluice opening days (*) Baseline

OB HP and GR Operated as on schedule of 2003 for May.

Closed in June.

For saline intake O1 HP and GR One day every week in May and June O2 HP and GR Two consecutive days every two weeks

in May and June O4 HP and GR Four consecutive days every four weeks

in May and June

OE HP and GR Every day in June

OI HP and GR Two directions automatically by tide one

day every week in May and June

OT HP and GR One day every week on the day with

highest difference in tidal amplitudes between the East and West seas For drainage

OD1 HP and GR One day every week at the lowest tidal

water level OD2 HP and GR Two consecutive days every two weeks

at the lowest tidal water level OD3 Sluices in the freshwater

zone HP and GR are closed.

One day a week at ebb tide.

a

Trang 6

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, 1 to 3 (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

2.4 Group 1: operation of HP and GR sluices

As presented inTable 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

intake is the same (SeeFig 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 di-rections 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 sce-nario 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 theflow 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 condi-tions 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

inFig 9a and b

2.6 Group 3: dredging canals in different zones The location and number of dredged canals every year are important factors in generating acidity (Phong, 2008) In this

Table 1b

Scenarios of canal widening combined with sluice operation (Group 2) a

Scenario Canal widening

W1 Widening NTL and QLCC canals connected to HP and GR sluices

(see Fig 9 a) with the same cross section (top width ¼ 50 m, canal

bottom ¼ 2.0 m below mean sea level)

W2 W1 plus widening more secondary canals connected to the West Sea

(see Fig 9 b) 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 Dredging canals in zone F (freshwater area)

DB Dredging in zones B1 and B2 (brackish-water area)

DS1 Dredging canals in zone S1 (saline-water area)

DS2 Dredging canals in zone S2 (high-saline-water area)

a Zone locations are shown in Fig 1 Sluice opening days in these scenarios are the

same as in scenario OT.

Trang 7

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:

Fig 6 Simulated water pH on 30 June under scenarios of saline water intake Note: Details of scenarios are presented in Table 1 a.

Trang 8

Effð%Þ ¼ TALre

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

dis-cussed in the next section in this paper because acidity on that day

represents the most severe acidic pollution in each year

3 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 6

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 6 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 6 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 6 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 5 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 6 dominates in almost all

zones and eliminates most severe acidity spots except some in zone S1 and in zone F Compared with the baseline scenario OB, sce-narios 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 6 becomes broader in zone S3 while canal water area with pH< 6 still remained in zones S1, S2, B1 and B2 This situation is explained by the different flow directions duringflood 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 con-trolling 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

Fig 7 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 1 a N.D Phong et al / Journal of Environmental Management 140 (2014) 14e25 21

Trang 9

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 5 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 4 g L1at 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 to-ward the East Sea In this scenario OD2, the areas with canal water pH around 6 (5.7e6.3) were broader in zones B1, B2, S2 and S3 (Fig 8b) but the area with canal water pH 6 in zone S3

is smaller than in scenarios O1, O2, O4 and OE (Fig 6a to d) Canal water pH 7 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 8 Simulated water pH on 30 June under scenarios for drainage b a Note: Details of scenarios are presented in Table 1 a.

Trang 10

The worst case is scenario OD3 when sluices along the

fresh-water 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 5 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 L1at 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 inTable 1b

Compared with scenario OT, scenarios W1 and W2 brought about a broader area of canal water with pH 6 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 canalﬂow and acidity conditions

3.1.4 Group 3: dredging canals

In general, compared to scenario OB, the epicenters of acidic pollution do 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 inTable 2 The results show that TAL decreases in the order of zones S1>S2>B1þB2>F in spite

Fig 10 Simulated water pH on 30 June under scenarios of dredged canals in different zones Note: Details of scenarios are presented in Table 1 c.

Table 2

Sum of total acidity load into canal (TAL) reduced by saline water (TAL re ) and effectiveness (Eff %) in acidity reduction under scenarios of canal dredging.

Scenario Sum of total acidity load (tons Hþ) in each zone

TAL TAL re Eff TAL TAL re Eff TAL TAL re Eff TAL TAL re Eff TAL TAL re Eff

DF 799.4 678.8 85 64.4 19.6 30 224.9 217.1 97 275.0 270.0 98 235.1 172.0.1 73

DB 802.7 692.4 86 55.0 18.8 34 237.5 231.1 97 275.0 270.3 98 235.1 172.3 73 DS1 788.0 680.4 86 55.0 18.2 33 224.9 216.8 96 280.1 275.2 98 228.0 170.2 75 DS2 803.5 693.1 86 55.0 18.4 34 225.0 217.8 97 278.3 273.6 98 245.2 183.3 75

Note: Zone locations are shown in Fig 1 Whole area is the total area of all zones (F, B1, B2, S1 and S2).

Định dạng
Số trang	12
Dung lượng	4,12 MB