Vos 7VIETNAM INLAND WATERWAY INTERNATIONAL BANK FORDEPARTMENT I PMU-SW RECONSTRUCTION

AND DEVELOPMENT

INLAND WATERWAYS ANDPORT MODERNIZATION PROJECT

VOLUME 5ANNEX 11-3 Soil Report

JUNE 1996

INEDECOINetherlands Engineering Consultants

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LIST OF REPORTS

VOLUME I EXECUTIVE SUMMARY

VOLUME li : INLAND WATERWAYS IMPROVEMENT PROJECT(Main Report)

Annex II - 1: Design ReportAnnex II - 2 Navigation AidsAnnex II - 3: Soil Report

VOLUME III : RESETTLEMENT ACTION PLAN

VOLUME IV : ENVIRONMENTAL IMPACT ASSESSMENT &ENVIRONMENTAL MANAGEMENT PLAN

VOLUME V TOPOGRAPHIC MAPS (scale 1:5000)

Book 1: Ho Chi Minh City - Cho Gao (Pl/0 to Pl/1 3)Cho Gao - Sa Dec (Pll/O to PIV/2 1)

Book 2 Sa Dec - Lap Vo (PIiI/OtoPIII/14)Rach Gia - Ha Tien (PIV/0 to PIV/1 9)Rach Soi - Ha Giang (PIV/20 to PIV32)

Book 3: Cho Lach - Mang Thit (PV/0 to PV/20)Can Tho - Ca Mau (PVI/0 to V1l38)

CONTENTS page

1. INTRODUCTION 1A. Oxidation of Pyrite 1B. Oxidation of pyritic sediments before the presence of

canals 4C. Accelerated oxidation of pyritic sediments after excavation

of canals 5D. Summary of impacts of canal excavation and functioning on

acidification of land and water 7

2. SOIL SURVEY 9A. Methodology 9B. Results 10

3. EXCAVATION OF POTENTIALLY ACID SEDIMENTS 14

4. ESTIMATION OF THE ACID RUN-OFF FROM THE SPOIL 16A. Methodology 16B. Calculation of the acid run-off from dredge spoils 18C. Acid run-off from spoils where potentially acid sediment is

covered with non-potentially acid sediment 19D. Period until complete depletion of the acidity of the spoil 22

5. ACID RUN-OFF FROM SURROUNDING LAND 23A. Methodology 23B. Results 24

6. ACID RUN-OFF FROM DREDGE SPOILS IN RELATION TO ACIDRUN-OFF FROM SURROUNDING LAND 27

7. ACIDIFICATION OF SURFACE WATER DURING THE DREDGINGOPERATION AND WITHIN THE CANAL 30

LIST OF FIGURES

Figure 1.01: Summary of the impacts of the presence of canals onacidification of water and land 8

LIST OF TABLES

Table 2.01: Physical and chemical characteristics of the soil layers in thecross-sections. 10

Table 2.02: Description and colour of the soil layers in the 23 cross-sections 11

Table 3.03: Total dredging volumes and volume pyritic sediments to bedredged. 14

Table 3.04: Potential acidity of the pyritic sediments 1 5Table 4.05: Potential acidity of freshly excavated pyritic sediments and

older dikes and raised bed. 17Table 4.06: Yearly acid run-off from pyritic dredge spoils with and

without bunds. 19Table 4.07: Possibility to cover potentially acid sediments with non-

potentially acid sediments 20Table 4.08: Yearly acid run-off from dredge spoils in which potentially

acid sediment has been covered with underlying non-potentially acid sediment. 21

Table 4.09: Period until depletion of the potential acidity of the spoil. 22Table 5.10: Calculation of the yearly acid run-off from land bordering the

canals. 24Table 6.11: Yearly acid run-off from the spoil and effect of mitigating

measures 27Table 6.12: Yearly acid run-off from dredge spoils as a percentage of the

yearly acid run-off from the bordering lands. 28Table 7.13: Results of the. sediment-water mixing experiment 31

June 1996/2KC1875.21/Annex 11-3/SPE/lb

1. INTRODUCTION

A1.01 In the framework of the Inland Waterway project, quantification ofthe expected acid run-off from dredge spoils after widening and deepening ofexisting canals together with a quantification of the expected acid run-off fromsurrounding land bordering these canals is needed in order to evaluate the impactof acid run-off from the dredge spoil in relation to acid run-off from adjacent land.

A1.02 The excavation of potentially acid material quickly and dramaticallygenerates acidity. In the late seventies and early eighties, when hundreds of canalswere excavated in the Plain of Reeds, Ha Tien plain and Ca Mau peninsular, severeacidification took place resulting in massive starvation of fish, shrimp, degradationof the aquatic environment damage to human health.

A1.03 With respect to the origin of the acid water, different sources canbe distinguished:

- rice cultivated land;- cultivation of perennial crops on raised beds;- dredge spoils (dikes).

A1.04 The highest inflow of acidity into the canal network occurs duringthe first months of the rainy season. During this time, acidity accumulated duringthe dry season in the topsoil of uncultivated land or at the surface of raised bedsor dredge spoil is washed into the canals and adjoining fields.

A 1.05 With the rising interest in the environmental aspects of acid sulphatesoils, several attempts have recently been made to quantify the acidifying impactof both of the afore mentioned sources of acidity (ref 2,3,4,5). Based on thesestudies an attempt is made to quantify the acidification due to the enlargement ofthe canal stretches as envisaged in the Inland Waterway project in relation to othersources of acidity along these stretches.

A1.06 Chapter 1 describes the mechanisms of pyrite oxidation andoxidation of pyrite before and after the excavation of the extensive network ofcanals in the Mekong Delta. Chapter 2 describes the results of the soil surveyalong the canal stretches executed during phase 11 by the Can Tho University.Chapter 3 describes the quantities of pyritic sediment to be excavated. In Chapter4 the amount of acid run-off from the dredge spoil is estimated. In Chapter 5 thebackground acidification as a result of rice cultivation or cultivation of crops onraised beds such as pineapple, eucalyptus, etc. is estimated based on a land-usesurvey. In Chapter 6 a comparison is made between the amount of acid run-offfrom the dredge spoil and the background acification. Chapter 7 discussesacidification of surface water during dredging operation and possible acidificationfrom canal slopes.

A. Oxidation of Pyrite

A1.07 The first step in quantification is the determination of the amountof acidity released after oxidation of Pyrite. The oxidation of Pyrite firstly involvesoxidation to ferrous iron and sulphuric acid (see reaction 1):

2

FeS2 + 7/2 02 + H20 --> Fe2+ + 2 SO42- + 2H+

(reaction 1)

A1.08 Under more oxidised conditions, Pyrite is directly oxidised to themineral Jarosite (K(Fe)3(S04)2(OH)6), see reaction 2. Most oxidised pyriticsediments in the Delta can easily be recognized by the presence of straw-yellowJarosite mottles.

FeS2 + 15/4 02 + 5/2 H20 + 1/3 K+ --> 1/3 K(Fe)3(S04)2 (OH)6 + 4/3so,2-+ 3H+

(reaction 2)

A1.09 Under the strongly oxidised conditions prevailing during the dryseason in the Mekong Delta, Jarosite is not stable and changes into ferric-hydroxide and sulphate. The total oxidation reaction of pyrite to ferric-hydroxideand sulphate with Jarosite as an intermediate product can be written as:

FeS2 + 15/4 02 + 7/2 H20 --> Fe(OH)3 + 2 So42- + 4H+(reaction 3)

Al1.10 As can be seen from the previous reactions, the production ofacidity is dependent on the end-product of the oxidation reaction. When the endproduct is ferrous iron: 2 mol acidity, Jarosite: 3 mol acidity and ferric-hydroxide:4 mol acidity are liberated per mol Pyrite.

A1.11 The end-product of Pyrite oxidation depends mainly on the redox-conditions in the soil together with time (ref 6). When the ground water level islowered in a badly drained area, rich in organic matter, for instance in someprofiles in the Ha Tien plain or Plain of Reeds, the end product of Pyrite oxidationwill be primarily ferrous iron and sulphate (ref 7). In most cases the amount oforganic matter is less and the end-product of Pyrite oxidation will be Jarosite.After a number of years (in the Mekong Delta 5-10 years) Jarosite will slowly behydrolysed to ferric-iron and sulphates hereby releasing one more hydrogen ion permole pyrite.

Al. 12 The theoretical amount of acidity to be released when 1 m3 of pyriticsediment is dug up and placed on the soil surface to dry can be calculated asfollows. Assuming that the wet sediment has a dry bulk density of 0.8 kg per dm3

undisturbed sediment, the total dry weight of the cubic meter is 800 kg. Assuminga pyrite content of 1 % by weight, the cubic meter contains 8 kg of Pyrite. 1 MolPyrite weighs 120 gr. 1 kg of Pyrite contains 8.3 mol Pyrite. The cubic metretherefore contains an amount of 66.7 mol Pyrite. Complete oxidation of one molPyrite to ferric-iron and releases 4 mol H+. Complete oxidation of all Pyrite withinone cubic metre wet sediment results in the liberation of 267 mol H+.

A1.13 Under the climatic and soil physical conditions prevailing in theMekong Delta, Pyrite is seldom directly oxidised to ferric-(hydr)oxide and sulphate.Usually Jarosite is formed which is gradually hydrolysed to ferric-(hydr)oxide andsulphate in the course of several decades. Since oxidation of Pyrite to Jarositereleases 3 mol H+ initially only 200 mol H+ is being released per cubic metre wetsediment. The remaining 67 mol Ho are gradually released in the followingdecades when Jarosite hydrolyses to form ferric(hydr)oxides and sulphate.

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A1.14 A part of the acidity released is directly neutralized by soil alkalinityfor instance CaCO3 present in the form of sea shells. Unfortunately the amount ofalkalinity in the Mekong Delta sediments is usually low. The amount of acidityneutralized by CaCO3 can be calculated as follows. Assuming a very low contentof 0.25 percent by weight, one cubic metre sediment contains 2 kg of CaCO3.One mol CaCO3 weighs 100 gram; 1 kg CaCO3 contains 10 mol CaCO3 . One cubicmetre of wet sediment contains 20 mol CaCO3. One mol CaCO3 is capable ofneutralising 2 mol H + . The total neutralising capacity by CaCO3 is therefore 40 molH+ per cubic metre. Fast buffering by 0.25 weight percent CaCO3 directly reducesthe amount of acidity released in one cubic metre wet sediment from 200 to 160mol H+.

A1.15 The remaining acidity is slowly neutralized by the weathering ofclay-minerals. Aluminum and Magnesium present in the clay minerals as Al- andMg-oxides are released by accelerated weathering of the clay-minerals in thepresence of the acidity (reaction 4):

Al(OH)3 Cl.y-.i,eraXs + 3 H+ --> Al3+ + 3H20 (reaction 4)

As a result, aluminum is being released in the soil solution. Aluminum isincreasingly soluble below pH 4 and its concentration increases -ten-fold with everypH unit decrease.

Al .1 6 When acidified water containing soluble aluminum is leached incanals containing near neutral water, soluble aluminum will precipitate as AI(OH)3,hereby releasing 3 mol H+ per mol soluble aluminum (see reaction 4). Whenferrous- and ferric-iron are leached into a canal, having a higher pH, 2 mol and 3mol H+ are released into the canal respectively (reaction 5 and 6).

Al8 ,0 + 3 H20 --> AI(OH)3 + 3H+ (reaction 5)Fe2+601 + 2 H20--> Fe(OH)2 + 2H+ (reaction 6)Fe3+ 3 H --> Fe(OH)3 + 3H+ (reaction7)

The total acidity of the soil water consisting of H+, Al3+s,, Fe2+. , Fe3+.,, ions, istherefore defined as the total amount of H+ released after increasing the pH of thiswater to pH 7.

Al .17 Since the concentration of H+, soluble aluminum and iron in the soilsolution contribute to the total acidity of the soil solution, total acidity isconveniently expressed in mol-equivalents per cubic metre, written as mol( + ).m-3.This is equivalent to mmol(+)/l. Following reactions 5-7, one mol of Al3+ isequivalent to 3 mol(+) Al3+. Similarly, one mol Fe2" is equivalent to 2 mol(+)Fe2'.

A1.18 A part of the acidity mentioned above is adsorbed to the soilexchange complex of the clay-soil. Assuming a CEC of 15 mmol per 100 gram drysoil, an amount of 0.15 mol can be adsorbed to the exchange complex per kg drysoil. The total neutralizing capacity of the soil complex for 1 cubic meter of wetsediment is estimated as 120 mol(+) acidity. The amount of acidity whichremained in the soil after fast buffering by CaCO3 is therefore further reduced from160 to 40 mol(+) per cubic metre wet sediment, about 20% of the originallyreleased amount of acidity.

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Al.19 Based on these exemplary calculations it can be assumed that 40mol(+) acidity is released within several months after digging up 1 m3 wetsediment. In the following decades, an additional 67 mol( +) acidity will bereleased when Jarosite hydrolysed. An additional 120 mol( +) acidity are graduallyreleased from the exchange complex. The total amount of acidity which will bereleased equals 227 mol(+).

A1.20 When the pyritic sediment however contains 2% Pyrite by weight,an amount of 246 mol(+) acidity is released within several months. In thefollowing decades 135 mol(+) will gradually be released upon hydrolysis ofJarosite. An additional 120 mol( +) will gradually be released from the exchangecomplex. The total amount released equals 500 mol(+)

A1.21 Measurements of total potential acidity (TPA) or total actual acidity(TAA) are often expressed per 100 gr dry soil. The theoretical amount of acidityreleased by oxidation of 100 gram pyritic sediment (1 % Pyrite) to Ferric-hydroxideand sulphate would be 33 mmol(+)/100 gr dry soil.

B. Oxidation of pyritic sediments before the presence of canals

A 1.22 Oxidation of Pyrite occurs when pyrite containing sediments becomeexposed to atmospheric oxygen. This happens when the ground water level islowered. Lowering of the ground water level in the Mekong Delta has firstly beena result of the burning of the natural forest together with the underlying peats,which destroyed the existing natural water buffering capacity of the land. In theabsence of the peat, the underlying clayey sediments were directly exposed to theair. As a result of the high evaporative demand in the 5 months dry season, theground water levels in most area's fell 0.5-1.5 meters below the soil surface.Computer simulations using the non-stationary-one dimensional model SWATRE,which describes the vertical water flow through the unsaturated zone indicatedthat in the Plain of Reeds, without the presence of a canal, the dry season groundwater table was lowered to 2.3 m. below the soil surface (ref 1). This was inaccordance to the observed depth of the permanently reduced Pyritic sediment.

A1 .23 During the dry season acidified soil moisture, present in the oxidisedpart of the pyritic sediment (sulphuric horizon) was transported upwards bycapillary action. The acidity accumulated in the alluvial topsoil, thereby acidifyingthis layer. In some cases acidity precipitated as acid salts for instance hydratedNaAI(S04 )2, MgAI2(SO4)4 , FeSO4, A12(S04)2.

A1.24 During the first rains of the rainy season these acid salts weredissolved and (in the absence of canals) transported by overland flow to localdepressions. This mechanism has probably resulted in leaching of higher placestogether with further acidification in depressions.

A1.25 During the flood period, acidity accumulated in the topsoil graduallydiffused into the standing floodwater, partly removing acidity both from higherelevated places as well as low depressions. In view of the enormous volume of theflood water, acid concentrations of this water remained low. At the end of theflood season, the flood drained naturally to rivers and creeks via overland flowremoving a part of the acidity.

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C. Accelerated oxidation of pyritic sediments after excavation of canals

A1.26 The excavation of canals which started in the second half of the1 9th century and continues up to now resulted in further acidification of waterand land. The excavation of canals by itself created a huge amount of acidity bydigging up pyritic sediments and exposing them to atmospheric oxygen.The change of the hydrological and transport conditions after canal constructionhowever also affected soil properties and landuse in the lands surrounding thecanals. It is therefore useful to look not only at the direct impact of canalexcavation but also at the impacts of canal "functioning" on oxidation of pyriticsediments on subsequent release of acidity.

A1.27 In the Plain of Reeds, the canals were connected upstream to theMekong river and downstream to the Vai Co rivers. In the Ha Tien Plain, the canalswere connected upstream to the Bassac river and downstream to the Gulf ofThailand. In the Ca Mau peninsular, the canals connected the Bassac river to thetidal creeks flowing into the Gulf of Thailand and South China sea. Other canalsconnected individual branches of the Mekong river. Perpendicular to these primarycanals large secondary canals were dug. Perpendicular to these tertiary canals,etc, increasing time by time the canal density in the delta.

The canals serve several purposes:

Transportation and settlement:

A1.28 Firstly the canals provided a way of transportation through theswampy lands, enabling the settlers to enter the area. Settlers settled on thehigher dredged spoils along the canals where they were safe for the annualflooding.

Drainage:

A1.29 The canals improved the drainage of the annual flood. Inundationdepth in most places decreased and depressions which previously remainedinundated throughout the year now fell dry during the dry season. The lengtheningof the dry period, especially in depressions that were previously flooded all yearround, resulted in lowering of the dry season ground water levels and thusoxidation of Pyrite. Since in these places the pyritic sediment comes close to thesoil surface, acidification was locally severe.

A1.30 During the first months of the rainy season, the presence of thecanals cut off the overland flow of acid water from higher places to localdepressions, draining it in the direction the sea. Furthermore, canals directlydrained acidified ground water present in large soil pores of the acidified pyriticsediments. As a result, during the first months canal waters were extremelyacidified.

Irrigation

A1.31 Throughout the year, canals carried fresh water from the Mekong,supplying irrigation water to farmers fields. The fresh water supply of the canalsis especially important during the dry season when evaporative demand exceeds

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rainfall. Especially towards the end of the dry season, canal water levels tend tobe higher than surrounding ground water levels resulting in infiltration of canalwater in surrounding lands rather than drainage. This phenomenon has beenobserved in the Plain of Reeds by Tran minh Thuan (ref 1), Bil (ref 2), Verburg (ref3) and in the Ca Mau Peninsular by Hanhart and Ni. It is not clear whether thisobservation is true for all area's where canals cut through acid sulphate soils.Hydrological models describing water flow through the canals and rivers of theMekong Delta, may shed more light on this matter.

A1.32 In most cases, it can be stated that canals do not contribute toadditional lowering of the lowest dry season ground water level and subsequentadditional oxidation of Pyrite. The presence of the canals however enables thefarmers to cultivate an irrigated dry season rice crop. The cultivation of theirrigated rice crop strongly promotes further leaching of the topsoil and underlyingoxidised pyritic sediments. The leaching is very efficient as a result of acontinuous head difference between the water layer in the rice field and the lowerdry season canal water level. Leaching is furthermore strongly promoted by thedense network of ditches dug by the farmers for irrigation and drainage purposes.

Al1.33 As a further remark on the irrigation function of the canals attentionshould be paid to the positive impacts on soil and land when irrigated dry seasonrice is cultivated along the canals. It should be noted that dry season irrigated ricecultivation aims at maintaining water permanently on the land. Farmers achievethis by:

- repeatedly puddling the field, generating a hard and badly permeable ploughlayer at the bottom of the topsoil,

- construction of small field bunds around the rice field,- often closing ditches in order to maintain a high drainage basis.

A1.34 Doing this farmers partly reinstall the buffering function of landwhich existed before the excavation of the canals and clearing of the forest. Thishas the following consequences for the acidity of soil and water:- all acidity is leached from the topsoil,- the presence of the plough layer, together with continuous downward

water flow prevents capillary rise of acidified water during the dry season,- the permanently reduced conditions in topsoil and oxidised part of the

pyritic sediment prevent further oxidation and acidification of these soillayers, stopping the previously continuous production of acidity during eachdry season.

- decrease of the ground water flow through macro-pores in the oxidisedpart of the pyritic sediment to the ditches and canals by decreasinghydraulic conductivity of the plough layer, closing ditches and maintaininga high drainage basis.

A1.35 As a final remark on the irrigation function of the canals it should benoted that instead of rice cultivation, farmers may also choose to grow perennialcrops like Eucalyptus trees, Cashew trees, pineapples, sugarcane, mung beans,vegetables, etc on raised beds. These crops must be cultivated on raised beds inorder to maintain a sufficiently aerated rootzone during the flood. Sugarcane doesnot require a thick aerated rootzone and can therefore be grown on lower raisedbeds. Eucalyptus, pineapple and cashew do not require dry season irrigation.

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Sugarcane, mungbeans and vegetables however do need dry season irrigation. Allcrops benefit from the drainage function of the canal which removes acid drainagewater and high floods.

A1.36 The construction of raised beds results in the exposure of pyriticsediments to atmospheric oxygen. In chapter 2 some further remarks are made ondifferent ways to construct the raised beds in order to improve crop growth andminimize release of acidity. Contrary to rice cultivation, growing of perennial cropson raised beds aims at optimal removal of water from the land together with anattempt to maintain oxidised conditions throughout the year. The effects ofcultivation of perennial crops on raised beds on land and water can be summarizedas follows:

- digging up (partly oxidised) pyritic sediments exposes these sediments toatmospheric oxygen, resulting in gradual oxidation of Pyrite and oxidationof the soil,

- bringing soil on the surface can be regarded as a lowering of the drainagebasis resulting in increased oxidation of the soil,

* the construction of the raised beds increases the surface exposed to rainfallresulting in runoff of acidified water throughout the rainy season,

- the presence of the dense network of ditches usually connected throughopen collector ditches with the canal network results in a massiveacidification of canal water and downstream aquatic environments.

D. Summary of impacts of canal excavation and functioning on acidificationof land and water

A1.37 In order to assess the combined impacts of the construction andfunctioning of the canal network it is useful to distinguish the impact on the landand the impacts on the water. The possibility to cultivate irrigated dry season riceor irrigated perennial crops on raised beds can be seen as indirect impacts of thepresence of the canals, which may increase or decrease the direct impacts of thecanals on water and land. Negative impacts are indicated by "-", positive impactsare indicated by " + ".

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Figure 1.01: Summary of the impacts of the presence of canals on acidificationof water and land

IMPACTS ON LAND:

* direct impact of canal construction on land:- digging or dredging of pyritic sediments exposes Pyrite to

atmospheric oxygen resulting in acidification of the sedimentand acid runoff from the "dikes" on adjacent land

* direct impact of canal functioning on land:- lengthening of the dry period resulting in additional lowering

of the dry season GWL and subsequent increased oxidationof Pyrite

+ improved leaching of acidity from the soil,

* indirect impact of canal functioning on land:+ water conservation management for dry season rice

cultivation reduces the dry period, increases the GWL andprevents further oxidation of Pyrite,

+ idem further improves leaching of acidity from the topsoil,- construction of raised beds exposes pyritic sediments to

atmospheric oxygen, resulting in acidification of thesediment,

+ water evacuation management associated with thecultivation of perennial crops on raised beds improvesleaching of acidity from the topsoil,

IMPACTS ON WATER:

* direct impact of canal construction on water:- acid runoff from the "dikes" acidifies canal water and

downstream aquatic environments

* direct impact of canal functioning on water:- increased drainage of acid surface- and groundwater

resulting in acidification of canal water and downstreamaquatic environment during the first months of the rainyseason,

+ fresh water availability in canals during the dry season.

* indirect impact of canal functioning on water:+ water conservation management for rice cultivation

decreases the drainage of acid surface- and ground waterdrainage into canals and downstream aquatic environment.

- water evacuation management associated with cultivation ofperennial crops on raised beds by the presence of densenetwork of ditches increases the drainage of acidified waterto the canals.

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2. SOIL SURVEY

A. Methodology

A2.01 As a part of the phase 11 activities, a soil survey was held along thestretches where canal enlargement was envisaged. The soil survey was executedby the Soil Department of the Can Tho University according to the terms ofreference included in the phase I report. The soil survey was executed inOctober/November 1995.

A2.02 The soil survey aimed at identifying where acid sulphate soils wouldbe encountered in the subsoil, if present to describe the relevant physical andchemical characteristics of the soil and to investigate if the pyritic sediment isunderlain by non-pyritic sediment. This is of special importance since it mayprovide the dredging staff with an opportunity to cover the pyritic sediment withunderlying non-pyritic sediment.

A2.03 Soil samples in 23 cross-sections of the stretches envisaged forcanal enlargement. The locations of the cross-sections are indicated in figure 2.02.The samples have been taken using a soil auger with 4 extension rods. Sampleshave been taken from the canal bank until a depth of 5 m. The soil profiles havebeen described. In some locations additional samples have been taken from thecanal bottom using a boat.

A2.04 The presence of potential acidic soil layers was identified using asimple field test. The soil samples were quickly oxidised using H202 (32%). Afterquick oxidation the pH of the soil sample was checked using a pH paper. Insamples containing a relevant amount of potential acidity (either pyrite or otherreduced organic sulphur-species) a pH of 1.0 or 1.5 was noticed. Samplescontaining potential acidity were stored in plastic bags and taken to the laboratoryfor further analysis. Samples where pH after quick oxidation remained above 1.5were not considered to contain a significant amount of potential acidity.

A2.05 In the laboratory the wet bulk density (WBD) in gr/cm3 was recordedfirst. After establishing the WBD the soil was oxidised rapidly using 32% H202. Asaturation extract was obtained by centrifugation. The saturation extract wastitrated with NaOH for the determination of the total potential acidity (TPA).Furthermore the aluminum content of the saturation extract was determined bycolorimeter.

A2.06 The quick oxidation results in complete oxidation of pyrite and otherreduced sulphur-species to ferric-hydroxide and sulphate under the release of 4mol H' per mol pyrite. A disadvantage of quick oxidation of the sample using apowerful oxidant as H202 is the fact that a part of the neutralising CaCO3 presentin the sample is oxidised as well and loses it's neutralising capacity. Due to thefact that possible neutralising CaCO3 in the sample is lost, the total potentialacidity of the sample maybe somewhat overestimated. As mentioned in chapter1, in samples containing up to 2% pyrite, neutralising capacity of CaCO3 is rathermodest compared to the huge amount of acidity released. A slow oxidation testby allowing the sample to dry for several months in the laboratory underestimatesthe amount of TPA since under the dry circumstances bacterial oxidation ishampered. The most accurate way to determine TPA would be to regularly wet

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the sample for several months. In the framework of this project, this methodwould be too time-consuming.

B. Results

A2.07 For a complete description of the results reference is made to the:"Report on soil and landuse for the main inland waterways Ho Chi Minh city - KienLuong, Ho Chi Minh city - Can Tho and Can Tho - Ca Mau", by the Dept. of SoilScience of the Can Tho University.

A2.08 The results of the soil survey are summarized in Table 2.01 whichdescribes various characteristics of the soil layers containing potential acidity(pyritic layers) and if present the characteristics of an underlying layer withoutpotential acidity (non-pyritic layer). For the soil layers the following characteristicsare described:

- depth and thickness of the soil layer;- wet bulk density (WBD) in g/cm3;- total potential acidity in mmolH+)/100 gram;- aluminum content in mmol(-i-)/100 gr.;- soil colour according to the Munsell colour charts:

Table 2.01: Physical and chemical characteristics of the soil layers in the cross-sections.

PYRITIC LAYER 1: PYRITIC LAYER 2: HOW FrRITIC LAYER

canal: pro. depth to 0 WBO TAwet Alt frm to 0 V60 TAat Alwt fro to Dfrom

J/cm3 mmo()_ol(+) 9/cm3 ol(+) ol(+)/ Ccm P.l00 gr 100 gr 100 gr 100 gr

Cho CSo Cl 335 357 0.2 1.63 Z3 4 n.r n.r n.r n.r n.r n.r

Rang Thit CZ n.r n.r h.r p.r n.r n.r

XaMgo C3 0 0 0.0 0 0 0 0 0 0.0 0 0 0Ca 160 360 2.0 1.42 30 S8 0 0 0.0 0 0 0 360 460 1CS 100 IS0 0.8 1.29 *9 12 0 0 .0 0 0 I6O 400 2.2

Tat Cay trm CS 95 230 1.4 1.53 93 6 0 0 0.0 0 0 0 230 ZlO 1.5

Tram Canh Den C7 40 IS0 1.1 1.08 144 18 1S0 320 1.7 1.48 81 7 320 420 1Ca 120 220 1.0 1S1 S0 S 0 0 0.0 0 0 0 220 350 1.3C9 50 120 0 7 I 51 43 8 120 290 1.7 1.5 16 4

Lap Wo-Sa Dec C12 268 430 1.6 1.43 36 5 0 0 0.0 0 0 0 430 S00 0.7

Rach Soi-hau Ciang C13 240 260 0.2 1.62 IS 2 0 0 0.0 0 0 0. 260 400 1.4C14 0 00.0 0 0 0 0 00.0 0 0 0c15 0 00.0 0 0 0 0 00.0 0 0 0C16 0 0 0 0 0 0 00.0 0 0 0t17 0 0 0 0 0 0 0 0 0.0 0 0 0

Rach Gia-Ha Tien ClI 0 0 0.0 0 0 0 270 400 1.3 I.S1 I 1C19 112 165 0.5 1.39 38 8 165 400 2.4 1.37 104 12CZ 190 400 2.1 1.55 9316 0 00.0 0 0 0C21 110 153 0.4 1.43 57 14 153 350 2.0 1.23 116 10 350 400 0.5C22 IS0 250 1.0 1.56 51 6 250 600 1.5 1.S5 43 5C23 250 320 0.7 3.69 34 5 0 0 0.0 0 0 0 320 400 0.0

_ ... .. ... .. _ .. , _ .... ., .._ .. _.... _-_-.-....-------_-.-.-.--..-_-_-_-.-..

1 1

Table 2.02: Description and colour of the soil layers in the 23 cross-sections

PYRITIC LAYER 1I PYPITIC LAYER 2 RON PYRITIC LAYER:

canal: pro.description pyritic colour description pyritic colour description eatrixsediment code sediment code colour

Cho Gao Cl n.r nfr not present

Mang Thit C2 n.r n r not present

Xa Wc C3 n.r n.r not presentCa dark grey heavy clay 2.5Y4/0 not present dark gry fire sandy clay 1OYR4/1CS very dark greyish brown heavy Clay 10YR3/2 not present grey clay SY5/I

Tat Cay tram C6 dark gry heavy clay 2.5Y4/0 not present dark grey clay SYR4/I

Tram Canh Den C7 dark reddish brown heavy clay SYR3/2 grey heavy clay IOYRS/I dark grey fine sandy clay 5Y4/ICB dark grey heavy clay 7.SYR4/0 not relevant dark grey fine sandy clay 7.5YR4/0Cg dark greyish brown heavy clay 2.514/2 greyish brown heavy clay 2.5YS/2 unknown

Lap Vo-Sa Dec C12 dark-very dark grey silty clay 2A5Y4/0-3/D not prevent dark grey fine sandy loaa 2.5Y/0

Rach Soi-Hau Giang C13 dark grey silty clay Z.SY4/0 not present grey silty clay SYS/ICIS not present

5I6 not presentCI7 not present

Rach Gia-Ha Tien tCS not presentCI9 black clay 2.5Y2/0 dark grey silty clay SY4/1C20 dark-very dark grey silty clay SY4/1-3/I not presentCAl reddish brown silty clay SYR4/3 dark grey clay loan 5Y4/I light grey-grey clay IYR6/1C22 grey-dark grey silty clay SY5/i-4/I light grey-gry 5Y5/1-6/1'R3 dark gry heavy clay SY4/I not present light brownish clay 2.5Y6/2

A2.09 The TPA of the soil samples is expressed in mmol( + )/1 00 grammesof wet soil. The TPA of the soil layers is classified into three classes:

TPA Classification:

0-25 slightly acid25-50 moderately acid> 50 severely acid

A2. 10 According to Table 2.01, in cross-section Cl in the Cho Gaocanal, a thin slightly acid soil layer is found at 3.35 m. depth from the river bank.This amount of acidity is not expected to do great harm after dredging. In cross-section C2 in the Mang Thit canal, no potential acid layers were encountered.

A2.1 1 In the first part of the Xa No canal, at cross-section C3, nopotentially acid subsoil was encountered.

In the middle of the canal at cross-section C4, a layer of 2.0 m thickness from160-360 cm depth was encountered, consisting of severely acid dark grey heavyclay. Below this sediment a dark grey fine sandy clay was found which containedno acidity. This fine sandy clay is probably deposited at a higher rate compared tothe pyritic heavy clay. At this rate there was not sufficient time for formation ofsignificant quantities of pyrite.

12

At the end of the canal, at cross-section C5, a layer of 0.8 m thickness isencountered from 100-1 80 cm depth, consisting of moderately acid dark greyishbrown heavy clay. Below this layer, a non-pyritic layer is found consisting of greyclay. In this case, the pyritic soil can be distinguished from the underlying non-pyritic soil by it's greyish brown colour.

A2.12 In the Tat Cay Tram canal one cross-section (C6) wasstudied. In this cross-section a pyritic layer of 1.4 m thickness was encounteredat 95 to 230 m, consisting of severely acid dark grey heavy clay. At 2.30 m.depth a non-pyritic sediment is fount consisting of almost identical dark grey clay.The difference between the two sediments is hard to see by the eye. In the fieldthe two layers can be identified using the quick oxidation test with 32% H202.

A2.13 In the Tram Canh Den canal three cross-sections wereinvestigated: C7, C8 and C9. In all three transects, severely and in C9 moderatelyacid sediments are found at shallow depth. The thickness of the pyritic sedimentsranges from 1.0 to 2.8 m. The severely acid sediments consist of dark reddishbrown heavy clay (having an extremely high TPA of 144 mmol( + )/1 00 grammeswet soil) and dark grey heavy clay. In C9 a dark greyish brown sediment is foundof moderate acidity.

Below the severely and moderately potentially acid sediments a non-acidicsediment is found consisting of dark grey sandy clay. This material can easily bedistinguished from the potentially acid sediments by it's sandy texture.

A2.14 In the Lap - Sa Dec canal, one cross-section was studied:Cl 2. a layer of 1.6 m thickness is found from 270-430 cm depth consisting ofmoderately acid dark to very dark grey silty clay. Below this sediment non-potentially acid dark grey sandy loam is found. This sediment can be distinguishedby it's different texture (sandy loam in stead of silty clay).

A2.15 In the Rach Soi - Hau Giang canal 5 cross-sections wereinvestigated Cl 3 to Cl 7. Only in the first transect (Cl 3) a thin 0.2 m layer wasfound at 240 cm depth consisting of slightly acid dark grey silty clay. Below thislayer non-potentially acidic layer was found having the same soil colour andtexture.

A2.16 In the Rach Gia - Ha Tien canal 6 cross-sections werestudied: Cl 8 to C23. In the first cross-section (Cl 8) no potentially acid layerswere encountered. In the remaining 5 cross-sections mostly severely potentiallyacid layers were encountered at shallow depth. As expected the potentially acidlayers were of great thickness. In three cross-section (Cl 9, C20 and C22) thepotentially acid layers extended to the maximum auguring depth of 4 m. In twocross-sections (C21 and C23) the potentially acid layers were underlain by non-acidic layers.

A2.17 The potentially acid sediments in cross-sections C19 and C20consist of moderately acid black clay and severely acid dark to very dark grey clay.In cross-section C21 the severely acid dark grey clay loam is overlain by 40 cmmoderately acid reddish brown silty clay. Below the severely acid grey clay loam,at 350 cm depth, a non-potentially acid light grey-grey clay is found (1 OYR6/1).This sediment is probably the old alluvium of Pleistocene age which surfaces in the

13

Plain of Reeds at the border with Cambodia. In cross-section C22 this material isfound at 250-400 cm depth but it now contains moderate potential acidity. incross-section C23, the moderately acid dark grey heavy clay is underlain by a lightbrownish old alluvial clay starting at 320 cm depth.

A2.18 The dark to very dark grey (sometimes black) heavy clay containssevere acidity in the cross-sections near Rach-Gia, gradually changing to moderatepotential acidity in the direction of Ha Tien. The dark grey clays are underlain bylight grey (sometimes light brown) to grey old Pleistocene clays. In some casesthis sediment may contain some potential acidity but far less compared to theoverlaying dark grey clays. In the direction of Ha Tien, the layer of dark greyseverely acid sediments become thinner and contain less potential acidity,Nevertheless, the light grey - light brown Pleistocene clays occur at gradually moreshallow depth.

14

3. EXCAVATION OF POTENTIALLY ACID SEDIMENTS

A3.01 The quantity of potentially acid sediments which will beexcavated depend on the depth of occurrence and thickness of the potentially acidlayers (pyritic layers) and the required canal dimensions.

A3.02 Based on the results of the soil survey, cross-sections of theexisting canal dimensions at 100 m interval and required canal dimensions, thetechnical staff of the Inland Waterways and Port Modernisation Project calculatedthe total dredging area in M2 and dredging area consisting of potentially acidsediment in m2 for each cross-section. The total area to be dredged and the areaconsisting of potentially acid sediment were multiplied with the distance for therepresentative canal distance (usually 80-1 20 m.) and accumulated for the entirecanal. In order to visualize the location of the potentially acid sediments in thecross-sections a number of representative cross-sections were presented. Thefollowing volumes were prepared:

- List of dredging and pyritic volume Xa No canal;- List of dredging and pyritic volume Tat Cay Tram canal;- List of dredging and pyritic volume Trem Canh Den canal;- List of dredging and pyritic volume Lap Vo Sa Dec canal;- List of dredging and pyritic volume Rach Soi Hau Giang canal;

- List of dredging and pyritic volume Cay Nhat creek- List of dredging and pyritic volume That Thu Ganh Hao river- List of Dredging and pyritic volume Rach Gia Ha Tien canal.

The total volumes of pyritic sediment dredged are listed in Table 3.03

Table 3.03: Total dredging volumes and volume pyritic sediments to be dredged.

canal: pro.canal length: Dredging volwme: length total voliue volume both P-layer

from to total pyritic laywr I pyritic layer 2: canal dr dging P-lsyer P-layer layersstart nd start nd start nd strteh volue 1. 2. dredging

volume. ' 1B33 Ml. lO. kV0O 03 I000 3 1000 B3 1000 m3 S

Cho GAD cl

Pang Thit C2

X& NO C3 0 23923 0 904241 0 0 0 0 23923 904 0 0 0 CC4 23923 36011 904241 1927581 0 473681 0 0 12018 1023 474 a 474 46CS 36011 39155 1927581 2113411 473681 511834 0 0 3144 186 3B 0 38 21

0Tat Cay C" U 0 4236 0 271953 0 71111 0 0 4236 272 71 0 71 26

True Cdnh Drn C7 0 0 0 0 0 0 0 0 0 0 0 0 0CS 0 33205 0 3021013 0 499942 1063615 33205 3021 S00 10604 1564 52C9 0 0 0 0 0 0 0 0 0 0 0 a 0

Lap vo-Sa Dee C12 0 3889 0 743752 0 361042 0 0 388916 744 361 0 361 49

Rach Soi-asu Siung C13 0 12193 0 200318 0 7768 0 0 12193 200 8 0 8 AC14 12193 24824 200318 83574S 7768 7B31 0 0 12631 635 0 0 0 DCIS 24824 33630 935745 1309207 7838 7838 0 0 Om06 473 0 0 0 0C16 33630 45762 1309207 1630426 783U 7838 0 0 1215Z 521 0 0 0 0C17 45782 53764 I830628 2019574 783 7938 0 0 79B2 IR9 0 0 0 0

Rach Gia-Ila ev n0C18 0 6626 0 245529 0 19672 0 124569 6626 246 20 125 144 59CIg 6626 19020 245529 804204 19672 46254 124569 640392 12394 859 27 516 542 97C20 19020 32958 804204 1569993 46254 697004 640392 641847 13938 7K6 651 1 652 85CZ1 32958 44401 J569993 2144(9 897004 750631 U41847 944604 11443 575 54 303 357 62C22 44401 53536 2144809 2613516 750831 859978 944604 1156139 9137 469 109 212 321 68C23 53538 62534 2613518 3000212 859978 927486 1158139 1186S87 9096 3B7 as 1 68 18------------------------------------------------------------------------------------------------------------------------------------------

15

A3.03 The calculation of the total dredging volumes and volumesof pyritic sediment to be dredged indicate that the largest amounts of pyriticsediments to be dredged are located in the Tram canh Den canal and Rach Gia toHa Tien canal. The calculations indicate that in large sections of the Rach Gia - HaTien canal the major part of the sediment to be dredged consists of pyriticsediments.

A3.04 The total quantities, based on 30 m bottom width (14.4 x1 06 mi

3) of potential acidity of the pyritic sediments to be dredged can be derived

from the results of the physical and chemical analysis of the soil samplespresented in Table 3.04.

Table 3.04: Potential acidity of the pyritic sediments

canal: pro. length volume ME0 TAwet acidity volum kw0 Tawet acidity aciditycanal P-layer P-layer P-layer P-layer P-layers

stretch 1. 1. 2. 2. 1i2

k 1000 *3 9/ca3 ol(1)/ lOOO ol(+ 1000 e3 9/c83 _ol(+)/ 1001Ol(+)1000m0l(*)100gr 100 ogr

-- _--- --- _-- ---- ,,-,-_ _ -- ,- -- _ _-- --- -- , ........... _,__,__,__.. ,,,..... .,__. ............ .... ..... .... .... . -- - -

Cho GAO cl 1.63 23.24 0 n.r n.r 0 0

ang Thit C2 0 n.r n.r 0 0

Xa No C3 23923 0 0 0 0 0 a 0 0 0C4 12088 474 1.42 30.31 203873 0 0 0.00 0 203873CS 3144 38 1.29 99.34 48902 a 0 0 0 48902

Tat Cay tram C6 4236 71 1.53 93.14 101395 0 0 0 0 101365

Tram Canh Den C7 0 0 1.03 144.35 0 0 1.48 81.47 0 0CS 33205 S00 1.5S S0.09 378136 164 0 0 0 378136C9 0 0 1.1 42.83 0 0 1.5 15.69 0 0

Lap Vo-Sa Dec C12 38898 361 1.43 36.41 187981 0 0 0 0 187981

Rach Soi-hau Giang C13 12193 8 1.62 15.0 1894 0 0 0 0 1894C14 12631 0 0 0.00 0 0 0 0 0 0Cis 8a06 0 0 0.00 0 0 0 0 0 0C16 12152 0 0 0 I 0 0 o 0 0C17 7982 0 0 0 0 0 0 a 0 0

REach Gia-ha Tien CIS 6826 20 0 0.00 0 125 1.S1 1.1 2069 2069Cl9 12394 27 1.39 37.92 14011 516 1.37 103.59 732047 746058C20 13938 651 1 55 93.18 939872 1 0 0 0 939372C21 11443 54 1.43 57.07 43928 303 1.23 115.56 430335 474263C22 9137 109 1.56 51.34 87076 212 1.55 42.7 140004 227080C23 9096 G8 1.59 33.65 36119 1 0 0 0 36119------------- -----------------------------------------------------------------------------------------------------------

A3.05 The calculations presented in table 3.04 indicate that thedredge spoils in the Rach Gia - Ha Tien canal contain the largest amount ofpotential acidity. The spoils in the Xa No canal and Tram Canh Den canal containsmaller amounts of potential acidity.

16

4. ESTIMATION OF THE ACID RUN-OFF FROM THE SPOIL

A4.01 The potential acidity contained in the spoil is not immediatelyreleased to the environment. Some spoils may still contain most potential aciditywithin the inner core of the spoil for many years. This phenomenon can beexplained by the fact that the pyritic sediments in the Mekong delta consist ofclays and silty clays of heavy to very heavy texture. Oxidation of pyrite within thespoil is strongly hampered by the slow rate of oxygen diffusion into the spoil.Secondly, once pyrite has been oxidised, diffusion of the acid end-products ofpyrite oxidation to the surface of the spoil is equally slow. In the heavy claystransport of acid end-products of pyrite oxidation such as H+, Fe2", Fe3+, Al3+ andS042- mainly takes place by mass-flow during capillary rise of soil moisture duringthe dry season or dry spells during the early rainy season. Once the acid end-products reach the soil surface they precipitate as acid salts. These acid salts aredissolved by rainfall and washed either into the canal or into the fields behind thespoil as acid run-off.

A. Methodology

A4.02 The determination of the concentration of the run-off fromraised beds or dikes is a very difficult issue. The overall process can be divided inthe following processes:

step 1: oxidation of Pyrite in the raised bed or dike,step 2: diffusion of acidity through capillary rise to the surface of

the raised bed or dike,step 3: washing the acidity from the dikes by rainfall.

A4.03 The rate of oxidation of Pyrite (step 1) is firstly determinedby the rate of diffusion of Oxygen into the soil. This rate is very slow. Mostoxidation of Pyrite however takes place below pH =4, with the very mobileferrous/ferric iron as an intermediate. The necessary oxidation of Fe2" to Fe3"needed for this reaction is catalysed at low pH by Thiobacillus Ferrooxidans (ref6). Since diffusion rates through the heavy clay soil are extremely slow, cracksand macropores in the soil play a very important role.

A4.04 The movement of acidity to the surface of the raised bed andmore importantly to the surface of cracks (step 2) takes place through mass flow(capillary flow) and diffusion. The velocity of this flow depends on the moisturecontent of the soil. When the moisture content is high, mass flow and diffusionare high. During the dry season however, the surface of the soil dries out,resulting in extremely low flow rates. This explains the phenomenon that theamount of acidity washed from dikes and raised beds during the first rainfall of therainy season is much lower than the amount of acidity washed out by the secondor third shower (ref 3).

A4.05 The complexity of the processes described in A4.0 1 to A4.04makes it very difficult to quantify the rate of the overall process. Quantificationof this overall process is the subject of a PhD study by Le Quang Minh (Can ThoUniversity) at the Wageningen Agricultural University. In general it can be said thatPyrite oxidation in dikes and raised beds is slow. Pyrite oxidation takes place nearthe surface of the soil. A 10 year old dike may contain large amounts of Pyrite at

17

say 50 cm below the soil surface. Table 4.05 shows amounts of potential acidityin samples taken from fresh pyritic sediments and older dikes in the Mekong Delta.

Table 4.05: Potential acidity of freshly excavated pyritic sediments and older dikesand raised bed.

Location Potential acidityin mmol(+)/100 gr

Plain of Reeds (Verburg):fresh black pyritic sediment (3 depths) 250-500dike, 5 years after construction, surface 120idem, 10 cm depth 90idem, 30 cm depth 58idem, 40 cm depth 75idem, 50 cm depth 170

Ha Tien Plain along Ha Tien to Rach Gia canal (NEDECO):fresh grey pyritic sediment, depth 6 m 76recently excavated sediment near Triton 103

Ca Mau peninsular along the Cai Tram canal:recently excavated pyritic sediment 27

A4.06 Verburg found that the concentration of total acidity fromrun-off from fresh spoil increased from an average of 3 mmol( + )/l during the firstshower in March to 1 20 mmol( + )/I after a heavy shower in June. He explains thegradual increase in concentration by the re-wetting of the soil after each showerenabling capillary rise during the following dry period. In a 5 years old spoil, hemeasured a concentration of 25 mmol(+)/l during the first shower in Marchremaining relatively constant throughout the remaining showers. The spoilconsisted of organic pyritic sediment having an average potential acidity in therange of 60-90 mmol( + )/1.00 gr dry soil or an estimated 30-45 mmol/1 00 gr wetsoil (moderate potential acidity).

A4.07 For the purpose of quantification, the followingconcentrations of run-off are assumed for dredge spoils consisting of sedimentsof potentially light, moderate and severe acidity:

type of sediment TPA est. concentrationof run-off

mmol(+ )/100 ar mmol(+ )/Illight pot. acidity 0-25 20moderate pot. acidity 25-50 40severe pot. acidity >50 60

A4.08 The yearly amount of run-off from dredge spoils can be estimatedusing a simple water balance. The average yearly rainfall in Rach Gia for instanceis 1464 mm/year. The average ETo Penman is 1603 mm/year. The crop coefficientfor a bare soil is 0.5, resulting in an estimated actual evaporation of 800 mm/year.The storage in the soil is zero over one year. The run-off of the raised bed cantherefore be estimated as 1464 - 800 = 664 mm/year.

18

A4.09 The total volume of run-off from the raised beds can becalculated by multiplying the run-off water layer with the surface of the spoilexposed to rainfall. The surface of the spoil exposed to rainfall is therefore a veryimportant parameter to control the amount of run-off from the spoil. When forinstance an amount of 1 00 m3/m of pyritic sediment is simply sprayed on the landallowing it to spread out evenly, the thickness of the spoil may as thin as 20 cm.The surface of this spoil exposed to rainfall is 500 m2/m. When the spoil is keptbetween bunds of 1 m height, the surface of the spoil is reduced to about 100m2/m. resulting in an amount of 20% of the run-off of the spoil without bunds.When the height of the bunds is increased to 2 m, the surface of the spoil isfurther reduced to about 50 m, and run-off is further reduced to 10% of the run-off of spoil without bunds.

B. Calculation of the acid run-off from dredge spoils

A4.10 Table 4.06 shows the calculation of the acid run-off fromthe dredge spoil. The acid run-off from the spoil is strongly determined by thesurface of the spoil exposed to rainfall. In the Mekong delta dredge spoil is usuallypumped on the canal bank without taking much measures to maintain the spoil.The resulting spoil is therefore quite low and the surface exposed to rainfall verylarge. Pyrite within the spoil is usually quickly oxidised and resulting acidity iseasily transported to the surface of the spoil where it is transported quickly tosurrounding fields and canals. This situation is represented in the first case(H =0.2 m.).

A4.11 When disposal sites along the canals are surrounded bybunds, the height of the spoil increases. The increase in height results in an equaldecrease of the surface exposed to rainfall and acid run-off. The heigth of the spoilis usually 1.5 meter. After gradual dewatering, the height usually decreases to1.2-1.3 m. This situation is represented in the second case (H = 1 m.).

A4.12 A further increase in the height of the dredge spoil to 2.0meter would result in a further decrease of the yearly acid run-off. Constructingdredge spoil higher than 1 meter will propably require dredging in two phases, thesecond phase taking place after settling of the spoil disposed during the firstphase. This option is probably not cost-effective. The situation is represented bythe third case (H = 2.0 m.).

19

Table 4.06: Yearly acid run-off from pyritic dredge spoils with and without bunds.

canal: pro. length total both total surface surface surface est. est. acid acid acidcanal dredging P-layers dred log spoil spoil spoil run-off conc. run-off run-off run-off

stretch volume dredging volume W.0.2 m. 1 . =2 . run-off h-0.2 m H-1 n H-2 m.vo lume

m. 1000 m3 10C0 113 m3/l .2/m 12/l mZ/im m/year mol(.)/o3 mol(+)/oml(+)/m .l(+)/m

Cho Gao Cl

PAng Thit C2

XIlNo C3 23923 904 0 38 189 38 19 0.6625 0 0 0 0Ca 12088 1023 474 85 423 85 42 0.6625 40 11217 2243 1122CS 3144 186 38 59 296 S9 30 0.6625 60 11747 Z349 1175

Tat Cay tran C6 4236 272 71 64 321 64 32 0.5625 60 12760 2552 1276

Tra Cann Den C? a a 0 0 0 0 0 0.6625 60 0 0 0CS 33205 3021 1564 91 455 91 45 0.6625 60 18082 3616 I80C9 0 0 0 0 0 0 a 0.6625 30 0 0 0

Lap Vo-Sa Dec C12 38898 744 361 19 96 19 10 0.662S 40 2533 507 253

RaCh Soi-Hau Giang C13 12193 200 8 16 82 16 8 0.6626 20 1088 218 109C14 12631 635 0 50 252 50 25 0.5625 0 0 0 0C15 8806 473 0 54 269 54 27 0.6625 0 0 0 0C16 12152 521 0 43 215 43 21 0.6625 0 0 0 0C17 7982 189 0 24 118 24 12 0.6625 0 0 0 0

Rach Gia-Ha Tien CIS 6626 246 144 37 185 37 19 0.6625 40 4910 982 491C19 12394 859 642 45 225 45 23 0.6625 60 8959 1792 896C20 13938 766 652 55 275 55 27 0.6625 60 10920 2384 1092C21 11443 SYS 357 S0 2SI S0 25 0.6625 60 9984 1997 998C22 9137 469 321 51 256 51 26 0.6625 50 8496 1699 8s0C23 9096 387 68 43 213 43 21 0.6625 40 5633 1127 563

A4.1 1 The estimation of the yearly amount of acidity washed from thedredge spoils in the canal stretches presented in Table 4.06 that the largestamount of acidity is being released in the Rach Gia - Ha Tien canal. Considerableamounts of acidity are being released in the Xa No, Tat Cay Tram and Tram CanhDen canals.

C. Acid run-off from spoils where potentially acid sediment is covered withnon-potentially acid sediment

A4.12 The amount of acid run-off can be strongly reduced by cappingpotentially acid sediments with a layer of 0.5 meter non potentially sediments. Inthose canal stretches where below the potentially acid sediments a non-potentiallyacid sediment is found at shallow depth, this can be achieved by the system ofselective dredging. In canal stretches where the lower limit of the potentially acidsediment lies below the required depth of dredging, selective dredging can not beused to obtain suitable material to cap the spoil In this case non potentially acidsediment may have to be transported to the area.

A4. 13 When widening a canal, the depth of the cutter can be adjusted tothe lower limit of the potentially acid sediment, in order to remove potentially acidsediment first (stepped box cut). In the system of selective dredging, potentiallyacid sediments and non-potentially acid sediments are be pumped into specialdisposal area's, separated by bunds. The non-potentially acid materials can betransported later to the disposal area's with potentially acid sediments using dryearth moving equipment.

A4.14 In most parts of the Mekong Delta the layer of potentially acidsediment is rather thin (0.5-2 m.). In these area's the potentially acid sedimentconsists of moderate to severely potentially acid grey to dark grey (sometimes

20

black) heavy clay. The underlying non-potentially acid sediment usually consistsof grey to very grey sandy or silty clay. The lighter texture can be explained by thefact that this sediment has been deposited under more turbulent marine orestuarine conditions. This situation is encountered in the Xa No, Tat Cay Tram,Tram Canh Den and Lap Vo Sa Dec canals.

A4.15 The potentially acid sediments in the Ha Tien Rectangular werehowever deposited in an earlier stage of Delta development. The thickness of thesediments is usually more than the above mentioned sediments: 3-6 m. Below thepotentially acid sediments, an old alluvial sediment is found of pleistocene age.The potentially acid sediments consist here of moderately to severely potentiallyacid grey to very grey (sometimes black) heavy clays. The underlying old alluviumconsists of light grey to grey heavy clay, usually with a very low organic mattercontent and greater resistance compared to the darker, softer and more organicpotentially acid sediment.

A4.16 As explained in chapter 2, the potentially acid and non-potentiallyacid sediments can be distinguished by colour, texture and firmness. During theprocess of dredging the potential acidity of the different sediments can easily bechecked using the quick oxidation test mentioned in chapter 2.

A4. 17 An evaluation has been made of the possibility to cover thepotentially acid sediment with non-potentially acid sediment. The results areshown in Table 4.07.

Table 4.07: Possibility to cover potentially acid sediments with non-potentially acidsediments.

canal: pro. total pi P2 NPtop ItPbott. Perc. effeCtivnessdging area area area area NPbott

area

-2 2 - *2 2 2 S

Cho Gao Il

Nang Thit C2

Xa lo C3C4 100.35 43.55 0 33.62 23.18 23 reasonableC5 87.85 19.77 0 16.8 51.28 58 good

Tat Cay tram C6 85.21 22.28 0 38.S 24.13 28 reasonable

Tram Canh Den C7cB 51.13 8.84 0 31.75 10.54 21 reasonableCs

Lap Wo-Sa Dec C12 not possible, the non-pyritic layer lies just belowthe required bottom

Rich Soi-Hau Giang C13 goodC14ClsC16Ci7

Rach Cia-Ha Tien CIB not possible, pyritic layer until required bottomCO9 not possible, pyritic layer until required bottomCZO nut possible, pyritic layer until required bottomC21 reasonableC22 not possible, pyritic layer until required bottomC23 56.93 10.62 0 33.8 12.41 22 reasonable

.-. .-.-.-,-..--,-,-,-,-,-,-.-...---.-.-.-.----, . ,-, , ,- ,, ,_-, , __-.. . . . .-. . .-.--..-.-...---.-.. .-. . . ---. _-..-...--. -.- _-..-. . .-

21

A4. 1 8 The evaluation of the possibilities as shown in Table 4.07 indicatesreasonable and good possibilities in the Xa No, Tat Cay Tram and Tram Canh Dencanals. The non-potentially acid sediment in the Lap Vo Sa Dec canal is locatedbelow the projected canal bottom and can therefore not be used to cover thepotentially acid sediment. Locally, where the non-acid layer is found at shallowerdepth covering may be possible. As expected, the thickness of the potentially acidsediment in the Rach Gia - Ha Tien canal is such that covering is not possible. Inthe north-western part of the canal (cross-sections C21, C22 and C23) the oldalluvial may be found at such depth that covering could be an option.

A4.19 When the potentially acid sediment is well covered with non-potentially acid sediment, both the diffusion of oxygen into the spoil as well as thecapillary rise of acid end-products to the soil surface are greatly reduced. In orderto estimate the run-off in a well constructed spoil the following reduction factorsof the surface of the spoil exposed to rainfall have been assumed:

Possibility to reductioncover the swoil: factor:

good 0.2reasonable 0.5not possible 1.0

A4.20 Table 4.08 shows the estimated run-off of acid water from spoilsin which care has been taken to cover the potentially acid sediment with non-potentially acid sediment. The results presented in Table 4.08 indicate that acidrun-off can be strongly reduced in the Xa No, Tat Cay Tram and Tram Canh Dencanal.

Table 4.08: Yearly acid run-off from dredge spoils in which potentially acidsediment has been covered with underlying non-potentially acid sediment.

c;asaI pro. length total both total surface surface surface *astimatedstimatad acid acid acidcanal dradging P-layurs dredging WIoil apoil spoil run-off conc. run-off run-off run-off

stretch volum dre4ging volum N40.2 *. H-1 *. h4.2 *. run-off hl40.2 * 4-1 a h-2 *.

a. 1000 3 1000 .3 d3/. *2/. V2/. m2/ */year mol(+)/.3 .ol()/ol(n)/ 1(+)/r

Cho Gao CI

Mang Thit C2

Xa No C3 23923 904 0 36 189 38 19 0.6625 0 0 0 0Ca 12088 1023 474 85 212 42 21 0.6625 40 5609 1122 561CS 3144 186 38 59 59 12 a 0.6625 60 2349 470 235

Tat Cay tras C6 4236 272 71 64 161 32 16 0.6625 60 6380 1276 638

Tram Canh Den c7 0 0 0 0 0 0 0 0.6625 60 0 0 0C8 33205 3021 1564 91 227 45 23 0.6625 60 9041 I108 904C9 0 0 0 0 0 0 0 0.6625 30 0 0 0

Lap Vo-Sa Dec C12 3883 744 361 19 96 19 10 0.6625 40 2S33 507 253

Rach Soi-Hau Glang C13 12193 200 a 16 8 2 1 0.6625 20 109 22 11C14 12631 635 0 50 0 0 00.6626 0 0 0 0C15 am06 473 0 64 0 0 0.6625 0 0 0 0C16 12152 521 0 43 0 0 00.6625 0 0 0 0C17 7882 189 0 24 0 0 00.6625 0 0 0 0

Rach Gia-Ha Tien CIS 6626 246 144 37 185 37 19 0.6625 40 4910 982 491C19 12394 559 S42 45 225 45 23 0.6625 60 8959 1792 896C20 13938 766 652 55 275 55 27 0.6625 60 10920 2164 1092C21 11443 575 357 S0 126 25 13 0.6625 60 4992 996 499C22 9137 469 321 51 256 51 26 0.6625 so 8496 1699 95CC23 9096 387 68 43 106 21 11 0.6625 40 2816 563 282

…---------------_ _ -_ -_ _ -. -- - ----_ --------- ------------------------------------------------------- -------------- _-_-_-_ -

22

D. Period until complete depletion of the acidity of the spoil

A4.2 1 The covering of potentially acid sediment with non-potentiallyacid reduces the yearly acid run-off. The potential acidity is maintained in the spoilfor a longer period. Table 4.09 shows the calculation of the period until completedepletion of the acidity present in the spoil. The period of depletion is presentedfor the following 6 cases:

case 0: without case, no bunds, H = 0.2 m., no coveringcase 1: no bunds, H = 0.2 m., coveringcase 2: low bunds, H = 1.0 m., no coveringcase 3: low bunds, H 1.0 m., coveringcase 4: high bunds, H = 2.0 m., no coveringcase 5: high bunds, H = 2.0 m., covering

Table 4.09: Period until depletion of the potential acidity of the spoil.

cana pro time to leach out acid trecm spoil:0 1 2 3 4 5

year year yuar y_ar y-ar year

cno coo cl

Nang 2Ait C2 - - - - - -

laio C3 0 - 0 - 0 -C4 2 3 * 15 is 30C5 1 7 7 33 13 66

rtCay tram C6 2 4 9 19 19 38

Tr euC.nb DMi C7 - - - _ _ _C8 1 1 3 6 6 13CS - _ _ _ _ _

Lap Vo-Sa Dan C12 2 2 10 10 19 19

Rach Soi-Ibu QiWgC13 0 1 1 7 1 14Cl4 0 - 0 - 0 -C15 - 0 - 0 -C16 0 - 0 - 0 -C17 a - a - 0 -

Rach cia-ha Tien CIS 0 - 0 - 1 -Cilg 7 7 34 34 67 67C20 6 6 31 31 62 62C2 4 6 21 42 42 93CR2 3 3 15 15 29 29C23 1 1 4 7 7 14

23

5. ACID RUN-OFF FROM SURROUNDING LAND

A5.01 The impact of yearly acid run-off from the dredge spoil onwater and land should be evaluated in relation to the yearly acid run-off from theland bordering the canals. When for instance, the canal cuts through an area withsevere actual acid sulphate soils (Sjl) which is used for the cultivation ofeucalyptus or pineapple on raised beds, the yearly acid run-off from the land isusually such that the pH of the canal water at the start of the rainy season is inthe range of 2.8-3.5. Even substantial additional acid run-off from dredge spoilsmay result in a drop in pH of 0.1 unit only. When however a canal cuts throughan area where potentially acid sulphate soils are limited to the subsoil and the landis used for rice cultivation, pH of the surface water could be close to neutral (6-8).The acid run-off from a dredge spoil may result in pH values in the range of 3.5 -4.0, a drop of 2 - 4.5 pH units.

A. Methodology

A5.02 The yearly acid run-off from the land bordering the canal isinfluenced by soil type and land-use.

A5.03 When the land is used for rice cultivation, the yearly acid run-off into the canal can be estimated assuming an average percolation rate of 0.1 5mm/day. This percolating water flows through the jarositic and pyritic subsoil intofield ditches and finally into the canal. This water is mixed with the soil moisturepresent in the macropores of the jarositic and pyritic horizons.

A5.04 The concentration of acidity in the oxidised part of the pyriticsediment (sulphuric horizon) is surprisingly constant throughout the year andthroughout the Delta. Hanhart and Ni (ref 4) measured an average concentrationof 40-60 mmol( + )/Il total titratable acidity consisting of 20-30 mmol( + )/l solublealuminum at a pH of 3.0-3.5 in the Ca Mau peninsular. Verburg (ref 3) measuredan average concentration of total acidity in the range of 25-30 mmol( + )/lconsisting of 10-15 mmol(+)/l soluble aluminum and a pH of 2.7-3.5 in the Plainof Reeds. Based on these data an average concentration of 40 mmol( + )lI can beassumed. The relative constant concentration of the soil moisture can be explainedby the buffering of the soil solution by acidity released from the soil complex. Incase of a moderately acid sulphate soil the concentration of the soil moisture isassumed to be 20 mmol( + )/I.

A5.05 Percolation of water through the subsoil takes place in theperiod where there is a pressure head between the field water level and canalwater level. Percolation is therefore restricted to the period from February to June,a period of roughly 180 days. Assuming a strip of 400 meter rice land on bothsides of the canal, the total amount of percolating water in this period isequivalent to 90 mm. The total volume of percolating water per meter canal is 72m3. The average concentration of the water entering the canal is similar to theconcentration of the soil moisture in the jarositic and pyritic horizons: 40 mol/m3 .The total amount of acidity yearly entering the canal per m. canal can becalculated as: 864 mol(+)/m/year in a severely acid soil (Sji and Sp1) and 432mol( + )/m/year in a moderately acid sulphate soil (Si2 and Sp2).

24

A5.06 When a canal is running along fields consisting of raisedbeds, acidity washed from the raised beds enters the canal. The yearly acid run-offfrom the raised beds can be calculated similarly as the acid run-off from dredgespoils. Assuming a strip of 400 meter on both sides of the canal under raisedbeds, the surface of these raised beds exposed to precipitation can be estimatedto be 640 m2/m.

A5.07 During the soil survey executed by the Can Tho University,land use along the canals was noted as well. Based on the recorded landuse andsoil type acid run-off from bordering lands was calculated.

B. Results

A5.08 The results of the calculation of the yearly acid run-off fromland bordering the canals is presented in Table 5.10.

Table 5.10: Calculation of the yearly acid run-off from land bordering the canals.RICE: RAISED BED3S

canal: pro.land use: Goil type: width period p-rc. conc. acid eat. surtace coOc. acid totalof of rate qround- run- acid raised run-oft Inflow acid

rice raised strip perc. water off run-oft beds inflowbeds no ass mod a* sevre ractor

I 9 4 9 aM. days u/day Mol(+4)/3 Dol(+)/m rf/year m/m Mol(+)/m3 aol(+)/m mol(+)/m

Cho Gao Cl 0 100 100 0 0 0.00 800 180 0.00015 0 0 0.8 0 0 0

Xanq Thit C1 90 10 0 80 20 0.60 800 160 0.00015 24 91O.4 0.6 36 0 467

Xa No C3 90 10 100 0 0 0.00 600 1t0 0.00015 0 0 0.6625 0.8 0 0 0C: 90 10 100 0 0 0.00 8oo 180 0.00015 0 o 0.6625 0.: 0 0 0CS 90 10 20 o 80 0.60 800 1I0 0.00015 32 691.2 0.6625 0.8 48 20352 2657

Tat Cay tram C6 50 50 0 0 100 1.00 900 10 0.00015 0 0 0.6625 0.8 60 25440 12720

Tram Canh Dan C7 75 25 0 s0 20 0.60 800 10 0.00015 24 516.4 0.6625 0.8 36 15264 4205CS 90 10 0 80 20 0.60 800 1'0 0.00015 24 516.4 0.6625 0.8 36 15264 1993C9 90 10 0 80 20 0.60 600 180 0.00015 24 519.4 0.6625 0.8 36 15264 1993

Lap Vo-Ss Do0 C12 80 20 90 10 0 0.05 800 180 0.00015 2 43.2 0.6625 0.8 3 1272 259900

Bach Soi-llau Glang C13 90 10 100 0 0 0.00 800 160 0.00015 0 0 0.6625 0.8 0 0 OC14 90 10 50 50 0 0.25 800 160 0.00015 10 216 0.6625 0.8 15 6360 830C15 90 10 0 100 O 0.50 900 180 0.00015 20 432 0.6625 0.8 30 12720 1661CI1 90 10 50 80 0 0.25 800 180 0.00015 10 216 0.6625 0.8 15 6360 930Cl7 90 10 100 0 0 0.00 600 180 0.00015 0 0 0.6625 0.8 0 0 0

Pach Gi0-Ba Tin CIO 90 10 100 0 0 e.00 800 10 0.00015 0 0 0.6625 0.8 0 0 0C19 90 10 0 50 50 0.75 800 180 0.00015 30 648 0.6625 0.8 45 19080 2491C20 90 10 0 50 I0 0.75 80 180 0.00015 30 648 0.6625 0.6 45 19080 2691C21 0 100 0 50 50 0.75 800 180 0.00015 30 648 0.6625 0.6 45 1.9080 19080C22 90 20 0 50 50 0.75 800 180 0.00015 30 648 0.6625 0.6 45 19080 4334C21 0 40 20 40 40 0.70 800 160 0.00015 28 604.0 0.6625 0.8 42 17808 7123__--_--_--_- - -_- - --_ - ---_ ----_ --_ --_ --_ -_--_ - ---_ - -_ ---___--___-- _ _-- ___---- _--- _- - _-- _-- _---- _ __ __- __-_- __- __ __-_- ___-_-_-_-_-_-_-_-_- __- _-_- _- __- __- _-_-_-__-__-__-_-___-__-__-__-__-__-__-__-_-

26

A5.09 According to the calculations presented in Table 5. 10,very high amounts of acid run-off is found in cross-section C6 in the Tat Cay Tramcanal and cross-section C21 in the Rach Gia - Ha Tien canal. The high values arecaused by the fact that the landuse consists for a large part of raised beds onpartly severe, partly moderately acid sulphate soils.

A5.10 Table 5.11 presents the yearly acid run-off from dredge spoilsunder the 6 cases described in the previous chapter. Table 5.12 presents theYearly acid run-off from dredge spoils as a percentage of the acid run-off frombordering land.

27

6. ACID RUN-OFF FROM DREDGE SPOILS IN RELATION TO ACID RUN-OFFFROM SURROUNDING LAND

A6.01 Table 6.1 1 summarizes the results of the calculation of the expectedyearly acid run-off from dredge spoil with varying height and with and withoutcovering the potentially acid sediment with a layer of non-potentially acid material.In cross-sections where there is insufficient seiment below the potentially acidsediment to cover the potentially acid sediment, it is assumed that non-potentiallyacid covering material is transported to the disposal site to cover the disposal sitewith a layer of 50 cm.

Table 6.11: Yearly acid run-off from dredge spoil of varying height, with andwithout covering.

…------------------ --------------------------------------------- …---------_ +

1 2 3 4 5 6canal: pro. acid acid acid acid acid acid

run-off run-off run-off run-off run-off run-offH=0.2 a H=0.2 E H=1 Hl D=1 H=2 x. H=2 i.

uol(+)/nol(+)/m uol(+)/1 rol(+)/m zol(+)/m Rol(+)/a

Xa No C3 0 0 0 0 0 0C4 11217 5609 2243 1122 1122 561C5 11747 2349 2349 470 1175 235

Tat Cay tran C6 12760 6380 2552 1276 1276 638

Tram Canh Den C7 0 0 0 0 0 0C8 18082 9041 3616 1808 1808 904C9 0 0 0 0 0 0

Lap Vo-Sa Dec C12 2533 1286 507 253 253 126

Rach Soi-Hau GiangC13 1088 109 218 22 109 11C14 0 0 0 0 0 0C15 0 0 0 0 0 0Cl6 0 0 0 0 0 0C17 0 0 0 0 0 0

Rach Gia-Ba Tien C18 4910 2455 982 491 491 245C19 8959 4479 1792 896 896 448C20 10920 5460 2184 1092 1092 546C21 9984 4992 1997 998 998 499C22 8496 4248 2699 850 850 425C23 5633 2816 1127 563 563 282

t----------- -t---…

In cross-sections Cl 2, C28, Cl 9, C20, C22 covering material is transported fromelsewhere.

28

A6.02 In order to obtain insight in the impact of the acid run-off from thedredge spoil on the aquatic environment, the yearly acid run-off from the dredgespoil is expressed as a percentage of the acid run-off from the surrounding land (seetable 6.12).

A6.03 The acid run-off from the spoil is presented for the following cases:

case 1: without case, no bunds, H = 0.2 m., no coveringcase 2: no bunds, H = 0.2 m., coveringcase 3: low bunds, H = 1.0 m., no coveringcase 4: low bunds, H = 1.0 m., coveringcase 5: high bunds, H = 2.0 m., no coveringcase 6: high bunds, H = 2.0 m., covering

Table 6.12: Yearly acid run-off from dredge spoils as a percentage of the yearlyacid run-off from the bordering lands.

came: 1 2 3 4 5 6

canal: pro- a %

Cbo GC C1 - - - - - -

Mang Thit C2 0 0 0 0 0 0

Xa No CS - - - - - -

C4 - - - - - -

CS 442 as 8s 18 44 9

Tat Cay tram C6 0oo 50 20 10 10 5

Tram Canb Don C7 -

Ca .907 454 le1 91 91 45C9 - - _ _ _ _

Lap Vo-Sa Dec C12 877 438 175 88 a8 44

Rackh 80i-Dau lang CIS - - - - - -

C14 0 0 0 0 0 0

CIS 0 0 0 0 0 0

C16 0 0 0 0 0 0C17 - - - - - -

Racb Gia--Ba Tien C18 - - - - - -

C19 360 1SO 72 36 36 18

C20 438 219 8s 44 44 22

C21 52 26 10 5 5 3

C22 196 98 39 20 20 10

C23 79 40 16 8 a 4

A6.04 Based on the figures presented in table 6.12, from a soilpoint of view no special dredging precautions need to be taken in the Cho Gaocanal, Mang Thit canal, north-eastern and central part of the Xa No canal, RachSoi - Hau Giang canal and first part of the Rach Gia to Ha Tien canal.

29

A6.05 In the south-western section of the Xa No canal, the run-offfrom the dredge spoil is expected to exceed the acid run-off from the borderingfields with a factor 4 when no bunds are constructed. When the potential acidsediment is covered with underlying non-potentially acid sediment (case 1), theacid run-off from the spoil lies in the same order of magnitude as the run-off fromthe land. When bunds are constructed to maintain the spoil (case 3) a similaramount of acid run-off can be expected. When bunds are constructed and thepotentially acid sediments are covered, acid run-off from the spoil is expected tobe only 20% of the acid run-off from the fields. In this case, increased acid run-offinto canals is not expected to have great impact on the aquatic environment.

A6.06 Construction of bunds is also required in the Tat Cay Tramcanal. Although the expected acid run-off from the spoil is high, the relativeimpact is limited to 20%, as a result of the high amount of acidity from the,sugarcane plantations. In view of the expected low additional costs of covering,it is recommended to dredge this stretch according to case 4 (low bunds andcovering), resulting in an increase of only 10% acid run-off.

A6.07 Construction of bunds and covering of the potentially acidmaterial in the Tram Canh Den canal will result in an 90% increase of acid run-offinto the canal. Despite the fact that acid run-off from surrounding area's is alreadyquite high, an 90% increase of acid run-off is significant. The negative effects ofacid run-off may be further mitigated by limiting dredging activities to the rainyseason or by construction of a higher spoil. The latter possibility may not be cost-effective.

A6.08 Construction of bunds in the Lap Vo Sa Dec canal as in case3 results in a 175% increase of the acid run-off. The relatively high increase of theacidity can be explained by the fact that in this area only few acid soils are foundat the surface. Acid run-off from surrounding land is therefore limited. Aftercovering the soil with non-potentially acid material, the increase of the acidity isreduced to 80%. Covering the spoil with non-potentially sediment is difficult sincethe lower limit of the potentially acid sediment lies just below the requireddredging depth. It may therefore be necessary to transport soil from neighbouringfields or area's to the the site to cover the soil. Althoug absolute quantities arelow, in view of the high relatively increase, limiting dredging activities to the rainyseason is recommendable.

A6.09 In the Rach Gia - Ha Tien canal, absolute quantities of acidrun-off are high. Construction of normal bunds alone (case 3) results in mostcross-sections in an increase of the acid run-off of 40-90%. The relative increaseof the acidity in cross-section C21 is quite low in view of the high acid run-offfrom the surounding land. In cross-sections C18, C19, C20 and C22, covering ofpotentially acid sediments with non-potentially acid sediments is not possible inview of the fact that the lower limit of the potentially acid sediment lies below therequired dredging depth. Covering of the potentially acid sediments brings therelative increase of the acid run-off to the acceptable level of 20-40%. Since mostsurface soil in this area consists of severely and moderately acid soil, nonpotentially acid sediment will have to be transported to the area by boat. In cross-sections C-22 and C23 possibilities for covering are reasonable. The relativeincrease in acidity in these cross-sections is such that covering is not required.

30

7. ACIDIFICATION OF SURFACE WATER DURING THE DREDGINGOPERATION AND WITHIN THE CANAL

A7.01 Besides acid run-off from the dredge spoil oxidation of pyriteand subsequent acidification of surface may occur during dredging operation.Furthermore after widening of the canal pyrite in the canal bank may becomeexposed to atmospheric oxygen.

A7.02 Acidification of pyrite in newly excavated canal slopes isprobably limited as a result of the relatively high hydraulic conductivity of thesubsoil of acid sulphate soils. The high hydraulic conductivity is usually caused bya system of interconnected soil pores from decomposing mangrove roots andtrees. As a result of the high hydraulic conductivity of the subsoil, fluctuation ofthe ground water level remains noticable at long distance from the canal. As aresult of the fluctuation pyrite has usually been oxidised until the level of meanlow spring tide. Pyrite present in the canal slope belwo this level will remain underpermanently reduced conditions.

A7.03 Canal excavation in the Mekong Delta usually takes placeusing cranes constructed on a floating platform. The platform is moved forwardthrough the canal through the pulling action of the crane. The crane places the soilin large lumps on the canal bank thereby creating "dikes' with an altitude rangingfrom 1 to 3 m depending on the size of the canal.

A7.04 In the IWT-project canal enlargement is however projectedto be done by dredging. In the dredging process, the rotating cutter scrapes theheavy clay sediment from the bottom and mixes it with the canal water. Thissediment-water mixture is pumped up and sprayed on the canal bank. Thesediment-water mixture usually consists of 1 part sediment and 1.5 parts water.After the mixture is sprayed on the canal bank, the sediment is deposited whilethe water flows back into the canal or adjoining fields. As discussed in theprevious chapters, in this case the thickness of the sediment may be as thin as0.2 m.

A7.05 This situation is strongly improved when bunds areconstructed to maintain the dredge spoil. In this case an outlet is created in thebunds on the canal side in order to allow the water to flow back into the canal.

A7.06 When the pipe spraying the sediment-water mixture is placedhigh above the spoil (say 2 m), mixing with oxygen and subsequent oxidation andacidification may be more intense compared to the situation where the tube isplaced just above or just below the spoil level.

A7.07 During the dredging process (scraping the sediment from thebottom off the canal, mixing it with oxygen-rich water and spraying it on the canalbank) oxidation of pyrite and other reduced sulphur-compounds may take placeresulting in acidification of the water flowing in farmers fields (no bunds) or intothe canal (bunds). Pyrite is known to oxidise quite slowly. Other reduced sulphur-compounds may oxidise more quickly.

31

A7.08 In order to investigate the speed of acidification of thesediment-water mixture after dredging a simple experiment was executed in theCan Tho University.

A7.09 In the experiment two sediment samples were taken from thepotentially acid sediment in cross-section C20 in the Rach Gia - Ha Tien canal. Thesamples were stored in plastic bags in order to avoid penetration of oxygen andsubsequent oxidation and acidification. The soil was divided in two pots. Each potreceived 3.2 kg of soil. The chemical properties of the sediment are shown below:

Treatment: pH TPA Al Fe203mmol(+)/100 gr

undisturbed sediment 5.06 92 0 0H202 oxidised soil 1.15 92 14 13

A7.10 Sample A was mixed with 4.81 litre distilled water. SampleA represents the situation in which the spraying tube is placed low above thespoil. Sample B was mixed with 4.81 litre of distilled water which was saturatedwith air by bubbling air through it for 80 minutes. Sample B represents thesituation where the spraying tube is placed well above the spoil. The chemicalcomposition of the water in the sediment-water mixture was checked at thefollowing time-intervals: 0, 1, 2, 4, 8, 24, 48, 72, 142 hours and 1 month aftermixing the sediment with the water. The results of the experiment are shown inTable 6.13.

Table 7.13: Results of the sediment-water mixing experiment

Treatment: pH totaL acidity Al Fe(hours) imro(+)/l ppm PPM

A B A B A 8 A B

0 4.8 4.81 0.84 1.22 0.37 0.55 1.55 12.421 4.96 4.96 1.08 1.62 0.5 0.65 3.8 16.472 5.07 4.98 1.23 1.74 0.55 0.67 8.6 21.784 5.18 5.15 1.29 1.78 0.64 0.68 11.7 17.468 4.33 4.34 1.34 1.84 0.72 0.67 12.69 18.9

24 4.67 4.67 1.44 1.98 0.71 0.75 24.21 35.4848 4.82 4.66 1.48 2.03 0.69 0.82 12.33 23.6672 4.12 4.17 1.64 2.17 0.75 5.85 8.55 14.04

142 3.07 3.02 2.31 2.93 2.98 4.42 25.5 23.73

A7.11 The results of the experiment indicate that the acidificationof the sediment-water mixture is rather slow. After 142 hours or 6 days, the pHof the mixture has decreased to a level of 3.0-3.1, the total acidity of the samplehas increased to 2.3-2.9 mmol( + )/Il and the concentrations soluble aluminum andiron increased to respectively 3-4 ppm and 24-25 ppm. There is little differencebetween pots A and B indicating no significant effect of the aeration of the water.

A7.12 After 6 days, the concentration of the water in the sediment-soil mixture averaged 2.6 mmol( + )/I. The total amount of dissolved acidity in eachpot therefore averaged 12.5 mmol(+). Each pot contained 3.2 kg of soil. Aftercomplete oxidation, each pot would contain 2944 mmol( +) acidity. The amount

32

of 1 2.5 mmol( +) of dissolved acidity amounts to only 0.4% of the total potentialacidity present in the pot. Considering the fact that from most spoils yearly 2%to 5 % of the potential acidity is being leached from the spoil, the amount of 0.4%during the actual dredging operation is not disturbingly high.

REFERENCES:

1. Tran Minh Thuan, 1989. A study of the lowering of the ground water level.A case study in the Plain of Reeds, Vietnam. Unpublished thesis presentedin partial fulfilment of the requirements for the degree of master of science.Agricultural University Wageningen, the Netherlands.

2. Bil, F. 1 994. Opening up the Plain of Reeds, a study on the hydrologicaleffects of making new canals in a catclay area. Agricultural UniversityWageningen, the Netherlands.

3. Peter Verburg. 1 994. Morphology and Genesis of soil & Evaluation of theside effects of a new canal in an acid sulphate soil area in the Plain ofReeds, Vietnam. Projet Recherche-Developpement de la Plaine des Joncs,Institute for Agricutural Sciences, Ho Chi Minh city, Fonds voorOntwikkelingssamenwerking, Brussel. Dept. of Soil Science and geology.Wageningen Agricultural University, the Netherlands.

4. Hanhart, K. and Duong Van Ni. 1995. Development of suitable soil andwatermanagement practices on severe acid sulphate soils, series ofexperiments in the Hoa An station, Hau Giang Province, Vietnam.Wageningen Agricultural University, Wageningen, the Netherlands.

5. G. Sterk. 1991. Leaching of acidity from the topsoil of raised beds on acidsulphate soils. Nuffic project VH-10, Dept. of Hydrology, Soil Physics andHydraulics, Wageningen Agricultural University, the Netherlands.

6. Van Breemen, N. 1976. Genesis and solution chemistry of acid sulphatesoils. Agricultural research Reports 848. Centre for Agricultural Publishingand Documentation, Wageningen, the Netherlands.

7. Mensvoort, M.E.F. and Le Quang Tri. 1986. Morphology and genesis ofactual acid sulphate soils without Jarosite in the Ha Tien Plain, Mekongdelta, Vietnam. In: Selected papers of the Dakar Symposiumm on AcidSulphate Soils, H. Dost editor. International Institute for Land Reclamationand Improvement, Publication 44, Wageningen, the Netherlands.

8. Working paper no. 3: Irrigation, drainage and flood control. 1991. MekongDelta Masterplan. NEDECO Consultants.

9. Khoa, L.V. 1991. Relationship between some soil physical characteristicsand morphology in acid sulfate clays in the Mekong delta - Viet nam. MScthesis, Wageningen Agricultural University, the Netherlands.

10. ILRI publication 16: Drainage Principles and Applications, volume IlIl,Chapter 24, (Published by the International Institute for Land Reclamationand Improvement, 1983, the Netherlands).