Aluminum-contaminant transport by surface runoffand bypass flow from an acid sulphate soil

L.Q. Minha,b, T.P. Tuongc,*, M.E.F. van Mensvoorta, J. Boumaa

aDepartment of Soil Science and Geology, Wageningen Agricultural University,

P.O. Box 37, 6700 AA Wageningen, The NetherlandsbFaculty of Agricultural Technologies, Can Tho University, Can Tho, Viet NamcCrop, Soil and Water Sciences Division, International Rice Research Institute,

DAPO 7777, Manila, Philippines

Accepted 13 March 2002

Abstract

Quantifying the process and the amount of acid-contaminant released to the surroundings is

important in assessing the environmental hazards associated with reclaiming acid sulphate soils

(ASS). The roles of surface runoff and bypass flow (i.e. the rapid downward flow of free water along

macropores through an unsaturated soil matrix) in transporting aluminum from three types of raised

beds (soil ridges formed by piling up soil materials excavated from adjacent ditches) were studied in a

Typic Sulfaquept in Can Tho, Vietnam. During the month of April, 1 h cumulative infiltration of the

low raised beds (made only of the topsoil materials) and high raised beds (made of topsoil and

jarositic layers) was significantly higher than that of the traditional raised beds (made of topsoil,

jarositic and pyritic materials). As the rainfall season progressed, infiltration in July decreased four to

seven-fold from the initial values in April, resulting in an increase in runoff. Due to surface crusting,

the traditional raised beds yielded the highest runoff (110 versus 50–60 mm in the other types in

July). Aluminum concentrations in the bypass flow (6–22 mmol l�1) associated with each of the

three bed types were higher than in the runoff (3–14 mmol l�1). In low and high raised beds, the

amounts of aluminum transported by bypass flow (15–16 kmol ha�1) was higher than in the runoff

(4–6.5 kmol ha�1), while in the traditional type, the two components were similar (11–12 kmol ha�1).

The total amount of aluminum released from the low raised beds was lowest. Low raised beds thus

pose less environmental hazards to the surroundings compared to the other two types. Interventions

that affect the amount of aluminum transport in runoff and in bypass flow are important in balancing

agricultural production and environmental protection in ASS areas. # 2002 Elsevier Science B.V. All

rights reserved.

Keywords: Leaching; Mekong river delta; Raised beds; Pollution

Agricultural Water Management 56 (2002) 179–191

* Corresponding author. Tel.: þ63-2-812-7686/845-0563; fax: þ63-2-891-1292/845-0606.

E-mail address: t.tuong@cgiar.org (T.P. Tuong).

0378-3774/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 3 7 8 - 3 7 7 4 ( 0 2 ) 0 0 0 3 3 - 1

1. Introduction

Acid sulphate soils (ASS) that result from oxidation of reduced S-compounds in pyritic

mud are characterized by low pH and high aluminum, iron, and sulphate concentrations

(van Breemen and Pons, 1978). Farmers often construct raised beds—i.e. piling up soil

materials excavated from adjacent lateral ditches to form ridges 0.3–0.6 m higher than the

original surface—to enhance the leaching of toxic compounds to grow upland crops

(Panichapong, 1982; Dent, 1992; Sarwani et al., 1993; Tri et al., 1993; Xuan, 1993).

Leaching of ASS results in transferring acidity to the surroundings and may pollute the

surrounding water and land. Minh et al. (1997b) reported that the monthly total amount of

aluminum released by the raised beds could be as high as 16 kmol ha�1 and was three to

five times greater than that from rice fields. Aluminum poses a high risk to the environment

because of its high toxic potential to plant roots and aquatic resources. A better under-

standing of the aluminum-contaminant transport in raised beds is useful in assessing

management options for ASS.

Depending on the environmental conditions and experiences of farmers, raised beds are

built in different ways. Tri et al. (1993) classified the raised beds in the Mekong delta,

Vietnam, into three types: low, high, and traditional (Fig. 1). Farmers use only the topsoil

excavated from shallow ditches (about 0.4 m deep) to form the low raised beds, which are

about 0.3 m higher than the original soil surface. The high raised beds (0.5 m higher than

the original soil surface, often found in the moderately-flooded area) are constructed with

soil materials taken from ditches of about 0.6 m deep. The topsoil is first excavated and set

aside. Then, sulfuric material (from the underlying jarosite layer) is excavated and spread

out on top of the original soil surface. Finally, the topsoil that had been set aside is spread

out to cover the sulfuric material on the raised beds. The traditional raised beds (0.7 m

higher than the original soil surface) are constructed by piling up the soil material in the

same order as it is excavated. Consequently, the excavated topsoil is placed at the bottom of

the raised bed, then the sulfuric materials, which are then covered by pyritic soil materials

Fig. 1. Schematic cross section of raised beds: low (a), high (b) and traditional (c). Letters T (for topsoil), J

(sulfuric material), and P (sulfidic material) indicate soil materials for construction of raised beds.

180 L.Q. Minh et al. / Agricultural Water Management 56 (2002) 179–191

excavated from the pyrite horizon. It is known that the types of raised bed construction

affect the chemical and physical properties of the soil, and thus the leaching process and the

amount of toxicity transferred to the surrounding areas.

Construction of raised beds invariably creates macropores in between soil lumps (Sterk,

1993; Minh et al., 1997a). During rainfall on a macropore-dominated soil, two major types

of water flow can occur, namely (a) surface runoff (hereafter called runoff) and (b) bypass

flow, i.e. the rapid downward flow of free water along macropores through an unsaturated

soil matrix (Falayi and Bouma, 1975; Bouma and Dekker, 1978; Bouma, 1990; Roth and

Helming, 1992). The amount of water absorbed into the soil matrix can be neglected

because it accounts for only about 10% of the bypass flow water after the first few rainfalls

and may become much smaller when the soil becomes wetter (Minh et al., 1997a). A better

understanding of the relative importance of bypass flow and runoff in transporting toxic

compounds is important in selecting the type of raised bed for a particular land-use in a

given environment.

Sterk (1993) and Minh et al. (1997a) highlighted the importance of bypass flow in the

removal of toxic substances in ASS. Their studies were, however conducted in the

laboratory using undisturbed soil cores and artificial rainfalls. The processes may differ

under field conditions. Sterk (1993) indicated that, under high rainfall intensity, as much as

one-third of the rainfall may be converted to runoff on the raised beds. In saline soils, runoff

may help remove salt that has accumulated in the topsoil (Pepper and Morrissey, 1985).

The amount of runoff and its role in transporting toxic substances in different raised beds of

ASS has not been studied. We hypothesize that the runoff component is higher in the

traditional raised bed compared to the other two types because the pyritic materials, being

unripe and without structure (Fanning and Witty, 1993) would easily form a crust, which

reduces the infiltration rate into the raised bed (Minh et al., 1997a).

This study was conducted to test the above hypothesis by assessing the amount of runoff

and bypass flow, and the effectiveness of each flow type in removing aluminum from

different types of raised beds in an ASS of the Mekong delta, Vietnam.

2. Methodology

2.1. Experimental site

The study was conducted at Hoa An station (108100 N, 1068150 E), in the Mekong delta,

Vietnam. According to soil taxonomy (Soil Survey Staff, 1992), the soil is classified as a

very fine Typic Sulfaquept with a high organic matter content in the top 30 cm. There is a

compacted layer with low saturated hydraulic conductivity at 30–40 cm depth. The sulfuric

horizon, with yellow mottles of jarosite, runs from 40 to 120 cm. From 120 cm downwards,

grey permanently reduced sulfidic materials are found. The soil was originally uncultivated

and covered by co nang reed (Chinese water chestnut, Eleocharis dulcis).

The experimental site has two distinct seasons: a dry season from January to May and a

rainy season from June to December which accounts for 80–90% of the 1800 mm annual

rainfall. Average evaporation rate varies from 4 mm per day in the rainy season to 6.5 mm

per day in the dry season.

L.Q. Minh et al. / Agricultural Water Management 56 (2002) 179–191 181

2.2. Experimental layout and condition

The treatments consisted of three types of raised bed: low, high and traditional. They

were built at the end of the 1993-dry season following farmers’ construction procedures

and dimensions (Tri et al., 1993) as shown Fig. 1. The treatments were arranged in a

complete block design, with three replications, totaling nine experimental plots. Each plot

was 10 m long, consisted of two raised beds (each was 4 m wide) and three lateral ditches,

and was isolated from the surrounding area by an earthen bund. The three ditches were not

interconnected.

The experimented raised beds were left fallow for 1 year as is commonly practised by

farmers in the Mekong delta. The fallow period stabilizes the raised bed and allows some

leaching of toxic compounds to a level, low enough for upland crops, like pineapple, yam,

and sugar cane, to be planted (Xuan, 1993). Monitoring of the experiment started at the end

of April 1994 and was completed at the beginning of August 1994. Table 1 characterizes

some physical and chemical properties of the top 15 cm of the three types of raised beds at

the beginning of the monitoring program. Total rainfall amount was 472 mm during the

monitoring period. In a month, there were on the average 14 days with rainfall exceeding

5 mm. The maximum daily rainfall during the study was 47.2 mm.

2.3. Soil physical properties

Bulk density of the topsoil of each plot was determined at the beginning of the

experiment with undisturbed soil cores sampled using 15 cm long and 20 cm diameter

PVC tubes. The large size of the cores was necessary to take into account the heterogeneity

of the macropore-dominated soils (Minh et al., 1997a). The soil moisture content under

which bulk density was determined varied from 0.1 to 0.17 Mg m�3. The same soil

samples were used for texture determination.

The saturated hydraulic conductivity (ks, in m s�1) of the topsoil layers was measured bi-

weekly based on soil cores, following the constant-head method described by Booltink and

Bouma (1991). It was expected that rainfall would create crusts on the raised bed surfaces

and wash fine particles into the macropores in between soil lumps. These may reduce the

overall saturated conductivity of the topsoil. To test this hypothesis, we expressed the

Table 1

Means � S:E:a of some chemical and physical properties of the top 15 cm layer of different types of raised beds

at Hoa An station: wet season 1994

Types of

raised bed

Alb

(cmol kg�1)

pHc Bulk density

(Mg m�3)

Clay (%) Silt (%) Sand (%)

Low 11.6 � 2.8 3.9 � 0.2 0.65 � 0.1 48.4 � 5.5 33.7 � 4.3 17.9 � 1.9

High 13.5 � 3.5 3.4 � 0.2 0.77 � 0.2 59.8 � 5.4 38.5 � 4.3 1.7 � 0.3

Traditional 21.8 � 5.7 3.1 � 0.1 0.82 � 0.2 52.7 � 6 42.6 � 4.7 4.7 � 0.3

a Means and S.E. were derived from three measurements.b Extracted by 1 M KCl (1:2.5).c Extracted by H2O (1:2.5).

182 L.Q. Minh et al. / Agricultural Water Management 56 (2002) 179–191

saturated ks as a function of cumulative rainfall (Rcu, in mm of water depth) as in Eq. (1),

following Edwards and Larson (1969):

ks ¼ ksiebRcu (1)

where ksi is the saturated hydraulic conductivity at the beginning of the experiment; b is a

fitting constant.

One hour cumulative infiltration was measured in each experimental plot at the end of

April (beginning of the experiment) and again in the middle of July (in the middle of the

rainy season), using the double-ring method.

2.4. Measurement of flow components

Water collected in the lateral ditches could be drained to the surrounding canal by

opening a cap attached to 20 cm diameter PVC culverts. A gauge for measuring water

depth was installed in the central ditch of each plot. The water gauge readings were

recorded daily at 07:00 h and before and after each drainage event. From the gauge

readings, the water volumes in the ditch could be derived using a volume–depth relation

previously determined for the ditch. Though, we did not monitor the water levels in the two

adjacent lateral ditches, we kept the water in the three ditches at the same level to minimize

the lateral flow among the ditches.

Fig. 2 shows the components of the daily water balance of the central ditch of each plot.

Besides rainfall (R), the central ditch also received the runoff (Ro) and bypass flow (By)

from the two halves of the adjacent raised beds. Outflows from the ditch comprised of the

drainage (Dr) and evaporation (E) from the water surface. During the experiment, the water

table was high and the percolation loss from the ditch was assumed negligible. We also

neglected the lateral flow between the central ditch and the other two in the plot because

they had the same water levels. Under these circumstances, the daily water balance of the

central ditch can be expressed by Eq. (2):

R þ Ro þ By � E � Dr ¼ We � Wi (2)

In this equation, all terms are in m3. R was calculated from the depth of precipitation and

surface area of the ditch, E from the class-A pan evaporation and the ditch surface area, Dr

Fig. 2. Water balance components of a ditch in between two raised beds.

L.Q. Minh et al. / Agricultural Water Management 56 (2002) 179–191 183

from the water depth gauge readings before and after the drainage. Wi and We are the water

storage volumes in the ditch at the beginning and at the end of the computed period, which

were derived from the daily water gauge readings.

We quantified the runoff by using the concept of micro-catchment (Falayi and Bouma,

1975; Bisonnais et al., 1989; de Roo and Riezebos, 1992). The micro-catchments were

constructed from 15 cm long, 40 cm diameter PVC tubes. They were sharpened, greased,

and hammered slowly into the soil to a depth of 10 cm. A 2 cm diameter overflow hole was

punched at a height of 2–5 mm above the soil surface. The amount of surface runoff after

each rain was collected into a bottle collector via a hard Tygon tube connected through the

overflow hole into the bottle. From this, we could calculate the runoff depth. The term Ro in

Eq. (2) could then be calculated by multiplying the runoff depths by the surface area of the

raised bed. With all other components known, By could be calculated from Eq. (2).

2.5. Measurement of water quality and aluminum balance

Water samples were taken daily from the central ditches at the time of reading the water

depth gauges. Drainage water was also sampled just before the completion of each drainage

event. All water samples (ditch, drainage, and from runoff collectors) were analyzed for

aluminum concentration and pH, using the method by Begheijn (1980).

Since, the pH of water in the ditches was low (from 2.5 to 3.8), precipitation of aluminum

(which could occur at pH >4) was not expected to occur. In the experiment, water from the

external sources was not allowed to flow into the ditches. The ditch water was free of

external sediments and surface complexation/adsorption of aluminum to suspended

materials (Schnoor, 1996) could be neglected. The daily aluminum mass balance for

the central ditch can be written as in Eq. (3):

CRR þ CRoRo þ CByBy � CEE � CDrDr ¼ CWeWe � CWiWi (3)

Where C designates aluminum concentration and subscripts R, Ro, By, E, Dr, We, Wi

designate the flow components as defined in Eq. (2). With the other terms in Eq. (3) known

(assuming aluminum concentration in rainwater and evaporated water was negligible), the

mass transport of aluminum by bypass flow ðCByByÞ can be calculated. Since, By can be

derived from Eq. (2), CBy can be computed.

3. Results and discussions

3.1. Infiltration and hydraulic conductivity of topsoil

Among the three types, the low raised beds had the highest, and the traditional the

lowest, cumulative infiltration values in April as well as in July (Fig. 3a and b). The topsoil

in the low raised beds was made up mainly with soil lumps from the topsoil material with

high organic matter content (Hanhart and Ni, 1993) and low bulk density (Table 1). This

condition is more favorable for infiltration process compared with the high type, composed

partly of topsoil material and partly of sulphuric material, with less organic matter and a

higher clay content. The traditional raised beds were covered with the muddy sulphidic

184 L.Q. Minh et al. / Agricultural Water Management 56 (2002) 179–191

materials from the pyrite layer. The mud might have blocked the interplanar pores formed

between adjacent soil lumps, hindering the infiltration process.

In all types of raised beds, the infiltration rate and the cumulative infiltration in July were

much less than those in April (Fig. 3). In general, there was approximately 80% reduction

in 1 h cumulative infiltration in between the two periods of measurement. At both

measuring times, the slopes of the cumulative infiltration were constant, indicating steady

infiltration rates were reached, after about 30 min from the start of the infiltration tests.

The steady infiltration rates in July were about 7–11 times smaller than those in April

(Table 2). The decrease in the steady infiltration rates was higher in the traditional

(0.5–0.04 mm min�1) and high (3.7–0.3 mm min�1) than in the low raised bed

Fig. 3. Cumulative infiltration in different types of raised beds (L, low; H, high; T, traditional) in April (a) and

July (b). Vertical and capped bars indicate standard errors of the means of three replications.

L.Q. Minh et al. / Agricultural Water Management 56 (2002) 179–191 185

(3.5–0.5 mm min�1). At the start of the experiment (April), there were no significant

differences among the initial saturated conductivity ksi of the three types of raised bed. As

the experiment proceeded, the saturated conductivity decreased rapidly in all three types of

raised beds, following Eq. (1) with highly significant coefficient of determinant (R2) and

factor b (Table 3). The fitted equations support Edwards and Larson (1969) who also found

an exponential decrease of saturated hydraulic conductivity with cumulative rainfall. The

absolute values of b, indicating the rate of decline of ks with respect to cumulative rainfall,

were highest in the traditional raised beds, followed by the high and low types. This implies

that conductivity of the traditional raised beds reduced most rapidly.

The overall saturated conductivity was predominantly controlled by the macropore

network (Bouma et al., 1978). The difference in macropore network geometry at the

beginning of the experiment was probably not adequate to bring about significantly

different saturated conductivity. As the experiment progressed, the decrease in infiltration

rate and saturated hydraulic conductivity was probably due to the blockage of the

macropore system by soil fragments, which were dispersed by the impact of raindrops,

moved with the water down the sides of clods, and were deposited at the points of contact

between clods (Falayi and Bouma, 1975). Consolidation with respect to time also

decreased the area and perimeter of water-conducting pores (Minh et al., 1997a). Crust

formation caused by the impact of raindrops may be another reason for the decrease in the

saturated conductivity (Falayi and Bouma, 1975; Sharma et al., 1983). Though, we did not

Table 2

Means � S:E:a of soil moisture content and steady-state infiltration rate measured in April and July 1994

Types of raised bed

Low High Traditional

Soil moisture content (%)

April 0.17 � 0.04 0.12 � 0.05 0.11 � 0.02

July 0.32 � 0.08 0.34 � 0.07 0.21 � 0.03

Steady-state infiltration rate (mm min�1)

April 3.9 � 1.2 3.7 � 1.2 0.5 � 0.3

July 0.5 � 0.3 0.3 � 0.14 0.04 � 0.01

a Means � S:E: were derived from three measurements.

Table 3

Saturated hydraulic conductivity measured at the beginning of the experiment (ksi), fitting parameter (b), and

coefficient of determinant (R2) in the regressione ks ¼ ksiebRcu

Types of raised bed Ksi (10�8 m s�1) b R2

Low 2.75 af �0.0029** 0.77**

High 1.74 a �0.0041** 0.93**

Traditional 3.1 a �0.0066** 0.96**

e ks ¼ saturated hydraulic conductivity of the top layer (0–15 cm); Rcu ¼ cumulative rainfall (mm).f In a column, means followed by a common letter are not significantly different at the 5% level by DMRT.** Significant at 1% level.

186 L.Q. Minh et al. / Agricultural Water Management 56 (2002) 179–191

quantify the crust formation, we noted that the traditional raised beds formed crust rapidly

and the crust in these raised beds were thicker than in the other two types. This must have

contributed to the more rapid decline in infiltration rate and saturated conductivity in the

traditional raised beds.

Due to the high organic matter content of the topsoil in the low raised beds, soil structure

was more stable, resulting in slower reduction in saturated conductivity compared with the

other types of raised bed.

3.2. Surface runoff

Runoff started on the traditional raised beds when cumulative rainfall was 166 mm

(about 50 days after the start of the experiment). On the other two types of raised bed,

runoff started about 20 days later, at a cumulative rainfall of about 210 mm (Fig. 4). Bypass

flow, on the other hand, started about 10 days earlier in the low raised beds (about 30 days

after the start of the experiment; data not shown) compared to other types of raised bed.

Cumulative runoff increased most rapidly in the traditional raised beds, followed by the

high raised beds and the low type (Fig. 4). By the end of the experiment, cumulative runoff

from low raised beds was 55 mm (corresponding to 12% of total rainfall) compared with

66 mm (14%) for the high, and 109 mm (23.1%) for the traditional raised beds. The

increase in the slope of the cumulative runoff curve with respect to rainfall (Fig. 4)

corresponded to the decrease with time in the infiltration rate and the hydraulic con-

ductivity of the topsoil. The increase in soil moisture content of the raised beds (Table 2)

further reduced soil intake capacity and increased runoff toward the end of the experiment.

Fig. 4. Cumulative runoff depth from different types of raised beds (L, low; H, high; T, traditional) as a function

of cumulative rainfall). Vertical and capped bars indicate standard errors of the means of three replications.

L.Q. Minh et al. / Agricultural Water Management 56 (2002) 179–191 187

Higher runoff in the traditional raised beds could be attributed to their lower infiltration

rate compared to the other types of raised beds (Fig. 3). The increase in runoff and the

corresponding decrease in cumulative infiltration with increased crust thickness were also

reported by Roth and Helming (1992).

3.3. Aluminum concentration in runoff and in bypass flow

Fig. 5 shows the 10-day mean values of aluminum concentration in the runoff and the

bypass water from different types of raised bed. Aluminum concentration in the bypass

water was consistently higher than that in the runoff. This is expected because bypass water

flowed through a longer and more torturous path than the runoff, thus increasing the time

and surface area between the flow and the soil and the amount of dissolved aluminum in the

solution (Minh et al., 1997a). Furthermore, an important part of the soluble aluminum,

which had accumulated on the bed surface due to capillary rise, might have been dissolved

by the first part of a rainfall event before the runoff took place (Minh et al., 1998). This

water with high aluminum content might infiltrate into the soil and later on move

downward as bypass flow.

In general, aluminum concentrations in the bypass flow tended to increase with time

(except for the last 20 days in the traditional raised beds). This was in agreement with Minh

Fig. 5. Aluminum concentration in the runoff and bypass flow from three types of raised beds (L, low; H, high;

T, traditional).

188 L.Q. Minh et al. / Agricultural Water Management 56 (2002) 179–191

et al. (1997a) who reported a significant increase of aluminum concentration of the bypass

as more water had infiltrated into the soil. They postulated that the increase in concentra-

tion might have been the result of the increase in contact surface areas between the soil

clods and the bypass flow as the total infiltrated water increased (Bouma and Dekker,

1978).

Among the three types and for most of the times, aluminum concentration in the bypass

flow was lowest in the low raised beds. This conforms to its lowest aluminum concentration

in the soil (Table 1), the traditional type always had the highest aluminum concentration in

runoff water (Fig. 5). This was probably due to the higher aluminum content in the top layer

of the traditional raised beds (Table 1) as it contained more newly oxidized pyritic material.

Aluminum concentration in runoff water in the three types of raised beds increased with

time until the end of July. Thereafter, there was a tendency for the concentrations to

decrease. By that time, a large proportion of the dissolved aluminum at the surface of the

soil might have been removed by leaching and flushing processes.

3.4. Aluminum transported by runoff and bypass flow

Table 4 shows the total amount of aluminum transported to the central ditch and its

components in the runoff and bypass flow during the experiment. The total amount of

transported aluminum from the low raised beds was significantly less than that from the

traditional raised beds but not significantly different from that in the high raised beds.

The traditional raised beds released more aluminum than the high raised beds but the

difference was not significant at 5% level. The amount of aluminum transported by runoff

in the low and high raised beds was significantly less than that in the traditional raised beds.

A reverse tendency was found for the amount of aluminum transported by the bypass flow:

the lowest values occurred in the traditional bed. The higher amount of aluminum

transported by runoff in the traditional raised beds was attributed to relatively higher

runoff from the traditional raised beds (Fig. 4) and to higher aluminum concentration in the

runoff (Fig. 5).

In each type of raised beds, the flow components contributed to the total transported

aluminum in different ways. In the low and high raised beds, the amount of aluminum in the

bypass flow was higher than that in the runoff. This reflected the higher aluminum

concentration in the bypass flow compared with that in the runoff (Fig. 5) and the greater

amount of water in the bypass flow in these raised beds (data not shown). In the traditional

raised beds; however, the difference between the amount of aluminum transported by

runoff flow and by bypass flow was not significant (Table 4).

Table 4

Total aluminum (kmol ha�1) transported by runoff and bypass flow from different types of raised bed

Types of raised bed Runoff Bypass Total

Low 4 be 14.9 a 18.9 b

High 6.5 b 16.2 a 22.7 ab

Traditional 11.6 a 11.8 b 23.4 a

e In a column, means followed by a common letter are not significantly different at the 5% level by DMRT.

L.Q. Minh et al. / Agricultural Water Management 56 (2002) 179–191 189

4. General discussions and conclusions

This study showed that the micro-catchment concept, in combination with water and

solute transport balance equations, was useful in quantifying the amount of water and

aluminum runoff and bypass flow, and their transport concentration from three types of

raised beds commonly used in the Mekong delta of Vietnam. The findings highlight the fact

that both chemical and physical soil properties played important roles in the total amount of

the transported aluminum and its components in runoff and bypass flow.

Because of its lesser amount of released aluminum, the low raised beds pose less

environmental hazard to the surroundings compared to the other two types. Runoff can

remove solutes only from the very top part (about 2 cm) of the soil surface (Ahuja and

Lehman, 1983). Bypass flow can be more effective than runoff in improving ASS because it

can remove more toxic substances from deeper layers. Soil quality can thus be improved

faster by bypass flow. In this respect, the high raised beds are more recommendable than

the traditional types, though both pose the same degree of environmental hazard to the

surroundings.

As the rainy season progresses, crust formation increases the runoff, at the expense of the

bypass flow, and decreases its effectiveness of removing aluminum from the root zone.

Reducing runoff from the raised beds (e.g. by hoeing) in the middle of the rainy season is

possibly a good measure to improve leaching efficiency of rainwater. In cases, however,

where environmental protection is the main concern, intervention should aim at reducing

the bypass flow. Embankments of newly dug canals and road construction in acid sulphate

areas are important sources of acid pollution (Tuong et al., 1998). Soil compaction may

help reduce the bypass flow in these structures and minimize their environmental hazards.

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