International Conference on Sustainability Science in Asia (ICSS-Asia), 2009

Arsenic in Irrigated Soil and Food Chain

Dr. M. Ashraf Ali

Professor of Civil EngineeringBangladesh University of Engineering and Technology,

BUET

Bangkok, Thailand, 23 November 20091

Presentation Outline

Background

Presence of As in Irrigation water and Irrigated Soil

Arsenic in Food Chain (Rice, Vegetables)

Conclusions

2

BACKGROUND:As Contamination and GW Irrigation

in Bangladesh

3

Arsenic Contamination of Groundwater in Bangladesh:

• Shallow aquifer (< 100 m) primarily affected, which is widely used for domestic purpose through use of hand tubewells as well as for irrigation

• Out of 465 upazilas (sub-districts), 270 seriously affected

• Over 30 million people exposed to As above 50 ppb

Groundwater is widely used for irrigation during dry season (December-May), primarily for growing dry season paddy, called boro; some vegetables also receive groundwater irrigation.

Groundwater irrigation covers about 79% of the total irrigated area (over 5 million ha in 2007-08); shallow irrigation wells covers about 63% of total irrigated area.

In 2007-08, over 1.3 million shallow tubewells were used to irrigate about 3.2 million ha of land.An estimated 1360 tons of As is added to arable soils each year in Bangladesh Widespread groundwater irrigation suggests that ingestion of irrigated crops could be a major exposure route for As, along with drinking water.

Groundwater Irrigation in Bangladesh

• Surface water irrigation: almost unchanged since 80s• Groundwater irrigation: 5 fold increase (mainly STW)

Source of water, irrigation equipment Source of water, irrigation equipment and irrigated area in Bangladeshand irrigated area in Bangladesh

Distribution Of Shallow Irrigation Wells In Bangladesh

High concentration of irrigation wells in N and NW regions of Bangladesh.

Whereas higher As concentration in S, SC, NE regions.

T h a n a w i s e

S T W

0

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6 4 4 6 4 4

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1 7 2 5

1 7 2 5

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2 9 5 0 2 9 5 0

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4 2 4 7

4 2 4 7

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6 2 0 0 6 2 0 0

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9 3 4 2

9 3 4 2

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1 2 8 1 1 N o

D a t a

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ARSENIC IN IRRIGATION WATER AND IRRIGATED SOIL

o EAWAG-ETH-BUET joint research

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Data on As in soils in Bangladesh indicate that 5–10 mg/kg is the background level.

Reports indicate that soil concentrations are increasing because of As input via irrigation water.

Available data suggest significant accumulation of As in top layer of irrigated soil; reported soil As concentrations vary from about 10 to 70 mg/kg.

Our research revealed significant spatial and temporal variation of As concentration in irrigated soil

Arsenic in Irrigated Soil

Fe(III) oxides present in soil and those produced by oxidation of Fe(II) in irrigation water promote adsorption and retention of As in soil.

However, desorption and dissolution of Fe-solids (e.g., during flooding) may reduce As accumulation in soil.

The conditions under which As is building up in the soil and the time frame involved are not certain.

Recent findings from a field trial demonstrated that increasing soil As levels resulted in a decrease in rice yield.

Arsenic in Irrigated Soil

Distribution and Variability of Arsenic in Irrigated Rice Field

EAWAG-ETH-BUET Joint Research on Arsenic in Irrigated Land and in Food Chain, funded by Swiss National Foundation

Spatial distribution and temporal variability of Arsenic in Irrigation water and paddy soilArsenic mobilization from paddy soilDynamics and trends of Arsenic Accumulation in paddy soil

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Field Site: Sreenagar, Munshiganj

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• 18 paddy fields• Total area: 3.16 ha• Total irrigation: 1 m/season

Irrigation Water:• As (total): 397 ± 7 g/L• As(III): 332 ± 27 g/L • Fe: 10.9 ± 0.2 mg/L • P: 1.92 ± 0.07 mg/L

Field site located at Munshiganj, about 30 km south of Dhaka

Field exclusively used for cultivation of boro rice, since 1990s, when local irrigation well was constructed.

The irrigation well extracts groundwater from a depth of 30-60 m, with a pumping rate of approximately 19 L s-1

Fields are irrigated 18-21 times between early January and late April. Local boro rice production requires 1 m of irrigation water per season.

Fields are irrigated for 2-4 h at a time until covered by 3-10 cm of water.

Field Site and Irrigation Practice

Fe Concentration in Irrigation Channel Water

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Dissolved Fe concentrations decreased by 76% along the channel, indicating that most Fe was oxidized and formed HFO colloids.

Roberts et al., 2007

As Concentration in Irrigation Channel Water

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Total As input to any field within the irrigation area is virtually unaffected by the length of channel stretch between well and field inlet.

Roberts et al., 2007

Fe Concentration in Rice Field Water

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All Fe(II) was oxidized and formed HFO colloids within a distance of 20 m from the field inlet.

Shortly (0-2 hr) after irrigation

Roberts et al., 2007

As Concentration in Rice Field Water

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As input to soil decreased strongly with increasing distance from the inlet.

2.75 hr after irrigation

Shortly (0-2 hr) after irrigation

Roberts et al., 2007

As Concentration in Field Water at Different Times

18

As concentrations in the stagnating field water decreased strongly, indicating transfer of As to soil.

Roberts et al., 2007

Major Findings Total As input to a rice field was independent of the length of the irrigation channel.But within each field As input to soil decreased strongly with increasing distance from the inlet.The pattern of As distribution and extent of lateral heterogeneity would depend both on the composition of the irrigation water (As/P, Fe/As ratios) and on the type of irrigation system usedPassage of the irrigation water through a designated treatment field or pond prior to its distribution might offer a simple means for reducing As input to soils used for crop production.

. Within the top 40 cm of soil, the mean As

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Arsenic in Paddy Soil

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Soil Sample Collection:• December 2004: before rice transplantation• May 2005: After harvest• December 2005: After

monsoon flooding

Distribution of As in topsoil (0-10 cm) in December 2004

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Median (12 fields) As in topsoil (0-10 cm): 12.5 – 18 mg kg-1

Median (12 fields) As in subsoil (30-40 cm): 7 – 14 mg kg-1

Dittmar et al., 2007

Total and Oxalate-extractable As in topsoil (0-10 cm) as a function of distance from Inlet

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Peak Soil-As concentration in May 2005, after harvest As concentration reduced in December 2005, suggesting

remobilization during monsoon due to reduction of submerges soil

Dittmar et al., 2007

Total and Oxalate-extractable As as a function of depth

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Peak Soil-As concentration in May 2005, after harvest As concentration reduced in December 2005, suggesting

remobilization during monsoon due to reduction of submerges soil

Dittmar et al., 2007

Difference in Soil As (in mg kg-1) in topsoil (0-10 cm) between Dec. 2007 and Dec. 2004

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Dittmar et al., 2009, Submitted

Area-weighted Soil As Concentration betweenDecember 2004 – December 2007

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Dittmar et al., 2009, Submitted

Soil As Mass at different depth ranges betweenDecember 2004 – December 2007

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Dittmar et al., 2009, Submitted

Major Findings Annual As input with irrigation water equaled 4.4±0.4 kg ha-1 a-1. Within the top 40 cm of soil, the mean As accumulation over three years amounted to 2.4±0.4 kg ha-1 a-1, implying that on average 2.0 kg ha-1 a-1 were lost from the soil. Estimated 1.05-2.1 kg As ha-1 a-1 were lost by diffusion into monsoon floodwater The remaining As-loss (up to 0.95 kg ha-1 a-1 was attributed to leaching with percolating irrigation water.

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Estimated changes in soil As (0 – 40 cm) according to three scenarios

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Significant risk of high long-term accumulation of As in soil

Dittmar et al., 2009, Submitted

ARSENIC IN FOOD CHAIN

o EAWAG-ETH-BUET joint researcho BUET study

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Rice mainly contains As(V), As(III) and DMA.

Reported As concentrations in rice range from 0.03 to 1.24 mg/kg; percentage of inorganic As ranged from 11 to 94%.

Compared to other countries, rice from Bangladesh (and India) had the highest percentage of inorganic As (80 percent), against 42 percent in rice from the USA.

All As present in vegetables, pulses and spices is reported to be present in inorganic form

Limited data on water-soil-crop (rice/vegetable) transfer of As

Arsenic in Rice and Food Items

Arsenic in Rice

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Plant Sample Collection:• May 2005• May 2006• May 2007

Dittmar et al., in preparation

Variation of Soil As with Distance from Inlet

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Dittmar et al., in preparation

0 5 10 15 20 25 30 35 40 45 50 55 60 65 700

2

4

6

8

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20A

Soil As (mg kg-1) in 0-40 cm depth/pot

Str

aw A

s d

ry m

ass

(mg

kg

-1)

0 5 10 15 20 25 30 35 40 45 50 55 60 65 700.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7 Field, May 2005Field, May 2006Field, May 2007

Pots, 15 mg kg-1 As

Pots, 25 mg kg-1 As

Pots, 43 mg kg-1 As

Pots, 15+28 mg kg-1 As

BG

rain

As

dry

mas

s (m

g k

g-1)

Soil As (mg kg-1) in 0-40 cm depth/pot

Variation of Straw-As with distance from Inlet

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Dittmar et al., in preparation

Variation of Grain-As with distance from Inlet

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Dittmar et al., in preparation

Variation of Grain-As with Soil-As (Pot Study)

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Dittmar et al., in preparation

Major Findings As concentrations in straw and grain of rice appear to be strongly correlated with As in soil As. Arsenic concentration in both straw and grain could vary over a wide range within a paddy field Increasing soil As could result in significantly elevated concentrations of As in straw and grain of rice

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As in Vegetables Three areas selected:

Two As affected areas: Sonargaon, Narayangonj Kachua, Chandpur

One unaffected area: Sherpur, Bogra

Selected Vegetables :

Potato, Tomato, Stem Amaranth, Red Amaranth, Cabbage and Cauliflower(grown the both As-affected groundwater and As-free surface water)

Study involved collection of irrigation water (both surface water and groundwater) soil core samples and vegetable plants samplesAnalysis of As concentration in irrigation water, soil and plant (root, stem, leaf, edible part) samples

As in groundwater used for irrigation:

Borga : < 1 ppb Chandpur : 73 to 132 ppb Narayanganj : 63 to 266 ppb

As in surface water used for irrigation: Bogra : < 1 ppb Chandpur : up to 6.7 ppb Narayanganj : up to 25.3 ppb

Arsenic in Vegetables

Arsenic in Vegetable Samples: PotatoGroundwater Irrigation

SamplingLocation

As in Water(mg/l)

As in Root Soil

Mean ± SD(mg/kg)

As in RootMean ± SD (mg/kg)

As in Stem Mean ± SD

(mg/kg)

As in Leaf Mean ± SD

(mg/kg)

As in Edible Part

Mean ± SD (mg/kg)

Bogra (n=6) < 1.0 2.55 ± 0.28 0.16 ± 0.04 0.09 ± 0.03 0.07 ± 0.03 0.02 ± 0.01

Chandpur (n=6) 95 – 132 4.09 ± 0.23 1.78 ± 0.57 0.34 ± 0.15 0.25 ± 0.10 0.23 ± 0.12

Narayanganj (n=6) 214 - 243 5.82 ± 0.93 2.62 ± 0.60 1.45 ± 0.32 2.06 ± 0.41 1.15 ± 0.31

Surface Water Irrigation

SamplingLocation

As in Water (mg/l)

As in Root soil

Mean ± SD (mg/kg)

As in Root Mean ± SD

(mg/kg)

As in Stem Mean ± SD

(mg/kg)

As in Leaf Mean ± SD

(mg/kg)

As in Edible Part

Mean ± SD (mg/kg)

Bogra (n=3) < 1.0 2.59 ± 0.51 0.35 ± 0.12 0.12 ± 0.05 0.25 ± 0.11 0.04 ± 0.01

Chandpur (n=3) 1.6 3.12 ± 1.08 0.45 ± 0.09 0.24 ± 0.10 0.25 ± 0.08 0.10 ± 0.03

Narayanganj (n=3) 25.3 4.90 ± 1.67 1.58 ± 0.46 0.85 ± 0.25 1.03 ± 0.45 0.51 ± 0.20

Arsenic in Vegetable Samples: TomatoGroundwater Irrigation

SamplingLocation

As in Water(mg/l)

As in Root Soil

Mean ± SD (mg/kg)

As in Root Mean ± SD (mg/kg)

As in Stem Mean ± SD

(mg/kg)

As in Leaf Mean ± SD

(mg/kg)

As in Edible Part Mean ±

SD (mg/kg)

Bogra (n=6) <1.0 3.03 ± 0.57 0.33 ± 0.28 0.13 ± 0.04 0.39 ± 0.16 0.27 ± 0.05

Chandpur (n=3) 73 4.53 ± 0.51 1.69 ± 0.47 0.99 ± 0.24 0.85 ± 0.21 1.05 ± 0.29

Narayanganj (n=6)

63 - 166 5.63 ± 1.68 1.99 ± 0.11 1.15 ± 0.35 2.82 ± 0.23 1.70 ± 0.15

Surface Water Irrigation

SamplingLocation

As in Water (mg/l)

As in Root soil

Mean ± SD (mg/kg)

As in Root Mean ± SD

(mg/kg)

As in Stem Mean ± SD

(mg/kg)

As in Leaf Mean ± SD

(mg/kg)

As in Edible Part Mean ±

SD (mg/kg)

Bogra (n=3) 2.5 3.67 ± 0.76 1.72 ± 036 0.55 ± 0. 13 0.33 ± 0.09 0.47 ±0.08

Chandpur (n=6) 2.0 - 3.2 3.53 ± 1.46 0.48 ± 0.16 0.23 ± 0.08 0.36 ± 0.07 0.78 ± 0.31

Narayanganj (n=3)

18.2 2.21 ± 0.51 1.03 ± 0.38 1.09 ± 0.27 0.93 ± 0.35 0.99 ± 0.25

Arsenic in Vegetables: Correlation with Water and Soil Arsenic

Potato

y = 0.0042x + 0.0558r = 0.775

0

0.4

0.8

1.2

1.6

2

0 75 150 225 300

Arsenic in irrigation water (µg L-1)

Ars

en

ic in

ed

ible

pa

rt

(mg

kg

-1)

y = 0.3303x - 0.9189r = 0.868

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0 2 4 6 8

Arsenic in root soil (mg kg-1)A

rse

nic

in e

dib

le p

art

(mg

kg

-1)

(p < 0.05) (p < 0.01)

MAJOR OBSERVATIONS

The As concentrations in edible parts of the vegetables are strongly correlated with As concentrations in both irrigation water and root-soil.

Among the selected vegetables, mean As concentrations have been found to be relatively higher for stem amaranth, red amaranth (both leafy vegetable) and tomato.

Total As concentrations in most vegetables irrigated with As contaminated groundwater exceeded the Chinese food safety standard of 0.05 mg/kg inorganic As (wet weight basis) by a large margin.

Food Consumption PatternAverage food consumption in “g/capita/day”

Hels et al. (2003)

- Rice : 450

- Vegetables : 178

- Roots & Tubers : 61

- Fish : 39

- Other animal products : 31

In rural areas, the main meal usually consists of boiled rice served with cooked vegetables.

Reported vegetable consumption varies from 130 to 500 g/capita/day (wet/fresh weight)

Exercise: Dietary Intake of Arsenic from Rice & Vegetables (g/day)

VegetableConsumption(g/capita/day)

Bogra VegetableMean As: 0.074

mg/kg

Narayanganj Vegetable

(SW irrigation)Mean As: 0.185 mg/kg

Narayanganj Vegetable

(GW irrigation)Mean As: 0.33 mg/kg

130 9.6 24 43

178 13.2 33 58

500 37 93 165

WHO’s Provisional Maximum Tolerable Daily Intake: 126 g/day (60 kg adult)

Intake from rice with As=0.2 to 0.5 mg/kg* (450 g/capita/day): 72-180 g/day

Intake from vegetables: 24-165 g/day

*80% inorganic As

Exercise: Dietary Intake of Arsenic from Drinking Water and Food Sources (g/day)

Source andConsumption(g/capita/day)

Bogra VegetableMean As: 0.074

mg/kg

NarayanganjVegetable

(SW irrigation)Mean As: 0.185 mg/kg

NarayanganjVegetable

(GW irrigation)Mean As: 0.33 mg/kg

Vegetables (178 g/capita/day)

13.2 33 58

Rice (450 g/capita/day)

As: 0.4 mg/kg*

144 144 144

Water from food(1.3 l/capita/day)

As: 100 ppb

-- 130 130

Drinking Water(3.0 l/capita/day)

As: 100 ppb

-- 300 300

As from Food 157 307 332As from Drinking

water-- 300 300

*80% inorganic As

Conclusions

Presence of As in irrigation water leads to elevated levels of As in soil, with significant spatial and temporal variation.Areas without pronounced monsoon flooding may be at greater risk of soil As accumulation than seasonally flooded fields. Long-term accumulation of soil As will result in a concomitant long-term decline in rice yields on As-affected paddy fields.Because only about 21% of the total area of Bangladesh is annually flooded more deeply than 0.9 m and because less intensely flooded areas are often multiply cropped, As-related yield reductions in these areas could seriously affect national food security.

ConclusionsHuman As uptake via rice and vegetables may represent a major As uptake pathway, especially where As intake via drinking water is limited (by provision of low-As drinking water)Increasing soil As levels cause an increase in the As content of rice straw. Since rice straw is used to feed cattle, this may result in increasing human exposure to As via milk and meat products.Among other measures, efforts should be made to develop measures (e.g., water treatment) to reduce As input to agricultural land, and agricultural practices (e.g., raised bed cultivation, maintaining aerobic condition, use of silicon fertilizer) that reduces As uptake by plants. Efforts should also be made to develop rice varieties less susceptible to adverse effects of high soil As.

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