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Spatial Dependency of Arsenic in Soils, Irrigation Water and Plants from Arsenic Contaminated Tube Well Used for Irrigation in Boro Rice Cultivation

Md. Shahjahan Kabir
PSO and Head
Agricultural Statistics Division, BRRI, Gazipur

Abstract/ Executive Summary:

The experiments were carried out during the Boro season 2003 and 2004 in four Shallow Tube Well (STW) command areas: B. Baria and Faridpur Sadar upazila, the smallest administrative unit in Bangladesh. The size of the command area varied from 1.54 ha (Faridpur) to 2.43 ha (B.Baria). Each command area was grided by 20m distance and the sampling points were geo-referenced by a global positioning system (GPS). Composite soil samples (0-15 cm), Plants and water samples were collected from the grids. Micro elevation model was created only for Faridpur command area. Geostasistical analysis was performed to analyse the data and create the elevation surfaces. In both the command areas, the concentration of arsenic in water flowing through the irrigation channel reduced with the distance from the tube well. A considerable amount of arsenic in water was absorbed in soils while flowing through the channel. As loading in the soil of a rice field from As contaminated irrigation water would be relatively higher in depressed areas than in elevated areas. Micro elevation may be an impacting factor for As loading in soil. The concentrations of Arsenic (As) in both the locations varied widely. The positive correlations among the soil, grain and straw As concentration are highly significant in both the locations. Which posed the cause of contamination by As in soil as well as grain and straw. Arsenic can enter into the edible plant parts and is influenced by soil As content. High concentration of As in rice straw can influence the translocation of As to grain. The spatial dependency of soil arsenic gives an indication that longer irrigation channels or ponded ground water prior to irrigation and discouraging standing water for long periods in rice fields could be positive measures in reducing arsenic loading in irrigated rice soil. In that case alternate wetting and drying (AWD) method can be a remedial measure to reduce As contamination in soil as well as food chain.

Keywords: Spatial variability, STW, Command area, Arsenic (As), Micro elevation, Geostatistics, Semivariogram, Krigging

Location: Faridpur and B.Baria Sadar Upazila

Start Date: 00-00-2003

End Date: 00-00-2004

Program Area: Risk Management in Agriculture

Commodity: Contamination of soil

Objectives:

This paper assesses the distribution of arsenic concentration in the soil surrounding of a single arsenic contaminated STW in two different locations in Bangladesh; identify the means to reduce As contamination in rice soils as well as food chain.

Methodology:

Study area: The study was conducted in two command area under two locations Faridpur and B.Baria Sadar Upazila. The area extends within northing of 23°29' to 23°44' N latitude and easting of 89°41' to 89°56' E longitude and northing of 24°03' to 23°58' N latitude and easting of 90°45' to 91°44' E longitude, respectively. Command area: Faridpur and B.Baria command areas are under Faridpur and B.Baria Sadar Upazila, respectively, where relatively high soil and water As exists. The studied areas are located within 23°31'23" to 23°31'29"N latitude and 89°46'10" to 89°46'15"E longitude and 23°59'12" and 23°59'19"N latitude and 90°59'38" and 90°59'48" E longitude, respectively. The command area maps were screen digitized using Arcview GIS from the mouza map and built in Arcinfo environment. The soil of the two sites was sampled at 101 and 96 points, respectively at 0-15 cm depth and geo-referenced using GPS. The average sampling interval was 12.35 m over an area of 1.54 ha in Faridpur and 15.9 m over an area of 2.43 ha in B.Baria; the smaller sampling intervals were chosen to resolve any short-scale variation that might be present. At the time of sampling, the crop was irrigated rice (winter rice). The soil and plant samples were air-dried and analyzed by Tri Acid method and the results of the soil elements (As, Fe, Mn, P, OC, pH and soil texture) were recorded accordingly. Data were analyzed using Arcview Spatial Analyst 3.2 GIS software. Geo-Statistical Analysis: Semivariograms were calculated to determine the spatial dependency of soil, water, grain and straw As, Fe, Mn and P for Faridpur by GS+ 5.3.2. Maximum lag distance and lag interval for the semivariance were determined iteratively to best fit the model having highest R², the lowest residual sum of squares (RSS) and spatial dependence close to unity. Kriging interpolation was performed to create As surfaces to spatially describe the distribution of As concentration in surface soils, irrigation water, grain and straw. Soil As was measured in the irrigation channels at 20-meter intervals from the irrigation source (STW). For assessing the spatial dependency of soil As, semi-variance analysis was performed.

Source of Information: BRRI Annual Report 2003-04, 89-90, BRRI, Gazipur.

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

The conclusions that follow from this study are

Micro elevation may be an impacting factor for As loading in soil.
Longer irrigation channels or ponded ground water prior to irrigation and discouraging standing water for longer periods in rice fields could be positive measures in reducing arsenic loading in irrigated rice soils.
Alternate Wetting and Drying (AWD) method can be a remedial measure to reduce As contamination in soil as well as food chain.

Knowledge Management: Report/Proceedings