Abstract
Several field, net-house and laboratory experiments were conducted with the objectives of (i) determining spatial variability of arsenic (As) in irrigation water, soil and rice in a shallow tube-well (STW) command area, (ii) examining As levels in soil porewater, (iii) understanding adsorption of As by soil, and (iv) evaluating the effect of added As on the uptake and growth of rice in the presence of P and Fe. The As concentrations in irrigation water and soil in the STW command area decreased as the distance from the pump source increased. In the distribution channel, As concentration in the standing water was almost three times lower than that in freshly pumped out flowing water. This could be explained by As co-precipitation with Fe oxides. More than 50% of soil samples within the STW command area had total As concentrations of greater than 20 mg kg1 in the plough layer. The total soil As generally decreased with depth vertically downward. The background level of soil As was 9.24 mg kg'. Chemical extraction results indicated that 36% of the total soil As was associated with poorly crystalline Fe- oxide compounds. The porewater As concentrations were higher in the dry season (Boro) than in the wet season (transplanted Aman), with As (V) as the predominant species, in spite of moderately reduced conditions (Eh = 28-34 mV). The porewater As concentration was positively correlated with P, Mn and Zn concentrations and negatively correlated with Fe concentration. The total As content in rice decreased in the order: straw > husk > grain, with mean values of 0.97, 0.71 and 0.30 ug g', respectively. The As adsorption study demonstrated that the increase of As adsorption was progressively smaller with subsequent additions of the element, showing a good fit to the quadratic equation. The Freundlich model provided a better fit compared to the Langmuir model. The plant culture experiment indicated that As markedly affected the grain yield and increased the As level in rice, especially in straw. The As concentrations in rice grain and straw ranged from 0.19 to 0.71 μg g1 and 0.30 to 18.65 μg g', respectively. The addition of P and Fe to the soil reduced the toxic effect of As which was probably due to Fe-plaque formation on the rice roots. The use of As contaminated irrigation water can result in elevated concentrations of As in soils and, ultimately, in plants, especially rice. High As in rice grains is potentially dangerous for humans. The levels of As found in rice straw could be a potential risk for cattle health, too. Spatial variability of As in irrigation water, soils and plants warrants location-specific strategies for managing the As problem in agriculture and food.