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 kg¹. Chemical extraction
results indicated that 36% of the total soil As was associated with poorly crystalline Feoxide 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 µg g¹, 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.