Spatial Distribution and Temporal Variability of Arsenic in Irrigated Rice Fields in Bangladesh. 2. Paddy Soil

Abstract

Arsenic-rich groundwater from shallow tube wells is widely used for the irrigation of boro rice in Bangladesh and West Bengal. In the long term this may lead to the accumulation of As in paddy soils and potentially have adverse effects on rice yield and quality. In the companion article in this issue, we have shown that As input into paddy fields with irrigation water is laterally heterogeneous. To assess the potential for As accumulation in soil, we investigated the lateral and vertical distribution of As in rice field soils near Sreenagar (Munshiganj, Bangladesh) and its changes over a 1 year cycle of irrigation and monsoon flooding. At the study site, 18 paddy fields are irrigated with water from a shallow tube well containing 397 +/- 7 microg L(-1) As. The analysis of soil samples collected before irrigation in December 2004 showed that soil As concentrations in paddy fields did not depend on the length of the irrigation channel between well and field inlet. Within individual fields, however, soil As contents decreased with increasing distance to the water inlet, leading to highly variable topsoil As contents (11-35 mg kg(-1), 0-10 cm). Soil As contents after irrigation (May 2005) showed that most As input occurred close to the water inlet and that most As was retained in the top few centimeters of soil. After monsoon flooding (December 2005), topsoil As contents were again close to levels measured before irrigation. Thus, As input during irrigation was at least partly counteracted by As mobilization during monsoon flooding. However, the persisting lateral As distribution suggests net arsenic accumulation over the past 15 years. More pronounced As accumulation may occur in regions with several rice crops per year, less intense monsoon flooding, or different irrigation schemes. The high lateral and vertical heterogeneity of soil As contents must be taken into account in future studies related to As accumulation in paddy soils and potential As transfer into rice.

Arsenic Limits Trace Mineral Nutrition (Selenium, Zinc, and Nickel) in Bangladesh Rice Grain

“…Originating principally from the Himalayas, this region also bears a signature associated with the peninsular Indian shield and from eroding terrace soils (32): typical background levels for As are between 5 and 10 µg g topsoil -1 (31,33,34). Gazipur soils in contrast are older, highly weathered and constitute part of the Madhupur tract; a terrace formed prior to Bangladesh's inclusion into the Himalayan drainage system (32).…”

Arsenic contamination in shallow groundwater and agricultural soil of Chakdaha block, West Bengal, India

“…Boro (dry season) rice requires approximately 1000 mm of irrigation water per season, which is promoting by 1 µg/g per year soil As concentration due to irrigation with As contaminated water (Meharg and Rahman, 2003). Although the long-term trends of As concentrations in irrigated paddy soils are difficult to assess due to large spatial heterogeneity and temporal variability (Dittmar et al, 2007). The average arsenic concentration in the groundwater of the contaminated area 122.57 µg/l, is much above the prescribed limits by WHO, i.e., 10 µg/L.…”

Soil Pollution Due to Irrigation with Arsenic-Contaminated Groundwater: Current State of Science

“…In terms of edaphic factors, mineralogy, soil texture, pH, and redox status interplay to control the speciation and fate of As in agricultural fields. In most irrigated soils, metal oxides are the primary As sinks, although clay, sulfide, and carbonate minerals may also sequester As, depending on the soil pH [14,17,[67][68][69]. In particular, As sorption to iron (Fe) and aluminum (Al) oxides immobilizes As within irrigated topsoil, thereby decreasing As leaching and plant uptake, unless conditions become favorable for As remobilization.…”

“…Rice roots readily absorb As, which can translocate through the plant and create elevated As concentrations within rice grains [46,71]. Arsenic mobilized to porewater can also leach through the soil/sediment profile, furthering groundwater contamination [68], or be carried away from the system with receding floodwater [17,72]. Research quantifying As mass balance indicates that groundwater-irrigated rice-field soils are net sinks for As [56••], but long-term As mobility and mechanisms of plant uptake are open areas of study.…”