Mass Balance of Arsenic Input, Release and Accumulation in Irrigated Paddy Fields in Munshiganj/Bangladesh

In Bangladesh, groundwater from shallow tube wells is extensively used for irrigation of dry season Boro rice. Due to often high arsenic (As) concentrations in irrigation water, approximately 1360 tons of As were estimated to be transferred to arable soils in Bangladesh each year (1). Potential accumulation of As in paddy soils may have adverse effects on rice yield and quality and consequently human health.

We studied the fate of As in paddy fields at a field site in Munshiganj, where Boro rice is grown since the early 1990's using irrigation water containing ~ 400 µg As L^-1. The growth season from January to May is followed by intense monsoon flooding until October. We found that irrigation leads to spatially heterogeneous As input into paddy fields. Soil As contents are highest in topsoils and decrease with increasing distance from the point where irrigation water enters the fields. Furthermore soil As is variable over the year, increasing during irrigation and decreasing during monsoon. The high spatiotemporal As variations complicate predictions of long term trends. First estimates suggested that As input may partly be counteracted by As loss through irrigation water percolation and/or As diffusion into monsoon floodwater (2, 3).

In order to asses the potential for long-term As accumulation, we determined the soil As contents in an individual field with high spatial and temporal resolution (38 soil cores, 4 sections down to 40 cm, 7 sampling campaigns over 3 years (4x after monsoon, 3x after irrigation)). The results from this monitoring campaign will be discussed with respect to As fluxes and the As accumulation potential in paddy fields.

References

(1) Ali et al., In Fate of Arsenic in the Environment; ITN: Dhaka, 2003, 7-20.

(2) Roberts et al., ES&T 2007, 41, 5960-5966.

(3) Dittmar et al., ES&T 2007, 41, 5967-5972.