Assessment of the potential for arsenic accumulation in rice field soils in Bangladesh and its implication on rice yield

Abstract

Widespread use of arsenic (As) contaminated water for irrigation to produce dry season crop (mainly boro rice) poses threats to sustainable agriculture and to human health in Bangladesh. Presence of As in irrigation water leads to elevated levels of As in soil, with significant spatial and temporal variation. Monsoon flooding removes significant fraction of As added to soil through irrigation each year. Reported soil-As data from non-flooded areas show much higher retention of As added through irrigation water. So areas without pronounced monsoon flooding are at greater risk of soil-As accumulation than seasonally flooded paddy fields. This study makes an assessment of the potential of arsenic accumulation through irrigation in soil where extensive boro rice cultivation is carried out; the potential of arsenic accumulation has been presented in the form of a “Arsenic Hazard Map”. The Arsenic hazard map has been prepared considering the hypothesis on As deposition/ mobilization in paddy soils, using data on arsenic distribution in groundwater, boro cultivation/ irrigation intensity, and flood inundation depth. This risk level of arsenic accumulation in soils has been divided into seven categories: (a) Very high-risk area, (b) High-risk area, (c) Moderate-risk area, (d) Moderate to low-risk area, (e) Low-risk area, and (f) Very low-risk area. According to the Arsenic Hazard Map, areas to the south-west and some areas in the north-east are at risk of higher accumulation of arsenic, because of higher concentration of arsenic in irrigation water, higher intensity of boro cultivation, and higher land elevation. “Query analysis” of the hazard map reveals that 282,267 ha of highland used for boro cultivation in 22 upazilas of the south-western region of Bangladesh are at “very high risk” of arsenic accumulation, because of higher arsenic in STW irrigation water and low flood inundation depth; and 382,001 ha of highland in 35 upazilas are at “high-risk” status, where flood inundation depth is low , but arsenic concentration in STW irrigation water is also relatively low. Areas in the northern parts of Bangladesh are at “low to no risk” because of lower concentration of arsenic in irrigation water, despite higher intensity of boro cultivation. Some areas in the south-central region of Bangladesh are at “moderate to low risk” of arsenic accumulation because of higher elevation of these areas (which results in lower flooding intensity). Soil-As data from primary (i.e., field sampling and analysis) and secondary sources qualitatively validate the Arsenic Hazard Map prepared in this study. Soil-As map prepared form collected primary and secondary data shows that in most of the south western region, soil-As level exceed 30 mg/kg. An assessment of the impact of arsenic accumulation in soil on rice yields in selected high-risk areas has been made utilizing available data/ information correlating arsenic in irrigation water and agricultural soil, and soil arsenic and crop yield. It appears that significant rice yield reductions could result in the future due to accumulation of arsenic in irrigated soil. Estimated yield reductions in nine high-risk upazilas varied for about 9 percent to 24 percent for the year 2020, and 12 percent to 53 percent for the year 2050. Possible yield reduction due to elevated soil arsenic appears to be a potential threat to food security and agricultural sustainability in Bangladesh, where rice is the main crop and staple food, and boro rice accounts for over 50 percent of total rice productions.