Arsenic budgeting in a long-term irrigation-contaminated rice field of the Bengal Delta Plain

Arsenic (As) contamination of groundwater-irrigated rice agroecosystems presents a major environmental and public health challenge across the Bengal Delta Plain (BDP). Although long-term irrigation is thought of as a dominant As input pathway, there is a lack of studies that have quantitatively reconciled field-scale As influx with measured soil accumulation. This study establishes a 33-year As mass balance for a rice field in Nadia district, West Bengal, integrating measured irrigation-water As concentrations, rice grain and straw As contents, crop yields and topsoil As levels. Annual groundwater irrigation introduced 207 mg As m-2 yr-1, whereas biomass removed only 3.66 mg As m-2 yr-1 (< 2 % of inputs). The resulting net accumulation of 203.4 mg m-2 yr-1 predicted an enrichment of 39.9 mg kg-1 in the 0-15 cm soil layer, aligning closely with the measured value of 37.9 mg kg-1. This agreement indicates minimal leaching or lateral losses and reflects strong Fe-mediated retention under flooded redox conditions. The findings confirm that groundwater irrigation alone accounts for long-term As build-up in BDP paddy soils and provide a quantitative basis for anticipating future soil burdens under continued reliance on As-bearing aquifers.