Seasonal variability of arsenic concentrations in irrigation water and its impact on arsenic accumulation in agriculture soils: A study from Bengal Delta

Arsenic (As) is a highly toxic geogenic contaminant present in groundwater globally. The Bengal Delta (jointly Bangladesh and West Bengal, India) is considered as one of the worst As affected areas. The present study investigates (i) the extent of As contamination in the irrigation wells, (ii) its seasonal variability, and (iii) understanding the As accumulation in agriculture soils in the Bengal Delta. For this purpose, 68 irrigation wells (34 shallow and 34 deep) were sampled during pre-monsoon, monsoon and post-monsoon seasons, and thus, a total of 204 water samples were collected. Additionally, top-soil samples (15 cm) and paddy plant (grain, leaf and stem) samples were collected from 6 paddy-cultivating agricultural fields and analysed for As concentrations. The samples were analysed for various physicochemical parameters, major ions and trace elements, including As and iron. Two-way ANOVA test suggests that significant variation in As concentration only across seasons and the As concentration ranges from <1 μg/L to 197 μg/L. This highlights the role of seasonal redox conditions in mobilizing As from sediments into the groundwater. In the Piper plot, both the deep and shallow groundwater falls in Ca-HCO₃ type and do not show any significant seasonal variation in hydrochemical facies. Further, silicate and carbonate weathering have been identified as major contributors to groundwater solute composition.

The As concentrations in the irrigation water and in different parts of the paddy plant were utilized for As budgeting in the agricultural soils for the last 50 years. The calculated (21.48 mg/kg) As values in soils closely align with the measured values (21.29 mg/kg) indicating a gradual build-up of As in the agricultural system through irrigation water over the last 50 years.