Impact of irrigation with As rich groundwater on soil and crops: A geochemical case study in West Bengal Delta Plain, India

The distribution of As was explored in soils and crops in order to investigate the influence of irrigation with As rich groundwater on the soil-plant system, and to determine its impact on the environment and human health. The study was carried out in an intensively cultivated agricultural area of the Bengal Delta Plain, West Bengal, India. Soils, plants, and irrigation water from adjacent rice and wheat fields were analysed for As and other elements. Irrigation water has concentrations of up to 780 μg L−1 As in the study area. Rice and wheat grains are not contaminated by As (about 0.3 and 0.7 mg kg−1, respectively), but concentrations in rice roots were found to be 169-178 mg kg−1 As, which is more than 20 times higher than value of 7.7 mg kg−1 measured at the uncontaminated reference site. This high content is due to an Fe-rich plaque which coats the rice roots. A significant increase of As concentration was registered also in the stems of the rice plants irrigated with As rich groundwater (6.55-7.06, relative to 0.36 mg kg−1 As in the reference plant). Arsenic concentration in the uppermost soil layers of the rice paddy field (38 mg kg−1) was found to be roughly twice as high as in the soil of the less intensively watered wheat field (18 mg kg−1) and more than 5 times higher than in the soil of a rice paddy irrigated with uncontaminated water (7 mg kg−1). In both soil sections As contents decreased downwards to 11 mg kg−1 at 100-110 cm depth, approaching the background value of 5-10 mg kg−1 measured in an unaffected reference area. Sequential extraction experiments show that most of the mobile As in soils is bound to Fe-oxides. Though no tight correlation was found between Fe and As in the bulk soil samples, these experiments coupled with μ-synchrotron radiation XRF analysis of single soil particles from the rice paddy also indicates that additional to the mobile fraction a substantial part of As is immobilized in (chiefly Fe bearing) silicates. In rice soil some As was also found in the sulphide-bearing phase of the extraction.