Dissolved arsenic in the shallow alluvial aquifers in North Brahmaputra Plain, India: a case study in and around lower Jia Bharali River basin

Abstract This study was carried out to investigate Arsenic (As) contamination in the alluvial aquifers of the lower Jia Bharali catchment and adjoining areas in Sonitpur district of Assam. Samples were collected twice a year (July and February) for three consecutive years from 50 monitoring wells spread into both older and younger alluvium between the Brahmaputra River towards south and Arunachal foothills towards north. The analytical results show that dissolved As content [both As(III) and As(V)] varies from below detection level (BDL) to 7.39 µg/L with a mean value of 1.92 µg/L and standard deviation of 1.37 µg/L during wet season (July). Thus, it remains within the WHO (2004) prescribed limit (10 µg/L) in the study area in the wet season. During the dry season, the range of variation is higher, from BDL to as much as 13.8 µg/L with a mean value of 2.57 µg/Land standard deviation of 2.23 µg/L. About 78% of the wells show a concentration between 1 and 10 µg/L in both the seasons. However, only one of the wells present in foothills of Arunachal Himalaya was found to have As content higher than the WHO limit in the dry season. Along with total As, examination of concentration levels of other key parameters, viz., Fe, Mn, Ca, Na, K, and Mg with pH and SO4 2− was also carried out. Most of the wells (~92%) showed Fe concentration much higher than the WHO (2004) permissible limit of (0.3 mg/L) particularly during the dry season and it is likely that high Fe concentration was responsible for keeping total As concentration at comparatively low levels. 34% of the samples in the wet seasons and 86% of the samples in the dry seasons have Mn above the permissible limit of 0.1 mg/L.