MONITORING AND SURVEILLANCE OF GROUNDWATER ARSENIC CONTAMINATION IN THE BRAHMAPUTRA FLOODPLAIN IN ASSAM

摘要

Based on the results of a Rapid Assessment program carried out in 2005, an initiative of arsenic screening program for the Brahmaputra Floodplain in Assam was undertaken by this group of authors, in collaboration with the Public Health Engineering Department (PHED), Assam and UNICEF. A rigorous three phase testing procedure, involving three different instruments, Arsenator, UV-1 Spectrophotometer and Atomic Absorption Spectrophotometer (AAS) was adopted for the program. The results yielded were reasonably consistent, showing good analytical performances. For a better understanding of the mechanism of arsenic release into the groundwater of the region, along with the wider monitoring and surveillance, study on the hydrogeochemical processes leading to arsenic release in the northern part of the Brahmaputra floodplain was conducted simultaneously. Data available from the three tire analysis were used for preparation of district level arsenic concentration maps using ArcGIS 9.2, from which the total vulnerable areas could be delineated for appropriate remedial measures, supported by the output of the focused research on release process and mechanism. Almost 30 % of about 56,180 sources showed elevated arsenic concentrations above the BIS and WHO permissible limits of 10 ppb. The study revealed that the concentrations were localized at certain pockets of the floodplain and is narrowly spread than the GBM floodplain in Bangladesh and West Bengal. The results of the hydrogeochemical study conducted showed that reductive dissolution processes was likely to be a major mechanism of arsenic release into groundwater along with major cations like Fe, Na and Ca playing an important role in leaching of arsenic. As a multi pronged solution to the problem, development of removal technologies through improvement of indigenous media based filtration as immediate measures and provision of drinking water from amply available surface water sources in the long run indicated promising mitigation options.