CHARACTERIZING HYDROGEOCHEMICAL REACTIONS AND ARSENIC DISTRIBUTION IN THE GANGES RIVER DELTA USING INVERSE REACTION PATH MODELING

The groundwater of the Ganges River Delta, which serves as the primary source of drinking water for millions of people in India and Bangladesh, has been found to be highly contaminated by arsenic (As), leading to what is regarded as the largest case of mass poisoning in human history. Previous studies have identified major sources and liberation processes of arsenic (As) into groundwater based on hydrogeochemical signatures. However, the regional-scale delineation of groundwater evolution through reaction pathways to support evidence of As liberation remains to be undertaken.

This study aims to characterize the major hydrogeochemical reactions contributing to the observed regional-scale arsenic distribution across the delta. Using PHREEQC, we are employing inverse reaction path modeling along hypothesized (a) nearly north-south regional-scale groundwater flow paths and (b) near-vertical local flow paths across the delta. Reaction path models were constructed for all possible depth classes [shallow (<70 m below MSL); intermediate (70-<150 m); deep (>150 m)] along 74 flow segments, connecting a total of 304 groundwater samples from 52 target locations along seven east-west transects. The results are interpreted with a special focus on iron oxides and sulfide phases, which are known to control arsenic dynamics within the delta. We hypothesize that the reaction path modeling will reveal the complex interplay between hydrogeochemical and redox signatures of the groundwater that dictates the variability of As occurrence within the delta.