A COMPOSITE APPROACH TO CHARACTERIZE DEEP GROUNDWATER OF THE ARSENIC-CONTAMINATED WESTERN GANGES-BRAHMAPUTRA-MEGHNA DELTA

The As poisoning of groundwater in the shallow Holocene aquifers of the densely populated Ganges-Brahmaputra-Meghna (GBM) delta is one of the most formidable environmental crises of the present day. A number of workers have suggested that the “deeper aquifer” has negligible As and is thus a potential alternative water source for the millions of GBM delta residents. Evaluating this hypothesis entails a thorough characterization of the hydrostratigraphic framework, groundwater flow, and chemistry of the “deeper aquifer.”

Since 2003, we have been studying the hydrogeology of a severely As-affected area (~ 21,000 km²) of the Indian part of the delta (eastern West Bengal). Based on 139 lithologs, we have developed conceptual and computer hydrostratigraphic models (at a resolution of 1 km x 1 km x 2 m) up to a depth of 300 m. The models show the presence of a single major aquifer (fine to coarse sand), which has been punctuated by discontinuous clay-sandy clay aquitards, mostly toward the south. The aquifer thickens from the north (80 m) toward the east (~150 m) and south (>200 m). There are also some small, isolated deeper aquifers. Based on this aquifer-aquitard framework, numerical flow models (at a resolution of 1 km x 1 km x 15 m) have been developed to understand the regional flow of groundwater and contaminant transport prior to extensive irrigation (pre-1970s) and at present. Initial results indicate that pumpage for irrigation has thoroughly disturbed the N-SSE regional flow system by forming small-scale flow cells. These results suggest mixing of contaminated shallower water with deeper water in a single aquifer system.

We collected 53 water samples from deep public-supply wells (47 in the main aquifer and 6 in the isolated aquifers) along the general regional flow path. Total As is ≥10 ppb in 61% of the samples. As(III) is the dominant oxidation state in 14 of 15 filtered samples from 2004 with detectable As. The main aquifer water is a relatively fresh, circumneutral, Ca-HCO₃ type, with high Fe(II), negligible NO₃^- and NH₄⁺, no detectable PO₄^3- or H₂S, and O₂ typically ≤2 ppm. Water from the isolated aquifers is high in Na⁺, Cl^-, HCO₃^-, and B, but has no detectable As. No definite relationship has been observed between As and Fe or between As and Mn. δ¹⁸O and δ²H analyses indicate that both fresh and brackish waters are meteoric in origin.