2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 3
Presentation Time: 8:45 AM


MUKHERJEE, Abhijit and FRYAR, Alan E., Geological Sciences, University of Kentucky, 101 Slone Building, Lexington, KY 40506-0053, amukh2@uky.edu

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 PO43- 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.