CONTROL OF DELTA SEDIMENT GEOMETRY ON ARSENIC DISTRIBUTION WITHIN THE GANGES RIVER DELTA, INDIA AND BANGLADESH

The sedimentation process within the Ganges river delta has resulted in the formation of a highly heterogeneous sediment complex which also acts a prolific aquifer system. Geomorphologically, the present delta can be divided into the paleo-delta, mature delta, and active delta from north to south, each having characteristic sediment geometry. Lithological modeling shows that underlying the paleo-delta to the north is a thick and continuous sand aquifer with little or negligible clay interlayers, which overlies a clay unit pinching out to the east and south. This thick sand layer breaks into thinner poorly connected aquifer units, intercalated by silt and clay interlayers in the mature and active delta regions, with increasing dominance of clay and silt towards the south and east. The spatial variance in geometry of these delta sediments imposes a strong control on the spatial and depth distribution of arsenic in the groundwater of the Ganges delta. Spatial interpolation maps of arsenic along different depth classes indicate that the interlayered aquifer units to the east and south of the delta show very high levels of contamination in the shallow depths (>70 m) with about 65%-85% of arsenic contaminated (<10 µg/L) tube wells, while in the deeper levels (<150 m) these regions show significantly high percentages (70%-100%) of arsenic safe wells. In contrast, depth-classed histogram of arsenic concentrations in tube wells within the continuous aquifer to the north shows the presence of about 70% arsenic contaminated tube wells, with significant percentages of contaminated tube wells even in the deeper depths. However, statistics show very high variability in groundwater arsenic concentrations even in the same depth, implying the significantly strong control of other geological, geomorphological, hydrogeological and biogeochemical factors on arsenic concentration within the delta.