UNDERSTANDING THE ARSENIC DISTRIBUTION PATTERNS IN THE AQUIFERS OF THE GANGES RIVER DELTA

The spatial variability of arsenic within the Ganges river delta is a function of a complex interplay between various geological, hydrogeological and biogeochemical factors at local to basinal scale. Out of the most strongly influencing factors on a basinal scale, delta physiography and aquifer geometry, statistically seems to be the most consistent and dominant factors controlling the spatial distribution of arsenic within the Ganges river delta. Physiographically, the Ganges delta can be divided into the inactive, active and tidal delta zones from north to south. The delta morphology shows a strong correlation with the aquifer geometry, indicating a significant control of the delta building processes on the sedimentation patterns and the resultant architecture of the delta aquifers.

The underlying aquifer architecture shows a complex geometry with thick sand sequence towards the north, which then breaks up into thinner sand layers, intricately intercalated by discontinuous clay units towards the south and east. Though the proportion of the confining clay units gradually increase towards south, on a simplistic estimate, the aquifer geometry changes from unconfined aquifer to the north to the semiconfined and confined aquifer units to the south along the N 23.1° latitude. The arsenic within each of this unconfined, semiconfined and confined delta aquifer units show significantly different distribution pattern, spatially and also along the depth. In the unconfined aquifer zone to the north, the percentage of arsenic contaminated wells does not show much variation over depth and remains high throughout the entire thickness of the aquifer. However, for regions with significant clay intercalation towards the south and east, percentages of high arsenic wells decline considerably with depth, with a significant percentage of arsenic contaminated wells in the deeper levels. It is to be understood that such correlations do not take into account the local factors which results to the well-to-well variability of arsenic on a local scale.