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Well testing in the floodplain of the Brahmaputra River in Golaghat and Jorhat districts of
Assam, India, shows that groundwater arsenic (As) concentrations increase with distance from
the river. To establish the origin of this pattern, an additional 900 wells <60 m deep were tested
for As and 9 sites were drilled along a 35-km transect perpendicular to the river. The field data
show no relation between groundwater As concentrations ranging from <1 to 660 μg/L along the
transect and (a) As concentrations of <1-5 mg/kg in cuttings of aquifer sand recovered while
drilling or (b) the degree of reduction of iron oxides in these cuttings. The drilling indicates,
however, a marked increase in the thickness of a clay layer capping the aquifer starting from <1-
5 m near the river to over 60 m at the most distant site towards the base of the Naga foothills.
Organic radiocarbon ages of 18-46 kyr obtained from all but one of 13 clay samples indicate pre-
Holocene deposition of the underlying sands across the entire transect. Radiocarbon ages of
dissolved inorganic carbon of 0.2, 4.7, and 17.8 kyr were measured in groundwater from 3
monitoring wells installed to 30-60 m depth at distances of 10, 20, and 40 km from the river,
respectively. A conceptual groundwater flow model consistent with monitored heads and
groundwater ages suggests that thick clay layers capping the aquifer further from the river
inhibited flushing of the aquifer and, as a result, preserved higher As levels in groundwater.
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