In contrast to high concentrations observed near the Brahmaputra River in Bangladesh, well testing in an upstream portion of floodplain in the Indian state of Assam has shown groundwater arsenic (As) concentrations increasing with distance from the river. To study the cause of this pattern, over 900 wells <60 m deep were tested for As and 9 sites were drilled manually over the same depth range along a 35-km transect perpendicular to the river. No relation was observed between groundwater As concentrations ranging from<0.04 to 660 μg L−1 along the transect and the As <1–5mg kg−1 content of recovered sand cuttings. Drilling showed a marked increase in the thickness of a clay layer capping the aquifer starting from <5 m closest to the river to >60 m at the most distant site near the base of the Naga foothills. Radiocarbon ages of 18–46 kyr indicate pre-Holocene deposition of the underlying sands across the entire transect. With the exception of two drill sites closest to the river, the cuttings were consistently grey and indication of reduced Fe oxides. Radiocarbon ages of DIC of 0.2, 4.7, and 17.8 kyr were measured in groundwater from 3 monitoring wells installed to 30–60m 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 inhibited the flushing of As towards the foothills and, as a result, maintained higher levels of As in the groundwater.