资源环境科技发展态势分析平台

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 nine 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 mu g/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 toward 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 three 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.

Plain Language Summary A large fraction of the rural population of South Asia living in river floodplain areas is exposed to toxic levels of arsenic by drinking well water. This study combines field data from the Indian state of Assam with a groundwater flow model to understand the mechanisms that control the distribution of arsenic in groundwater. A field kit was used to document groundwater arsenic concentrations that increase from the banks of the Brahmaputra River to the foothills of nearby mountains. Drilling showed that the sandy aquifer becomes isolated from replenishment with local surface water along the same transect by an increasingly thick impermeable capping clay layer. A simple groundwater flow model of the area tuned to match available data indicates that aquifers near the Brahmaputra River were flushed of much of their initial arsenic content by recharge with low-arsenic surface water over thousands of years. With increasing distance from the river, this flushing was progressively inhibited by a layer of clay layer capping the aquifer and elevated arsenic remained high as a result. Mitigation efforts to reduce arsenic exposure should take into account that the distribution of arsenic in well water evolves slowly and is therefore unlikely to change very soon.