Arsenic (As) is widespread in alluvial aquifers along rivers draining the Himalayas but has received little attention along the Mekong River upstream of its delta. This study investigates linkages among groundwater recharge, flow, chemistry, and river stage at two sites along the Mekong River in northeast Thailand. Hydraulic head and chemistry were monitored in January and June 2014. In addition, hydraulic head and electrical conductivity were continuously logged at one site from January 2014 to March 2015. During the dry season, groundwater tends to flow toward the river, and saline water from shallow evaporites can intrude the alluvial aquifer. As stage rises, hydraulic-gradient reversals can result in groundwater-river water mixing, thereby promoting geochemical disequilibrium. Groundwater chemistry reflects silicate, carbonate, and evaporite weathering; multiple redox reactions; and spatial and temporal variability in recharge. Concentrations of As and manganese (Mn) in groundwater exceeded drinking-water guidelines by as much as an order of magnitude. Results suggest As is mobilized in groundwater by reduction of ferric (oxyhydr)oxides, consistent with the findings elsewhere in South and Southeast Asia. Upwelling of saline water can increase the solubility of (oxyhydr)oxide phases that sequester Mn and As, can complex Mn and can mobilize adsorbed As oxyanions. Conversely, influxes of oxic river water could promote (oxyhydr)oxide precipitation.
|Journal||Environmental Earth Sciences|
|State||Published - Mar 2021|
Bibliographical noteFunding Information:
The authors acknowledge the National Research Council of Thailand for permission to conduct this project (# 2774); staff of the Groundwater Resources Institute at Khon Kaen University for logistical support; land owners at the study sites for access; the Mekong River Commission and Thai Meteorological Department for data; Joshua Benton for drafting Fig. 1 ; and the journal reviewers for their constructive suggestions, which improved the manuscript.
© 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.
- Mekong River
ASJC Scopus subject areas
- Global and Planetary Change
- Environmental Chemistry
- Water Science and Technology
- Soil Science
- Earth-Surface Processes