CONTROLS ON ARSENIC AND MANGANESE OCCURRENCE IN THE MEKONG RIVER ALLUVIAL AQUIFER (THAILAND) (Invited Presentation)

During the last two decades, numerous studies have addressed the occurrence of As and Mn in alluvial aquifers of major river valleys draining the Himalayas. However, such studies have been relatively scarce along the Mekong River upstream of its delta in Vietnam and Cambodia. We investigated how river-aquifer interactions may affect the spatial and temporal distributions of As, Mn, and other solutes at two sites (Sri Chiang Mai and That Phanom) along the Mekong River in northeast Thailand. Hydraulic head and chemistry were monitored in January and June 2014, and hydraulic head and electrical conductivity were continuously logged at That Phanom from January 2014 to March 2015.

Concentrations of total As and Mn exceeded drinking-water guidelines by as much as an order of magnitude. A broadly-increasing trend between total Fe (above 1.0 mg/L) and total As points to As mobilization by reductive dissolution of Fe(III) (oxyhydr)oxides, which is consistent with other studies in alluvial aquifers along rivers draining the Himalayas. However, occurrences of (1) elevated As in groundwater from both gray and brown sands and (2) a shallow saltwater-freshwater interface upstream of the delta are unusual relative to other As-affected alluvial aquifers in South and Southeast Asia. The interface, which is associated with shallow evaporite beds at That Phanom, rises during the dry season and falls during the monsoon as river stage rises. Upwelling of saline water can promote the formation of metal(loid)-chloride complexes and increases in ionic strength, thus increasing the solubility of (oxyhydr)oxide phases that could sequester Mn and As. Salinization can also mobilize adsorbed As oxyanions via the counterion effect. Conversely, stage fluctuations may result in localized influxes of oxic river water, thereby promoting Mn and Fe (oxyhydr)oxide precipitation and As retention. Climate change and river management associated with installation of dams are likely to perturb groundwater flow and chemistry along the Mekong River. Therefore, we recommend longer-term, spatially detailed monitoring of groundwater flow and chemistry in the Mekong River alluvial aquifer.