SPATIAL AND SEASONAL VARIABILITY IN ARSENIC AND OTHER SOLUTES IN GROUNDWATER ALONG THE MEKONG RIVER, NORTHEAST THAILAND

Previous studies have identified elevated As concentrations in aquifers beneath the delta of the Mekong River (Vietnam and Cambodia) and other major rivers draining the Himalayas. However, apart from the Ganges valley, there have been few studies of As in upstream floodplain aquifers in South and Southeast Asia. We sampled groundwater along transects perpendicular to the Mekong in northeast Thailand during the dry season (January) and early monsoon season (June) of 2014 to evaluate spatial and seasonal variability in As and other solutes. Sampling sites included two monitoring wells each along four transects in Si Chiang Mai district, Nong Khai province, and four piezometer nests along a transect in That Phanom district, Nakhon Phanom province. Wells and nests were laterally offset by tens to hundreds of meters from each other and the river. Screened intervals in piezometers (two to three at each nest) were vertically offset by 3-6 m. Logging transducers and electrical conductivity (EC) sensors were hung in the two nests closest to the river at That Phanom.

Initial results indicate that As concentrations in groundwater are spatially patchy, even in sediments with similar lithologies. Wells and piezometers are screened in brown and gray sands and gravels that are typically overlain by a surficial clay layer. Arsenic ranged from 0.5 to 103.8 ppb, with values > 10 ppb (the WHO limit) in four of eight wells at Si Chiang Mai and four of nine piezometers at That Phanom. Such values usually occurred in gray sands and gravels, consistent with dissolution of As from Fe and Mn (oxyhydr)oxides under reducing conditions. All but one sample with Fe > 300 ppb and most samples with Mn > 100 ppb had As > 5 ppb. Solute concentrations were more temporally variable in That Phanom, where brackish water occurs locally at depths < 20 m. Logged EC at the nest nearest the Mekong varied inversely with river stage, fluctuating between 0.845 and 0.885 mS/cm from mid-January to late February, rising over 2 weeks to 1.14 mS/cm in mid-March, attaining a plateau of 1.14-1.18 mS/cm from mid-March to mid-June, then decreasing to 1.01 mS/cm over 8 days in mid- to late June. These EC fluctuations suggest that as river stage falls, groundwater discharges from greater depth, which may explain observed variability in As between January (10.0 ppb) and June (41.7 ppb) at this nest.