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