Assessment of a ground water flow model of the Bangkok Basin, Thailand, using carbon-14-based ages and paleohydrology

Abstract

A study was undertaken to understand the groundwater flow conditions in the Bangkok Basin, Thailand, by comparing 14 C-based and simulated groundwater ages. 14 C measurements were made on about 50 water samples taken from wells throughout the basin. Simulated ages were obtained using 1) backward-pathline tracking based on the well locations, and 2) results from a three-dimensional groundwater flow model. Comparisons of ages at these locations reveal a large difference between 14 C-based ages and ages predicted by the steady-state groundwater flow model. Mainly, 14 C and 13 C analyses indicate that groundwater in the Bangkok area is about 20,000 years old, whereas steady-state flow and transport simulations imply that groundwater in the Bangkok area is 50,000–100,000 years old. One potential reason for the discrepancy between simulated and 14 C-based ages is the assumption in the model of steady-state flow. Groundwater velocities were probably greater in the region before about 10,000 years ago, during the last glacial maximum, because of the lower position of sea level and the absence of the surficial Bangkok Clay. Paleoflow conditions were estimated and then incorporated into a second set of simulations. The new assumption was that current steady-state flow conditions existed for the last 8,000 years but were preceded by steady-state conditions representative of flow during the last glacial maximum. This “transient” paleohydrologic simulation yielded a mean simulated age that more closely agrees with the mean 14 C-based age, especially if the 14 C-based age corrected for diffusion into clay layers. Although the uncertainties in both the simulated and 14 C-based ages are nontrivial, the magnitude of the improved match in the mean age using a paleohydrologic simulation instead of a steady-state simulation suggests that flow conditions in the basin have changed significantly over the last 10,000–20,000 years. Given that the valid age range of 14 C-dating methods and the timing of the last glacial maximum are of similar magnitude, adjustments for paleohydrologic conditions may be required for many such studies.