GroMoPo Metadata for Bangkok Basin groundwater age model

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 13C analyzes 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 14C-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 14C-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.

本研究旨在通过对比基于碳14（14C）的地下水流年龄与模拟地下水流年龄，探究泰国曼谷盆地的地下水流状态。研究人员从全盆地范围内的钻井中采集约50份水样，开展了碳14测量工作。模拟年龄通过两种方式获取：一是基于钻井位置的反向迹线追踪（backward-pathline tracking），二是三维地下水流模型（three-dimensional groundwater flow model）的计算结果。对这些点位的年龄进行对比后发现，基于碳14的年龄与稳态地下水流模型预测的年龄存在显著差异。具体而言，碳14与碳13分析结果显示，曼谷地区的地下水年龄约为20000年，而稳态水流与运移模拟则表明该区域地下水年龄为50000至100000年。模拟年龄与碳14年龄之间存在偏差的一个潜在原因，是模型中采用了稳态水流的假设。约10000年前的末次冰盛期（last glacial maximum），由于海平面更低且表层曼谷黏土层（surficial Bangkok Clay）尚未形成，该区域的地下水流速可能更高。研究人员对古水流条件进行了估算，并将其纳入第二组模拟中。新的假设为：过去8000年以来当前的稳态水流条件一直存在，而在此之前则是代表末次冰盛期水流状态的稳态条件。相较于稳态模拟，该瞬态古水文模拟（transient paleohydrologic simulation）得到的平均模拟年龄与基于碳14的平均年龄更为吻合，尤其是在基于碳14的年龄经黏土层扩散（diffusion into clay layers）校正后。尽管模拟年龄与碳14年龄均存在不可忽视的不确定性，但相较于稳态模拟，采用古水文模拟后平均年龄的匹配度得到了显著提升，这表明该盆地的地下水流状态在过去10000至20000年间发生了显著变化。鉴于碳14测年（14C-dating）方法的有效年龄范围与末次冰盛期的时间跨度大致相当，许多类似研究可能需要对古水文条件进行校正调整。