Detecting groundwater inputs into Bangkok canals via radon and thoron measurements

Abstract

Naturally-occurring radon (222Rn) is very concentrated in groundwater relative to surface waters and thus serves as an effective groundwater discharge tracer. Conductivity is also typically present in groundwaters at different levels than associated surface waters and thus may also be used as a tracer of-interactions between these water masses. Previous studies by our group using radon and conductivity as groundwater tracers suggested that there is shallow groundwater seeping into the man-made canals (klongs) around Bangkok. Furthermore, the groundwater was shown to be an important pathway of nutrient contamination to the surface waters. In the present study, we have re-examined some of the same canals and added thoron (220Rn) measurements in order to evaluate if this would provide more site-specific information. Thoron is a member of the natural 232Th decay chain, has exactly the same chemical properties as radon, but has a much shorter half-life (56 s) than radon (3.84 days). Because of its rapid decay, if one detects thoron in the environment, there must be a source nearby. Thus, thoron is potentially an excellent prospecting tool. In the case of measurements in natural waters, sources of thoron (as radon) could indicate groundwater seeps. During our surveys in the canals of Bangkok, we did successfully measure thoron and its distribution was more variable than that of radon, suggesting that seepage into the canals is not uniform. Areas of higher ground elevation, often in areas where Thai temples are located, were particularly high in thoron. © 2011 Springer Japan.