This study aims to introduce thoron ((220)Rn), a naturally occurring isotope, as a new groundwater tracer for detecting groundwater seepage into Bangkok canals. Previous studies by the group using radioactive radon ((222)Rn) 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. Thoron is a member of the natural (232)Th decay chain, has exactly the same chemical properties as radon, but has a much shorter half-life (56 s) than radon (3.84 d). By using its advantage of 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 should indicate the point of groundwater discharges more precisely than radon. During the surveys in the canals of Bangkok, thoron was successfully measured and its distribution was more variable than that of radon, suggesting that seepage into the canals is not uniform.