As well as reducing groundwater recharge by precipitation, urbanization creates a new important source of water for groundwater recharge, that is mains leakage. Although it has been realized by a water balance method that urban groundwater recharge is often as high, or higher, than pre-urbanization rates owing to the downward percolation of mains leakage, the increases have rarely been quantified in a direct manner. In this study, contribution of mains leakage to the discharge of springs in the Kurome River basin and Shakujii River basin, both in Tokyo, were evaluated through a comparison among the δD and δ18O of spring water and mains leakage (tap water). In the moderately-urbanized Kurome River basin, all the springs represented almost 100% precipitation contribution. This is also the case in the more urbanized upper reaches of the Shakujii River basin, where precipitation alone could account for 85-100% of the discharge of springs. Precipitation was found to play a less important role in spring discharge in the highly-urbanized lower reaches of the Shakujii River basin (impermeable surface ratio: some 0.8 and more), were some springs contained substantial proportions of mains leakage of as much as 30-70%. On the other hand, it is worthy of notice here that, even in central Tokyo, the isotopic composition of most spring waters was indicative of predominant contribution of precipitation, which is contrary to the results of the previous studies on the basis of a water balance method.
