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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.

Urbanization affects quantitatively and qualitatively groundwater environment in urban area. Especially, groundwater environment in large urban area has been changed more than several decades. However, there are few studies about evaluation of secular change of groundwater environment with developing of urban area. In this study, long term change of groundwater environment in the Kanto plain and the Nobi plain which had the very large urban areas were summarized from the view point of regional groundwater flow.Present groundwater flows in these plains were significantly different from the groundwater flows in the periods when the effects of urbanization were small, even though more than 30 years have passed since groundwater extraction had been regulated. Also, three dimensional distributions of chemical properties of groundwater quality and environmental isotopes have changed. These results show that the effects of human activities were still strongly remained. On the other hand, demands for groundwater for public use and private use have become increasing in recent years because of recovery of hydraulic potentials of confined aquifers that had depleted from the 1950s to 1970s by huge amount of groundwater abstraction. Therefore, it is important to keep monitoring and evaluating the groundwater environment in these urban areas for sustainable management of groundwater resources.

Environmental tracers become a common tool for the groundwater study and a number of methods have been presented in order to understand groundwater flow processes, water budget, origins, chemical reaction processes and retention time. Tracers often used are selected and reviewed for their various methods and advantages as follows; 1) stable 18O, D in water, 2) stable 13C and radioactive 14C in DIC, 3) noble gases such as He, Ne, Ar, Kr, Xe and their isotopes, 4) radioactive 36Cl in dissolved chloride and some heavier isotopes, and 5) inert gaseous species such as CFCs.If they are less reactive species, they likely preserve information at the time of recharge or their origin. Use of D, 18O and the d-value of water is the powerful tool to determine the recharge area because recharged meteoric water have their inherent isotopic ratios correlated with the recharge elevation, distance from the coast, or the local topography. Carbon-bearing species are more reactive though, use of stable isotopes of DIC leads to identify its origin and helps to analyze the chemical reaction between minerals and water or gas addition processes during the groundwater flow in aquifers. Radioactive 14C has been used to estimate groundwater age however special attention should be paid for, i.e., the origin of DIC, before applying the method. Noble gas tracers are the useful species to presume recharge temperature from their concentrations in water using their temperature dependence of solubilities. Radiogenic 4He concentration can be used for the very long-term groundwater dating since the 4He is produced in the crust and is accumulated in the deep aquifers, if the local accumulation rate of 4He is known. Radioactive 36Cl has been used to determine the age of very old saline waters up to million years. This isotope will also be convenient for the dating of very younger waters, by the use of bomb-produced 36Cl resulted from surface nuclear experiments near the seawater in the 1950s. Chlorofluorocarbons (CFCs) are the gas species produced by the recent human activity and dissolve in water during the recharge, therefore, the affected younger groundwater will have equivalent CFCs concentrations with the atmospheric CFCs concentrations at the time of the recharge. As these species are easy to detect with very high sensitivity, this tracer has now been applied not only for the age determination but for the mixing or contamination of shallow young water to a deep old groundwater. As an individual method listed above is valid only for the very simple flow system, appropriate assumptions or coupling of using different tracers is necessary to understand natural complex groundwater flow system where mixing of groundwaters of different origin or age occurs. Combination of tracers helps us simulating the complex system in detail and is being a growing trend in groundwater study.