Spatial distribution of oxygen-18 and deuterium in stream- and groundwaters across the Japanese archipelago

The stable isotope signatures of local stream- and groundwater are commonly used for the hydrological processes assuming that these tracers reflect the characteristics and history of the recharge water, i.e., local precipitation. To validate this assumption, one important step is to compare the signature of local precipitation with that of stream- and groundwater. In this study, the spatial distribution of oxygen-18 and deuterium in stream- and groundwaters across the Japanese archipelago is considered. The streamwater samples were collected at 1278 forest catchments during the summer season of 2003. As the groundwater samples, we collected 142 commercial bottled waters with as many different sources across Japan as possible. The precipitation data were collected from previous studies and unpublished data kindly offered by many researchers, as well as new data for this study. We found a clear liner relationship for the streamwater as; deltaD = 6.85 delta18O + 6.11 (r2= 0.89), and for the groundwater, i.e., deltaD = 7.32 delta18O + 9.90 (r2 = 0.93). The delta18O in the streamwater, as well as in the groundwater, had negative correlation with the latitude and the ground surface elevation at the sampling point. The regression equations to the latitude were similar each other; y=-0.35x+3.4 (r2=0.43) for streamwater and y=-0.35x+3.6 (r2=0.45) for groundwater, respectively. The equations to the elevation were also similar each other; y=-0.0028x-8.0(r2=0.30) for the streamwater and y = -0.0032x-8.1(r2 = 0.35) for the groundwater. Thus, the latitude and altitude effects, which are commonly observed in precipitation, are also reflected to the stream- and the groundwater. Moreover, the isotope signature of the recharge water is well conserved along the infiltration processes. The observed d-excess in precipitation were commonly higher in winter and lower in summer. The values in both stream- and groundwater were clearly divided along the backbone mountains; higher at Japan Sea side, and lower at Pacific Ocean side of the Japanese archipelago. These spatial patterns may be caused by the seasonality of monthly precipitation. At Japan Sea side it is much snowfall and seasonality in precipitation are rather small. In contrast, it is much rainfall during summer with small precipitation during winter at Pacific Ocean side. Comparing the mean weighted values of the precipitation with the arithmetic mean values of the stream- or groundwater, these were well agreed at Pacific Ocean side, however, less agreed and the values of the rainfall were smaller at Japan Sea side. Therefore, the lower values at Pacific Ocean side reflect the summer plentiful precipitations, and the higher values at Japan Sea side are affected by the delayed snowmelt. The spatial distribution of isotope signature in stream- and groundwaters clearly reflected the geographical features and precipitation patterns of the Japanese archipelago. Therefore, the stream- or groundwater dataset is valuable as a proxy for the isotopic compositions of precipitation. Especially, the commercial bottled waters are, if available, very easy to collect and very powerful tool for the hydrological researches.