Inter-regional comparison of the effects of long-term trends in water balance on groundwater isotope composition in the Japanese archipelago

We clarify the long-term trends of temperature, precipitation, and recharge in various regions of Japan, and discuss how these changes affect the trends of δ18O in groundwater observed at three sites in Japan. We divided the Japanese archipelago into six regions and obtained climate data from a total of 84 sites. The time series data of δ18O were obtained since 2011 or 2013 from Yamanashi, Toyama, and Tottori prefectures using commercial bottled water.

The long-term trends of groundwater δ18O at each site showed an increasing trend of 0.017‰/year in Toyama and 0.059‰/year in Yamanashi, and a decreasing trend of -0.024‰/year in Tottori. The MAP in Yamanashi was 1135.8 mm, which is characterized by higher in summer and lower in winter, and the influence of summer has a significant effect on δ18O. Average summer temperatures tended to increase. These results suggest that in Yamanashi, the rise in summer temperatures increased δ18O in precipitation and δ18O in groundwater.

The MAP in Tottori, located on the Sea of Japan in western Japan, is 1780.3 mm, which is relatively low in winter and high in summer, so the impact on groundwater is estimated to be greater in summer. In summer, water vapor is transported from the Pacific Ocean side over the mountain ranges. In the western Japan Pacific region, an increase in summer precipitation and an increase in average temperature were observed, while in Tottori, a decrease in summer precipitation was observed. Tottori, which is affected by the remaining vapor-derived precipitation, shows a decreasing trend in groundwater δ18O, as it is recharged by precipitation with lower δ18O.

Toyama has a MAP of 2279.9 mm, which is significant in both winter and summer. The summer precipitation process in Toyama, located on the Sea of Japan side, is similar to that in Tottori, but the summer precipitation has not decreased, and the groundwater is considered to be similar to Yamanashi-type fluctuations. Furthermore, in this region, winter snow accumulation due to water vapor from the Sea of Japan affects the summer season as snow melting. The number of days of snowfall remains the same, and the effect of rising temperatures is not clear at this time. In other words, the effects of summer and winter will cancel each other out, resulting in an increase in groundwater δ18O, but at a smaller rate of increase than in Yamanashi.