Abstract
In the present paper, the water temperature at 30 spring sites in the Tokyo metropolis was investigated with a research focus on its change over the last 20 years. The temperature, pH, and electrical conductivity of spring water were measured in situ in both the dry season and wet season for the years 2005-2006. In addition, the concentration of silica was determined to estimate the recharge area of each spring. By compiling the past records on spring water temperature observed by the Tokyo Metropolitan Government, consecutive observation data from 1987 to 2006 were arranged for analysis. As the first step, water temperature was classified according to each spring site and each season. Subsequently, the Mann-Kendall rank statistic method was applied to clarify the trend of long-term changes in spring water temperature with due regard for the effect of outliers. As a result of analysis, with a significance level of 5%, a rising tendency of water temperature was detected in 13 spring sites during the dry season and 11 sites during the wet season.
A negative correlation, statistically significant at the 5% level, was found between the silica concentration of spring water and the difference in water temperature between the dry and wet seasons. This result suggests that spring sites with higher silica concentration have relatively longer residence times and more extensive recharge areas. Under such conditions, it is probable that the changes in air temperature and other environmental factors might have little effect on the water temperature due to the comparatively large thermal capacity of the springs.
