ESTIMATION OF MEAN TRANSIT TIME OF SUBSURFACE WATER AND STREAM WATER IN A SMALL CRYSTALLINE BEDROCK CATCHMENT IN KOREA

Mean transit time (MTT) of soil water, groundwater, and stream water is the average time span from recharge to the collection or discharge point. Estimation of MTT provides fundamental insights into hydrological processes including flow paths and storage in a catchment and response of a watershed to land-use or environmental changes. Temporal variations of stable isotope compositions (δ¹⁸O and δ²H) of precipitation, soil water, groundwater, and stream water have been used to determine MTT. In this study, water samples were collected weekly for 13 months between August 2014 and August 2015 in a small crystalline bedrock catchment (3.9 km²) with thin soil in Korea. The seasonal variations of the deuterium excess (d-value) of precipitation and soil and stream waters showed more pronounced sinusoidal changes than those of δ¹⁸O and δ²H alone. As the d-value represents the seasonal changes in the source of water vapor, it can be useful to estimate MTT, especially in the regions of East Asian monsoonal climate. Precipitation in the study area exhibited the strong seasonal variation of d-values ranging from -4 ‰ to 35 ‰, with the amplitude of 14 ‰. However, the amplitude was significantly attenuated to 2 to 10 ‰ in soil water and stream water, while groundwater showed negligible of sinusoidal change. Sine-wave approach with exponential model (EM) for the observed seasonal changes of d-values, MTT was determined as 51 and 110 days for soil water at depth of 40 cm and 80 cm, respectively, and 173 to 323 days for mainstream and tributaries of the catchment. These results indicate that stream water in the study area is significantly contributed by the discharge of groundwater which flows through fractures in the bedrock aquifer.