SOIL WATER MOVEMENT IN KANTO LOAM AS TRACED BY ENVIRONMENTAL TRITIUM

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Behavior of the soil water in Kanto loam, a volcanic ash layer covering diluvial uplands in Kanto district, is made clear by using environmental tritium as a tracer. The study area is located at Musashino upland with a surface gradient of 7.5‰ (Fig. 1). The top 6meter of the upland is covered with Kanto loam (Fig. 3), which has soil water characteristics as shown in Fig. 2. The tritium concentration of precipitation as the input of environmental tritium into Kanto loam has greatly varied both secularly and seasonally (Fig. 4). If the soil water moves down in a form of piston flow as suggested by Zimmermann .et al. (1966, 1967), then the recharge rate could be estimated from profiles of tritium concentrations in soil waters. Soil waters were extracted from soil samples collected twice on October 1 in 1976 and April 5 in 1978 at an interval of 0.5 meter down to a depth of 6 meters. From the first soil samples collected in 1976, three stages of soil waters with different matric suctions were extracted by; using the centrifuging and the vapor pressure depression methods. Tritium concentrations of these waters are shown in Table 1. Since there were no marked differences in concentrations, the only one sample collected in 1978. The study spot is completely flat and no surface runoff was observed even when the rainfall intensity was as high as 30mm·hr-1. As the top soil has saturated hydraulic concluctivities of about 2_??_5 × 10 -3cm·sec-1, no ponding occurred even during such an intensive rain. Therefore, the amount of groundwater recharge, I, was estimated by subtracting evapotranspiration calculated from Penman's method, E, from precipitation, P, as shown in Table 2. Comparisons of the tritium concentration profiles calculated by the piston flow model with the observed ones are shown in Figs. 5 and 6. From the mean soil water contents and tritium concentrations, tritium balances were calculated for samples collected on October 1 in 1976 and April 5 in 1978. In the former case, the total amount of tritium input is 66. 40 TU-m whereas the total amount of tritium in soil waters within Kanto loam of 6-meter thick is 76. 8 TU-m. In the latter case, they are 51. 7 TU-m and 55. 2 TU-m respectively. From the data mentioned above, the following conclusions are obtained. 1. The amount of evapotranspiration calculated by Penman's method may be a reason able value, because variations in calculated tritium concentrations of soil waters within the top 1_??_2 meters coincide well with the observed ones. 2. The mean recharge rate of the groundwater at the study spot is 885mm yr-1, which is about twice or three times larger than the mean recharge rate in Japan. 3. Though mixing process prevails between soil waters with different matric suctions, , the soil water with high matric suction is slow to move comparing with the gravity water. This may be a main reason for the differences in tritium balances. 4. Quick response of the free groundwater level to rainfall is explained by a piston flow model. According to this model, a water recharged to the groundwater table immediately after a rain has an age of 4_??_5 years. 5. Dispersion is predominant in the soil water due to differences in velocity of water parcels.
The Association of Japanese Geographersの論文

SOIL WATER MOVEMENT IN KANTO LOAM AS TRACED BY ENVIRONMENTAL TRITIUM

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