LAKE WATER MIXING AND TRITIUM BALANCE ON LAKE CHUZENJI

Abstract

Both mixing processes in a fresh-water lake and residence time of groundwater in its drainage basin have been made clear by using environmental tritium as a tracer on Lake Chuzenji. The lake covers an area of 11.97 km² at an altitude of 1, 269m, having a large drainage basin of about 119.68 km². The mean depth is 95m and the maximum 172m.
A field survey was carried out from July 29 to August 3, 1978. Water samples for tritium analysis were collected at eleven stations, as shown in Figure 1. The sampling of lake water was made every 20m vertically. Temperature, ply, and electrical conductivity of the water were determined on the spot. The values obtained are indicated in Figure 2. Tritium concen-trations of lake water are shown in Figure 3 and those of stream water and groundwater in Figure 4. The mean tritium concentration of lake water was 34.7 TU, and a significant variation was not found throughout the all vertical profiles of the stations. Concerning the vertical profiles of tritium concentration in lake water, the lake water would be well mixed at least vertically. The tritium concentration of Shirakumo Falls, 37.8TU, showed nearly the same value to the lake, which shows that the Falls would be poured out as the seepage of the lake. Spring water from elevator shaft, which was drilled near Kegon Falls for the sight-seeing of the Falls, showed 33.3 TU. This figure was also the same as that of the lake. Those of spring water in Jigoku-zawa and of the well near the lake shore were 21.5 and 26.9 TU, respectively, showing lower than the lake water. This fact seems to reflect the influence of recent precipitation. Yukawa River, 37.9 TU, showed higher tritium concentration than both precipitation and the stream water flowing into Senjuga-hama.
The annual water balance of the lake was calculated in the period from 1952 to 1977 (Table 3). Meteorological data at the Chugushi Meteorological Station were used as basic data in the calculation, and evapotranspiration from the drainage basin and evaporation from the lake were calculated by Penman's method. The tritium concentrations of precipitation in Tokyo and Tsukuba from 1952 to 1977 were used to estimate the mean tritium concen-tration of the lake water. The calculation was carried out on the assumption that lake water is completely mixed every year.
As a result, the following conclusions were obtained.
1. The mean inflow to the lake is about 1.92×10⁸m³•yr^-1, that is, 6.09m³•sec^-1, and the mean residence time of the lake water is determined to be about 5.9 years. In spite of the huge volume of lake water, the mean residence time is shorter than expected, because the area of drainage basin is about ten times as large as that of the lake.
2. The discharge from the lake depends upon groundwater outflow which is about 4.76m³•sec^-1 and 79% of the total outflow. The propotions of evaporation and surface outflow are about 4% and 17%, respectively.
3. The vertical profile of tritium concentration in lake water and the result of the simula-tion of tritium balance support that the lake water would be well mixed in a year.
4. As a component of precipitation on the drainage basin, groundwater flow into the lake takes an average time of less than 4 years.