RADIOISOTOPES

In order to reveal the quality and isotopic characteristics of lagoon waters and precipitations in Niigata City in recent years, oxygen and hydrogen stable isotope ratios (i.e., δ¹⁸O, δD) and tritium (T) concentrations as well as the concentrations of several major ions in water samples were measured. Lagoon water samples were regularly taken at the fixed sampling points from Sakata and Toyanogata, and precipitations were collected at Niigata University. Consequently, the following matters have been mainly clarified. (1) The water quality of Sakata is considered to be formed by the mixture of Uwakata (Mg-SO₄ (NO₃) type), the groundwater of southern Shitakata (Ca-SO₄ (NO₃ type), and precipitation (Na-Cl type). In case of the water quality of Toyanogata, a remarkable Na-Cl type was found at the outlet point in Toyanogata. (2) Each concentration of T is about 0.33 Bq/kg for precipitation, 0.29 Bq/kg for lagoon water (downstream point of Sakata), and 0.24 Bq/kg for groundwater on average. (3) In precipitation at Niigata City, d-excess (δD-8×δ¹⁸O; the parameter to estimate the origin of precipitation) was low in summer and high in winter.