酸素・水素安定同位体比（δ18O, δD），トリチウム（T）濃度および各種イオン濃度から見た近年の新潟市の湖沼水系と降水の特徴

Kohei Oishi; 大石皓平; Takayuki Enomoto; 江野本貴之; Syunki Sato; 佐藤舜起; Takahide Morohashi; 諸橋峻秀; Naoto Miyamoto; 宮本直人; Naoki Kano; 狩野直樹; Hiroshi Imaizumi; 今泉洋; Naoki Watanabe; 渡部直喜

doi:10.3769/radioisotopes.69.299

酸素・水素安定同位体比（δ¹⁸O, δD），トリチウム（T）濃度および各種イオン濃度から見た近年の新潟市の湖沼水系と降水の特徴Characteristics of Lagoon Water Systems and Precipitation in Niigata City in Recent Years from Oxygen and Hydrogen Stable Isotope Ratios (δ¹⁸O, δD), Tritium (T) Concentration and Various Ion Concentrations

大石皓平¹，江野本貴之¹，佐藤舜起²，諸橋峻秀²，宮本直人²，狩野直樹²，今泉洋²，渡部直喜³Kohei Oishi¹, Takayuki Enomoto¹, Syunki Sato², Takahide Morohashi², Naoto Miyamoto², Naoki Kano², Hiroshi Imaizumi², Naoki Watanabe³

¹ 新潟大学大学院自然科学研究科Graduate School of Science and Technology, Niigata University

² 新潟大学自然科学系（工学部）Faculty of Engineering, Niigata University

³ 新潟大学災害・復興科学研究所Research Institute for Natural Hazards and Disaster Recover, Niigata University

受付日：2019年12月17日Received: December 17, 2019

受理日：2020年4月25日Accepted: April 25, 2020

発行日：2020年9月15日Published: September 15, 2020

新潟市の環境水の特徴を把握するため，新潟市内における湖沼水と降水中の酸素・水素安定同位体比及びトリチウム濃度，各種イオン濃度の測定を行った。その結果，（1）佐潟（下潟）の水質は，上潟（Mg-SO₄（NO₃）型），下潟南部の地下水（Ca-SO₄（NO₃）型），降水（Na-Cl型）が混合して形成されており，一方，鳥屋野潟の水質では，流出部で顕著なNa-Cl型が見られた。（2）トリチウム濃度の平均値は，降水で0.33 Bq/kg，佐潟（下潟）で0.29 Bq/kg，地下水で0.24 Bq/kg程度であった。（3）降水の同位体的特徴として，d-excess（δD-8×δ¹⁸O；降水の起源を推定する指標）は夏に低く冬に高い傾向が見られた。

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.

Key words: stable-isotopes; d-excess; tritium; precipitation; lagoon water; water quality; Niigata City

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