日本地球惑星科学連合2019年大会

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[E] ポスター発表

セッション記号 A (大気水圏科学) » A-HW 水文・陸水・地下水学・水環境

[A-HW22] 流域の物質輸送と栄養塩循環-源流域から沿岸海域まで-

2019年5月30日(木) 17:15 〜 18:30 ポスター会場 (幕張メッセ国際展示場 8ホール)

コンビーナ:小林 政広(国立研究開発法人森林研究・整備機構 森林総合研究所)、吉川 省子(農業・食品産業技術総合研究機構 農業環境変動研究センター)、安元 純(琉球大学 農学部 地域農業工学科)、Adina Paytan(University of California Santa Cruz)

[AHW22-P07] Evaluation for temporal variation in groundwater inflow to the lagoons connected to Lake Biwa by radon (222Rn) tracer analysis

*齋藤 光代1小野寺 真一2友澤 裕介2Wang Kunyang2伴 修平3奥田 昇4 (1.岡山大学大学院環境生命科学研究科、2.広島大学大学院総合科学研究科、3.滋賀県立大学環境科学部、4.総合地球環境学研究所)

キーワード:地下水、内湖、琵琶湖、ラドン

Primary production in freshwater lakes and reservoirs are usually controlled by phosphorus (P). Lacustrine groundwater discharge (LGD), direct discharge of subsurface water to lake, is regarded as a potential nutrient paths, including phosphorus from a catchment to lake water environment. There are more than 20 lagoons around Lake Biwa which were originally a part of the lake. These lagoons are regarded to have important functions as settling ponds of inflowing loads from the catchments to Lake Biwa, refuges for shallow water ecosystems and so on. Because these lagoons are connected to Lake Biwa, evaluation of nutrient budget is important for the nutrient cycle in the lake. Some researchers have pointed out the presence of LGD in littoral area of Lake Biwa. However, it has not been evaluated for the lagoons around the lake. In the present research, we aimed to evaluate the temporal variation in groundwater inflow to a lagoon connected to Lake Biwa by radon (222Rn) tracer analysis.

The field campaigns were conducted in December 2017, August and October 2018 for the lagoons (Noda-Numa and Hasu-Ike) located on northeastern shore of Lake Biwa. The volume of inflow and outflow, water temperature, electric conductivity, chlorophyll-a and radon (222Rn) concentration were measured in the lagoon. 222Rn is a radioactive element of uranium (238U) series with 3.8 day of half-life. 222Rn is a useful tracer of groundwater discharge because it generally enriched in groundwater than surface water. To examine the temporal variation in groundwater inflow, the time-series 222Rn concentration in water was obtained. Water samples were collected at the inlet, outlet and near the center of the lagoons and natural springs and groundwater wells around the lagoons. Water and 222Rn balance suggest that groundwater inflow is larger in Noda-Numa than Hasu-Ike. The catchment topography may influence on this difference. 222Rn balance-based residence time of lagoon water was estimated 10-20 days in Noda-Numa, and 4-5 days in Hasu-Ike, were similar with that based on water balance. These results suggest the groundwater inflow is a potential phosphorus path to Noda-Numa.