森林流域内の水移動にともなう地中水・渓流水の水質変化

Abstract

森林流域における水質形成の過程を水循環と対応させて把握することを目的とし, 滋賀県南部の山地に位置する桐生水文試験流域内小流域において, 雨水－不飽和土壌水－飽和帯地下水－湧水－渓流水の各水質の分析を, 雨量－土壌水分－地下水位－流域流出量の水文観測と対応させて実施した。1990年9月初旬の乾燥期, 9月13日から9月24日にかけての雷雨, 台風を含む数回の降雨イベント, 9月下旬から10月初旬にかけての流出逓減期の観測, 分析結果を中心に水移動にともなうpH, 電気伝導度, 8種のイオン濃度の変化を考察し, 以下の知見を得た。1) 降雨に対する出水の特徴から分類した観測井戸 (斜面直下型, 飽和帯型, 遷移帯型) ごとに, 水質が異なり, 変動の特性も異なる。2) 斜面直下型井戸の水質は, 降雨イベントに応答して変動する。無降雨時, 表層土の不飽和土壌水中に滞留しているイオンが雨水の浸透によって移動し, 斜面直下型の井戸に水位を発生させる一時的飽和側方流の水質に影響していると考えられる。3) 飽和帯型井戸の水質は, 渓流水の水質に対応しており, 飽和地下水帯の上流部, 下流部を問わず一様な水質を示している。4) 各イオン濃度の流域での水移動にともなう変化傾向は次の2パターンに大別できる。(1) : 雨水の濃度は低く, 内部で飽和帯まで移動する間にある濃度まで上昇し流出する。斜面直下型の井戸では変動が大きい。(2) : 雨水の濃度は低く, 斜面直下型井戸では高濃度が現れるが, 飽和帯型井戸まで移動する過程で再び濃度が低下し流出する。雨水に対して渓流水はわずかに濃度が高い。アニオンではCl_-とSO_4_<2->がパターン (1), NO_3_-は (2) に属し, カチオンでは, Na_+が (1) に属し, 他は (2) に分けられる。5) 各水移動過程におけるpH値の形成に影響するイオンを特定することはできなかったが, HCO_3_-イオンの影響を検討する必要性が示唆された。

To explain changing process of water qualities in forested watershed which correspond to water cycling, a research of water qualities which have been combined with hydrological observations have been started on a small forested watershed belong to the Kiryu experimental watershed in Shiga prefecture. The reserch have been operated by two main ways related each other : one is the quality analysis of rainwater, unsaturated soil water, saturated ground water, spring water and stream water, the other is the measurements of precipitation, soil water suction, groundwater level and discharge from the watershed. This report shows mainly the data collected since early September to early October 1990. This period includes some stage; severe dry, storm event with typhoon and recession of discharge. The analysis items of water qualities are pH, electric conductivity, concentrations of Cl-, NO3-, SO42-, Ca2+, Na+, Mg2+, K+, and NH4+. Discussions about variations in water qualities related with water cycling lead us to following knowledge : 1) There are three types groundwater wells which are classified by characteristic of responses to storm events : 1. slope side, 2. saturated zone, 3. transitional zone. Each type has different water quality, and fluctuation of it are also different. 2) The water qualities of the slope side wells fluctuate corresponding with the storm events. It means that the suspended ion in unsaturated soil water for non-rainfall stage is moved by infiltration of rain water, and influences to the water qualities of temporary saturated lateral flow. 3) The water qualities of the saturated zone wells correspond with that of stream water, and are distributed over the zone uniformly. 4) The variational patterns of concentration of eight ions are classified into two types: 1. concentration of rainwater is low, and that of groundwater in saturated zone is high and same of stream water. That of the slope side well fluctuates keenly, 2. concentration of rainwater is low, and that of saturated zone is also low in spite of high concentrations of slope side wells. That of stream water is slightly higher than that of rain water. Cl-, SO42-, and Na+ are type 1. Others are type 2. 5) The ion which influence the variation of pH value could not specify on each hydrological stage. And it is conjectured that an unknown factor, HCO3- is closely related to the variation of pH.