新疆カシュガルデルタにおける地下水硫酸塩の化学的特徴と発生源．

[Chemical Characteristics and Sources of Groundwater Sulfate in the Kashgar Delta, Xinjiang].

• Xing Wei
• Jin-Long Zhou
• Wei-Hua Nai
• Yan-Yan Zeng
• Wei Fan
• Bin Li

抄録

新疆ウイグル自治区のカシ県では、水質の悪さが水資源不足の主な原因の一つとなっている。カシュガルデルタは、カシ県西部に位置し、面積は13329kmに及ぶ。硫酸塩濃度が地下水水質クラスⅢの基準を超えているのは、未確認地下水域の73.2％、地下水閉じ込め域の53.2％で、地下水水質クラスⅢの基準を超えています。また、TDS含有量や硬度も高い。地下水の硫酸塩の化学的特性と発生源をD,O-HO,S-SOの同位体法で分析した。その結果、①石灰質珪岩、石灰質砂岩、石膏などの塩類鉱物の溶出が流域の水質組成を制御し、SOを主成分とする表層水と地下水を形成していることが明らかになった。地下水の化学種の進化傾向は、HCO-SO→SO→SO-Clであった。ピエモンテ斜面の沖積-沖積砂利平原は主な溶出-流出帯であり、河床平原は主な流出蓄積帯であった。地下水の化学組成は、上流から下流にかけて塩析傾向にあり、②地下水は主に大気中の降水に由来し、蒸発の影響を受けていた。水理地質学的に異なる単位の地下水に含まれるDとOの分布は、上流から下流にかけて、同位体の値が低いものから高いものへと変化し、蒸発量は弱いものから強いものへと変化していることが明らかになった。また、河川沖積平原の地下水は、非集中型地下水の混合効果の影響を受けて、同位体の分布が比較的不連続であった。河川沖積平野の未確認地下水の硫酸塩の発生源は、陸域堆積物蒸発物の溶出と肥料浸出による汚染であった。また、河川沖積平野の地下水の硫酸塩発生源は、陸域堆積物の蒸発物の溶出と細菌による硫酸塩還元の混合効果の影響を受けていた。

Poor water quality is one of the main reasons for a shortage of water resources in the Kashi Prefecture of Xinjiang. The area of the Kashgar Delta is located in the western Kashi Prefecture and covers an area of 13329 km. Sulfate concentrations exceed the groundwater quality class Ⅲ standard in 73.2% of the unconfined groundwater area and 53.2% of the confined groundwater area. In addition, the TDS content and the hardness of the water are high. The chemical characteristics and sources of groundwater sulfate were analyzed using the isotope method of D, O-HO, and S-SO. The results showed that:① The dissolution of salt minerals such as calcareous siltstone, calcareous sandstone, and gypsum in denuded mountainous areas controlled the hydrochemical compositions in the basin, and formed surface water and groundwater dominated by SO. The evolution trend of groundwater chemical types was HCO·SO→SO→SO·Cl. Alluvial-proluvial gravel plains on the piedmont slopes were the main dissolution-runoff zones and the fluvial plain was main runoff accumulation zone. The chemical composition of the groundwater tended towards salinization, from upstream to downstream; ② The groundwater was mainly derived from atmospheric precipitation, and was affected by evaporation. The distribution of D and O in the groundwaters of different hydrogeological units was clear; from upstream to downstream, isotope values varied from low to increasingly higher, and evaporation varied from weak to strong, respectively. The distribution of isotopes in the confined groundwater in the river alluvium plain was relatively discrete, which was affected by the mixing effect of unconfined groundwater; ③ The sources of sulfate in the unconfined groundwater in the southern and northern alluvial-proluvial gravel plains on the piedmont slopes were the dissolution of marine-terrestrial and terrestrial deposit evaporates, respectively. The sources of sulfate in the unconfined groundwater in the river alluvium plain were the dissolution of terrestrial deposit evaporates and pollution from fertilizer leaching. The confined groundwater was affected by the dissolution of evaporates and the mixing effect of the unconfined groundwater and bacterial sulfate reduction.