報告書番号

U97098

タイトル（和文）

流域の地質・水文条件を考慮した陸水酸性化予測手法

タイトル（英文）

PREDICTION METHOD FOR INLAND WATER ACIDIFICATION COCERNING HYDROGEOCHEMICAL PROPERTIES IN WATERSHEDS

概要 （図表や脚注は「報告書全文」に掲載しております）

東アジア地域の酸性物質排出量の増大により，日本国内においても河川水，湖沼水が酸性化するのではないかと危惧されている。本報では，当所で提案した地域全体の物質収支と，鉱物の風化反応式を用いて風化作用による中和反応を定量的に評価する手法を国内の条件の異なる地域や，アメリカ国内の酸性化が進んだ地域に適用し，各流域の風化速度と水質について検討した。その結果，酸性化したアメリカの流域に比べて，日本の流域では鉱物の化学的風化速度が大きく，酸性降下物を中和していると考えられる。また，この結果を用いた水質予測計算によると，対象とした３流域では風化による中和作用が長期間継続するため，酸性降下物が増大した場合でも水質の変化は小さいと考えられる。さらに酸性化の恐れのある流域の条件としては，風化に対して安定な鉱物を多く含む地層が分布している，地形が急峻である，などが考えられる。

概要 （英文）

ACID DEPOSITION IN EAST ASIA WILL INCREASE AND EXCEED THAT ENCOUNTERED IN NORTH EUROPE AND NORTHEAST AMERICA. CONSEQUENTLY,FRESHWATERS IN JAPAN ARE LIKELY TO BECOME ACIDIFIED. TO PREDICTTHE LONG-TERM ACIDIFICATION,IT IS NECESSARY TO EVALUATE ACID NEUTRALIZATION MECHANISMS IN JAPANESE WATERSHEDS. IN THE PREVIOUS REPORT,A METHOD FOR ESTIMATING ACID-NEUTRALIZATION BY CHEMICAL WEATHERING WAS PROPOSED AND VERIFIED BY APPLICATION TO A JAPANESE MOUNTAINOUS WATERSHED. THISMETHOD IS COMPOSED OF QUANTIFYING THE INPUT-OUTPUT BUDGET,AND ESTIMATING THE CHEMICAL WEATHERING REACTION OF MINERALS IN THE WATERSHED. THE PRESENT REPORT COMPARES THE ACID-NEUTRALIZATION MECHANISM BETWEEN ACIDIFIED AND NON-ACIDIFIED WATERSHEDS USING THIS ESTIMATION MEHTOD. BASED ONTHE ESTIMATED ACID-NEUTRALIZATION CAUSED BY CHEMICAL WEATHERING,FRESHWATER CHEMISTRIES IN JAPANESE WATERSHEDS WERE PREDICTED USING THE ILWAS MODEL. THE PRINCIPAL RESULTS ARE: (1) JAPANESE NON-ACIDIFIED WATERSHEDSRECEIVE APPROXIMATELY THE SAME AMOUNT OF ACID DEPOSITION AS ACIDIFIED WATERSHEDS IN USA. ON THE OTHER HAND,THERE ARE MORE BASE CATIONS SUPPLIED IN JAPANESE NON-ACIDIFIED WATERSHEDS THAN IN AMERICAN ACIDIFIED WATERSHEDS. (2) IN JAPANESE NON-ACIDIFIED WATERSHEDS,ACID DEPOSITION IS NEUTRALIZED BY CHEMICAL WEATHERING OF PRIMARY MINERALS. ON THE OTHER HAND,IN ACIDIFIED AMERICAN WATERSHEDS CHEMICAL WEATHERING RATES OF PRIMARY MINERALS ARE MUCH LOWER THAN IN JAPANESE NON-ACIDIFIED WATERSHEDS. THEREFORE,ACID DEPOSITION IS NOT NEUTRALIZED. (3) A MATHEMATICAL MODEL PREDICTS THAT FRESHWATERS IN JAPANESE WATERSHEDS WILL NOT BE ACIDIFIED EVEN IF ACID DEPOSITION INCREASES. (4) THE MATHEMATICAL MODEL SHOWS THAT STREAMS BECOME ACIDIC WHEN HYDROLOGICAL CONDITIONS FORCE PRECIPITATION TO FLOW TO THE STREAM AS LATERAL FLOW THROUGH A SHALLOW SOIL HORIZON,AND THE PRIMARY MINERALS HAVE LOW CHEMICAL WEATHERING RATES. TYPICAL WATERSHED DIESCRIPTIONS FOR THESE CONDITIONS ARE: 1. PRIMARY MINERALS WITH LOWWEATHERING RATES DOMINATE WATERSHED SO

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