新潟平野，宍道低地帯における温泉水中のヒ素濃度 : 地下水ヒ素汚染問題におけるヒ素の供給源の検討　その2 | Arsenic　concentration in hot　spring　waters　from　the　Niigata Plain and　Shinji　Lowland, Japan. : Source supply of arsenic in arsenic contaminated ground water problem, Part 2

The purpose of this study is to investigate the distribution of arsenic concentration in hot spring waters from the Niigata Plain, central Japan and the Shinji Lowland, western Japan and to consider the origin of arsenic supply for groundwaters in the modern sedimentary basins. The hot springs distributed in the inner area of the Niigata Plain have low temperature (11 to 40℃) and low As content (0.1 to 10ppb). However, hot springs in the inner area of the Shinji Lowland have middle to high temperature (40 to 85℃) and middle to high As content (10 to 114ppb). The As contents in the Shinji Lowland hot springs are about ten to hundred times higher than those in the Niigata Plain. The hot spring distribution pattern, geology, and the arsenic content of hot spring waters closely depend on geological structure under the both plains. In the granitoid and green tuff areas, the temperature and arsenic concentration of hot spring waters are high compared to the Tertiary and Quaternary sediment areas. The hot springs are distributed along deep fracture zones at the margins of the plains. It is considered that the hot spring waters is the mixture of underground geothermal water from the basement fractures and ground waters of meteoric origin. Arsenic is eluted from the basement rocks into the geothermal water. In the eastern part of the Shinji Lowland hot spring waters with high temperature and high concentration of arsenic appear at hot springs upwelling from depth near the basement rocks. This shows that the arsenic concentration is controlled by the mixing rates of the meteoric water and geothermal water. The low arsenic concentration in the Niigata Plain compared with the Shinji Lowland is mainly due to the low arsenic content of the geothermal water. It is also due to the tick sedimentary rocks overlying basement rocks which reaches 7,000m or more below the surface.

本論は,新潟平野と宍道低地帯を対象に,温泉水中のヒ素濃度分布と平野の地質構造との関係を検討し,深部熱水に由来するヒ素の供給について考察した.新潟平野内に分布する温泉水は,低温(11〜40℃未満),低ヒ素濃度(0.1〜10ppb未満)であるのに対し,宍道低地帯内では,中〜高温(40〜85℃),中〜高ヒ素濃度(10〜114ppb)で,新潟平野のヒ素濃度の10〜100倍に相当している.両地域とも,花崗岩およびグリンタフ火山岩類分布域では新第三系〜第四系の堆積岩分布域に比べ,泉温・ヒ素濃度とも高い傾向にある.温泉は平野縁辺部に分布し,そこに推定される深部断裂に規制されているとみられる.ヒ素は,両地域とも,深部熱水が基盤花崗岩の断裂を上昇する過程で含まれたもので,天水との混合・希釈によって,温泉中のヒ素濃度が規制されたものと考えられる.宍道低地帯東部では,温泉の採水位置が基盤花崗岩ないしはその直上の場合は,高温-高ヒ素濃度であるが,採水位置が基盤から離れると,低温-低ヒ素濃度になる.これは,採水位置と基盤との距離が離れるほど,天水の混入による希釈率が大きくなることによるものであろう.新潟平野のヒ素濃度がきわめて低い理由は,基盤花崗岩を上昇する熱水中のヒ素濃度が元々低いことに加え,新潟堆積盆地の基盤深度がきわめて大きく,取水位置と基盤岩までの距離が大きいことに起因するためと考えられる.