Abstract
Many accidents due to oxygen deficiency were reported from 1960 to 1980 during the period of decreasing groundwater level in the Tokyo area. The compressed air passed easily through the aquifers and the oxygen in it was consumed to oxydized the ferrous ions to ferric ions. The accidents occurrred not only at the sites in underground constructions, but also in residences in Tokyo. The recovery of both the groundwater level and the pore-water pressure in the aquifers has continued to rise because of the restrictions on groundwater withdrawal. The accidents due to oxygen deficiency have become less frequent with these recoveries since 1972.
During the recovery of the groundwater level and the pore-water pressure in the aquifers, two types of fires and explosions by gush out of methane occurred in the Tokyo Lowland area since 1973. One type of methane explosion (Asakusa type) has not happened since these recoveries in alluvial and Pleistocene sediments was completely finished.
As results of penetration of the compressed air into the reducing marine sediments, oxidation of sulfide minerals in them causes the formations ofdeoxygenated air and acidic water, and generation of heat. Similar phenomenon has observed in situations where the marine clayey sediments turned into farmland and housing lots.
This paper describes the geological and geochemical characteristics of the places wherethese two kinds of accidents have occurred and discusses the mechanisms of the accidents and acidification associated with the formation of acid sulfate soils.
