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The leaching behavior of arsenic (As), lead (Pb), nickel (Ni), iron (Fe), and manganese (Mn) was investigated from subsurface core sediment of marine and nonmarine depositional environments in central Kanto Plain, Japan. A four-step sequential extraction technique was adopted to determine the chemical speciation, potential mobility, and bioavailability of metals under natural conditions in variable depositional environments. In addition, a correlation of these properties with pore water and total metal content was carried out. The concentration of As in pore water was found to be 2&ndash, 3 times higher than the permissible limit (10 &micro, g/L) for drinking water and leachate in fluvial, transitional, and marine environments. The trend of potential mobile fractions of As, Pb, and Ni showed Fe&ndash, Mn oxide bound &gt, carbonate bound &gt, ion exchangeable bound &gt, water soluble in the fluvial environment. However, in the marine environment, it showed Fe&ndash, water soluble &gt, ion exchangeable bound for As. The leaching of As in this fluvial environment is due to the organic matter-mediated, reductive dissolution of Fe&ndash, Mn oxide bound, where Mn is the scavenger. The amount of total content of As and sulfur (S) in transitional sediment reflects an elevated level of leachate in pore water, which is controlled by S reduction. However, the leaching of As in marine sediment is controlled by pH and organic matter content.

The Nakagawa lowland is located in the central part of the Kanto Plain; the largest plain in Japan. During the 1960s, major land subsidence occurred, exceeding 10 cm/year, as a result of over pumping of shallow and deep groundwaters in this area. In recent times, following the regulation of pumping by national and local governments, the amount of land subsidence in this area has decreased sharply. However, in the Nakagawa lowland, subsidence exceeding 1 cm/year is still ongoing in some areas, and is more pronounced during dry years in which the amount of groundwater use increases. In this study, we examined the local characteristics of land subsidence through persistent scatterer interferometry using ALOS/PALSAR data obtained from April 2006 to April 2011. On the basis of this analysis, we confirmed that subsidence was concentrated along the western margin of an incised buried valley, in which average displacement rates exceed 10 mm/year. The characteristics of this subsidence zone are suggestive of a geological structure that is buried beneath the alluvial plains, which is affected by groundwater pumping that targets the basal gravel layers of buried valley-fills. In addition, we detected a patched zone of around 0.3 km2 in the study area, in which average displacement rates exceed 15 mm/year. We estimated that this zone was formed by consolidation settlement of soft ground as a result of residential landfill.

This paper addresses the influence of sea level changes on the stratigraphy and basal topography of latest Pleistocene–Holocene incised valley fill of the Arakawa and Menuma Lowlands in central Japan by using sedimentary drill cores and columnar geologic sections. In the study area, four buried fluvial terrace surfaces (I to IV) formed in response to sea-level fall during the last glacial period. Surface III is covered with As-YP tephra (15–16.5 ka) and extends at least 65 km landward from the present mouth of the Arakawa River. A buried valley named surface V that extends from at least 85 km upstream from the present river mouth to about 120 m below present sea level on the continental slope was incised in response to the sea-level lowstand of the Last Glacial Maximum. The incised valley fill can be divided into three units: unit A (gravelly river channel sediments including valley-bottom gravels), unit B (fluvial sediments), and unit C (marine sediments), which intertongues within unit B and pinches out up to 60 km upstream from the present river mouth. Unit C marks the landward limit of Holocene transgression. The lower part of unit B is a fining-upward succession formed during rising sea level and indicates retrogressive aggradation of floodplain sediments during transgression, whereas the upper part of unit B was formed during a sea-level highstand. The floodplain migrated more than 85 km upstream from the modern shoreline in response to sea-level rise until the middle Holocene and extends over several tens of kilometers beyond the landward limit of the Holocene marine transgression.

荒川低地中・上流域と妻沼低地において，ボーリング柱状図資料およびボーリングコアを解析して，沖積層に埋積されている埋没地形面群の区分と対比・編年を試みた．本地域の埋没地形面群は高位よりI～V面に区分され，I～IV面は最終氷期後半に形成された埋没段丘面であり，V面は埋没谷の基底に堆積する河成礫層の頂面である．これらの埋没地形面群は，最終氷期後半の海面低下に応じて順次形成された．V面は荒川低地から妻沼低地を経て利根川右岸まで連続することから，V面形成当時の利根川は荒川とともに現在の荒川低地を流下していたと考えられる．荒川低地と妻沼低地の境界付近には，深谷断層が伏在している．ここでは，深谷断層の活動により埋没地形面の段丘化や変形が引き起こされた可能性が高い．

The northern Ayasegawa fault is a part of the Fukaya fault system, which is the longest active fault in the Kanto district. The paleoseismology of the northern Ayasegawa fault was revealed by a combination of arrayed boring and ground penetrating radar (GPR) survey. The northern Ayasegawa fault produced a fold scarp with the NW-SE direction running along the boundary between the Oomiya 2 (O2) surface and fluvial lowland. The O2 was formed in Marine Isotope Stage 5a, and was slightly deformed with a wide warping zone. Sixteen sediment cores arrayed across the warping zone contain a series of tephra layers such as Hk-TP (ca. 60-65 ka), KMP, AT (26-29 ka), As-BP group (20-25 ka), and As-YP (15-16.5 ka). These key beds except Hk-TP were deposited and deformed parallel to each other, suggesting that no faulting events occurred between KMP and As-VP fall. The timing of the last faulting event is after the As-YP fall, and is probably younger than 10 ka based on an interpretation of GPR profiles and 14C ages. KMP should be deposited horizontally because it intervened in the peaty silt layer, which accumulated conformably on lacustrine deposits overlapping the fold scarp. Thus, the KMP horizon roughly indicates the vertical offset produced by the events occurred after the As YP fall. The events were probably singular, and the last one formed a vertical offset of more than 4 m. The older event occurred at around 70 ka between Hk-TP fall and O2 formation. Vertical deformation of the O2 was at least 7 m, indicating the possibility that the vertical offset caused by the penultimate event is at least 3 m. The vertical slip per event might reach 5 m, and the average vertical slip rate is nearly 0.1 mm/yr because the warping zone detected by the arrayed boring above is within the flexure zone shown by the P-wave seismic profile. The northern Ayasegawa fault is considered to be a single behavioral segment because of its longer recurrence interval and lower slip rate of 0.1 mm/yr in comparison with those of the other part of the Fukaya fault system.

数値標高モデル (DEM) を用いた地形計測手法とその応用例に関する文献調査を行った. DEMを用いた地形計測の現状と今後期待される応用項目および問題点は, 以下のとおりである.1) 既往研究において計測された地形特性値は, 局所地形量, 区域地形量および流域地形量に分類される. さらに, 区域地形量は区域統計量, 区域計測量および空間変動分析により求められる値に分類され, 流域地形量は流域統計量と流域計測量に分類される.2) DEMによる地形計測の応用例としては, (1) 地形の判読と分類への応用, (2) 地形発達論や地形変化モデルへの応用, (3) 地質構造や岩相分布の把握への応用, (4) 自然災害評価の試み, などがある.3) DEMによる地形計測は, 地形学, 地質学および自然災害評価などへの応用が期待できるが, 各計測に関して必要なDEMの格子間隔や標高の精度および有効な地形特性値を明らかにすることが今後の課題である.

The weathering rates of bedrock of the dated erosional marine terraces were examined for two definitions: (1) the rates at which the thicknesses of weathered zones with different strengths increase with weathering time and (2) the rates at which the strength of weathered materials at an arbitrary depth decrease with time. The bedrock of terraces in the study area consists mainly of Tertiary sandstone and mudstone. The weathering profiles were observed from drill cores on three levels of terrace surfaces, i.e. 2850, 290 and 70 years B.P. in the emergence age of terraces, which is regarded as the weathering time of bedrock. The degree of weathering grade was described by the residual strength ratio, which is defined as the ratio of the needle penetration hardness of weathered part to that of fresh part of bedrock. The weathering rates of the first definition do not decrease monotonically, but decelerate logarithmically with time. Zones with lower weathering grades have faster weathering rates than those with higher weathering grades. With respect to the weathering rates of the second definition, mudstone starts to weather earlier than sandstone, but is exceeded by the latter after a certain time has elapsed, because mudstone has a higher slaking susceptibility.