Atsunao Marui

Various-scale electromagnetic investigations of high-salinity zones in a coastal plain

Three different-scale electromagnetic (EM) measurements have been performed in the Kujukuri coastal plain, southeast Japan, to investigate the distribution of saline groundwater. The three techniques were audio-frequency magnetotelluric (AMT), transient electromagnetic (TEM), and small loop-loop EM measurements. The resistivity sections estimated from these data sets reveal three independent resistivity distributions extending to different depths. The AMT method reveals a regional-scale resistivity distribution across the plain to a maximum depth of approximately 500 m and the existence of deep conductive zones, which are inferred to be associated with fossil seawater trapped in a Pleistocene formation. The TEM results show a medium-scale resistivity distribution to depths of approximately 100 m, in which two shallow conductive zones are recognized. It is concluded that these features are caused by present seawater intrusion and high-salinity salt-marsh deposits formed during sporadic marine regressions. The small loop-loop EM method provided a shallow resistivity profile that highlights the conductive salt-marsh deposits and resistive sandy ridges. Although these resistivity sections correspond to different depth ranges, the overlapping portions of the sections are very consistent with one another. These EM methods are useful in detecting and interpreting important resistivity features. Taking the geologic evolution of the coastal plains into consideration is crucial when interpreting resistivity profiles such as these, and our results suggest that the presence of fossil seawater is an important factor controlling resistivity at a variety of depths.

Review: Deep groundwater research with focus on Germany

While research focuses mainly on the intensively used shallower aquifers, only a little research has addressed groundwater movement in deeper aquifers. This is mainly because of the negligible relevance of deep groundwater for daily usage and the great efforts and high costs associated with its access. In the last few decades, the discussion about deep geological final repositories for radioactive waste has generated strong demand for the investigation and characterization of deep-lying aquifers. Other utilizations of the deeper underground have been added to the discussion: the use of geothermal energy, potential CO2 storage, and sources of potable water as an alternative to the geogenic or anthropogenic contaminated shallow aquifers. As a consequence, the fast growing requirement for knowledge and understanding of these dynamic systems has spurred the research on deep groundwater systems and accordingly the development of suitable test methods, which currently show considerable limitations. This review provides an overview of the history of deep groundwater research. Deep groundwater flow and research in the main hydrogeological units is presented based on six projects and the methods used. The study focuses on Germany and two other locations in Europe.RésuméAlors que la recherche se focalise principalement sur les aquifères superficiels intensivement sollicités, seules quelques recherches portent sur l’écoulement de l’eau dans des aquifères plus profonds. Ceci principalement en raison de la faible adéquation de l’eau de nappe profonde à l’usage quotidien et aux coûts élevés associés à son accès. Durant les quelques dernières décades la discussion au sujet du stockage profond des déchets radioactifs ultimes a généré une forte demande pour l’investigation et la caractérisation des aquifères profonds. D’autres utilisations du sous-sol profond ont été ajoutées à la discussion : utilisation de l’énergie géothermale, potentiel de stockage de CO2 et sources d’eau potable comme alternative aux aquifères superficiels à pollution d’origine géologique ou anthropique. Comme conséquence, le besoin rapidement croissant de connaissance et de compréhension de ces systèmes dynamiques a incité à la recherche sur les systèmes aquifères profonds et conséquemment au développement de méthodes test adéquates, qui montrent actuellement des limitations considérables. Cette revue fournit une vue d’ensemble sur l’histoire de la recherche portant sur l’eau profonde. L’écoulement profond et la recherche dans les principales unités hydrogéologiques sont présentés basés sur six projets, ainsi que les méthodes utilisées. L’étude est focalisée sur l’Allemagne et sur deux autres emplacements en Europe.ZusammenfassungDer Schwerpunkt in der hydrogeologischen Forschung liegt vorwiegend auf den intensiv genutzten flacheren Grundwasserleitern, während Strömung in tieferen Grundwasserleitern bis zum heutigen Tage nur in sehr geringem Maße untersucht wurde. Die Hauptursache hierfür ist die vernachlässigbare Relevanz von tiefem Grundwasser im täglichen Gebrauch, sowie der mit der Förderung verbundene große Aufwand und die hohen Kosten. Die in den letzten Jahrzehnten aufgekommene Diskussion über die tiefe geologische Endlagerung von radioaktivem Abfall zog einen dringenden Bedarf der Erforschung tiefer liegender Grundwasserleiter nach sich. In der Folge wurden auch weitere Anwendungen für die Nutzung des tiefen Untergrundes diskutiert: Die Nutzung von Geothermie, die mögliche Speicherung von CO2 und die Nutzung tiefen Grundwassers als alternative Trinkwasserquelle zu geogen oder anthropogen kontaminierten flachen Grundwasserleitern. Der stetig wachsende Bedarf nach Kenntnis und Verständnis dieser dynamischen Systeme hat die Erforschung tiefliegender Grundwassersysteme und die Weiterentwicklung von bisher nur begrenzt anwendbaren Testmethoden vorangetrieben. Dieser Beitrag gibt einen Überblick über die geschichtliche Entwicklung der Erforschung tiefer Grundwässer. Die tiefe Grundwasserströmung und deren Erforschung in hydrogeologischen Haupteinheiten werden auf der Basis von sechs Fallstudien dargestellt und die angewandten Methoden erläutert. Die Studie legt hierbei den Schwerpunkt auf Deutschland. Außerdem werden zwei weitere europäische Standorte vorgestellt.ResumenMientras que la investigación se enfoca principalmente en los acuíferos pocos profundos usados intensivamente, solo una pequeña parte de la investigación se ha ocupado del movimiento del agua subterránea en los acuíferos profundos. Esto es principalmente debido a la escasa relevancia del agua subterránea profunda para el uso diario y los grandes esfuerzos y los altos costos asociados para su acceso. En las últimas décadas la discusión acerca de la geología profunda de repositorios finales para residuos radiactivos ha generado una fuerte demanda de la investigación y caracterización de acuíferos profundos. Otras utilizaciones del subsuelo más profundo han sido sumadas a la discusión: el uso de energía geotermal, almacenamiento potencial de CO2 , y fuentes de agua potable como una alternativa a la contaminación geogénica o antropogénica de los acuíferos poco profundos. Como una consecuencia, la exigencia de un rápido crecimiento del conocimiento y compresión de estos sistemas dinámicos ha estimulado la investigación de los sistemas de agua subterránea profunda y de acuerdo con esto, el desarrollo de métodos de ensayos apropiados, los cuales en la actualidad muestran considerables limitaciones. Esta revisión proporciona un panorama general de la historia de la investigación del agua subterránea profunda. El flujo subterráneo profundo y la investigación de las unidades hidrogeológicas principales es presentada sobre la base de seis proyectos y de los métodos usados. El estudio se enfoca en Alemania y otros dos sitios en Europa.摘要以往研究主要集中在强烈开采的浅层含水层,而对深层含水层中地下水运动的研究较少。这主要是因为用于日常使用的地下水不取自深层,且它们的开采难度和费用高。在过去二三十年间,放射性废弃物深层地质处置的讨论需要对深层含水层进行调查和刻画。对深层地下水的其他利用也加入到讨论中:地热能的利用,CO2储存潜力评价,作为饮用水源替代自然或人为污染的浅层含水层。因此,对于有关动力系统的知识与认识的需求推动了深层地下水系统的研究以及相应的检测方法的发展。但当前仍显示相当大的局限性。这篇综述对深层地下水研究的历史进行了回顾。基于六个工程及其采用的方法对深层地下水径流及其主要水文地质单元的研究进行评述。本文主要集中在德国以及欧洲的另外两个地区。概要深部地下水挙動に関する研究は,浅部地下水を対象とした研究より少ない。これは,深部地下水の利用が,日常的には無視できるほど小さい上に,利用のためには多大な労力とコストを要するためである。しかし,ここ数十年,地下深部への放射性廃棄物の最終処分に関する議論が,深部地下水の調査と理解を深めてきた。加えて,地下深部の帯水層は,地熱エネルギー源,二酸化炭素貯留,浅部の汚染された帯水層の代替水源として,その潜在的価値が見出されつつある。結果として,これらのシステム化に必要な知識や解釈が急速に要求されるようになり,深部地下水研究とそれに伴う手法開発が加速されている。そこで本論は,これまでの深部地下水研究を概観し,ドイツおよびその周辺の6つのプロジェクトに基づいて,主要な水理地質ユニットにおける深部地下水流動の特徴を整理した。ResumoSe por um lado a investigação se centra principalmente nos aquíferos subsuperficiais intensamente explorados, por outro lado, apenas uma parcela pequena da investigação se dedicou ao movimento de água subterrânea em aquíferos profundos. Este facto deve-se sobretudo à relevância negligenciável da água subterrânea profunda para o uso diário e os enormes esforços e elevados custos associados ao acesso à água subterrânea profunda. Nas últimas décadas a discussão em torno dos repositórios finais geológicos profundos para resíduos radioativos gerou uma intensa necessidade de investigação e caracterização de aquíferos profundos. Outras utilizações do meio subterrâneo profundo também contribuíram para a discussão: o uso de energia geotérmica, o potencial armazenamento de CO2 e origens de água potável como alternativa aos aquíferos subsuperficiais contaminados por processos geogénicos ou antrópicos. Consequentemente, a demanda acelerada de conhecimento e compreensão destes sistemas dinâmicos estimulou a investigação de sistemas de água subterrânea profunda, concomitantemente com o desenvolvimento de métodos de ensaio apropriados que atualmente apresentam limitações consideráveis. Esta revisão proporciona uma visão geral da história da investigação de água subterrânea profunda. O fluxo de água subterrânea profunda e a investigação das principais unidades hidrogeológicas são apresentados com base em seis projetos e nos métodos usados. O estudo centra-se na Alemanha e outros dois locais na Europa.

The aftermath of the Fukushima nuclear accident: Measures to contain groundwater contamination

Several measures are being implemented to control groundwater contamination at the Fukushima Daiichi Nuclear Plant. This paper presents an overview of work undertaken to contain the spread of radionuclides, and to mitigate releases to the ocean via hydrological pathways. As a first response, contaminated water is being held in tanks while awaiting treatment. Limited storage capacity and the risk of leakage make the measure unsustainable in the long term. Thus, an impervious barrier has been combined with a drain system to minimize the discharge of groundwater offshore. Caesium in seawater at the plant port has largely dropped, although some elevated concentrations are occasionally recorded. Moreover, a dissimilar decline of the radioactivity in fish could indicate additional sources of radionuclides intake. An underground frozen shield is also being constructed around the reactors. This structure would reduce inflows to the reactors and limit the interaction between fresh and contaminated waters. Additional strategies include groundwater abstraction and paving of surfaces to lower water levels and further restrict the mobilisation of radionuclides. Technical difficulties and public distrust pose an unprecedented challenge to the site remediation. Nevertheless, the knowledge acquired during the initial work offers opportunities for better planning and more rigorous decisions in the future.

Managing Groundwater Radioactive Contamination at the Daiichi Nuclear Plant

The Great East Japan Earthquake and tsunami of March 2011 severely damaged three reactors at the Fukushima Daiichi nuclear power station, leading to a major release of radiation into the environment. Groundwater flow through these crippled reactors continues to be one of the main causes of contamination and associated transport of radionuclides into the Pacific Ocean. In this context, a number of strategies are being implemented to manage radioactive pollution of the water resources at the nuclear plant site. Along with water treatment and purification, it is critical to restrict the groundwater flow to and from the reactors. Thus, the devised strategies combine walls containment, bores abstraction, infiltration control, and the use of tanks for the temporary storage of contaminated waters. While some of these techniques have been previously applied in other environments, they have never been tested at such a large scale. Therefore, their effectiveness remains to be seen. The present manuscript presents an overview of the methods being currently implemented to manage groundwater contamination and to mitigate the impact of hydrological pathways in the dispersion of radionuclides at Fukushima.

Spatial variations of tritium concentrations in groundwater collected in the southern coastal region of Fukushima, Japan, after the nuclear accident

Spatial variations in tritium concentrations in groundwater were identified in the southern part of the coastal region in Fukushima Prefecture, Japan. Higher tritium concentrations were measured at wells near the Fukushima Daiichi Nuclear Power Station (F1NPS). Mean tritium concentrations in precipitation in the 5 weeks after the F1NPS accident were estimated to be 433 and 139 TU at a distance of 25 and 50 km, respectively, from the F1NPS. The elevations of tritium concentrations in groundwater were calculated using a simple mixing model of the precipitation and groundwater. By assuming that these precipitation was mixed into groundwater with a background tritium concentration in a hypothetical well, concentrations of 13 and 7 TU at distances of 25 and 50 km from the F1NPS, respectively, were obtained. The calculated concentrations are consistent with those measured at the studied wells. Therefore, the spatial variation in tritium concentrations in groundwater was probably caused by precipitation with high tritium concentrations as a result of the F1NPS accident. However, the highest estimated tritium concentrations in precipitation for the study site were much lower than the WHO limits for drinking water, and the concentrations decreased to almost background level at the wells by mixing with groundwater.