Ádám Tóth

Basin-scale conceptual groundwater flow model for an unconfined and confined thick carbonate region

Application of the gravity-driven regional groundwater flow (GDRGF) concept to the hydrogeologically complex thick carbonate system of the Transdanubian Range (TR), Hungary, is justified based on the principle of hydraulic continuity. The GDRGF concept informs about basin hydraulics and groundwater as a geologic agent. It became obvious that the effect of heterogeneity and anisotropy on the flow pattern could be derived from hydraulic reactions of the aquifer system. The topography and heat as driving forces were examined by numerical simulations of flow and heat transport. Evaluation of groups of springs, in terms of related discharge phenomena and regional chloride distribution, reveals the dominance of topography-driven flow when considering flow and related chemical and temperature patterns. Moreover, heat accumulation beneath the confined part of the system also influences these patterns. The presence of cold, lukewarm and thermal springs and related wetlands, creeks, mineral precipitates, and epigenic and hypogenic caves validates the existence of GDRGF in the system. Vice versa, groups of springs reflect rock–water interaction and advective heat transport and inform about basin hydraulics. Based on these findings, a generalized conceptual GDRGF model is proposed for an unconfined and confined carbonate region. An interface was revealed close to the margin of the unconfined and confined carbonates, determined by the GDRGF and freshwater and basinal fluids involved. The application of this model provides a background to interpret manifestations of flowing groundwater in thick carbonates generally, including porosity enlargement and hydrocarbon and heat accumulation.RésuméL’application du concept de modélisation régionale d’écoulements d’eau souterraine contrôlé par la gravité (MRESCG) au système hydrogéologique complexe carbonaté épais de la Chaîne Transdanubienne (CT), Hongrie, est justifiée sur la base du principe de continuité hydraulique. Le concept MRESCG informe sur l’hydraulique du bassin et sur les eaux souterraines en tant qu’agent géologique. Il apparaît clair que l’effet de l’hétérogénéité et de l’anisotropie sur le mode d’écoulement peut être dérivé des réactions hydrauliques du système aquifère. La topographie et la chaleur en tant que forces motrices ont été examinées à l’aide de simulations numériques de l’écoulement et du transport de la chaleur. L’évaluation de groupes de sources, en termes de phénomènes de débits associés et de distribution régionale des chlorures, révèle la prédominance des écoulements associés à la topographie lorsque l’on considère les flux et les modes connexes de distribution de la chimie et des températures. En outre, l’accumulation de la chaleur sous la partie captive du système influence également ces modes de distribution. La présence de sources froides, tièdes et thermales et de zones humides associées, criques, précipités minéraux, et de grottes épigéniques et hypogéniques valide l’existence du MRESCG dans le système. Vice versa, les groupes de sources reflètent l’interaction eau–roche et le transport de chaleur par advection et informent sur l’hydraulique du bassin. A partir de ces résultats, un modèle conceptuel MRESCG généralisé est proposé pour une région carbonatée libre et captive. Une interface a été mise en évidence à proximité de la marge des carbonates libres et captifs, déterminée par le MESRCG, où eau douce et des fluides du bassin sont impliqués. L’application de ce modèle fournit une base pour interpréter les manifestations d’écoulements d’eau souterraine dans des carbonates épais, comprenant généralement l’élargissement de la porosité et l’accumulation d’hydrocarbure et de chaleur.ResumenLa aplicación del concepto de flujo subterráneo regional forzado por gravedad (GDRGF) a los sistemas de potentes carbonatos potentes hidrogeológicamente complejos de las Transdanubian Range (TR), Hungría, se justifica basado en el principio de continuidad hidráulica. El concepto de GDRGF informa acerca del agua subterránea y de la hidráulica de la cuenca como un agente geológico. Es evidente que el efecto de la heterogeneidad y la anisotropía en el modelo de flujo podría ser derivada de las reacciones hidráulicas del sistema acuífero. Se examinaron la topografía y el calor como forzantes mediante simulaciones numéricas de flujo y de transporte de calor. La evaluación de los grupos de manantiales, en términos relacionados a fenómenos de descarga y la distribución de cloruro regional, revela el predominio de la forzante del flujo por la topografía al considerar los flujos y los patrones químicos y de temperatura relacionados. Por otra parte, la acumulación de calor debajo de la parte confinada del sistema también influye en estos patrones. La presencia de manantiales fríos, templados y calientes y los humedales relacionados, arroyos, precipitados de minerales, y las cuevas epigénicas y hipogénicas verifica la existencia del sistema GDRGF. Viceversa, los grupos de manantiales reflejan la interacción roca–agua y el transporte de calor advectivo e informa sobre el sistema hidráulico de la cuenca. Basado en estos resultados, se propone un modelo conceptual GDRGF generalizado para una región de carbonatos no confinada y confinada. Una interfaz se reveló cerca del margen de los carbonatos no confinados y confinadas, determinada por el GDRGF y por el agua dulce y los fluidos de la cuenca en cuestión. La aplicación de este modelo proporciona generalmente un antecedente para interpretar las manifestaciones del flujo del agua subterránea en carbonatos potentes, incluyendo la ampliación de la porosidad y la acumulación de hidrocarburos y de calor.摘要根据水力连续性原则,对重力驱使区域地下水流概念在匈牙利Transdanubian山脉水文地质条件复杂的厚层碳酸盐系统的应用进行了调整。重力驱使区域地下水流概念就是将流域水力学和地下水作为一个地质营力。很明显,不均匀性各向异性对水流模式的影响来自于含水层系统的水力反应。通过水流和热量传输数值模拟对作为驱动力的地形和热量进行了调查。群泉相关的排泄现象和区域氯化物分布评价揭示,鉴于水流和相关的化学和温度模式,地形驱动水流占主导优势。此外,系统承压部分之下的热量积累也影响这些模式。冷泉、微温泉和热泉的出现以及相关湿地、溪流、矿物沉淀物和外成、深成洞穴的出现确认系统中存在着重力驱使区域地下水流。反过来也是如此,泉群反映了岩-水相互作用和平流热量传输,揭示了流域水力学。根据这些发现,提出了非承压和承压碳酸盐地区概化重力驱使区域地下水流概念模型。揭示了非承压和承压碳酸盐边缘附近的界面,这个界面是由重力驱使区域地下水流、淡水和所涉及到的流域流体确定的。这个模型的应用通常提供了解译这个厚层碳酸盐地层中出现流动地下水,包括孔隙度扩大及碳氢化合物和热量积累的背景。ResumoA modelagem do sistema carbonácio espesso e hidrogeologicamente complexo da Região da Transdanúbia (RT), na Hungria é realizada por meio da aplicação do conceito do escoamento regional de água subterrânea por gravidade (ERASG), com base no princípio da continuidade hídrica. O conceito ERASG está relacionado com a hidráulica da bacia levando-se em conta as águas subterrâneas como um forçante geológica. Obtiveram-se evidências de que o efeito da heterogeneidade e anisotropia sobre o padrão de escoamento é determinado a partir de respostas hidráulicas do sistema aquífero. O trabalho analisou o comportamento da topografia e do calor como forçantes por meio de simulações numéricas de escoamento e transporte de calor. A avaliação de grupos de fontes, em termos de fenômenos de descarga e distribuição de cloreto, revelou a predominância de forças gravitacionais no escoamento ao considerar conjuntamente ao escoamento a distribuição de constituintes e o padrão de temperaturas. De modo específico, observou-se que o calor acumulado na porção confinada do aquífero influencia tais padrões. A presença de fontes de diferentes temperaturas (frias, mornas e quentes) e corpos de água conectados a este sistema, como pântanos, riachos, precipitações minerais e cavernas epigênicas e hipogênicas validam a existência de ERASG no sistema. Por outro lado, grupos de fontes refletem a interação água–rocha e o transporte advectivo de calor e evidenciam sobre a hidráulica da bacia. Com base nestas evidências, o trabalho propõe um modelo conceitual ERASG generalizado para uma região carbonácia livres e confinadas. Próximo à margem dos carbonatos livres e confinados foi encontrada uma interface determinada pelo ERASG, a água superficial e os fluidos basais presentes. A aplicação do modelo produz um arcabouço de conhecimento suficiente para interpretar de maneira geral fenômenos de escoamento subterrâneo em carbonatos espessos, incluindo-se porosidades variáveis e acumulação de hidrocarbonetos e calor.

Fluid Flow Systems and Hypogene Karst of the Transdanubian Range, Hungary—With Special Emphasis on Buda Thermal Karst

Carbonate regions have great economic importance for water supply, oil and gas reservoirs, geothermal fluids and also Mississippi Valley-type ore deposits. Therefore, the understanding and consequences of flow pattern in carbonates require special interest. The hypogene and epigene karst areas of carbonate sequences were distinguished and associated with different orders of groundwater flow. However, the effect of confinement on flow pattern of carbonate aquifers was not fully considered in previous studies. We demonstrated the most important prerequisites and consequences of the application of the gravity-driven regional groundwater flow concept for carbonate sequences at different degrees of confinement. The results put into a frame the distribution of different springs and caves (epigene and hypogene) of the carbonate system of the Transdanubian Range, Hungary, and provide insights for better understanding of the hydrogeology of areas with similar unconfined and confined settings. Relationship among different flow regimes, distribution and character of springs and hypogene karstification processes, in addition to natural discharge-related phenomena, such as mineral and microbial precipitates, were recognized in the area of Buda Thermal Karst. This area is a natural laboratory where the connection between groundwater flow and karstification processes can be studied.

Prediction of the Process of a Slowly Moving Loess Landslide by Electrical Resistivity Tomography

A slowly moving loess landslide along the River Danube in South Hungary was studied using electrical resistivity tomography (ERT). The aim of the research was to determine the fracture system of the study site. It seems to be the only possibility to get information about the landslide and its further evolution due to the homogeneous composition of the loess. The mass movement was expected to occur in the direction of the identified crack openings. The applicability of the ERT technique for such a supposedly dense fracture system was studied by numerical modelling and the results have been verified in the field. It was shown that it is especially important to carry out the field measurements following dry periods; otherwise the interpretation may become extremely difficult if not impossible. The dip of the fractures could not be observed and they could not be explored deeply. It was possible to map their surface projection to get the desired information about the structure of the landslide. Fracture zones could be especially well localized enabling the prediction of the positions of future rupture surfaces and thus also the delineation of the endangered zone. Although the area outside of the one that already subsided is not endangered yet, the area which has already started to move is going to break into two. Parts of the about 5 m wide blocks at the front of the landslide may fall or slide down anytime. The area below the buildings was assumed to move as one unit. Most of our predictions have been verified by the mass movements which occurred about one and half years after the measurements. The ERT method proved to be a good tool to characterize the fracture system of such a landslide area, enabling the prediction of future rupture surfaces and also delineation of the endangered area. Its use is therefore highly recommended to monitor landslides.

Mapping the Fracture System of a Landslide by Pressure Probe Method

Mechanically weak zones produced by rock realignment in the subsurface may give very useful information, but may not be directly seen from the surface. A new method, the so-called pressure probe (Pre-P) method was developed for such problems. Here an application of this method is presented on the example of a landslide. The fracture system of the investigated landslide area could have been investigated by very large resolution. We know from these investigations among others that: 1. there are consolidated zones on the slope side of the fractures whose broadness correlates with the broadness of the given fracture; 2. both the large and the small fractures follow each other periodically; 3. the future slide faces already exist. The here presented Pre-P method seems to be the most favourable tool to map the fracture system of such kind of landslides according to its resolution capacity (even 2-3 cm wide fracture proved to be detectable by this method), speed, simplicity of its application, the interpretation of the data and its costs. The fracture system map made using Pre-P data offers the possibility to better understand landslides and to delineate the not-yet-endangered areas before other methods could do it.

Topographically Driven Fluid Flow at the Boundary of Confined and Unconfined Sub-basins of Carbonates: Basic Pattern and Evaluation Approach on the Example of Buda Thermal Karst

The main focus of the study was to help in the application of the gravity-driven regional groundwater flow (GDRGF) concept for the evaluation of groundwater flow circulation in unconfined and confined sub-basins of carbonates. The concerns of the applicability of GDRGF could be released with focusing not on an individual aquifer but on a carbonate basin and interpret its heterogeneities based on the principle of hydraulic continuity. The approach and the application were demonstrated on the example of the Buda Thermal Karst (BTK), Transdanubian Range (TR), Hungary. The hydraulic connectivity of the system was proved based on the experiences of a long-term “pumping test” for the NE part of the TR. The intense connectivity of thick carbonates could be explained by the higher hydraulic diffusivity compared to siliciclastic regions. The original (before intense water production) spring data of the BTK were grouped based on the results of statistical analysis, and their critical parameters (chloride content, temperature and volume discharge) were displayed in the function of the discharge elevation and grouping of springs. In addition, a 2D flow and heat transport simulation were carried out by EPM approach based on topographic, hydrostratigraphic and structural settings of the BTK (Southern system). Based on the results, the location and chemical and temperature character of one-component thermal springs of the Southern system of BTK could be explained. The thermal springs receive water from regional flow system with additional basinal fluid contribution from the confining Paleogene and Neogene siliciclastic sediments of Pest side. In addition, heat accumulation under the Pest side of BTK also could be revealed. This approach can be used for the understanding of GDRGF of carbonate regions on all over the world.