Abstract
The Xinchang site in northwest China has been selected as a potential deep geological repository for spent high-level radioactive waste (HLW) due to its low-permeability granite bedrock. Predicting the characteristics of long-term flow systems is of priority for improving the suitability assessment for a HLW disposal repository. In this study, a three-dimensional saturated–unsaturated groundwater flow numerical model, using the software TOUGH3/EOS9 with an improved evaporation module, was developed. It was subsequently used to reveal the regional groundwater flow patterns in the bedrock fracture network and to provide scientific support for the safety assessment of the potential HLW disposal repository at the Xinchang site. The regional groundwater flow system associated with the Xinchang site can be characterized by three hydrogeological units. The groundwater at Xinchang site cannot flow into the Hexi Corridor due to the high surface topography and low-permeability F11 fault, but it flows southward to Yuanyang Ditch and then discharges from the east and south boundaries. Changes in precipitation, fault permeability and fault connectivity only cause fluctuation of the water table in local areas, and do not impact the regional groundwater flow patterns. Topography is the main factor determining regional groundwater flow. The proposed model is expected to serve as a powerful tool for studying groundwater flow patterns in fractured bedrock regions and provides the foundation for deeper analysis of nuclides migration in terms of distance and time in the Xinchang area.
Résumé
Le site de Xinchang, dans le nord-ouest de la Chine, a été choisi comme site potentiel de stockage en couches géologiques profondes de déchets hautement radioactifs (DHR) dans un substratum granitique fracturé et de faible perméabilité. La prévision des caractéristiques des systèmes d'écoulement à long terme est une priorité pour améliorer l'évaluation de l'adéquation d'un dépôt de déchets DHR. Dans cette étude, un modèle numérique tridimensionnel d'écoulement des eaux souterraines en zones saturée et non saturée, utilisant le logiciel TOUGH3/EOS9 avec un module d'évaporation amélioré, a été développé. Il a ensuite été utilisé pour révéler les schémas régionaux d'écoulement des eaux souterraines et pour apporter un soutien scientifique à l'évaluation de la sûreté du dépôt potentiel de DHR sur le site de Xinchang. Le système régional d'écoulement des eaux souterraines associé au site de Xinchang peut être caractérisé par trois unités hydrogéologiques. Les eaux souterraines du site de Xinchang ne peuvent pas s'écouler dans le corridor de Hexi en raison de la topographie de surface élevée et de la faille F11 peu perméable ; mais elles s'écoulent vers le sud jusqu'au fossé de Yuanyang, puis se déversent à partir des limites est et sud. Les changements des précipitations, de la perméabilité des failles et leur connectivité n'entraînent que des fluctuations du niveau piézométrique dans des zones locales et n'ont pas d'impact sur les schémas d'écoulement des eaux souterraines à l'échelle régionale. La topographie est le principal facteur déterminant l'écoulement régional des eaux souterraines. Le modèle proposé devrait constituer un outil puissant pour l'étude des schémas d'écoulement des eaux souterraines dans les régions à substratum rocheux fracturé et fournir les bases d'une analyse plus approfondie de la migration des nucléides en termes de distance et de temps dans la région de Xinchang.
Resumen
El emplazamiento de Xinchang, en el noroeste de China, ha sido seleccionado como posible depósito geológico profundo de residuos radiactivos de alta actividad ("HLW", por sus siglas en inglés) almacenados en un basamento de granito fracturado de baja permeabilidad. La predicción de las características de los sistemas de flujo a largo plazo es prioritaria para mejorar la evaluación de la idoneidad de un depósito de HLW. En este estudio, se desarrolló un modelo numérico tridimensional de flujo de agua subterránea saturada-no saturada, utilizando el software TOUGH3/EOS9 con un módulo de evaporación mejorado. Posteriormente se utilizó para determinar los patrones de flujo de las aguas subterráneas regionales y proporcionar apoyo científico a la evaluación de la seguridad del posible depósito de HLW en el emplazamiento de Xinchang. El sistema regional de flujo de aguas subterráneas asociado al emplazamiento de Xinchang puede caracterizarse por tres unidades hidrogeológicas. Las aguas subterráneas del emplazamiento de Xinchang no pueden fluir hacia el corredor de Hexi debido a la elevada topografía de la superficie y a la falla F11 de baja permeabilidad, pero fluyen hacia el sur hasta la zona de la Fosa de Yuanyang y luego se descargan por los límites este y sur. Los cambios en las precipitaciones, la permeabilidad y la conectividad de las fallas sólo provocan la fluctuación del nivel freático en zonas locales y no afectan a los patrones regionales de flujo de las aguas subterráneas. La topografía es el principal factor que determina el flujo regional de aguas subterráneas. Se prevé que el modelo propuesto sirva de potente herramienta para estudiar los patrones de flujo de aguas subterráneas en regiones de basamento fracturado y proporcione la base para un análisis más profundo de la migración de nucleidos en términos de distancia y tiempo en la zona de Xinchang.
摘要
中国西北地区新场场址已被选为低渗透性断裂花岗岩基岩中高放射性废物（HLW）的潜在深地质库。预测长期水流系统的特征对于提高HLW处置库的适应性评估至关重要。本研究开发了一个三维饱和-非饱和地下水流数值模型，采用改进蒸发模块的TOUGH3/EOS9软件。随后利用该模型揭示了新场场址区域地下水流模式，并为新场场址潜在HLW处置库的安全评估提供科学支持。与新场场址相关的区域地下水流系统可以划分为三个水文地质单元。由于地表地形高差和低渗透性的F11断层，新场场址的地下水无法流入河西走廊，但会向南流向鸳鸯沟，然后从东部和南部边界排出。降水量、断层渗透性和断层连通性的变化只会导致局部地区水位的波动，不会影响区域地下水流模式。地形是决定区域地下水流的主要因素。该模型预计将成为研究断裂岩基区域地下水流模式的有力工具，并为在新场场址以距离和时间为基础进行更深入的核素迁移分析奠定基础。
Abstrato
O local de Xinchang, no noroeste da China, foi selecionado como um potencial repositório geológico profundo para resíduos radioativos de alto nível (RRAN) em leito rochoso de granito fraturado de baixa permeabilidade. Prever as características dos sistemas de fluxo de longo prazo é prioritário para melhorar a avaliação de adequação para um depósito de descarte de RRAN. Neste estudo, foi desenvolvido um modelo numérico tridimensional de fluxo de águas subterrâneas saturadas-insaturadas, usando o software TOUGH3/EOS9 com um módulo de evaporação aprimorado. Posteriormente, foi usado para revelar os padrões regionais de fluxo de águas subterrâneas e fornecer suporte científico para a avaliação de segurança do potencial repositório de descarte de HLW no local de Xinchang. O sistema regional de fluxo de águas subterrâneas associado ao local de Xinchang pode ser caracterizado por três unidades hidrogeológicas. A água subterrânea no local de Xinchang não pode fluir para o Corredor Hexi devido à alta topografia da superfície e à falha F11 de baixa permeabilidade, mas flui para o sul até a vala de Yuanyang e depois descarrega dos limites leste e sul. Mudanças na precipitação, permeabilidade de falhas e conectividade de falhas causam apenas flutuação do lençol freático em áreas locais e não afetam os padrões regionais de fluxo das águas subterrâneas. A topografia é o principal fator determinante do fluxo regional de águas subterrâneas. Espera-se que o modelo proposto sirva como uma ferramenta poderosa para estudar os padrões de fluxo de águas subterrâneas em regiões rochosas fraturadas e forneça a base para uma análise mais profunda da migração de nuclídeos em termos de distância e tempo na área de Xinchang.
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Acknowledgements
Thanks to Beijing Research Institute of Uranium Geology for the in-situ hydraulic tests data. The constructive comments and suggestions from the anonymous reviewers are greatly appreciated and have helped improve the manuscript.
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This research was sponsored by the special project for Decommissioning of Nuclear Facilities and Radioactive Waste Treatment by the State Administration of Science, Technology and Industry for National Defense under Grant Number: 2017-1405.
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Li, L., Zhang, Q., Zhou, Z. et al. Regional groundwater flow pattern in fractured bedrock with a high-level radioactive waste repository: numerical simulation of the Xinchang site, northwestern China. Hydrogeol J 31, 2159–2176 (2023). https://doi.org/10.1007/s10040-023-02714-6
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DOI: https://doi.org/10.1007/s10040-023-02714-6