Abstract
Groundwater recharge source and flowpaths, along with aquifer medium characteristics, are identified based on nitrate distribution and dynamics. These are significant factors in long-term water-resources management of the Sichuan-Yunnan-Guizhou lead-zinc mining deposits, southwestern China. The results show that high nitrate concentration correlates with lower temperature and heavy isotope enrichment, indicating groundwater recharge from the eastern karst depression area (situated at higher elevation), which has intensive agriculture. Higher NO3- concentration corresponds to higher Ca/Mg ratio in the Carboniferous karst aquifer, both indicating the preferential groundwater flowpath, while the lateral nitrate distribution signifies hydraulic connection with the northern Devonian aquifer passing through the aquitard via NE–SW and N–S trending faults. Nitrate pollution has reached deep sections of the karst aquifers and generated a wide vertical distribution, with the lowest elevation of significant nitrate concentration at –28 m relative to sea level, far lower than the recharge area by over 1,500 m. The variations in groundwater temperature, electrical conductivity and partial pressure of CO2 in the early rainy season are associated with enhanced dissolution capacity of the carbonate minerals, controlled by the soil CO2 effect, hence characterizing the mining area as a fissure-dominant karst groundwater system. Under such conditions, nitrate concentration rises and recedes subsequent to recharge events, and these processes characterize nitrate leaching. Overall, the current nitrate distribution is the result of increased agricultural productivity, intensive mine dewatering works and intrinsic media heterogeneity. Nitrate functions as an artificial, stable and cost-effective tracer for groundwater-system characterization in this mining area.
Résumé
Les zones de recharge et d’écoulement d’eau souterraine, parallèlement aux caractéristiques aquifères, sont identifiés par la distribution et dynamique du nitrate. Ces facteurs sont importants pour la gestion à long terme des ressources en eau des dépôts miniers de plomb et zinc du Sichuan-Yunnan-Guizhou, dans le sud-ouest de la Chine. Les résultats montrent que les fortes concentrations en nitrate sont corrélées avec les basses températures et enrichissement en isotopes lourds, indiquant une recharge des aquifères depuis la zone de dépression karstique à l’est (située à plus haute altitude), zone d’agriculture intensive. Les concentrations en NO3– les plus élevées correspondent à un rapport Ca/Mg élevé au sein de l’aquifère karstique du Carbonifère. Ces deux paramètres informent sur l’écoulement préférentiel d’eau souterraine, alors que la distribution latérale du nitrate indique une connexion hydraulique avec la partie septentrionale de l’aquifère du Dévonien passant au travers de l’aquitard via les failles d’extension NE–SW et N–S. La pollution en nitrate a atteint les parties profondes de l’aquifère karstique et a créé une large distribution verticale, avec les plus faibles concentrations en nitrate à 28m sous le niveau de la mer, bien au-dessous de la zone de recharge située à plus de 1,500 m. Les variations de température de l’aquifère, de la conductivité électrique et de la pression partielle de CO2 au début de la saison des pluies sont associées à une capacité accrue de dissolution des minéraux carbonatés, contrôlée par l’effet du CO2 des sols et caractérisant ainsi la zone minière comme un système aquifère karstique dominé par des fissures. Sous de telles conditions, les concentrations en nitrate augmentent et baissent suite aux épisodes de recharge, et ces processus caractérisent le lessivage du nitrate. Au global, la distribution actuelle du nitrate est le résultat de l’augmentation de la productivité agricole, des travaux de dénoyage intense des mines et de l’hétérogénéité intrinsèque du milieu. Le nitrate fonctionne comme un traceur artificiel, stable et économique pour caractériser le système aquifère d’une zone minière.
Resumen
Las fuentes de recarga de agua subterránea y las trayectorias de los flujos, junto con las características del medio acuífero, se identifican en base a la distribución y dinámica de los nitratos. Estos son factores importantes en la gestión a largo plazo de los recursos hídricos de los yacimientos mineros de plomo-zinc de Sichuan-Yunnan-Guizhou, en el suroeste de China. Los resultados muestran que una alta concentración de nitratos se correlaciona con una temperatura más baja y un fuerte enriquecimiento de isótopos, lo que indica la recarga del agua subterránea de la zona de la depresión kárstica oriental (situada a mayor altitud), que tiene una agricultura intensiva. Una mayor concentración de NO3– corresponde a una mayor relación Ca/Mg en el acuífero cárstico carbonífero, lo que indica la trayectoria preferencial del flujo de agua subterránea, mientras que la distribución lateral de nitratos significa una conexión hidráulica con el acuífero Devónico del norte que pasa a través del acuífero a partir de las fallas de tendencia NE–SW y N–S. La contaminación por nitratos ha alcanzado secciones profundas de los acuíferos cársticos y ha generado una amplia distribución vertical, con la menor altura de una concentración significativa de nitratos a los –28 m en relación con el nivel del mar, muy por debajo del área de recarga ubicada a más de 1,500 m. Las variaciones en la temperatura del agua subterránea, la conductividad eléctrica y la presión parcial de CO2 en el inicio de la temporada de lluvias se asocian con una mayor capacidad de disolución de los minerales carbonatados, controlada por el efecto del CO2 en el suelo, por lo que se caracteriza a la zona minera como un sistema de agua subterránea de cárstica que predomina en la fisura. En tales condiciones, la concentración de nitratos aumenta y disminuye después de los eventos de recarga, y estos procesos caracterizan la lixiviación de los nitratos. En general, la distribución actual de nitratos es el resultado del aumento de la productividad agrícola, los trabajos intensivos de desagüe de minas y la heterogeneidad intrínseca del medio. El nitrato funciona como un trazador artificial, estable y rentable para la caracterización del sistema de aguas subterráneas en esta área minera.
摘要
本文基于地下水硝酸盐分布与动态特征识别了其补给来源、径流途径与含水介质结构特征,它们是影响中国西南地区川-滇-黔铅锌矿集区长期水资源管理的重要因素。结果表明研究区硝酸盐浓度较高的地下水同时具有低水温、富集重同位素等特征,指示地下水来源于东侧农业活动频繁、海拔相对较高的岩溶洼地区。石炭系岩溶含水层中高硝酸盐浓度对应着较高Ca/Mg比值,指示了其优先水流通道,而硝酸盐平面分布特征显示石炭系含水层与泥盆系北侧含水层经由北东向、南北向断层构成了穿越隔水层的直接水力联系。硝酸盐污染已深入岩溶含水层深部,垂向分布范围较大,最低在海拔–28 m仍可检测到显著硝酸盐浓度,与其补给区高程相差1,500 m以上。雨季初期在土壤CO2效应控制下,碳酸盐岩矿物溶解能力增强,水温、电导率及CO2分压的变化与此密切相关,均表明矿区仍是以裂隙为主的岩溶水系统。在这样的介质条件下,硝酸盐浓度随着次降雨补给升高而后下降,表现出淋滤效应的动态特征。总体而言,当前地下水硝酸盐分布是农业活动增强、矿区长期集中排水与含水介质本身非均一性的共同结果。硝酸盐作为一种人工、稳定、高效的示踪剂可以用于矿区地下水系统的识别与研究。
Resumo
A origem de recarga das águas subterrâneas e seus caminhos de fluxo, ao longo das características do meio do aquífero, são identificados com base na distribuição e dinâmica do nitrato. Esses fatores são significativos na gestão dos recursos hídricos a longo prazo nos depósitos de mineração de chumbo e zinco Sichuan-Yunnan-Guizhou, no sudoeste da China. Os resultados mostram que a alta concentração de nitrato se correlaciona com temperatura mais baixa e com o enriquecimento do isótopo pesado, indicando a recarga de água subterrânea na área de depressão cárstica oriental (situada em altitudes mais elevadas), caracterizada por agricultura intensiva. Concentração mais elevada de NO3– corresponde a maior razão Ca/Mg no aquífero cárstico Carbonífero, ambos indicando o caminho preferencial de fluxo da água subterrânea, enquanto a distribuição lateral de nitrato demonstra conexão hidráulica com o aquífero Devoniano, do norte, passando pelo aquitardo via falhas de direções NE–SW e N–S. A poluição por nitrato atingiu zonas profundas dos aquíferos cársticos e produziu uma ampla distribuição vertical, com a menor elevação da concentração nitrato significativa a –28 m em relação ao nível do mar, muito inferior à área de recarga superior por mais de 1,500 m. As variações de temperatura, condutividade elétrica e pressão parcial de CO2 na água subterrânea no início da estação chuvosa estão associadas à maior capacidade de dissolução dos minerais carbonatados, controlada pelo efeito de CO2 do solo, caracterizando a área de mineração como um sistema de águas subterrâneas cárstico fissural dominante. Sob estas condições, a concentração de nitrato aumenta e regride após os eventos de recarga, e, esses processos caracterizam a lixiviação de nitrato. Em geral, a distribuição de nitrato atual é o resultado do aumento da produtividade agrícola, operações intensivas de drenagem das minas e da heterogeneidade intrínseca do meio. O nitrato funciona como um marcador artificial, estável e econômico para a caracterização do sistema de águas subterrâneas nesta área de mineração.
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Acknowledgements
We appreciate the valuable comments raised by the two reviewers and the editor. The authors are really grateful to guidance by Zihua Zhu in the field hydrogeological investigation, and the sampling assistance given by Zhongming An and Junhong Lian. We are also thankful for some of the language modifications suggested by Dr. Hamza Jakada.
Funding
This research was funded by China University of Geosciences (Wuhan) and Yiliang Chihong Mining Co., Ltd. (No. 2016046177).
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Huang, H., Chen, Z., Wang, T. et al. Nitrate distribution and dynamics as indicators to characterize karst groundwater flow in a mined mineral deposit in southwestern China. Hydrogeol J 27, 2077–2089 (2019). https://doi.org/10.1007/s10040-019-01987-0
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DOI: https://doi.org/10.1007/s10040-019-01987-0