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Sensitivity of Percolation Estimates to Modeling Methodology: A Case Study at an Unlined Tailings Facility with Limited Monitoring Data

  • Spencer K. WhitmanEmail author
  • Ronald J. Breitmeyer
Technical Article
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Abstract

Calculation of water budgets is critical to developing long-term management and remediation plans for legacy mine waste facilities. The effects of several modeling decisions commonly made by practitioners constructing numerical hydrologic models can significantly alter estimates of percolation and resultant management strategies. We investigated the effects of variations in several common modeling decisions for an unlined legacy tailings emplacement in Lincoln County, Nevada using predictions from HYDRUS 1D and 2D simulations. Modeling decisions investigated include choice of hydraulic property model, hydraulic properties, numerical mesh discretization, boundary condition representation, dimensionality, and geometry. Hydrologic parameterization was informed by measurement of field-saturated hydraulic conductivity, in-situ density, field moisture content, bulk (dry) density, and water retention curve by hanging column, pressure plate, and chilled mirror hygrometer. Numerical hydrologic models resulted in percolation estimates from 0 to 24 mm/year through the tailings. Choice of hydraulic properties and temporal resolution of boundary condition data contributed the most to variations in percolation estimates. For comparison, percolation was predicted with a simple water balance model (HELP), resulting in a percolation rate of 51 mm/year. Such a wide range of percolation rate predictions demonstrates the potential for significant variability in model output within a range of defensible modeling decisions, assumptions, and parameterization methods. To appropriately bound this uncertainty, several model assumptions and iterations should be considered.

Keywords

Vadose Unsaturated Water balance Drainage Closure Reclamation Hydrus Modeling Simulation Mine waste 

Die Sensitivität von Versickerungsabschätzungen für die Modellierung - Eine Fallstudie an einem ungedichteten Tailingsbecken mit begrenzten Monitoringdaten

Zusammenfassung

Die Berechnung der Wasserbilanzen ist essenziell für die Entwicklung der Pläne für das Langzeitmanagement und die Sanierung von Bergbauhinterlassenschaften. Die Effekte von Entscheidungen bei der Modellierung, die in der Regel durch die Entwickler numerischer, hydrologischer Modelle getroffen werden, können signifikant die Abschätzung der Versickerung und die entsprechende Managementstrategie beeinflussen. Wir untersuchten die Effekte einiger üblicher Modellierungsentscheidungen für ein ungedichtetes Tailingsbecken in Lincoln County, Nevada unter Benutzung von Simulationen mit HYDRUS 1D und HYDROS 2D. Die untersuchten Modellierungsentscheidungen betrafen: hydrologisches Parametermodell, hydrologische Parameter, numerische Netzdiskretisierung, Repräsentation der Randbedingungen, Dimensionalität und Geometrie. Die hydrologische Parametrisierung beruhte auf Feldmessungen der gesättigten hydraulischen Leitfähigkeit, der in-situ-Dichte, des Wassergehaltes, der Bulk-(Trocken-)Dichte und der Wasserrückhaltungskurve, bestimmt mittels hängender Wassersäule, Druckplatte und gekühltem Spiegelhygrometer. Die numerischen hydrologischen Modelle lieferten Versickerungsraten von 0–24 mm/a durch die Tailings. Die Wahl der hydrologischen Parameter und die zeitliche Auflösung der Randbedingungsdaten trugen den größten Teil zur Variabilität der Versickerungsabschätzungen bei. Zum Vergleich wurde die Versickerung mit einem einfachen Wasserbilanzmodell (HELP) mit 51 mm/a abgeschätzt. Ein so weiter Bereich ermittelter Versickerungsraten verdeutlicht das Potenzial für die Variabilität von Modellergebnissen im Rahmen vertretbarer und begründeter Modellierungsentscheidungen, Annahmen und Parametrisierungsmethoden. Um diese Unsicherheit angemessen zu handhaben, sollten unterschiedliche Modellannahmen und Iterationen berücksichtigt werden.

Sensibilidad de las estimaciones de percolación a la metodología de modelado: un estudio de caso en una instalación de relaves no forrados con datos de monitoreo limitados

Resumen

El cálculo de los presupuestos de agua es crítico para el desarrollo de planes de gestión y remediación a largo plazo para instalaciones de residuos de minas heredadas. Los efectos de varias decisiones de modelado hechas comúnmente por los profesionales que construyen modelos hidrológicos numéricos pueden alterar significativamente las estimaciones de percolación y las estrategias de manejo resultantes. Investigamos los efectos de las variaciones en varias decisiones de modelado comunes para un emplazamiento de colas sin alineación en el condado de Lincoln, Nevada, utilizando predicciones de HYDRUS 1D y simulaciones 2D. Las decisiones incluyen la elección del modelo de propiedad hidráulica, las propiedades hidráulicas, la discretización numérica de la malla, la representación de condiciones de contorno, la dimensionalidad y la geometría. La parametrización hidrológica se informó mediante la medición de la conductividad hidráulica saturada en el campo, la densidad in situ, el contenido de humedad del campo, la densidad (seca) y la curva de retención de agua mediante la columna colgante, la placa de presión y el higrómetro de espejo frío. Los modelos hidrológicos numéricos dieron como resultado estimaciones de percolación de 0–24 mm/año a través de los relaves. La elección de las propiedades hidráulicas y la resolución temporal de los datos de condición de contorno contribuyeron más a las variaciones en las estimaciones de percolación. Para comparación, la percolación se predijo con un modelo simple de balance de agua (HELP), lo que dio como resultado una tasa de percolación de 51 mm/año. Este amplio rango de predicciones de tasa de percolación demuestra el potencial de una variabilidad significativa en el rendimiento del modelo dentro de un rango defendible de decisiones de modelado, supuestos y métodos de parametrización. Para limitar adecuadamente esta incertidumbre, se deben considerar varios supuestos e iteraciones del modelo.

渗透参数对建模方法的灵敏度—以有限监测数据的无衬砌尾矿处置场为例

抽象

水均衡计算对制定废弃矿山废物处置场的长期管理和修复计划至关重要。但是,从业者们常用的几种建立水文数值模型的建模方法却能大幅改变入渗参数估算值,进而最终影响决策管理。我们利用HYDRUS 1D和2D模拟方法研究了几种常用建模方法变量对内华达州林肯县未砌衬尾矿堆处置场水文模型建模的影响。讨论的建模方法包括水文参数模型选择、水力学特性、数值网格离散化、边界条件代表性、维度和几何形状。利用现场饱和渗透系数、原位密度、田间含水量、容重(干燥)密度和持水曲线(由悬挂柱、压力板和冷镜式湿度计获取)等野外测量值进行水文参数识别。水文数值模型得出的尾矿渗透参数为0–24 mm/yr。水力学特性选择和边界条件时间分辨率使渗透参数估计值变化最大。为便于对比,我们用简单的水量平衡模型(HELP)预测的渗透参数为51mm/yr。渗透率预测值如此宽的变化范围说明,即使一定范围内的可选择建模方法、假设条件和参数识别方法引起的模型输出变化也可能很大。为适当限制此类不确定性,应该考虑几个模型假设和迭代处理。

Notes

Acknowledgements

Financial and logistical support for portions of this study were provided by the Greenfield Environmental Multistate Trust, LLC—Trustee of the Multistate Environmental Response Trust, the Nevada Mining Association, Brown and Caldwell, Broadbent and Associates, and the Nevada Division of Environmental Protection, Abandoned Mine Lands Branch.

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Department of Geological Sciences and EngineeringUniversity of Nevada RenoRenoUSA

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