NDLI: Estimating the Decant Rate from a Rehabilitated Opencast Colliery Using a Water Balance Approach

Content Provider	Springer Nature Link
Author	Fourie, François Daniel
Copyright Year	2015
Abstract	The long-term average decant rates from rehabilitated opencast collieries in South Africa are often estimated by assuming effective recharge rates through the spoils. However, large uncertainties are associated with these assumed recharge rates. Furthermore, this approach assumes that groundwater inflows and pit water outflows are negligible compared to the volumes of water recharged through the spoils. To obtain an estimate of the decant rate at a particular colliery, I used rainfall figures, pumping rates, and water elevations measured over a period of 6 months as well as estimated evaporation rates to construct a water balance. I then calculated a decant rate, independent of assumed recharge rates, that was significantly higher than a previous long-term estimate, despite the lower than average rainfall experienced during these 6 months. This discrepancy suggested that groundwater inflow was indeed contributing to the decant volumes. The decant rate during years of average rainfall was subsequently calculated by adding the estimated groundwater inflow to the recharge volumes found with the method based on assumed recharge. This decant rate was approximately 49 % greater than a previous estimate obtained by assuming negligible groundwater inflow. This study shows that the decant rates at rehabilitated opencast collieries could be significantly underestimated if the decants are assumed to be recharge-driven without considering the possibility of groundwater inflow. Underestimation of the decant rates will lead to flawed water management strategies, which could result in adverse environmental impacts.Die langfristigen, durchschnittlichen Füllraten von sanierten Tagebauen in Südafrika werden oft auf Grundlage der angenommenen Neubildungsraten geschätzt. Allerdings sind große Unsicherheiten mit diesen angenommen Neubildungsraten verbunden. Darüber hinaus wird davon ausgegangen, dass dieser Ansatz Grundwasserzuflüsse und Grubenwasserabflüsse vernachlässigbar im Vergleich zu den Mengen Flutungswassers ist. Für die Schätzung der Füllgeschwindigkeit einer bestimmten Zeche wurden Niederschlagszahlen, Pumpraten und Wassererhebungen über einen Zeitraum von sechs Monaten sowie geschätzte Verdunstungsraten bestimmt um eine Wasserbilanz zu erstellen. Es wurde darauf basierend eine Flutungsrate berechnet. Trotz unterdurchschnittlicher Niederschlagsmengen lag die Neubildungsrate deutlich über den bisher angenommenen langfristigen Schätzungen. Diese Diskrepanz deutet auf einen bisher unterschätzten Beitrag des Grundwasserzuflusses zur Grundwasserneubildung in diesem Bereich hin. Es wurde für ein Jahr der Grundwasserzufluss der Flutungsrate hinzu addiert. Die Flutungsrate ist damit 49 % höher als die bisherigen Schätzungen ohne Berücksischtigung des Grundwasserzuflusses. Die Studie zeigt, dass bei einer Vernachlässigung des Grundwasserstromes die Kosten deutlich unterschätzt werden können. Diese Unterschätzung der Flutungsraten aufgrund von fehlerhaften Wassermanagementsystemen kann nachteilige Auswirkungen auf die Umwelt haben.Las velocidades de filtración promedio a largo plazo en una mina de carbón a cielo abierta rehabilitada en Sudáfrica, son usualmente estimadas suponiendo una efectiva velocidad de recarga a través de las escombreras. Sin embargo, existen muchas incertezas con estas velocidades de recarga. Además, esta aproximación supone que el influjo de agua subterránea y la salida de agua del pit son insignificantes comparadas con los volúmenes de agua recargada a través de las escombreras. Para obtener una estimación de la velocidad de filtración en una mina de carbón específica, se usaron datos de precipitaciones, velocidades de bombeo y elevaciones de agua medidas durante un período de seis meses así como velocidades de evaporación para construir un balance de agua. Luego se calculó una velocidad de filtración, sin la suposición de velocidades de recarga, que fue significativamente mayor que a un estimado previo, a pesar de que las precipitaciones fueron menores al promedio durante esos seis meses. Esta discrepancia sugirió que el influjo de agua subterránea contribuyó significativamente a los volumes de filtración. La velocidad de filtración durante años de precipitaciones promedio fue calculada adicionando el influjo estimado de agua subterránea a los volumenes de recarga encontrados con el método original. Esta velocidad de filtración fue aproximadamente 49 % mayor que un estimado previo suponiendo despreciable el influjo de agua subterránea. Este estudio muestra que las velocidades de filtración en minas de carbón a cielo abierto podrían ser significativamente subestimadas si se suponen manejadas por la recarga sin considerar la posibilidad de influjo de aguas subterráneas. Esta subestimación conduciría a estrategias defectuosas en el manejo del agua que resultarían en impactos ambientales negativos.在南非,露天矿坑复垦后的长期平均溢出排水量是在假设露天矿坑主要通过回填矸石堆渗透获得有效补给水量的前提下进行计算的。但是,该假设补给水量仍包含诸多不确定因素。同时,该方法假定矿坑地下水补给量和排水量相对于回填矸石堆渗透补给量忽略不计。文章利用6个月的降雨量、泵水量和观测水位及估算蒸发量数据建立了矿坑水均衡方程,实现了某矿坑溢出排水量计算。虽然这六个月经历了比平均降雨量还要低的枯水期,但是溢出排水量计算结果明显高于之前的长期估算值,受假设补给量影响较小。这种差异表明,矿坑地下水补给量构成了溢出排水量的重要部分。在平均降雨量年份,应该在渗透补给量基础上增加地下水补给量,计算复垦露天矿坑的溢出排水量,计算结果较忽略地下水补给量的估算值大49 %。研究表明,如果假设矿坑溢出排水完全由补给驱动而不考虑地下水流补给,复垦露天矿坑溢出排水量将被低估,最终导致水资源管理失当和环境造负面影响。
Starting Page	199
Ending Page	213
Page Count	15
File Format	PDF
ISSN	10259112
Journal	Mine Water and the Environment
Volume Number	35
Issue Number	2
e-ISSN	16161068
Language	Chinese
Publisher	Springer Berlin Heidelberg
Publisher Date	2015-08-07
Publisher Institution	International Mine Water Association (IMWA)
Publisher Place	Berlin, Heidelberg
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Effective recharge Groundwater inflow Salt balance Geology Hydrogeology Water Quality/Water Pollution Energy Economics Ecotoxicology
Content Type	Text
Resource Type	Article
Subject	Water Science and Technology Geotechnical Engineering and Engineering Geology

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