Groundwater age, mixing and flow rates in the vicinity of large open pit mines, Pilbara region, northwestern Australia

Age des eaux souterraines, mélanges et débits d’écoulement autour de vastes mines à ciel ouvert de la région de Pilbara, nord-ouest de l’Australie

Edad del agua subterránea, flujo y mezcla de caudales en el entorno de grandes minas a cielo abierto, región de Pilbara, noroeste de Australia

澳大利亚西北部皮尔布拉地区大的露天矿周边地区地下水年龄、混合和水流量

Idade, misturas e taxas de fluxo das aguas subterrâneas nas proximidades de grandes minas à céu aberto, região de Pilbara, Noroeste da Austrália

Abstract

Determining groundwater ages from environmental tracer concentrations measured on samples obtained from open bores or long-screened intervals is fraught with difficulty because the sampled water represents a variety of ages. A multi-tracer technique (Cl, 14C, 3H, CFC-11, CFC-12, CFC-113 and SF6) was used to decipher the groundwater ages sampled from long-screened production bores in a regional aquifer around an open pit mine in the Pilbara region of northwest Australia. The changes in tracer concentrations due to continuous dewatering over 7 years (2008–2014) were examined, and the tracer methods were compared. Tracer concentrations suggest that groundwater samples are a mixture of young and old water; the former is inferred to represent localised recharge from an adjacent creek, and the latter to be diffuse recharge. An increase in 14C activity with time in wells closest to the creek suggests that dewatering of the open pit to achieve dry mining conditions has resulted in change in flow direction, so that localised recharge from the creek now forms a larger proportion of the pumped groundwater. The recharge rate prior to development, calculated from a steady-state Cl mass balance, is 6 mm/y, and is consistent with calculations based on the 14C activity. Changes in CFC-12 concentrations with time may be related to the change in water-table position relative to the depth of the well screen.

Résumé

La détermination de l’âge des eaux souterraines à partir des concentrations de traceurs environnementaux mesurées sur des échantillons prélevés dans des puits ouverts ou à diverses profondeurs de crépines est difficile du fait que les eaux échantillonnées représentent divers âges. Une technique multi-traceurs (Cl, 14C, 3H, CFC-11, CFC-12, CFC-113 et SF6) a été appliquée afin de différencier les divers âges d’eau souterraine échantillonnée au niveau de longues crépines de forages de production captant l’aquifère régional autour de la mine à ciel ouvert de la région de Pilbara au nord-ouest de l’Australie. Les changements de concentration des traceurs du fait d’un dénoyage continu de la mine depuis sept ans (2008–2014) ont été étudiés et les résultats des différents traceurs comparés entre eux. Les concentrations des traceurs laissent penser que les échantillons d’eau souterraine représentent des mélanges d’eau jeune et ancienne; l’eau jeune étant la recharge par un proche ruisseau et l’eau ancienne représentant la recharge diffuse. Une augmentation de l’activité 14C avec le temps dans les puits proches du ruisseau indique que le dénoyage de la mine à ciel ouvert effectué pour obtenir des conditions d’exploitation à sec a entrainé un changement de la direction d’écoulement; la recharge localisée à partir du ruisseau formant ainsi une plus forte part des eaux souterraines pompées. Le taux de recharge avant l’exploitation, calculé à partir du bilan de masse du chlorure à l’état stationnaire, est de 6 mm/an, et est en accord avec les calculs basés sur l’activité 14C. Les changements temporels des concentrations du CFC-12 peuvent être mis en relation avec les changements du niveau de la nappe par rapport à la profondeur de la crépine.

Resumen

La determinación de edades del agua subterránea a partir de concentraciones de trazadores ambientales medidos en muestras obtenidas de perforaciones abiertas o a lo largo de los intervalos filtrados está llena de dificultades debido a que la muestra de agua representa a diversas edades. Una técnica multi-trazador (Cl, 14C, 3H, CFC-11, CFC-12, CFC-113 y SF6) se utilizó para descifrar las edades del agua subterránea muestreada a lo largo de los filtros de pozos de producción en un acuífero regional en el entorno de una mina a cielo abierto en el región de Pilbara en el noroeste de Australia. Se examinaron los cambios en las concentraciones de los trazadores debido al drenaje continuo durante siete años (2008–2014), y se compararon los métodos de los trazadores. Las concentraciones de los trazadores sugieren que las muestras de agua subterránea son una mezcla de agua joven y vieja; la primera se infiere por representar la recarga localizada de un arroyo adyacente, y la segunda por ser una recarga difusa.

Un aumento en la actividad del 14C con el tiempo en los pozos cercanos al arroyo sugiere que la extracción de agua en la minería a cielo abierto para lograr explotación en seco ha dado lugar a un cambio en la dirección del flujo, por lo que la recarga localizada de la quebrada constituye actualmente la mayor proporción del agua subterránea bombeada. La tasa de recarga previo al desarrollo, calculada a partir de un balance de masas Cl en estado estacionario, es 6 mm/año, y es consistente con los cálculos basados en la actividad de 14C. Los cambios en las concentraciones del CFC-12 con el tiempo pueden estar relacionados con el cambio de posición del nivel freático con relación a la profundidad de los filtros del pozo.

摘要

根据从开口井或滤水管长的间隔地段获取的水样测量环境同位素含量来确定地下水的年龄困难重重,因为所取水样展现出年龄差别很大。采用多重示踪剂技术((Cl、 14C、 3H、 CFC-11、 CFC-12、 CFC-113 和 SF6)解译了澳大利亚西北部皮尔布拉地区露天矿周边区域含水层长滤水管生产井中水样的地下水年龄。调查了由于连续七年排水导致的示踪剂含量变化(2008–2014年),对示踪剂方法进行了比较。示踪剂含量表明地下水是年轻水和年老水的混合水;推断前者是来自毗邻小溪的本地化的补给水,而后者为弥散补给水。小溪最近处的井水14C活性随时间增加表明,露天矿坑排水实现采矿条件干燥导致了水流方向的改变,因此,抽取的地下水中很大一部分就是来自小溪的本地化补给水。通过稳定态Cl质量平衡计算得到的开采前补给量为6 mm/y,与基于14C活性的计算结果一致。CFC-12含量随时间的变化可能与相对于水井滤水管的深度来说水位的变化有关。

Resumo

Determinar a idade das aguas subterrâneas a partir das concentrações de traçadores ambientais medidas em amostras obtidas de furos de sondagem ou longas seções filtrantes é repleto de dificuldades, pois a água amostrada representa uma variedade de idades. Uma técnica de multi-traçador (Cl, 14C, 3H, CFC-11, CFC-12, CFC-113 e SF6) foi usada para decifrar a idade das águas subterrâneas amostradas em poços de produção com filtros longos próximos a uma mina a céu aberto na região de Pilbara, Noroeste da Austrália. A mudança nas concentrações do traçador devido à retirada continua de água ao longo de sete anos (2008–2014) foi investigada, e os métodos de traçadores foram comparados. As concentrações de traçadores sugerem que as amostras de águas subterrâneas são uma mistura entre águas jovens e antigas. Foi inferido que a primeira representa a recarga localizada de um córrego adjacente, e a última a recarga difusa. Um aumento na atividade de 14C com o tempo em poços próximos ao córrego sugerem que a drenagem das minas para alcançar uma condição seca de mineração resultou em uma mudança na direção do fluxo, de modo que a recarga localizada do córrego agora constitui uma grande proporção das águas subterrâneas bombeadas. A taxa de recarga anterior ao desenvolvimento, calculada através de um balanço de massa de Cl em regime permanente, é 6 mm/ano, e é coerente com os cálculos baseados na atividade de 14C. Mudanças nas concentrações de CFC-12 com o tempo podem estar relacionadas a mudanças na posição do nível freático relativos à profundidade do filtro do poço.

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Acknowledgements

This work was undertaken as part of a collaborative project between the National Centre for Groundwater Research and Training (NCGRT) and Rio Tinto Iron Ore. Funding was provided by the Australian Research Council, through Linkage Grant LP150100395 and by Rio Tinto Iron Ore. We thank several members of our teams at RTIO and Flinders University whose work made this study possible, especially Glenn Kirkpatrick for assistance in the field, Maree Swebbs and Thomas Linklater for help with the map preparations.

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Cook, P., Dogramaci, S., McCallum, J. et al. Groundwater age, mixing and flow rates in the vicinity of large open pit mines, Pilbara region, northwestern Australia. Hydrogeol J 25, 39–53 (2017). https://doi.org/10.1007/s10040-016-1467-y

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Keywords

  • Groundwater age
  • Mining
  • Groundwater recharge/water budget
  • Australia