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

Cook, Peter; Dogramaci, Shawan; McCallum, James; Hedley, Joanne

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

2017

Cook, Peter | Dogramaci, Shawan | McCallum, James | Hedley, Joanne

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.

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