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. 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. 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. 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. The changes in tracer concentrations due to continuous dewatering over 7 years (2008–2014) were examined, and the tracer methods were compared. A multi-tracer technique (Cl, 14C, 3H, CFC-11, CFC-12, CFC-113 and SF 6) 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. 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.
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