Many gigalitres of groundwater have been extracted from the Condamine River Alluvial Aquifer (CRAA) since the 1960s. These groundwater withdrawals have stressed the system and locally altered the groundwater flow paths. Isotopes can provide powerful insights into recharge pathways, flow direction, and the sustainability of groundwater withdrawals from alluvial aquifers. To address some of the regional groundwater concerns we must characterise alluvial groundwater residence time.
A total of 31 groundwater samples were collected from privately owned irrigation bores and Qld-DNRM government monitoring bores in the region between Condamine Plains and Dalby. Parameters analysed included: 3H, 14CDIC, 222Rn, 87Sr/86Sr, δ13CDIC, water δ2H and δ18O, sulfate δ34S and δ18O as well major, trace and REE elements.
Distance from primary recharge areas (rivers) provides the main control on groundwater residence time in the CRAA. This is supported by the following observations:
1) Groundwater between the Condamine River and its northern branch has low TDS (~400 mg/L), is Na-HCO3-type and has detectable 3H, indicating a proportion of modern recharge (<70 years);
2) Groundwater east of the northern branch has higher TDS (~700 mg/L) and is Na-HCO3- -type with increasing eastern inputs. No 3H is detected and 14C shows sub-modern groundwater (~500 years);
3) Groundwater along the eastern and western boundaries of the alluvium or samples retrieved from the Walloon Coal Measures (WCM) have high TDS (1,250-19,770 mg/L) and are Na-Cl-type. Residence times in the upper WCM increase along the flow path to the west from modern to 32,000 years on the western side.
Groundwater residence time distributions provide a visualisation of recharge processes and delineate areas where groundwater withdrawals are less sustainable within the CRAA.