1.1.4 Hydrogeology and groundwater quality

Summary

The behaviour of groundwater within the Pedirka Basin is poorly understood; the availability of data is too limited to determine if the formations act as a single flow system or as separate, discrete aquifers. The quality of groundwater extracted from the Crown Point Formation is fresh to brackish, but is generally of reasonable quality. Salinities from drill stem test data closer to the centre of the Pedirka Basin provide slightly higher salinities. Groundwater from the Crown Point Formation aquifer forms two distinct groups with respect to major ion composition.

In the vicinity of the Pedirka subregion, there are two recognised main aquifer units within the Great Artesian Basin (GAB). The Mackunda and Winton formations form an upper aquifer unit (K aquifer), while a deeper aquifer (J aquifer) occurs within the Cadna-owie Formation and Algebuckina Sandstone/DeSouza Sandstone. The J aquifer forms the only regionally extensive water resource of generally fresh to brackish quality in the Pedirka subregion.

Regional groundwater flow direction in the Pedirka Basin and the J aquifer is broadly to the south-east, with flow emanating from the north-west margin of the basins. Groundwater recharge to Pedirka Basin aquifers is postulated to occur in focused areas such as rivers that flow over outcrop and via direct infiltration of rainfall. On a regional scale, the Dalhousie Springs complex may act as a point of discharge for Permian aquifers as well as GAB aquifers, while at a local scale small waterholes may act as points of discharge. Discharge is also likely to occur as cross-formational flow. It is currently thought that significant recharge to the J aquifer along the western margin occurred in wetter climatic periods of time than what is currently being experienced. The present day volume of recharge is considered to be significantly less than natural discharge; although modern recharge may have localised significance for the maintenance of springs. Diffuse recharge rates in the Pedirka subregion have been determined to be negligible. In contrast, locally significant active ephemeral river recharge has been identified. Discharge from the J aquifer occurs in the vicinity of GAB springs located to the south and south-east of the Pedirka subregion and potentially via upward leakage into the K aquifer, but only where faulting has deformed overlying confining layers.

Overlying the GAB, important local scale water resources occur in shallow, disconnected Cenozoic aquifers. Water quality and recharge processes in the Cenozoic aquifers are highly influenced by proximity to surface water systems. Little is known about groundwater discharge from the shallow aquifers, though it is likely to be via semi-permanent waterholes and via evapotranspiration.

Little is known about the prevalence and nature of intra-basinal aquifer connectivity within the Pedirka Basin, however the Purni and Crown Point formations may act as regional-scale aquifers in at least partial connectivity. Additionally, the significant faulting and folding identified may have formed pathways for groundwater to migrate between aquifers or conversely led to the formation of hydrogeological sub-basins.