Practical assessment of fractured rock hydrogeology in the open cut area at Northparkes Mines, NSW.
Thesis DisciplineEnvironmental Sciences
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
As the open pit mines at Northparkes near the end of their productive life, a better understanding of site hydrogeology is needed in order to set achievable aims for mine rehabilitation. This has been accomplished in this thesis by defining an improved conceptual hydrogeologic model for the open cut area, using an environmental framework to expound the interconnections between water in each component of the physical environment. Groundwater around the open pits is contained within secondary porosity features of trachyandesitic and intrusive lithologies. The water table is approximately 40 m below the land surface, overlain by regolith of variable thickness that has formed from a long history of chemical weathering. The hydrogeology is strongly influenced by the semi-arid environmental conditions, with long recharge and discharge pathways. Forming a conceptual understanding of the system has required rational analysis of sporadic data from short monitoring programs, which are prone to a high degree of error because of the heterogeneous and isotropic nature of the groundwater medium. Low mine inflows and subtle piezometric gradients are evidence for very slow groundwater movement in the area. There is a steep piezometric gradient surrounding each open-pit mine and less than 50 m from each pit perimeter the water table remains at its pre-mining position. The hydrogeologic data is consistent with a model whereby the bulk of groundwater occurs within a layer of oxide rock (saprock), situated beneath the intensely weathered saprolite and above fresh sulphide rock. The saprock represents a layer of enhanced permeability within the profile; it follows the undulating weathering front, and it has variable depth and thickness. In the sulphide rocks, hydraulic conductivity is strongly dependent on the type of fracture infill and the existence of open fracturing. Drilling data indicates that open fracturing declines with depth and is not encountered below a depth of 300 m. Classical pumping test data for the area is limited; estimates of hydraulic conductivity are <0.5 m/day in fractured sulphide rock. The results of slug test testing are consistent with this, indicating that hydraulic conductivity is variable but generally very low. Two new piezometers in the open cut area with short screened sections revealed that hydraulic conductivity in the oxide zone is at least an order of magnitude higher than in the sulphide zone. This research has shown that the position of the water table is controlled by rock fracture characteristics, the thickness of the weathered profile, and proximity to areas of shallow bedrock that receive direct rainfall recharge. Recharge in most areas is hindered by thick swelling clay soils and regolith units. Data from monitoring bores across the Upper Bogan valley showed that the water table responds very slowly to rainfall trends and land use changes. Groundwater quality in the area is moderate to poor, small pockets of low salinity water are confined to shallow depths near recharge outcrops. Groundwater composition is strongly influenced by the reactivity of fracture infill, mineralisation history, and the intensity of weathering processes. There is no evidence that mining at Northparkes has markedly altered groundwater chemistry. Analysis of groundwater samples from monitoring bores and pit wall seepage showed there are three general hydrochemical facies across the mine site. These represent consecutive stages in the evolution of groundwater chemistry with distance from recharge and with residence time in the rocks, according to a classic Chebotarev sequence. Groundwater with relatively low salinity (<3000 mg/l TDS) and a calcium-bicarbonate signature occurs close to bedrock outcrops, where direct rainfall recharge is thought to primarily occur. A short distance from recharge outcrops, TDS increases to 3000-10000 mg/l and groundwater composition is calcium sulphate or calcium chloride type due to wall-rock reactions and reducing conditions in the formation. Groundwater with a sodium-chloride signature and high TDS (> 10000 mg/l), is found deep in the profile and in areas overlain by thick, confining regolith material. Rapid evolution to sodium-chloride type groundwater is ensured by the nature of fracture infill in the crystalline lithologies, and high cation exchange capacity of the extensively weathered material. The hydrochemical data is consistent with geological evidence that groundwater in the Northparkes area is compartmentalised by lithologic, structural and weathering features in the profile. Exploration drilling has shown there are small basins of deeply weathered material isolated by faults and areas of low permeability, unaltered rock. Groundwater within each compartment is essentially stagnant and saline, and as a result it has very low resource potential. This research has provided a conceptual framework for the hydrogeologic system, which will facilitate further work to help to ensure environmental sustainability of open cut rehabilitation plans. Future work should be directed towards quantifying components of the system that are still poorly understood, such as recharge and vadose zone processes.