Groundwaters in wet, temperate, mountainous,sulphide-mining districts : delineation of modern fluidflow and predictive modelling for mine closure(Rosebery, Tasmania).

There are as yet few studies of the hydrogeology of sulphide-mining districts in wet, temperate, mountainous areas of the world. This is despite the importance of understanding the influence of hydrogeology on the evolution and management of environmental issues such as acid mine drainage (AMD). There is a need to determine whether the special climatic and geological features of such districts result in distinct groundwater behaviours and compositions which need to be considered in mining impact studies. The present study addresses this information gap by broadly delineating the groundwater regime within the temperate Rosebery catchment, western Tasmania, Australia. The mountainous Rosebery catchment contains the large, active, underground, polymetallic Rosebery mine, based on a sulphide deposit hosted in the Cambrian Mount Read Volcanics. Rainfall in the Rosebery region far exceeds evapotranspiration and supports a dense cover of vegetation. The Rosebery groundwater system provides an example of an area in which groundwater management and hydrogeological research is in its infancy, as it is in many regions with similar settings. Although the area has been glaciated historically, mine data, particularly rock quality designator (RQD) values, provides clear evidence of: (i) a weathering induced increase in permeability within 100 m of the natural ground surface; (ii) an increased permeability associated with shear zones and faults; and (iii) beyond a depth of 100 m, a uniform decrease in permeability with depth below the ground surface. Insights into groundwater flow have been provided by qualitative and quantitative observations of piezometric level in 29 exploration drillholes, 8 piezometers, discharging groundwaters, and surface water flow monitoring points. Together piezometric heads, flow rates, and calculated material properties have provided the framework for developing a conceptual model of the groundwater regime within the Rosebery catchment. The groundwater flow system is typified by a deep fractured aquifer (which contain the mine voids), overlain by surficial glacial deposits and weathered material. Significant interaction between surface waters and groundwater was observed throughout the catchment. Geochemistry provided support for conceptual flows and justification of the modelling approaches. Potential acid-generating and neutralising minerals were identified by examining whole-rock geochemistry. The net acid generation and acid consuming potential of Rosebery materials were quantified. The results indicated that mined materials at Rosebery have the potential to produce a significant volume of acid mine drainage. At the Rosebery mine, metal contaminated waters originate from localised point sources of sulphides, such as tailings dams, waste rock, and mine workings. Waters are contaminated by AMD, resulting in elevated levels of H2SO4 as well as the elements Pb, Zn, Cu, Fe, Mn, Mg Cd, Al, and Ca. The Mg content of AMD indicates that neutralisation is occurring, most likely through the dissolution of the hypogene minerals chlorite, ankerite, and dolomite. Although background surface waters and contaminated mine waters are acidic, regionally the groundwaters sampled are nearneutral. The quantification of the important flows in the conceptual model of the Rosebery catchment allows the construction of a water balance, which provides a reasonable estimation of annual flows. The average precipitation rate across the catchment is estimated at 8.4 m3/s. The water budget for the Rosebery catchment is: (i) 42% of precipitation runs off to become true surface water flow (including interflow); (ii) 24% of precipitation is lost to evapotranspiration; (iii) 17% of precipitation becomes groundwater and is discharged as baseflow into creeks and rivers within the catchment; and (iv) 18% remains as groundwater discharging into the regional groundwater system outside of the catchment or into the Pieman River system. The water balance highlights the importance of groundwater in the catchment, with a groundwater to surface water flow rate ratio of 1:1.2. The water balance was applied to three scenarios: (i) a quantification of the contribution of the open pit to underground flows; (ii) the filling of the southern exploration decline; and (iii) the filling of the mine after decommissioning. This work indicated that the open cut makes only a minor contribution to underground water flow and that the mine is expected to fill to a decant point after six years. A steady-state, 3D numerical groundwater flow MODFLOW model with a geometry representative of the hydrogeologic environment around the Rosebery mine and surrounding catchment was produced. To satisfactorily represent the dual aquifer system, deep fracture flow on a regional scale was represented by a continuum approach to couple with true porous media flow (and shallow fracture flow) in the near surface aquifer. The groundwater model was calibrated to the local observations and water balance to represent the mine whilst in operation. A predictive closure scenario was undertaken using the calibrated model in a non-operational state by eliminating pumping to simulate a flooded mine. Particle tracking was used to plot potential contaminant flow pathways from the mine in the Rosebery catchment. Numerical modelling identified that the potential hydrogeological area of influence of the mine was chiefly controlled by the topography of the host catchment. The topographically-driven western flows off Mount Black are redirected south, primarily by the conduits of the mine workings. Conceptualisation of the groundwater regime improved using computer generated 3D visualisation, and through the numerical modelling exercise. The numerical modelling suggested that the potential area of discharge for contaminated mine waters is far more limited in extent than was previously believed by mine personnel. This area is limited to: (i) areas along the Stitt River and Rosebery Creeks, which are already experiencing significant acid mine drainage contamination; and (ii) a very limited area south and north of the Pieman River’s confluence with the Stitt River. The implications of understanding the area of potential influence are: (i) resources for future monitoring investigations can be focussed in this discrete area; (ii) the scale of future modelling efforts can be restricted to this area, improving detail and limiting computational requirements; and (iii) background monitoring beyond this area can be used to further test the model and provide data for future model calibration and validation. The present study at Rosebery has wider implications for researching groundwater in wet temperate mountainous sulphide mining terrains. The characteristic feature of such terrains is the local spatial variation in precipitation, evapotranspiration, and therefore recharge. Estimating a representative recharge remains the pivotal quantification for undertaking groundwater investigations in such climates. The significant interaction of surface waters and the groundwater system in wet, temperate, mountainous environments requires modelling to be capable of accounting for this interaction. Surface water and groundwater interactions present an opportunity to investigate the groundwater regime at the surface and near surface in shallow drillholes. Gauging stream flow for baseflow contribution provides important quantification of groundwater flow where suitable drillhole flow data is scarce, a common state in mined settings. Mine outflows also provide an important quantification of groundwater flow on a large scale. The present study proposed a method for amalgamating these data which is applicable in other similar environments. The challenges encountered in the present study provide insight into planning for similar mining projects. Prior to the present study, there was no general knowledge of the groundwater patterns of the region, and no clear idea of whether the Rosebery mine had an influence on the regional groundwater regime. There was no historical information of the type that is usually associated with groundwater resource investigations, and the nature of exploration drillholes meant that some typical groundwater research methodologies were not practical at Rosebery. This resulted in the investigation taking a different research approach compared to that typically used for groundwater evaluation in, for instance, agricultural districts. Although the present study attempted to endure without standard hydrogeological data, there remains a significant gap in the reliabilty of the modelling. This is a situation that faces most mines nearing closure, when cash flow is likely to be reduced. Time-series piezometric levels, and a larger hydraulic property dataset were the key gaps identified in the present study. Appropriate budgeting for the acquisition of such information should be considered a priority for groundwater investigations of wet, temperate, sulphide-mining districts elsewhere.