4 Hydrogeological Field Investigations at Mine Sites

Field investigations to characterize groundwater conditions at a proposed mine site serve many purposes; studies will be undertaken at several candidate sites for development of a tailings storage facility, at the proposed locations of waste rock stockpiles, in the vicinity of the open pit or underground mine workings, and in the area of the proposed plant site. At the site of a proposed tailings storage facility, hydrogeological studies are usually linked with complementary geotechnical investigations that focus on the definition of the foundation conditions for dam design. Field programs are normally undertaken as phased studies, providing information requirements suited first for conceptual design, and then pre-feasibility study, feasibility design and finally detailed engineering. A similar approach is adopted in the design for a dewatering system at an open pit or in underground workings. An appropriately detailed characterization of the geologic setting is essential in advancing the hydrogeological studies. At each stage of the investigation, decisions on the number and location of boreholes are based on obtaining data required to answer specific questions that underpin the conceptual model of the groundwater flow system, or issues that must be resolved for advancing the design concepts. An inadequate site investigation program can lead to costly re-designs of containment structures, or the requirement in the future to implement perhaps costly mitigation measures.

There is no guidance that defines the number of boreholes that should be installed as part of a site characterization program at each stage of the design process. Each mine site presents a unique geologic setting. For sites with greater geologic complexity, reliable design will require a greater effort to define site conditions at each stage of the site characterization. For large mining projects, it is common that by the time detailed engineering studies are complete, and construction is underway, many tens to several hundred boreholes completed with piezometers will have been installed to meet information requirements.

In comparison to hydrogeologic studies for water supply purposes or at contaminated sites where open standpipe piezometers are common, more extensive use is made in the mining industry of vibrating wire piezometers (VWP) placed in sealed boreholes. A VWP uses a tensioned steel wire and an electromagnetic coil to convert pore pressure on a diaphragm to a frequency signal that can be recorded and related to the pore pressure. Each VWP is connected to a cable that connects to a manual readout or data logger on surface. The use of VWP’s reflects a greater focus in the mining industry on definition of subsurface fluid pressures for geotechnical design. In comparison to an open standpipe, they have a rapid response time in lower-permeability soils. Multiple VWP’s placed at different depths in a single borehole allow definition of vertical hydraulic gradients, which can be an important attribute in characterization of the groundwater flow system. However, sealed boreholes with VWP’s preclude the opportunity to monitor groundwater quality.

Doubek and others (2013) provide a comprehensive description of the site investigation process, hydrogeologic and geophysical testing programs, approaches to data interpretation, and design of monitoring systems used when advancing a design for an open pit from the conceptual phase to detailed engineering. The discussion there is also relevant to field programs undertaken in the siting and design of the tailings and waste rock management systems at a mine site.

Environmental tracers such as oxygen and deuterium isotopes can aid in refining the understanding of a groundwater system and in testing conceptual and numerical model formulations. An overview of the application of environmental tracers in assessment of coal-mining projects can be found at the following website: www.iesc.environment.gov.au/system/files/resources/269a12ba-09a5-424f-bda3-d5f9679317c7/files/environment-water-tracers-factsheet.pdf.