By Darrel Dunn, Ph.D., PG, Hydrogeologist
Mining hydrogeology is the application of hydrogeology to the successive stages of mining projects. These stages may include:
Scoping feasibility study,
Preliminary feasibility study,
Detailed feasibility study,
During the feasibility stages, hydrogeologic studies should be performed if the cost of dealing with water issues is likely to be significant during the permitting and later stages. These issues may include:
Mining impacts on water quality,
Mining impacts on water wells,
Mining impacts on streamflow,
Mine dewatering requirements,
Acid mine drainage,
Ground subsidence effects.
Adequate feasibility assessment can be very important, because a misleading assessment can result in rejection of a viable mining project or, conversely, further expenditure on a non-viable project.
During the scoping feasibility study, the hydrogeologic assessment of requirements for dealing with water issues may be based on pre-existing data and comparison with similar existing mines, if any. For dewatering assessment, a "large well" analog might be used along with hydraulic conductivities derived from existing well specific capacity data.
If the scoping feasibility study does not eliminate the project, the ensuing preliminary feasibility study may be based on recommendations for data collection and analysis in the scoping study. Some hydrological data may be collected in conjunction with early geological exploratory work. More realistic hydrologic modeling might be used, such as analytic element modeling and analytic stream depletion calculations.
If the preliminary feasibility study does not eliminate the project, a detailed feasibility will include any hydrologic data collection recommended in the preliminary feasibility study, such as:
Pumping or injecting testing in geology exploration holes to acquire data on hydraulic conductivity and other hydraulic properties, especially the effects of fracturing,
Testing wells constructed specifically to fill in critical groundwater data.
The detailed feasibility study may include numerical groundwater flow modeling to evaluate dewatering scenarios and surface water depletion.
If the detailed feasibility study does not eliminate the project, pre-mining hydrogeologic activities may include:
Design and installation of groundwater monitoring systems,
Pre-mining water quality data collection and assessment,
Pre-mining monitoring of groundwater levels,
Pre-mining monitoring of surface water flow,
Establishment of weather stations to help separate hydrologic effects of weather from the effects of mining,
Identifying and quantifying water supply sources,
Designing water control systems including dewatering alternatives that minimize cost and provide depressurization,
Permit application support including an environmental impact assessment.
During the mine construction and the extraction phase, hydrogeologic activity may include:
Updating groundwater modeling to track the hydrologic effects of the mine and modify dewatering operations,
Implementing in-mine drainage boreholes to adapt to fractures, faults, and permeable rock encountered,
Surface water monitoring.
Mine closure may include: