Conceptual Isotope Hydrostratigraphic Models For North American Sedimentary Basins: Understanding Water Sources In Deep Subsurface Mines
Goodman, William Mario (1); Phillips, Stephen (2); Van Dyk, Deon (1); Grobler, Nico (1)
1: Sovereign Hydro, United States of America; 2: Phillips Mining, Geotechnical and Grouting LLC
Deep underground mines have operated successfully in North America since the mid-eighteenth century. Many mines have encountered water and either pump for the life of the mine and/or conduct grouting programs to manage inflows.
Mines in crystalline rock may be less vulnerable to catastrophic flooding events from deep bedrock aquifer systems because of the typically low transmissivity of most igneous and metamorphic rocks and their resistance to erosion and dissolution. Deep mines in sedimentary basins are more vulnerable to catastrophic inflows because of potential connections to regional aquifers, the vulnerability of some sedimentary rocks to slake/weaken, and evaporite dissolution. Furthermore, multiple aquifers in the stratigraphic sequence can communicate via man-made connections or natural, cross-formational features.
Routine, vertical profiling of water seepage in mine shafts is recommended to catalogue isotopic and major ion water signatures in each major and minor water-bearing zone penetrated, starting at the time of sinking and continuing periodically over the life of the mine. Gradual changes suggestive of mixing between shallower and deeper aquifers can be detected through routine sampling and analysis.
Mines operating in ancient basins with long, multi-chapter fluid migration histories, including Pleistocene northern hemisphere glaciation may benefit significantly from the recommended shaft water profiling. Dynamic changes in flow regimes around mines in basins that have experienced glacial loading and loading, post-glacial basin margin erosion, and cold water karstification can be detected over time that would not be possible from sampling at the time of shaft excavation alone. Isotopic signatures of glacial water recharge into the deeper basinal aquifers over the life of a mine would stand out because of striking contrasts between the glacial fluid signatures and those of the highly evolved formation waters deep in the basins. A gradual change in isotopic signatures indicative of increased percentages of shallow meteoric water in a mix with deep formation waters may reflect mining-induced damage to surrounding strata. Routine sampling and analysis of mine shaft fluids and any waters discharging from roof and rib strata are recommended over the operational life of the mine to monitor for changes in fluid migration that could portend broadened hydrogeologic connections and greater risk of more substantial water ingress. Stable isotopes of oxygen and hydrogen, in combination with tritium and major ion chemistry, are helpful geochemical tools for monitoring such impacts. Based on findings, proactive grouting programs may be considered to minimize potential for sudden increases in water inflow.