Mine subsidence induced hydraulic connection tested by geochemical and geophysical tracing techniques.

Waring, CL; Peterson, MA

Mine subsidence induced hydraulic connection tested by geochemical and geophysical tracing techniques.

Date

2007-11-27

Authors

Waring, CL

Peterson, MA

Publisher

Mine Subsidence Technical Society

Abstract

Changes in surface and groundwater hydrology induced by mine subsidence near surface fracturing can also subtly alter the geochemistry. More rapid drainage of surface and groundwater along fracture pathways may lower the standing water level allowing air to penetrate further, accelerating rock weathering and leaching reactions. However, these sulphide oxidation and carbonate dissolution reaction products are normal constituents of groundwater and their presence does not necessarily indicate an impact of mine subsidence. Changes to the groundwater recharge – discharge regime due to fracturing will also affect the age of the groundwater. Whilst the groundwater age is not a significant water quality parameter it may unambiguously distinguish changes to the hydrology, where conventional chemistry may be difficult to interpret. Mine subsidence fracturing may also cause hydraulic connection between previously isolated aquifers or ultimately between the surface and the mine. Isotopic geochemical dating (35S, 3H, 14C) and tracing (δ13C, δ2H, δ18O, δ34S) techniques are used to distinguish the origin of the groundwater and potential isolation or fracture connections. Isolated groundwater may show a dissolved gas signature similar to petroleum gas within the Bulgo Sandstone. To identify the geochemical impact of mine subsidence fracturing, pre and post mining profiles are being assembled for comparison. Groundwater age profiles also help to constrain and verify groundwater flow models. However, to produce useful 14C dates from carbon dissolved in groundwater differences between carbon species (DIC dissolved inorganic carbon, DOC dissolved organic carbon and dissolved methane / ethane) caused by interaction with siderite in the aquifer are recognised and addressed. A nuclear geophysical logging technique, Prompt Gamma Neutron Activation Analysis (PGNAA) is also used to trace the flow of an injected salt solution into fractures and into the porous and permeable sandstone surrounding the borehole. The variable distance the salt tracer moves into the porous rock under a known pressure increase (above standing water level), over a known time and tracer volume allows calculation of hydraulic conductivity at 20cm increments along the length of the borehole. If there is significant flow of the tracer into fractures and beyond the PGNAA measurement range a relative tracer movement distance is provided by the PGNAA log, rather than hydraulic conductivity. Other relevant lithological and hydraulic parameters such as porosity may be derived from measured Si, H, Cl, ±Fe, ±Al elemental abundance provided by PGNAA borehole logging. Borehole sampling of aquifer water at narrow discrete intervals for geochemical profiling requires isolating the aquifer segment or individual fracture flow from the rest of the borehole. A dual packer apparatus is used to take narrow (2, 5, 10m) discrete interval samples or measure individual fracture flows.

Keywords

Boreholes, Aquifers, Ground water, Age estimation, Ground subsidence, Mines

Citation

Waring, C. L., & Peterson, M. (2007). Mine subsidence induced hydraulic connection tested by geochemical and geophysical tracing techniques. Paper presented to the 7th Triennial Conference of the Mine Subsidence Technical Society, 26th – 27th November 2007. In G. Li & D. Kay (Eds.), Proceedings of the 7th Triennial Conference of the Mine Subsidence Technical Society, (pp. 263-268). Newcastle, Australia: Mine Subsidence Technical Society.