Research in Fractured Rock Hydrogeology

Dr. Tom Burbey

Earth tides and barometric loading to characterize fractured-rock aquifers
Measuring & analyzing strain & deformation to quantify aquifer properties in fractured-rock systems
Evaluating factures that influence recharge in the Blue Ridge Province
Hydrogeology of the Coles Hill Uranium site in Pittsylvania County, Virginia
Hydrogeology of Mountain Lake, VA

Earth tides and barometric loading to characterize fractured-rock aquifers

Fractured-rock aquifers typically behave as confined low-storage systems so that small stresses from barometric loads or Earth tides result in measurable water-level fluctuations. These fluctuations have an amplitude and phase that can be compared with theoretical tides that allows for aquifer properties (hydraulic conductivity and storage) to be calculated. Such calculations are a relatively simple way to obtain often difficult to measure parameters of fractured systems and preclude the need for complex packer testing.
We are using and developing methods to characterize aquifer properties from stress-strain induced water-level fluctuations at the Fractured Rock Research Site in Floyd County, VA. Strain responses provide valuable data that reflect the fracture and hostrock properties.

Figure shows the correlation between the tidal potential and measured water levels at a well located at the Fractured-Rock research site in Floyd County, VA.

Figure shows (a) water level and barometric pressure head response to a well, (b) the detrended water-level and pressure response is used to obtain (c) the relative change response between fluid and barometric pressure heads that reflect the barometric efficiency of the fracture intersecting the well.

Related References

Burbey, T.J., and Zhang, M., 2010, Evaluation of hydrofracing success using Earth tide and barometric response: Ground Water Journal, v. 48, no. 6, p. 825-835.
Burbey, T.J., 2010, Fracture characterization using earth tide analysis: Journal of Hydrology, v. 380, issue 3-4, p. 237-246, doi:10.1016/jjhydrol.2009.10.037.
Burbey, T.J., 2008, Evaluation of fracture properties using Earth-tide analysis, EOS Trans, American Geophysical Union, H41A-0826.
Burbey, T.J., 2008, Non-traditional approaches to aquifer characterization: Proceedings of the 1st International Conference on Watershed Hydrology and

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Measuring and analyzing strain and deformation to quantify aquifer properties in fractured-rock systems

Storage properties tend not to be sensitive to water-level changes. Rather, they are highly sensitive to strain or deformation changes of the fractures or hostrock. Measuring these deformations is challenging because of the low compressibility of the rocks in this environment. Typical deformations associated with pumping is on the order of microns so instruments must be highly sensitive to small-scale stain and must be free from temperature changes that may affect the results. We employ borehole extensometers to monitor small-scale changes from pumping, and strain meters to measure tilt of either the fracture or land surface (tilt measured in nanoradians).

Figure shows borehole extensometer before being lowered into the borehole at the Fractured Rock Research Site. Dr. Larry Murdoch (white teeshirt) oversees the operation, while Dave Hisz checks the connections before deployment.

Figure shows how a shallow tiltmeter can measure tilt at the landsurface due to compression of a fracture from pumping-induced stress.

Figure above shows the stress (drawdown) stain (displacement) relation for three different locations within the borehole obtained from the extensometer. The top two show compression cycle of the fracture, while the lower plot shows the extension of the hostrock adjacent to the fracture.

We couple the strain and deformation measurements from aquifer pumping to obtain storage and porosity values of the fracture. We continue to develop techniques that will allow us to characterize the fracture and hostrock properties. Understanding these parameter values is key to providing estimates of long-term sustainability of fractured systems.

Related References

Burbey, T.J., Hisz, D., and Murdoch, L.C., Zhang, M., 2011 (in review), Quantifying fractured crystalline-rock properties using well tests, Earth tides and barometric effects: Journal of Hydrology
Burbey, T.J., and Murdoch, L.C., 2010, Using extensometer and earth tide data to characterize fractured crystalline-rock properties: in Land Subsidence, Associated Hazards and the Role of Natural Resources Development; Carreón-Freyre, D, Cerca, M., and Galloway D.L., eds.; IAHS Publication 339, p. 319-325.
Burbey, T.J., 2008, Non-traditional approaches to aquifer characterization: Proceedings of the 1st International Conference on Watershed Hydrology and Slope Stability, Taipei, Taiwan, Nov. 4-5, 2008. p. A1-A3.
Burbey, T.J., 2011, Using reverse water levels to characterize fracgtured-rock aquifers, EOS Trans, American Geophysical Union, Annual Meeting, San Francisco, Dec 5-9, 2011.
Burbey, T.J., 2008, Non-traditional approaches to aquifer characterization: Proceedings of the 1st International Conference on Watershed Hydrology and Slope Stability, Taipei, Taiwan, Nov. 4-5, 2008. p. A1-A3.

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Evaluating factors that influence recharge in the Blue Ridge Province

The hydrogeologic setting of the Blue Ridge Province, a complex highly metamorphosed region extending from Georgia to New York, can be characterized as a two-aquifer system. One shallow saprolite aquifer that rests on top of the underlying fractured-rock aquifer. The saprolite aquifer is typically highly heterogeneous, discontinuous, and intermittent because it typically disappears during summer months when ET is high. The fractured rock aquifer system that underlies the saprolite aquifer has typically been viewed as a system dependent on weathering of near surface fractures and flow is thought to decrease greatly with depth as weathering decreases. We have found, perhaps contrary to this philosophy, that the flow systems within the fractured-rock aquifer are more highly dependent of the tectonic history of the region. En echelon thrust fault slivers are ubiquitous throughout the region and tend to (1) compartmentalize flow, and (2) create preferential flow paths above the fault planes. While ductile deformation is common within the fault plane, brittle deformation and fracturing is common above and perhaps below the fault plane, creating flow paths that can be significantly deeper than fracture openings created by weathering.
Numerous studies have been undertaken at the Fractured Rock Research Site to characterize the flow patterns, recharge and active pathway migration of flow and contaminants in this complex metamorphic system. Below is a conceptualization of this system.

Electrical resistivity profiling has aided greatly our ability to characterize the fault zone aquifer. The fault extends to land surface where recharge can easily infiltrate the aquifer. A tracer study was conducted to identify the flow pathway of the recharge front. The Figure below shows the migration of a KCl tracer put in at the land surface at the subcrop of where the fault reaches the land surface. As you can see (below) the tracer follows the fault zone downgradient to a nearby spring.

We continue to investigate fracture characteristics using borehole geophysics, packer tests, extensometers and tiltmeters. Many opportunities are available for students who desire to investigate fractured-rock aquifers.

Related References

Rugh, D.E., and Burbey, T.J., 2008, Using Saline Tracers to Evaluate Preferential Recharge in Fractured Rocks, Floyd County, Virginia, USA: Hydrogeology Journal, v. 16, no. 2, p. 251-262.
White, B.A., and Burbey, T.J., 2007, Evidence for structurally controlled recharge in the Blue Ridge Province, Virginia, USA: Hydrogeology Journal, v. 15, no. 5, p. 929-943.
Lauer, R.M. and Burbey, T.J., 2006, Characterizing fractured rock aquifers of the Blue Ridge Physiographic Province using surface and downhole geophysical methods: Geol Soc. of Am., Abstracts with Programs, v. 38, no. 7, p. 25.
Rugh, D.F., and Burbey, T.J., 2006, Using tracers to evaluate preferential recharge pathways in the Blue Ridge Province: Geol Soc. of Am., Abstracts with Programs, v. 38, no. 7, p. 470.
Rugh, D.F., and Burbey, T.J., 2005, Using tracers to estimate recharge in a Blue Ridge fractured rock aquifer system: Geol. Soc. Of Am., Abstracts with Programs, v. 37, no. 7.
Seaton, W.J., and Burbey, T.J., 2005, Influence of ancient thrust faults on the hydrogeology of the Blue Ridge Province: Ground Water, v. 43, no. 3, p. 301-313.
Gentry, W.M, and Burbey, T.J., 2004, Characterization of Groundwater Flow from Spring Discharge in a Crystalline-Rock Environment: Jour. Amer. Water Resour. Assoc., v. 40, no. 5, p. 1205-1217.
White, B.A., and Burbey, T.J., 2003, Evaluation of Ground-water Recharge in the Blue Ridge Physiographic Province: Proceedings of the Virginia Water Resources Symposium, 2003, p. 177-180.
Burbey, T.J., 2003, Potential Ramifications of Compartmentalized Flow
in the Blue Ridge Province: Proceedings of the Virginia Water Resources Symposium, 2003, p. 172-176.
Seaton, W.J., and Burbey, T.J., 2002, Evaluation of two-dimensional resistivity methods in a fractured crystalline-rock terrane: Jour. of Applied Geophysics, v 51, no. 1, pp. 21-41.
Seaton, W.J., and Burbey T.J., 2000 Aquifer characterization in the Blue Ridge Physiographic Province using resistivity profiling and borehole geophysics: Jour. of Environ. and Eng. Geophysics, v. 5, no. 3, pp. 45-58.
Burbey, T.J., and Seaton, W.J., 2000, Evidence for a new hydrogeologic model for the Blue Ridge and Piedmont Provinces: Proceedings of the Virginia Water Research Symposium, Richmond VA, Nov 15-16, 1999, pp. 20-24.

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Hydrogeology of the Coles Hill Uranium site in Pittsylvania County, Virginia

Characterizing the fluid flow in and around the Coles Hill uranium ore body is necessary before any type of mining operation can be carried out in the future. What complicates this crystalline rock system is the Chatham Fault, which separates the Precambrian granitic rocks to the west from the metasediment Triassic basin deposits to the east. What role does this fault have on the hydrogeology of the ore body, which is located adjacent to the fault in the granitic rocks?

We use a multi-disciplinary approach to understanding this system. 2D electrical resistivity surveys using an AGI SuperSting 8 channel system with 64 electrodes helped us to locate fluid filled fractures associated with the Chatham Fault. In addition, we employed aquifer testing, geochemistry, and age dating of waters from the different units to help characterize this fractured rock system. We have found in our preliminary investigation just south of the south ore body that only a limited quantity of water is crossing the fault and that the water in the Triassic basin appears to be significantly older than the water in the granitic rocks to the west and may actually be coming from a deeper source along the fault.

Additional studies are needed to further understand the role of the Chatham fault in influencing fluid flow in the area and to identify other fracture systems that may be important to any mining operations. We will continue to apply a multi-disciplinary approach to understanding this complex system.

Picture shows Dr. Burbey and graduate student JP Gannon conducting resistivity imaging at the Coles Hill Uranium site near Chatham, VA. The Coles home, built in 1800, can be seen in the background.

Related References

Gannon, J.P., Burbey, T.J., Bodnar, R.J., and Aylor, J., 2011 (in review) Hydrogeologic characterization of the Chatham fault at the Coles Hill Uranium Deposit, Virginia: Hydrogeology Journal.

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Hydrogeology of Mountain Lake, VA

Mountain Lake in Giles County, Virginia has a documented history of severe natural lake-level changes involving groundwater seepage that extend over the past 4200 years. Featured in the 1986 movie Dirty Dancing, the natural lake dried up completely in September 2008 and levels have not yet recovered. An ongoing hydrogeologic investigation is being done in an effort to determine the factors influencing lake level changes. A daily water balance, dipole-dipole electrical resistivity surveying, well logging and chemical sampling have shed light on: 1) the influence of a fault not previously discussed in literature regarding the lake, 2) the seasonal response to precipitation of a forested first-order drainage system in fractured rock, and 3) the possibility of flow pathways related to karst features.
Geologic controls on lake level are being investigated using several techniques. Geophysical surveys using dipole-dipole resistivity have located possible subsurface flowpaths both to and from the lake. Well logs, lineament analysis, and joint sampling are being used to assess structural controls on lake hydrology. Major ions have been sampled at wells, springs, streams, and the lake to evaluate possible mixing of different sources of water in the lake. Groundwater levels are being monitored for correlation to lake levels, rainfall events, and possible seismic effects. The hydrology of the lake has been quantified with a water balance on a daily time step.

Current findings from the water balance indicate steady net drainage and significant recharge when vegetation is dormant, particularly during rain-on-snow melt events. The resistivity survey reveals discrete areas that represent flow pathways from the lake, as well as flowpaths to springs upgradient of the lake located in the vicinity of the fault. The survey also suggests that some flowpaths may originate outside of the topographic watershed of the lake. Chemical evidence indicates karst may underlie the lakebed. Historical data suggest that artificial intervention to mitigate seepage would be required for lake level recovery in the near future.

Figure showing the lake stage and the contribution of the other components of the hydrologic budget of the Mountain Lake watershed.

Related References

Roningen, J.M, and Burbey, T.J., 2011 (in review) Hydrogeologic controls on lake level: A case study at Mountain Lake, Virginia, USA: Hydrogeology Journal
Roningen, J.M., and Burbey, T.J., 2011, Hydrogeologic controls on lake level at Mountain Lake, Virginia, Southeastern GSA, March 23-35, 2011.

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