Characterization, Modeling, Monitoring, and Remediation of Fractured Rock (2020) / Chapter Skim
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5 Methods for Site Characterization and Monitoring
5 Methods for Site Characterization and Monitoring
Pages 68-93
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From page 68... ...
The focus of these latter sections is on areas that show great promise in the characterization of the fractured rock environment. GEOMECHANICAL CHARACTERIZATION Geomechanical characterization of fractures is important to understand the geometry, flow, and transport properties of existing natural fractures, as wells as the coupling of fracture flow and transport properties with rock stress and deformation.
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From page 69... ...
Because rock fracture geometry and properties are a major component of rock mass behavior, it is at least theoretically possible to utilize these empirical rock characterization techniques to estimate fracture geometry and properties. Even if the fundamental processes that govern fracture generation are known, the geometric characteristics of the void space within individual fractures and the three-dimensional spatial distribution of fractures cannot be mapped over large volumes deterministically.
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From page 70... ...
. GEOMETRIC CHARACTERIZATION Systematic fracture mapping is an essential tool to delineate the geometries of discrete pathways and the nature of both fracture pathways and rock matrix.
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From page 71... ...
Judicious and strategic use of coring to recover rock samples for physical and chemical analyses and fine-scale investigation of lithology and fractures can inform site hydrostructural models. Porosity and mineralogy can be analyzed from core samples, and mineral precipitates or weathering may indicate groundwater flow in specific fractures.
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From page 72... ...
, then a borehole itself does not necessarily conflict with test objectives because contributions of individual fractures are not being tested. In contrast, at sites where groundwater is contaminated, or if the goal is to determine the suitability or effectiveness of the geologic environment for waste isolation, characterization of individual fractures and the groundwater flow paths through multiple connected fractures is critical.
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From page 73... ...
Contaminant redistribution and longevity in previously uncontaminated fractures was due to groundwater moving through fractures as well as contaminant diffusion into the rock matrix. Furthermore, pumping in boreholes open to multiple permeable fractures can lead to geochemical mixing from multiple sources in the water withdrawn from the borehole.
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From page 74... ...
Perturbing hydraulically (e.g., injecting or pumping water) and monitoring the hydraulic response in the packed-off interval allows estimation of the transmissivity within that interval through a simplified interpretation of the groundwater flow regime (quasi-steady, radial flow)
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From page 75... ...
Estimating fracture transmissivity is based on the same conceptual model of groundwater flow in the vicinity of the borehole (quasi-steady, radial flow) as is used in the interpretation of flow meter logging and single-hole packer tests.
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From page 76... ...
. Characterizing Flow Paths at Different Scales The hydraulic properties of complexly connected fracture systems are sometimes inferred through interpretation of single-hole hydraulic tests using methods that assume fractional flow dimensions (i.e., dimensions of groundwater flow between linear, radial, and three-dimensions)
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From page 77... ...
. With the advent of numerical algorithms that solve groundwater flow equations in fractured rock systems (e.g., using discrete fracture networks or heterogeneous continua with spatially variable hydraulic properties)
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From page 78... ...
Durable technologies that allow transmission of data from depth need consideration. The concepts below are of particular importance: • Larger and more complex stress states can strongly influence conductive fracture pathways at great depths.
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From page 79... ...
, and the influence of natural groundwater sources or sinks that mask the perturbation. Explicit characterization of groundwater flow paths through hydraulic testing is dependent on the spatial distribution of monitoring wells.
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From page 80... ...
. Hydraulic Tomography Methods to interpret spatially distributed hydraulic properties from multiple hydraulic perturbations -- referred to as hydraulic tomography -- have been successfully applied in unconsolidated porous media.
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From page 81... ...
Determining the hydraulic significance of less transmissive fracture networks and the rock matrix requires hydraulic monitoring in fractures over a wider transmissivity range and longer duration than currently possible. Groundwater flow likely occurs over a much wider range of transmissivities than hydraulic testing can identify.
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From page 82... ...
. Tritium has been used with great success to define the depth of active groundwater flow in fractured clayey tills (Ruland et al., 1991)
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From page 83... ...
With two or more logs collected during heating or cooling, an estimate of thermal conductivity is obtained. In the absence of groundwater flow in or around the borehole, variations in the thermal conductivity of the rock are due largely to variable water content, and the ALS log provides a reasonable surrogate for a neutron porosity log (Pehme et al., 2007)
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From page 84... ...
Further development and testing will be required to determine whether these methods have significant value for testing fractured rock systems and to transition from ex situ, core-based NMR to downhole NMR approaches. Electrical and Electromagnetic Imaging at the Tens of Meters Scale Electrical and electromagnetic geophysical methods respond to contrasts in electrical properties across interfaces and have been used to image fractured environments in numerous studies.
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From page 85... ...
, and seismic and electrical data to map fracture locations and flow paths (Heincke et al.,
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From page 86... ...
Although less-permeable fractures may not contribute significantly to the volume of groundwater flow, contaminants will migrate from them to more permeable groundwater flow paths as a result of diffusion and slow advection. Similarly, dissolved contaminants diffuse between water in fractures and the rock matrix.
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From page 87... ...
It may be decided that rock matrix characterization of this sort is necessary for a site only if there is compelling reason to understand the partitioning between pore water and solid phases. If such a reason exists, then the porosity and organic carbon content of core samples need to be analyzed to differentiate the organic compounds in aqueous and sorbed phases and to develop reasonable rate estimates of sorption and desorption (Sterling, 1999; Sterling et al., 2005; Kennel, 2008)
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From page 88... ...
. In addition, because many fractured rock aquifers have small fracture porosities, extended pumping to remove standing water in the borehole may draw water from a large volume of the rock through interconnected fractures and therefore the resultant geochemistry may not be representative of ambient conditions in the fractures intersecting the borehole.
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From page 89... ...
active fracture orientation; (3) direction of groundwater flow in each fracture plane using tracers; (4)
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From page 90... ...
Those processes are affected by physical properties of the void space that control groundwater flow, as well as chemical processes such as diffusion and sorption that affect the concentration, spatial distribution, and magnitude of chemical fluxes associated with contaminants in the groundwater flow regime. Characterizing the processes is important in evaluating the fate and transport of groundwater contaminants.
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From page 91... ...
BIOLOGICAL CHARACTERIZATION OF FRACTURED ROCK Microorganisms inhabit all niches of the subsurface environment, including fractured rocks. The activities of subsurface microbial communities can exert significant effects on both physical and geochemical characteristics and may be responsible for a variety of dynamic processes including mineral formation and dissolution, as well as changes in redox chemistry, fluid surface tension, and pH.
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From page 92... ...
that target either universal bacterial or archaeal DNA, or that focus on one specific species, strain, or even functional gene. The rapid expansion in the past two decades of molecular techniques available to study microbial communities without the need for laboratory culturing has greatly expanded understanding of subsurface microbial communities and the importance they have influencing flow, fate, and transport within fractured rock.
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From page 93... ...
(2013) employed molecular techniques combined with a packer sampling method as a mechanism to evaluate indigenous microbial communities in water drawn from geochemically distinct bedrock fractures in a deep aquifer in Eastern Finland, demonstrating that community structure and geochemistry are strongly linked.
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