Accounting for important fractured rock features and processes is necessary and possible for characterization, modeling, monitoring, and remediation. Tools to analyze and characterize fractured rock have developed rapidly in the past 20 years and can provide a basis to solve or avoid problems related to contaminants in fractured rock near surface and at depth. Discrete groundwater flow pathways, multiple porosities, and the hydraulic and biogeochemical communication between different characteristic porosities can often be accounted for at reasonable levels of uncertainty and reliability if the right questions about them are asked. Ineffective engineering approaches may then be avoided, and realistic solutions can be designed within technical, logistical, and economic constraints.
This chapter synthesizes the recommendations found throughout this report and identifies particularly promising areas for research and development related to fractured rock systems. These recommendations are all applicable in some degree to practicing engineers, regulators, infrastructure owner and operators, and researchers. They are intended to improve engineering practice given today’s tools and knowledge, guide a framework for research that can inform future practice, and inform the decision maker who will benefit from a more realistic understanding of current capacities and limitations.
An integrated approach to engineering in the fractured rock environment is essential, but all too often engineering decisions are made by engineers and infrastructure managers without an integrated perspective. Integrating geologic, geophysical, hydrodynamic, and geomechanical information is essential to understanding fractured rock sites. Geologic features (e.g., fractures, discrete beds, sealing faults, rock matrix) that dominate fluid flow, transport pathways, and storage need to be understood to characterize fractured rock sites correctly and predict fluid and contaminant fate and transport.
Flow and transport are influenced by issues of scale for both fractures (μm to km) and rock matrix porosities (μm to cm) that, in turn, affect advective, diffusive, chemical, biological, capillary, and filtering processes over different timescales. Changes in flow and transport behavior can result from the combined effects of these processes (e.g., dissolution and precipitation can change preferential flow pathways) and therefore need to be considered from the earliest stages of analysis. Because a few fractures are likely to control the majority of groundwater flow, minor perturbations in fracture surfaces and connectivity resulting from geologic and biogeochemical processes are important in contaminant transport and distribution. This does not imply that large and complex geologic and biogeochemical analyses are always necessary—but does require that such issues be recognized and evaluated at appropriate qualitative or quantitative levels.
Recommendation 1. Take an interdisciplinary approach to engineering in fractured rock and use site geologic, geophysical, geomechanical, hydrologic, and biogeochemical information to conceptualize transport pathways, storage porosities, fate-and-transport mechanisms, and the coupled processes that control rock fracture-matrix interactions.
To facilitate this recommendation, engineers who are characterizing, modeling, monitoring, or managing engineering project sites need to
- Delineate at the onset the processes that can affect contaminant transport, storage, and transfer between the rock matrix and fractures, particularly over the life of a remediation project and follow-on monitoring. Determine the importance and potential for hydrologic, thermal, biological, chemical, and mechanical processes—and the coupling between those processes and their resulting feedback effects—in both fracture and matrix porosities.
- Use available geologic, hydrogeologic, and geophysical information to conceptualize flow pathways, fluid and contaminant storage, alteration or attenuation through geochemical reactions, and degradation via biological processes.
- Conceptualize fractured rock hydrogeology by first assuming dual porosity/single permeability discrete fracture network (DFN) (i.e., primary storage of contaminant in the rock matrix and primary flow of fluid in the fracture network), and then modify to simpler (single effective porosity and permeability equivalent porous media) or more complex (dual porosity/dual permeability hybrid DFN) assumptions when justified by site-specific evidence.
- Recognize the limitations of advection-dispersion-diffusion approaches to solute transport solutions, and consider which approach is most appropriate. An advection-dispersion model may be good at fitting transport measurements at a specific scale, but the anticipated groundwater velocity variability in fractured rock aquifers is inconsistent with the underpinnings of the advection-dispersion model. Therefore, the model parameters obtained in that fitting process often are not appropriate for use at other scales. Consider, for example, the potential for gravity-driven flow, heterogeneous fingering, and rock matrix imbibition processes for multiphase flow.
- Recognize the frequently dominant role of discrete fractures in flow and transport. Distinguish hydraulically active fractures from those that provide only fluid storage through hydraulic testing, passive monitoring of pressures and chemistry, and geophysical and geomechanical measurements. Design monitoring systems to refine site conceptual models, account for discrete flow and transport, and aid engineering decision making. Do not rely solely on integrated measures across large sections such as long, open boreholes. Recognize how fractures and rock matrix properties and processes change with depth, stress, and geochemical and geobiological conditions.
Understanding the interactions between rock matrix and rock fractures is critical to understanding contaminant fate and transport. The potential for storage of contaminants in the rock matrix, and the slow diffusive transfer from the matrix to flowing fluid in adjacent fractures, can result in remediation time frames of decades or centuries. It is important to understand such subsurface interactions at the onset of infrastructure development to inform appropriate mitigation measures. Engineers responsible for site selection, characterization, project permitting and licensing, project
construction, and infrastructure management need to be aware of the potential of contaminant release and post-release behavior—not only during the early stages of project development, but also for the lifecycle of the infrastructure. Failure to understand storage and release mechanisms can lead to gross miscalculation of subsurface contaminant distribution and effectiveness of remedial measures (e.g., pump and treat).
Recommendation 2. Estimate the potential for contaminant to be transported into, stored in, and transported back out of rock matrix over time.
- Recognize that fractures and fracture networks can be discrete flow pathways or flow barriers or provide only storage porosity. Develop and implement strategies to characterize mobile porosity geometries (e.g., structurally controlled flow pathways such as those defined by faults and karst features) and immobile porosities (e.g., low-permeability fractures, fractures not connected to other fractures, and rock matrix). Recognize uncertainties in geometry of heterogeneities, pathways, and barriers. Conceptualize boundary conditions, sources, sinks, and disturbed rock zones.
- Determine how to quantify and differentiate contaminant in mobile porosities from that in immobile porosities (generally rock matrix porosities) at various locations (e.g., source zones, impact zones, upgradient/downgradient zones).
- Evaluate the contaminant stored in and transferred between rock fractures and matrices, recognizing the various processes that might occur (e.g., advection, diffusion, sorption, biodegradation, filtration, and capillary processes). Consider the effect of fracture and rock matrix void sizes on capillary and wetting processes, bioactivity, and multiphase flow.
- Develop methods to better delineate the distribution of non-aqueous phase liquids within mobile porosity. Apply advances of the past 20 years (e.g., fracture image logging, borehole flow logging, partitioning tracers, and wireline geochemical monitoring) to characterize conductive discrete features. As appropriate, develop and confirm hypotheses concerning transport pathways and geometries using geologic, geomechanical, and geophysical techniques.
Chemical, biological, thermal, mechanical, and hydraulic processes can individually and in combination change how fluids containing contaminants, migrating fines, and colloids interact with the rock matrix and fracture infillings and coatings over time. The importance of coupled processes in fate and transport is not always recognized or addressed by engineers, infrastructure managers, and regulators. Furthermore, the timescales of changes associated with some processes may be longer or shorter than the life of the engineering project.
Recommendation 3. Characterize chemical, biological, thermal, mechanical, and hydraulic processes, their interactions, and the conditions that can lead to their coupling to better understand transport through discrete fractures and contaminant transfer between fractures and rock matrix.
- Consider the possibility of flow localization within fractures, channels within fracture planes and intersections, the role of hydro-mechanical coupling on changes in fracture and
- channel geometry, and the effect of these changes on advection, dispersion, and diffusion in the interpretation and prediction of transport rates.
- Evaluate the potential role of mineral fines to promote contaminant transport or clogging.
- Evaluate fluid properties (e.g., miscibility and mutual solubility, density, viscosity, and acidity) and reactivity with rock or infilling material. Assess timescales of fluid-mineral reactions to determine their impact in terms of the life of the engineered project.
- Recognize that immiscible fluids behave differently from water and understand the pore size-dependent capillarity reductions in permeability that result from phase interference.
- Consider the role of bioactivity—and limiting environmental factors (e.g., pore size and availability of nutrients)—on dissolution, precipitation, biofilm formation, and contaminant transformation.
- If thermal changes are anticipated, then assess subsurface heat conduction and temperature changes under static- and advective-flow conditions in consideration of flow localization.
- Consider differences in fracture and rock matrix properties, and mechanical, chemical, and biological conditions with depth, particularly with changes in stress, temperature, and groundwater chemistry.
Current remediation technologies are limited in their ability to reduce contaminant concentrations in fractured rock to desired levels within short time frames. Because of the potential longevity of contaminants retained within the rock matrix, time frames associated with reaching water quality standards may range from decades to millennia. Biological processes are known to be important in certain situations, but the magnitude of that importance in contaminant fate, transport, and remediation is not quantifiable in a general way. Their importance can be quantified for a specific situation given enough information. Characterization of biological communities and activities in fractured rock will allow for better characterization and predication of fluid and contaminant fate and transport. It is likely that biological processes will be relied on to reduce and control the further spreading of in situ contamination at most contaminated rock sites because no other alternative is likely to prove practical in many situations. The single biggest research need is, therefore, to understand better biological processes at fractured rock sites, and especially rock at great depth where there is a dearth of information about biological processes.
Improvements of molecular tools in the past two decades have advanced knowledge of biological processes. Genome sequencing at the individual cell level, for example, is now possible. However, subsurface microbial characterization is still hampered by difficulties studying in situ biological systems that allow the processes and rates of degradation to be understood. Reliable, reproducible, quantitative, and statistically valid experimental information on the spatial and temporal dynamics of biological communities is needed so that valid, quantitative assessments can be made of the biodegradation of contaminants in fractured rock. Application of advanced molecular tools (e.g., advanced sampling techniques and high-throughput metagenomics/metatranscriptomics technologies) coupled with computational modeling should make it possible to systematically address fundamental microbial questions not possible previously because of difficulties sampling and cultivating microorganisms from fractured rock environments.
Recommendation 4. Expand research to define and quantify microbial influences on fluid and contaminant fate and transport in fractured rock over timescales relevant to contaminant remediation processes.
Specific research topics could include
- The extent of phylogenetic, genetic, and functional diversity of microbial communities in fractured rock environments;
- The influence of microbial populations on hydraulic and hydrogeochemical characteristics of fractured rock environments;
- Changes of phylogenetic and functional structures in microbial communities, and associated relationships with hydrogeochemistry and the anoxic, high-salinity, high-temperature, and high-stress conditions of the deep environment across various spatial and temporal scales; and
- Microbial community response to environmental perturbations within fractured rock environments.
Site characterization methods need improvement. At present, site characterization includes application of tools and techniques for geologic, geophysical, geochemical, hydraulic, biological, and geomechanical observation, testing, and sampling. However, most of these methods are limited in terms of their ability to map fracture characteristics at appropriate scales. Borehole sampling and testing tend to be expensive and provide limited information on a relatively small scale, but surface-based methods such as seismic may not have appropriate resolution to map fractures directly. Cross-hole tracer testing is frequently needed to understand large-scale issues such as fracture connectivity and flow dimension, but it does require boreholes—often a limiting factor. Engineers need to better understand the benefits and limitations of characterization data from various sources, and project managers and regulators need to understand what constitutes an honest appraisal of the data. All need to remain aware of innovations in site characterization technologies and be willing to incorporate those developments into practice, particularly borehole- and surface-based technologies from the petroleum and mining industries for characterizing rock at depth.
Supporting text for the recommendation below includes contributions that the research community could make that could be supported by the industry and regulatory communities.
Recommendation 5. Improve characterization and monitoring through new and expanded research in surface- and borehole-based geologic, geophysical, geochemical, hydraulic, biologic, and geomechanical technologies.
Improve characterization and monitoring by advancing, for example
- Research on technologies such as geophysical tools, innovative tracer tests, and flexible liners capable of measuring contaminants in situ to identify, characterize, and monitor the volumes, spatial distribution, and transfer of contaminants between rock matrix and fractures;
- Joint inversion methodologies for fracture and process parameterization;
- Seismic, microseismic, and hydraulic tomography, hydraulic interference, and tracer testing techniques that allow for characterizing flow paths at a range of scales, depths, and in situ conditions;
- Fracture mapping techniques (e.g., microseismic, thermal, electrical resistivity, and geochemical) to identify conductivity and flow in fractures, calculate in situ gradients, and determine flow directions at a range of scales, depths, and in situ conditions;
- Techniques for tracking tracers or certain contaminants, including, for example, microseismic, electrical, nuclear magnetic resonance, and radar geophysical techniques; and
- Development of high-throughput molecular techniques for understanding fractured rock biology and bioremediation possibilities.
Current numerical model applications used by practicing engineers and researchers often do not account for discrete fracture flow pathways and fracture-matrix interactions. Budget, data, personnel, and time frequently constrain the choice of modeling methods, and practice tends to focus on building and calibrating a single porosity, equivalent continuum numerical model of flow and transport. Although it is frequently possible to calibrate such models to measurements, their predictive powers can be limited because they do not incorporate discrete features and fracture-matrix interactions. They also inform in a limited way the conceptual and parametric sensitivities and uncertainties associated with model assumptions. A variety of powerful preliminary scoping calculation methods can be used by practicing engineers and researchers to quantify the importance of specific processes and assumptions. Regulators need to be aware of the appropriate use of conceptual and numerical modeling methods to inform their decisions about the suitability of site engineering design and operations.
Recommendation 6. Develop appropriate hydrostructural conceptual models for fracture and rock matrix geometries and properties, and perform preliminary calculations (e.g., analytic or simple numerical) to better inform and allocate resources for site characterization, modeling, and remediation.
- Follow a systematic and well-documented hydrostructural conceptual model development approach so that underlying assumptions and simplifications are well understood, especially in the absence of supporting data. An appropriate hydrostructural model includes definitive fracture and matrix porosities, spatial and temporal uncertainties, and variabilities for a site.
- Begin with generic geologic and geomechanical conceptual models and parameterizations informed but not limited by site-specific direct measurements.
- Develop a broad, semi-quantitative understanding of important processes and parameters, refined with simplified scoping calculations that define and assess
- alternative multi-porosity fracture flow and transport conceptual models;
- model uncertainties;
- the appropriateness of simplified versus more complex modeling approaches; and
- how proposed site characterization, analysis, and modeling activities may reduce uncertainties and improve engineering decisions.
- Quantitatively integrate field data into models through model analysis tools such as parameter estimation, sensitivity analysis, and uncertainty quantification.
If numerical models do not represent adequately the hydrostructural conceptual models developed for a site, as is common in current practice, then they will inadequately predict site response
to in situ conditions and remedial actions, including contaminant transport and retention. For near-surface applications, and those at depths, it is essential to incorporate appropriate hydrostructural features (e.g., discrete fracture pathways and matrix storage) into numerical models—even in a simplified manner.
As described earlier, single porosity continuum-based approaches inadequately represent the effects of both fractures and rock matrix porosities on contaminant pathways and storage. Discrete fracture models that properly simulate flow, transport, and coupled processes can be difficult and complex to develop, but they may be required for adequate projections of contaminant fate and transport.
Recommendation 7. Base numerical models on an appropriate hydrostructural model, ensuring that simplification and upscaling of the hydrostructural model maintain those features, properties, and processes that dominate contaminant transport, fate, and storage. Evaluate impacts of uncertainties introduced by simplification and upscaling.
During analysis and modeling
- Understand the limitations and advantages of numerical models and analysis tools to represent the hydrostructural model and fractured rock environment. Choose tools that synthesize data, confirm or refute the validity of the conceptual model, and quantify the range and uncertainties of expected system behaviors.
- Consider the scale of the problem of interest relative to the scales of the rock fractures and matrices. Apply equivalent continuum models only when the problem scale is much greater than the scale of the fractured rock mass. Equivalent continuum and fracture network analysis upscaling techniques can produce anisotropic and heterogeneous continua, but not necessarily correct discrete pathways.
- The implications of the use of simplified and effective medium modeling approaches;
- The need for risk- and uncertainty-based modeling approaches, even where calibrated and conditioned models match available measurements;
- Approaches capable of addressing multiple porosities, discrete pathways, spatial heterogeneity, and decision making under uncertainty;
- Non-deterministic modeling techniques applicable when there are no current means to identify groundwater paths and response functions; and
- Modeling at multiple scales, for example near well and over the full pathway from depth to surface of boreholes.
Expectations related to site remediation often are unrealistic. The long-term goal of remediation at most hazardous waste sites in the United States is to meet drinking water requirements associated with Maximum Contaminant Levels (MCLs) at monitoring points or at actual or potential receptors. However, meeting MCLs at fractured rock sites can take decades or centuries. Because long time frames for remediation are becoming more acceptable to U.S. regulatory bodies, alternate approaches to remediation, particularly at fractured rock sites, are becoming more accepted.
Much information regarding innovative contaminated site remediation is available through, for example, an Environmental Protection Agency–sponsored website.1 Although information about fractured rock site remediation has been published, it is less widely available. Lessons learned from remediation and monitoring activities in the fractured rock setting are not well documented. As a result, there is risk of duplicated effort and wasted resources in practice. A significant impediment to the remediation of fractured rock settings is the lack of common framework, understanding, or expectations regarding objectives, assessment, and realistic remediation endpoints.
Recommendation 8. Develop and communicate realistic expectations related to remediation effectiveness through realistic goal setting and through explicit consideration of uncertainties in design, realistic use of natural attenuation, comprehensive monitoring programs, and dissemination of performance data to the technical community.
Practitioners and regulators should
- Assume contaminant must be remediated in rock matrix as well as in rock fractures.
- Incorporate appropriate estimates of plume longevity, based on sound characterization, into remedial action plans.
- Develop and embrace realistic regulatory frameworks for setting remedial objectives and formalizing the transition from active remediation to long-term monitoring.
- Design monitoring programs based on sound characterization and realistic remedial objectives, ensuring they are dynamic and informed by the remediation process itself.
- Include natural attenuation in all remediation designs, ensuring designs are based on a sound conceptual site model and realistic performance expectations (e.g., over time frames that may range from decades to centuries).
- Produce detailed, publicly accessible, research-level documentation regarding the application of different remediation technologies in a variety of fractured rock settings. This should be practitioner driven and government facilitated. Analysis of long-term performance of alternate contaminant remediation and transport control would benefit practice.
- Increase the degree and efficiency of monitoring at fractured rock sites, particularly during and following active remediation. Incorporate resulting data into feasibility studies at other sites.
- Develop better information communication and transfer mechanisms within and between agencies responsible for addressing fractured rock issues. Shorter-term activities requiring fewer resources include developing sets of common attributes to incorporate into individual agency databases that allow for easier data mining. Longer-term activities requiring greater resources include creating a centralized data bank of fractured rock-related data and information.
Remediation in some fractured rock settings can be a decades-to-centuries process. Because monitoring over the duration of remedial activity is necessary, monitoring systems need to be not only robust and cost-effective, but also replaceable. Furthermore, advances in sensor technologies and reduction in their costs have created opportunities to increase the frequency and volume of data collection. This, in turn, creates opportunities for increased data feedback and refinement of site models and remediation plans. Thoughtful data analysis can suggest additional data collection
needs and drive field observations through a quantitative decision-analysis process. Large data volumes, however, can complicate data management and informative feedback.
Recommendation 9. Incorporate long-term performance into monitoring system design.
Engineering practitioners and regulators should expect monitoring systems to accommodate long-term performance and be
- Durable and able to accommodate expected operation and maintenance;
- Inclusive of meaningful sampling frequencies to monitor trends;
- Designed to accommodate feedback so that monitoring strategies can be appropriately refined in response to new trends or findings;
- Designed to accommodate long-term variations in climate, water levels, temperatures, and other site conditions expected over the remediation period;
- Designed to require the minimal amounts of analytes to quantify performance effectiveness;
- Designed to be cognizant of the implications of discrete pathways, rock matrix contaminant storage, and issues of geologic heterogeneity and anisotropy when point source concentration measurements are used; and
- Capable of data storage and management such that data can be accessed in meaningful ways.
Research on automation of data collection, archiving, and retrieval and on the triggering of alarms when data values surpass tolerance levels is recommended. Research is also recommended on alternative monitoring approaches, in particular those for remote monitoring of field-assessable parameters that trigger additional sampling and analysis when needed. Best-practice protocols need to be established and communicated to future practitioners responsible for monitoring systems that may have been put in place decades or centuries earlier.
Adaptive and observational approaches to characterization of fractured rock sites are more effective than prescriptive, linear approaches. Site and contaminant characterization, remediation, and monitoring are most effective when data feedback allows for informed modifications in approach. Adaptive or observational methods recognize the value of information gathered once an engineering process is under way and formally integrate this information into engineering decision making. Guidelines for and examples of use of observational and adaptive approaches at fractured rock sites would facilitate greater use of these methods in practice. Specific steps in the process are available in the literature and discussed in Chapter 7.
Recommendation 10. Use observational methods and adaptive approaches to inform engineering decisions made for fractured rock sites.
Regular adjustments to monitoring plans and to fractured rock site engineering more generally, including modifications to performance criteria and data processing approaches, should be made as appropriate in response to new information. Engineering decisions should be informed by
- Risk-based and performance-based criteria, as appropriate, in prioritizing the components of contaminated site management;
- Monitoring and remediation approaches and criteria that test model assumptions concerning transport pathways and chemical and biological remediation processes; and
- Systems analysis approaches to integrate the value of information and support decisions made under uncertainty.
Better engineering, better use of resources, and improved outcomes will result if the use of oversimplified site conceptual models are avoided and realistic expectations regarding outcomes are adopted. The recommendations provided in this report are high level and intended to help the practitioner, researcher, and decision maker embrace a more interdisciplinary approach to engineering in the fractured rock environment. Although the fractured rock environment can be complex, it is subject to predictable laws of nature. This report describes how existing tools can be used to increase the accuracy and reliability of engineering design and management given those interacting forces of nature. With interdisciplinary and adaptive approaches to fractured rock site characterization and management, it is possible to conceptualize and model the fractured rock environment with acceptable levels of uncertainty and reliability and to design systems to maximize remediation potential and monitor long-term performance. Advances in technology and science need to be incorporated into engineering practice as they develop.