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To construct an initial conceptual site model, existing data related to the subsurface should be examined. These data would include determining the type of bedrock that is present, log data from existing wells in the area, and any aerial photographs available. If the rock is covered by overburden, then the aerial photographs may provide evidence of lineaments. For heavily vegetated sites, an overflight using Lidar might be useful. The Lidar can differentiate between ground and vegetative cover reflections, thus enabling an evaluation for lineaments (Doe 2010a).
Additional information on Lidar

An initial site visit will allow for the ground truthing of the lineaments as well as the mapping of any fractures at bedrock outcrops. Note that fractures identified with lineaments may or may not be capable of transmitting water.

Jump to a Subsection Geophysical Characterization Methods | Other Downhole Methods | Coring | Groundwater Flow | Construction of Monitoring Wells | Sampling and Analysis | Other Resources

Geophysical Characterization Methods

Surface geophysical surveys that depend upon the electrical or acoustic characteristics of the rock also are a consideration that can be helpful in planning a drilling program. These surveys are generally good for identifying top-of-rock contours and large fracture zones or faults (> 10 meters Doe 2010). They usually lack the resolution to identify small fractures. The type and thickness of the overburden also will affect the resolution of electromagnetic instruments. Good discussions of geophysical survey techniques are given in Appendix B of Doe 2010b and Chapter 4 of NRC 1996.

A recently developed tool (AquaTrack™) can locate many preferential flow paths of groundwater in bedrock. It can be used in the saturated and unsaturated zone. Electrodes are placed in boreholes that can be from 500 to thousands of feet apart, and a current is induced using an alternating current source. Because water is a good conductor, the current flows through it, producing a primary magnetic field. This field is much stronger than most geophysical instruments that rely on secondary fields or signal decay. The strength of the field is related to the amount of water flow (effective porosity); hence, areas with low flow show much weaker signals. The instrument measures the strength of the field at the ground surface where measurements on a pre-selected grid are taken. Anthropogenic structures such as electrical lines will cause interference. The instrument does not provide flow direction or quantitative flow rate. For further information see Black 2006 and Kofoed 2008.

Most geophysical surface instruments have an equivalent borehole design that allows measurements to be made downhole. These are useful for identifying different rock matrices, fracture zones, flow pathways, and avoiding overburden issues. Downhole geophysical surveys using a single borehole or borehole to borehole techniques are generally very useful in refining the site conceptual model and deciding whether additional monitoring wells are needed and where they should be placed (Lane et al. 1996). A discussion of borehole techniques can be found in NRC 1996 (beginning at page 186); Johnson (2002), Singha (2000); Williams (1998), Williams (2010), and Appendix B of Doe (2010b).

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Other Downhole Methods

Calipers are a wireline instrument that might be useful for bedrock characterization to identify enlarged zones around fractures resulting from erosion of weak rock following drilling.

Three hundred sixty-degree images of the borehole wall can be obtained with either an optical televiewer or an acoustical televiewer. The optical televiewer provides digital pictures of the wall that can be displayed as a roll or as a core that can be rotated. This viewer will not work in turbid water. The acoustic televiewer digitizes the return of sound waves (travel time and amplitude) that bounce off the borehole wall. It will work in turbid water but not in air. Doe (2010a & b) regards the optical televiewer as more accurate than the acoustical televiewer because the acoustical instrument "is more likely to miss some fractures and produce some 'false' fractures than core or optical tools. Because the image depends on elastic properties of the rock, heterogeneous zones of high or low energy absorption may appear as fracture-like features, though they may be part of the solid rock." Televiewers also provide fracture orientation information, which can be difficult and expensive to obtain from coring.

An important consideration in fractured rock characterization drilling is that fracture systems encountered vertically in a borehole may not be connected. If one of the fracture systems is contaminated, then leaving the borehole open for any length of time could result in contaminating parts of the aquifer that were clean. Once downhole logging has been accomplished, consideration should be given to stopping flow in the boreholes with known contamination through use of a device such as a blank temporary borehole liner. Some liners can also provide evidence of the presence of non-aqueous phase liquids if they are contaminants of concern at the site. The liner can be kept in place until a long-term monitoring system is installed. Note also that some liners may leach chemicals of concern into the groundwater. Before using a liner, consult the vendor about potential leaching issues.

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Coring

Since coring is a relatively expensive part of a bedrock drilling program, the decision to take cores and how many is site specific. Many of the functions of coring have been replaced by borehole imaging and other tools. Doe (2010b) provides a table comparing coring to optical and acoustical imaging. Coring does have some advantages over imaging especially if contaminant back diffusion is a concern. Research by USGS (2010) has shown that TCE concentrations in fractured shale bedrock core matrices can differ by orders of magnitude over very short distances. Sampling and analysis of these cores is important in developing a remedial strategy from the characterization data.

If multiple boreholes are expected at a site, core drilling might be considered for one of the initial boreholes to determine the value of the core, and provide a set of samples for later calibration of logging results if coring is not used further (Doe 2010b). Generally, in bedrock drilling, the decision of where to place the next borehole and whether to core it is made after a careful examination of available hydraulic and geophysical data from previous boreholes. Goode (2010) terms this process "Iterative synthesis of multiple investigations or Continuous Characterization."

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Groundwater Flow

Groundwater flow in bedrock is investigated using static methods in individual boreholes, pumping within a borehole to determine transmissivity, and cross-borehole pumping to determine connectivity. Tracers, combined with borehole geophysics can also be used to evaluate connectivity and transmissivity (Lane 1996). A detailed discussion of pumping techniques is found in Chapter 5 of NRC (1996). The USGS maintains several websites on work they have and are doing to characterize groundwater flow in fractured rock USGS (2008) and USGS (2010).

Water in different fracture zones can have different temperatures, which can be used to identify water entry points into the borehole. Pehme et al. (2007) make the case that temperature readings made in an open borehole can be misleading due to vertical flow; to obtain accurate readings, only the temperature around the contributing fracture should measured. This measurement can be accomplished using a blank temporary borehole liner. For each fracture zone, water temperature inside the liner will reflect the temperature of the water in contact with the liner at the fracture.

Shallow karst terranes present a somewhat different set of groundwater flow issues in that they generally are subject to precipitation events that can increase their flow volume dramatically. With conduit flow, the conductivity can be orders of magnitude larger than other types of fractured rock. Additionally, they typically discharge to springs. Tracer tests using dyes are commonly used to map the flowpaths by placing the dye in sinking streams and sinkholes and then monitoring the springs to see where the dye appears. See Karst Resources for references on investigating and sampling contaminated karst aquifers.

Vertical flowmeters (spinner, heat pulse, electromagnetic) can be used to determine vertical flow and rate in the borehole. They should be used under both ambient and pumping conditions. Together the two provide fracture inflow/outflow information and fracture transmissivity (Doe 2010b).

Horizontal flowmeters deployed with double packers can provide both direction and rate of flow for a given fracture zone.

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Construction of Monitoring Wells

Unless the rock has a relatively high primary porosity, flow generally will occur in a few major fracture zones. While more expensive, a better understanding of the flow system and contaminant distribution can be obtained when these fracture zones are monitored separately. A monitoring system can be constructed in two ways: use separate wells to monitor each fracture zone or use a multilevel well deployed in a single borehole with ports set at the depth of each fracture zone. A short discussion of three multilevel systems is found in CL:AIRE (2002).

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Sampling and Analysis

Sampling and analysis techniques and procedures for bedrock monitoring wells are the same as those for similarly constructed monitoring wells in unconsolidated soils. The exception is for shallower karst terranes at the facility level where primary recharge is from sinking streams, losing streams, and sinkholes. Since the water in the solution channels is flowing at a much greater rate than is typically found in other formation types, the water quality can change relatively rapidly, and contaminants found in it are not necessarily site related. A sampling plan for karst areas will often include sampling at springs using tracer dyes as well as monitoring wells. See Karst Resources for references on investigating and sampling contaminated karst aquifers.

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Other Resources

Analysis of Cross-Hole Tests in Fractured Systems
Roberts, R.M. and D.O. Bowman II.
Fractured Rock Conference: State of the Science and Measuring Success in Remediation, September 24-26, 2007, Portland, Maine. 16-30(2007)

Analysis of Selected Geophysical Logs at North Penn Area 6 Superfund Site, Lansdale, Montgomery County, Pennsylvania
Conger, R.W. and D.J. Low.
Fractured Rock Conference: State of the Science and Measuring Success in Remediation, September 13-15, 2004, Portland, Maine. 492-505(2004)

Borehole Geophysical Investigation of a Formerly Used Defense Site, Machiasport, Maine, 2003-2006
Johnson, C.D., R.A. Mondazzi, and P.K. Joesten.
U.S. Geological Survey Scientific Investigations Report 2009-5120, 333 pp, 2011

This report assesses the effectiveness and success in combining various downhole geophysical and hydraulic techniques to characterize highly fractured bedrock aquifers that have been contaminated with chlorinated solvents. In addition, each geophysical method is evaluated for effectiveness for potential application for further aquifer characterization and/or evaluation of remediation efforts.

Characterization of Crystalline Bedrock Contaminated by Dense Nonaqueous Liquid (Abstract)
Cho, H.J., R. Fiocco, and M. Daly.
Ground Water Remediation and Monitoring 28(2):49-59(2008)

Characterizing a VOC Plume Migrating From Fractured Shale into a Karst Limestone Aquifer
Landry, P.G., B.L. Hoke, and P.R. Stone III.
Fractured Rock Conference: State of the Science and Measuring Success in Remediation, September 24-26, 2007, Portland, Maine. 420-432(2007)

Characterizing Hydraulic Properties and Ground-Water Chemistry in Fractured-Rock Aquifers: A User's Manual for the Multifunction Bedrock-Aquifer Transportable Testing Tool (BAT3)
Shapiro, A.M.
U.S. Geological Survey Open-File Report 2007-1134, 136 pp, 2007

Conceptual Flow Model of Hydrocarbon Impacted Ground Water in an Undifferentiated Gneiss
Zuidema, S. and J.R. Hale.
Fractured Rock Conference: State of the Science and Measuring Success in Remediation, September 24-26, 2007, Portland, Maine. 372-384(2007)

Detailed CVOC Source Area Investigation in the Context of a Fractured Bedrock Conceptual Site Model
Vernon, J.H., P.C. Shattuck, M.D. Kauffman, D.M. Clemens, R.A. Leitch, and D.M. Maynard.
Proceedings of the Annual International Conference on Soils, Sediments, Water and Energy 12(36) 11 pp(2007)

Detailed Pumping Test to Characterize a Fractured Bedrock Aquifer
Gernand, J.D. and J.P. Heidtman.
Ground Water 35(4)632-637(1997)
Abstract

Developing Remedial Strategies in a Mixed Porous Medium/Fractured Rock System: Lemberger Site, Whitelaw, Wisconsin
Wedekind, J.E., K.R. Bradbury, P.M. Chase, M.B. Gotkowitz, E. Gredell, K.D. Krause, and J.M. Rice.
Fractured Rock Conference: State of the Science and Measuring Success in Remediation, September 24-26, 2007, Portland, Maine. 389-402(2007)

Factors Affecting Specific-Capacity Tests and Their Application: A Study of Six Low-Yielding Wells in Fractured-Bedrock Aquifers in Pennsylvania
Risser, D.W.
U.S. Geological Survey Scientific Investigations Report 2010-5212, 44 pp, 2010

This report assesses the affects of factors such as pumping rate, duration of pumping, aquifer properties, wellbore storage, and turbulent flow on the application of specific-capacity well tests.

Field-Scale Effective Matrix Diffusion Coefficient for Fractured Rock: Results from Literature Survey
Zhou, Q., H.-H. Liu, F.J. Molz, Y. Zhang, and G.S. Bodvarsson.
Journal of Contaminant Hydrology 93:161-187(2007)

Fractured Bedrock Aquifer Hydrogeologic Characterization for a Bioaugmentation Pilot Study
Jeffers, P. and V. Wittig.
Fractured Rock Conference: State of the Science and Measuring Success in Remediation, September 13-15, 2004, Portland, Maine. 148-157(2004)

Geophysical Characterization of Fractured Rock Aquifers: Accounting for Scale Effects and Putting Hydrology in the Geophysics
Paillet, F.L.
Fractured Rock Conference: State of the Science and Measuring Success in Remediation, September 13-15, 2004, Portland Maine. 14-26(2004)

Handcart Gulch: Integrated Headwaters Research on Hydrogeologic and Geochemical Processes and Monitoring of Environmental Change
U.S. Geological Survey Web page, 2010

Hydraulic Characterization of a Fractured Bedrock Aquifer
Murray, W.A. and D.R. Farnsworth.
Fractured Rock Conference: State of the Science and Measuring Success in Remediation, September 24-26, 2007, Portland, Maine. 50-65(2007)

An Illustrated Handbook of DNAPL Transport and Fate in the Subsurface
Environment Agency, United Kingdom.
R&D Publication 133, 67 pp, 2003

Investigating Contaminated Sites on Fractured Rock Using the DFN Approach
Parker, B.
Fractured Rock Conference: State of the Science and Measuring Success in Remediation, September 24-26, 2007, Portland, Maine. 150-168(2007)

Multi-Method Geophysical Approach for Characterizing a Deep Fractured Bedrock Aquifer, Anniston Army Depot, Anniston, Alabama
Murray, B.S. and M.B. Vest.
Fractured Rock Conference: State of the Science and Measuring Success in Remediation, September 13-15, 2004, Portland, Maine. 464-478(2004)

Multiple Well-Shutdown Tests and Site-Scale Flow Simulation in Fractured Rocks (Abstract)
Tiedeman, C.R., P.J. Lacombe, and D.J. Goode.
Ground Water 48(3):401-415(2010)

A New Depth-Discrete Multilevel Monitoring Approach for Fractured Rock
Cherry, J.A., B.L. Parker, and C. Keller.
Ground Water Monitoring & Remediation 27(2):57-70(2007)
Abstract

This article describes the FLUTe multilevel system.

Pumping Test Analysis in a Fractured Crystalline Bedrock
Cho, H.J., R.J. Fiacco, and M.H. Daly.
Fractured Rock Conference: State of the Science and Measuring Success in Remediation, September 13-15, 2004, Portland, Maine. 161-172(2004)

Site Characterization Technologies for DNAPL Investigations
EPA 542-R-04-017, 165 pp, 2004

Use of a Geophysical Toolbox to Characterize Ground-Water Flow in Fractured Rock
Haeni, F.P., John W. Lane, Jr., John H. Williams, and Carole D. Johnson
USGS, 5 pp.

Use of the In Situ, Inc. MP Troll 9000 to Locate Fractures Contributing to Ground Water Flow in Bedrock Wells
Sernoffsky, R., G. Robbins, and R. Mondazzi.
Fractured Rock Conference: State of the Science and Measuring Success in Remediation, September 13-15, 2004, Portland, Maine. 341-349(2004)

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Karst Resources

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