Project Summary

United StatesEnvironmental ProtectionAgency

National Risk ManagementResearch LaboratoryCincinnati, Ohio 45268

Research and Development

Hydrogeologic Characterizationof Fractured Rock Formations:A Guide for GroundwaterRemediators

Andrew J. B. Cohen, Kenzi Karasaki, Sally Benson, Gudmundur Bodvarsson,Barry Freifeld, Pascual Benito, Paul Cook, John Clyde, Kenneth Grossenbacher,John Peterson, Ray Solbau, Bhaskar Thapa, Don Vasco, and Peter Zawislanski

Abstract

A field site was developed in thefoothills of the Sierra Nevada, California,to develop and test a multidisciplinaryapproach to the characterization ofground-water flow and transport infractured rocks. Nine boreholes weredrilled into the granitic bedrock, and awide variety of instruments andmethodologies were tested. Fractureproperties were measured on outcropsand in boreholes using acousticteleviewer, digital borehole colorscanner, and by down-hole camera logs.Conventional geophysical logs werecollected. In addition, thermal-pulse andimpeller flowmeter logging, fluidreplacement and conductivity logging,packer-injection profiling tests, andordinary open-hole pumping tests wereconducted. Transmissive fractures wereidentified by integrating results fromhydrologic and geophysicalmeasurements, and the hydrogeologicstructure of the formation washypothesized. Cross-hole seismicsurveys yielded tomograms of inter-borehole rock properties. Visualizationsoftware was used in combination withgeophysical logs to interpolate inter-borehole properties, and a detailed 3-Dmodel of the subsurface wasconstructed. Other referenced work atthe site includes cross-hole hydrologic

EPA/600/S-96/001 May 1996

tomography, tracer tests, fracture-specific morphology studies, anddevelopment of an automated dataacquisition system used to collect dataand monitor and control test parametersduring borehole testing. A novel aspectof the project report is its guidebookformat. A description of each tool andmethodology, the strengths andshortcoming of each, how they comparewith one another, and suggestions ofhow best to analyze and integrate dataare presented.

The Project Summary was developedby the National Risk ManagementResearch Laboratory's SubsurfaceProtection and Remediation Division,Ada, OK, to announce key findings of theresearch project that is fully documentedin a separate report (see Project Reportordering information at the back). Thereader is encouraged to visit the InternetWeb page for further information:

http://www.epa.gov/ada/kerrlab.html

1 Introduction

Fundamental to every ground-waterremediation effort is a description of thesubsurface distribution of contaminants andfluid flow properties. The hydrogeologiccomplexity of fractured formations makestheir characterization very difficult. This

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Canada, Ltd., and by the U.S. EnvironmentalProtection Agency. The site was establishedto develop and test a multidisciplinaryapproach to the characterization of ground-water flow and transport in fractured rocks.No ground-water contamination waspresent. Research began in 1992 and isongoing.

The site is situated in the western foothillsof the central Sierra Nevada, California,approximately 60 km (37 mi) south ofYosemite Valley and about 5 km (3 mi)southeast of the town of Raymond (Figure1). Nine vertical boreholes penetratefractured granitic bedrock to depths rangingbetween 75 and 90 m. They are arrangedin a triangular pattern and are spaced nomore than 61 meters apart (Figure 2).

difficulty is the primary reason that themajority of fractured portions ofcontaminated formations have not yet beeninvestigated. Knowledge of new and existingsite characterization tools andunderstanding of analysis methods arenecessary first steps toward improvedremediation of fractured formations.

1.1 Format of Project Report

The project report describes the workand findings of a hydrogeologiccharacterization study of a saturated,fractured, granitic rock aquifer in the foothillsof the Sierra Nevada, California. A widevariety of new and traditional hydrologicand geophysical characterization tools andmethodologies were tested. The numerousfield experiments and analyses haveprovided many insights that should greatlybenefit the remediation community. Theproject report was designed in a guidebookformat. First, an overview of the problemsassociated with remediating fracturedaquifers is presented. Case histories arereferenced as examples. Second, a briefdescription of the methods and results ofthe characterization effort at theexperimental field site are presented. Thefollowing chapters comprise the bulk of thereport. Each chapter describes a particularcharacterization phase, and a generalstrategy for hydrogeologic characterizationis presented. Each tool and method isdescribed in detail. Descriptions of thetools, how they are used, what are thestrengths and shortcomings of each, howthey compare with one another, as well ashow best to analyze and integrate the datacollected, are discussed. Findings obtainedat other remedial sites where fracturedmaterials are present are also referenced.Determination of subsurface flow propertiesis the emphasis of this project rather thancontaminant sampling and plumedelineation. However, issues regarding theeffect of incorrect characterization of flowproperties on prediction of contaminantbehavior are addressed. The findings andrecommendations are not necessarilylimited to fractured crystall ine rockformations; many apply to fracturedsubsurface formations in general.

1.2 Experimental Field Site

Studies were conducted at the RaymondField Site, which is operated by the E. O.Lawrence Berkeley National Laboratory andthe U.S. Geological Survey. Sponsorshiphas been provided by the Office of CivilianRadioactive Waste Management of theDepartment of Energy, Atomic Energy of

Figure 1. Location of Raymond Field Site

The conceptual model of the sitesubsurface is based on the integration of allwork conducted at the site. Boreholegeophysical logs indicate that hundreds offractures intersect each well. However,borehole flow logs reveal there are onlyseveral distinct hydraulically conductivefractures in each. Many of the transmissivefractures are subhorizontal and westwardlydipping, and are associated with pegmatitedikes. A few are subvertical or of differentorientation. Most occur within or near oneof two zones of relatively low electricalresistivity and increased borehole diameter,both indications of altered rock. Thesezones dip gently to the west and areseparated by about 25 meters. Fractureswith minimal transmissivities occur in otherportions of the well. The conceptual modelconsists of two subhorizontal andhydraulically conductive fracture zones thatbehave as confined units imbedded within arelatively impermeable rock matrix(Figure 3).

1.3 Data Acquisition System

An innovative, automated data acquisitionsystem was developed for the field site.The new system was built around a 486 PCwhich was used for controlling a samplingtable, opening and closing borehole valves,and for logging pressure and chemicalconcentration data from all themeasurement locations throughout the site.Pressures are monitored in up to 29 differentpacked-off intervals while flow rates andwater chemistry are monitored and analyzedsimultaneously. The output from thepressure transducers and chemicalconcentration measuring devices areavailable in real-time on the computer. Inaddition, controlled parameters such asflow and sampling rates can be adjustedduring the test. A schematic of the new dataacquisition system is shown in Figure 4.Pressure transducers, tracer injection ortracer recovery instruments, and pumpswere placed within isolated zones duringvarious hydrologic tests. Flowmeters and aspectrofluorometer were located at thesurface. The packers have feed-throughsso that both fluids and electric signals canbe passed to the surface. Data can berecorded at a rate of 1 Hz. Electrical signalsfrom the well heads are transferred inunderground conduits to terminal blocks inthe computer room. Analysis of fluorescenttracers is accomplished using a flow-throughcell in a fluorometer, and ionic tracerconcentration is determined using ionspecific electrodes. A computer-controlled,144 bottle sampling table was built so thatsamples could be taken back to the lab forfurther analysis. In several months of fielduse, the fully automated data collectionsystem has proven to be highly reliable,even for extended multi-week tests.

2 Methods and Results

2.1 Surface FractureCharacterization

Physical properties of fractures exposedon outcrops in the area within a severalhundred meter radius from the well fieldwere measured. The report describesfindings from several hundredmeasurements of fracture orientation, tracelength, spacing, weathering, secondarymineralization, and relative displacement.Other measurements included fractureaperture, roughness, and planarity, detailedmeasurements of fracture spacing made atnine outcrops, and mapping of a largefracture pavement.

Regional and site-specific fracture setswere identified. Regional sets consist of

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to be planar and laterally extensive, whichjustified the use of extrapolating thesefractures linearly to other boreholes as ameans of hypothesis testing and modeldevelopment.

2.2 Well Drilling

Boreholes were drilled using an air-percussion rotary drill. Boreholes werecased at shallow depths where the rock wasweathered and weak, and left uncased andunscreened below these zones. Detailedlogs of the wells were made by a geologistas drilling proceeded. The location of somefractures was identified by observing drillbehavior and cuttings. Drilling ratesincreased in fracture zones. Fluid-bearingintervals were encountered, and changesin fluid flow with depth were measured.These observations, in combination withchanges in dril l ing rate and cuttingcharacteristics, indicated the possiblelocations of transmissive fracture zones.Total fluid flow observed at the final drillingdepth was measured and used as anindication of the relative discharge capacityof each well. Comparison of the geologist'slogs with conventional geophysical logs andfluid flow logging results show that severalof the dominant transmissive fracture zoneswere identified during drilling.

2.3 Conventional GeophysicalLogging

Three-arm caliper, fluid conductivity, 16-and 64-inch normal resistivity, naturalgamma, temperature, single-pointresistance, spontaneous potential, andlateral logs were collected in each well.Results and uses of these logs are describedin Section 3.6.

2.4 Pumping Tests

Constant rate pumping tests wereperformed for several well combinations.Pressure transducers that can detect waterlevel changes on the order of 0.1 mm wereused to measure drawdown. Measurementsin the pumping well and observation wellswere recorded as often as every 10 seconds.Wells that exhibited low yields during drillingcould not be sustained since water levelswould fall beneath the upper fracture zone,and well bore storage effects almostcompletely dominated the pumping wellpressure response. Observation wellpressure transients typically deviated froman ideal confined aquifer response at earlypumping times. Some fit the ideal curvealmost exactly. Transmissivities calculatedfrom standard semi-log pressure transientanalysis were very similar from well to well,

Figure 2. Plan and cross-sectional view of boreholes at the Raymond Field Site.

two orthogonal and steeply dipping tectonicfracture sets, and unloading fractures thatare subparallel to the topographic surface.The spatial distribution of fracture density,aperture, and infilling characteristics of thesesets is, in general, highly heterogeneous onthe scale of the well field, and there is noapparent systematic structure. Thecharacteristics of these sets were heavilyaltered due to their exposure at the surface.Other fracture sets exposed in the vicinity ofthe well field are associated with aplite andpegmatite dikes. Pegmatites withthicknesses on the order of centimeterswere often observed to contain adiscontinuous open fracture near the centerand chemical alteration associated with fluidflow. The average continuity of pegmatitic

dikes is on the order of 10 m, with someextending 30 meters or more.

Surface fracture characterization providedinformation useful to other phases of thesite characterization. Integration ofconventional geophysical logs, hydrologiclogs, and borehole fracture detectionmethods revealed that the uppertransmissive zone in many wells isassociated with a pegmatite dike. Theobservation that pegmatite dikes at thesurface are laterally continuous for tens ofmeters and are often fractured and exhibitsigns of fluid flow provided support forinference that these intervals are portionsof a continuous band of fracturing associatedwith a westwardly dipping pegmatite.Similarly, tectonic fractures were observed

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Figure 3. Conceptual model of hydrogeologic structure at the Raymond Field Site.

and did not reflect the large degree ofheterogeneity found from borehole flowlogging tests. The report describes howhighly heterogeneous fractured formationscan respond like homogeneous porousformations. Potential misinterpretations ofpressure transients, in general, are alsodiscussed. Vertical flow profiling inobservation wells shows that the boreholesact as short-circuiting fluid pathways.

2.5 Detection and Measurementof Subsurface Fractures

Several technologies were explored as ameans to observe and measure subsurfacefractures. Acoustic televiewer (ATV) logsprovide images of the acoustic reflectivity ofthe borehole wall. Fracture signatures areobservable and enable measurement offracture depth, orientation, dip, and apparentaperture. ATV logs were collected for allnine wells. Color television camera logswere obtained for all nine wells and wereused to observe fracture alteration andmineralization not observable on the ATVlogs. Seven wells were probed with a newdigital borehole scanner (DBS). The scannerdigitally records the reflected intensity ofthe red, blue, and green light wavelengthsfrom the borehole wall as a white light

source rotates and simultaneouslyilluminates the borehole wall. High resolutioncolor images of the borehole wall wereconstructed from the scanner data. Fractureorientation, surface roughness, aperture,identification of fracture mineralization, andother microscale properties are measurable.

A stereonet of measured fractures fromthe ATV logs shows that at least threefracture sets are present within the wellfield. Comparison of the ATV logs with thecolor television and DBS logs revealed thatfractures can be divided into more thanthree sets, as differences occur inmineralization that are not observable onthe ATV logs. In general, these comparisonsrevealed that in the ATV logs: 1) closelyspaced fractures or zones of altered rockthat were gouged out during drilling appearas a single, large fracture zone with nodistinguishable orientation, dip, or aperture;2) mineral infilled fractures that were slightlygouged-out during drilling appear as openfractures; 3) sealed and mineral infilledfractures not affected by drilling were notdetected; 4) partially sealed or very smallfractures were not detected or were difficultto identify; and 5) the top and bottom ofsteeply dipping fractures were gouged outand appear to have greater dip. Subsequentborehole hydrologic tests showed that only

several of the hundreds of observedfractures in each borehole are transmissive.Accordingly, spacing distribution of boreholefractures did not correlate with the hydrologicproperties around each borehole.

Quantitative measurements from thetelevision camera logs were not possible,but qualitative determinations of whether ornot fractures were infilled were possible.Rock discoloration around individualfractures could be seen, and thereforehelped locate fractures that might behydrologically altered. The location of manypegmatite dikes was identified because theirlarge, reflective crystals were easilyrecognizable on the image. This was veryvaluable since it was later found that thesedikes were one of the significant conductivefracture sets. Orientation and dip ofpegmatites were determined by identifyingtheir trace on ATV logs.

The DBS provided borehole wall imagesof the greatest detail and quality. Manymore fractures were observable comparedto the ATV logs. The photographic qualityof the logs enabled more precisemeasurement of fracture aperture,orientation, and dip. Mineral infilling infractures was clearly visible, and enableddistinction between open and sealedfractures. Fluid alteration such as oxidationstains around some fractures was clearlyobservable. Individual fractures within zonesthat were gouged out during drilling weremeasurable. Several disturbed zones werehighly transmissive, and the DBS enableddetection and measurement of the individualfractures within these zones.

2.6 Borehole Flow Logging

Impeller flowmeter logging, thermal-pulseflowmeter logging, fluid replacement andconductivity logging, and straddle-packerinjection profiling were performed todetermine the location of fluid-bearingfractures and their respective trans-missivities. Impeller flowmeter profiling wasperformed in three wells. All nine wells wereprofiled with a thermal-pulse flowmeter.Fluid conductivity logging was performed inseven wells, and straddle-packer injectiontests were conducted in all nine wells.Results were integrated with the boreholescanning logs described above, and withthe conventional geophysical logs todetermine the specific transmissive fractureswithin an observed borehole flow interval.

The impeller flowmeter was used to profilethe vertical flow in a well while it was pumped.The instrument is composed of an impeller-type flowmeter mounted above a hollowshaft that passes through the center of aninflatable packer. The well was pumped ata constant rate by a downhole submersible

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Figure 4. Schematic of data acquisition and control system at the Raymond Field Site.

pump situated near the upper portion of thewell. The assembly was successivelyinflated at different depths and vertical flowmeasurements were taken (Figure 5a). Thedrawdown transient throughout the entiretest duration was measured by a pressuretransducer situated near the upper portionof the well. Flow rate into the borehole froma particular interval was calculated from thedifference in vertical flow measured overthe interval. Apparent formationtransmissivity was determined from thetransient drawdown, which responded as aconfined aquifer. Transmissivities ofparticular flow intervals were calculated asthe product of fractional inflow and formationtransmissivity. The minimum measurableflow rate of the impeller was 4 L/min. Thisrelatively high stall velocity prohibited theprofiling of low-yielding wells. Hence, theprocedure was only conducted in threerelatively high yielding wells. In addition,flow intervals near the lower portions of thewells could not be defined because verticalflow rates were below the stall velocity ofthe instrument (Figure 5a).

The thermal-pulse flowmeter (alsoreferred to as heat-pulse flowmeter) wasused to profile multiple wells during thepumping of a single, high yielding well (Figure5b). The instrument consists of a heatinggrid and temperature sensors emplacedwithin a central tube that passes through aninflatable packer. Vertical flow velocity isdetermined by heating a small parcel ofwater at the heating grid and observing thetime of arrival of the parcel at temperaturesensors located above and below the grid.Response of one of the sensors indicates

flow direction. Differences in vertical flowwith depth indicated fluid flow into or out ofa borehole, but no quantitative estimate ofthe conductivity of such intervals could bedetermined. The thermal-pulse flowmetermethod detected more flowing intervalsthan the impeller method. This is consistentwith the fact that it has a lower minimumdetection limit. Vertical flows on the orderof 0.05 L/min were measurable. In theory,the thermal-pulse flowmeter could be usedin the same manner that the impeller wasused in this study, thereby allowingcalculation of interval conductivities. Forsome wells, measurements at the samedepth at different times were different dueto the evolving transient flow field. Thismade the interpretation of some portions ofsome logs difficult. In one well, the thermal-pulse flowmeter did not detect a flowinginterval identified by the impeller method.This is most probably an artifact of the testmethodology, not the instrument. Aparticular fracture may contribute flowdifferently when a well other than the onethe fracture intersects is being pumped. Inaddition, the vertical flow transient in anobservation well associated with flow into aconductive fracture could diminish tounobservable levels by the time the well isprofiled.

Fluid replacement and conductivitylogging was done in seven of the wells. Themethod consists of replacing the fluid in thewell bore with deionized water, which has amuch lower electrical conductivity than theformation water. Replacement wasachieved by simultaneously pumpingformation water near the upper portion of

the well and replacing with deionized waterat the bottom using downhole tubing. Afterreplacement was complete, the well waspumped at a low and constant rate and atime sequence of upward and downwardlogs of fluid electric conductivity werecollected. Figure 5c shows the downwardlogs for well SE-1. The conductivity logs forthe seven wells exhibit noticeable peaks atconductive fracture locations. Therefore,determination of the particular fracturecontributing to flow was straightforward,requiring only a brief inspection of theacoustic televiewer and/or television logs.Many more conductive fractures wereidentified by this method than by the impelleror thermal-pulse flowmeter. Equallysignificant is that the measurements weremore precise and enabled quick andconfident assessment of the locations andrelative magnitudes of the conductivity ofparticular conductive fractures. The resultsof the tests are amenable to analysiswhereby the transmissivity of particularfractures is determined, although this wasnot done in this study. The fluid conductivitylogs have higher resolution in general, whichresults in part because measurements weremade at many more depths compared tothe other methods.

Straddle-packer injection tests were donein all nine wells. Water was injected at aconstant rate into a 6 meter packed-offinterval in a borehole. Fluid pressure withinthe interval was measured with a pressuretransducer. Two transducers were alsomounted above and below the test intervalto monitor for possible flow leakage aroundthe packers and/or flow short-circuiting dueto interconnecting transmissive fractures.For each test interval injection was continueduntil a pseudo-steady flow was attained.This was on the order of minutes to tens ofminutes. This procedure was repeated atdifferent depths, and the transmissivity ofeach interval was calculated using theobserved flow rates and injection pressures,and assuming radial flow. Short-circuitingoccurred in high conductivity intervals andtransient injection pressure and flow ratespersisted because of flow-line resistanceeffects. Therefore, the conductivities of thehighest conductivity intervals wereunderestimated. The results are consistentwith the other techniques but in some casesindicate very low transmissivity intervalswhile the others do not.

2.7 Integration of Geophysicaland Hydrologic Logs

Conventional geophysical logs were usedin conjunction with flowmeter logs in orderto identify the particular hydraulically con-ductive fractures and/or fractured zones

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Figure 5. Example results from the various flow logging techniques performed in well SE-1. The impeller measurements were taken during constant ratepumping of approximately 15 L/min just above the most shallow measurement. Measurements below 31 m were not possible because of the highstall velocity of the instrument. Heat-pulse measurements were taken during pumping in well 0-0. Negative flow indicates downward flow.Upward and downward flow is converging and exiting borehole near 30 m. Fluid conductivity profiles exhibit peaks where formation water isflowing into well through discrete fractures while the well is pumped above 10 m. Measurements are in micro siemans per centimeter. Straddle-packer injection tests show the relative magnitudes of transmissivity for 3 m intervals. Values are given as flow rate achieved per given injectionpressure expressed as equivalent hydraulic head.

intersecting the boreholes. ATV, DBS, tele-vision, 16-inch normal resistivity, caliper,and natural gamma logs were the mostuseful for this purpose. After specific trans-missive fractures were identified in eachwell, fractures and fracture zones that inter-sect multiple wells were identified by inter-polating fracture geophysical propertiesusing 3-D visualization software.

Figure 6 shows geophysical and flowmetermeasurements, and the identification of theparticular transmissive fractures in wellSE-1, for example. Comparison of thegeophysical logs with one another indicatedthat caliper logs show the locations ofincreased borehole diameter. Zones ofintense fracturing such as in areas wherethere are many closely spaced, sub-horizontal fractures were easily detected.Some caliper anomalies are associated withindividual fractures. Peaks on the naturalgamma logs indicated the presence of

pegmatitic dikes. Regions of resistivity lessthan 700 ohm-m are associated withhydraulically conductive fractures. Claysand ferric oxides associated withhydraulically altered fractures exhibit highelectrical conductivities compared to theparent rock.

In general, several different types offractures are transmissive: fracturedpegmatitic dikes, closely spacedsubhorizontal fractures, subvertical tectonicfractures, and to a minor degree variouspartially infilled aplite veins which are veryweakly transmissive. The fracturedpegmatite dikes are often found within ornear the zones of subhorizontal and closelyspaced fractures. Two subhorizontalfracture zones comprised of the westwardlydipping, subhorizontal altered fractures andpegmatites were delineated based onsimilarities in fracture geophysical propertiesin adjacent boreholes. These two zones

define general hydrologic structures withinthe well field (Figure 3). Other importantfeatures include subvertical tectonicfractures which may connect these zones.The report describes how specifictransmissive fractures were identified byintegration of different combinations ofgeophysical and flowmeter logs.

2.8 Computer Visualization

Visualization software enabled viewing ofthe well field and how particular fracturesare positioned relative to boreholes.Borehole lateral logs were used inconjunction with surface mapping todetermine the x,y,z coordinates of the wellfield. The true 3-D perspective of all wellswas constructed from these coordinates.This formed the initial 3-D model uponwhich to superimpose other features. Onevery useful feature of the model was

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Figure 6. Conventional geophysical logs and borehole fluid replacement and conductivity log of well SE-1. Fractures identified as transmissive shown atright.

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Figure 7. Concatenated seismic velocity tomograms from adjacent well pairs. Image representscross-section between well SW-3 and SE-3. Negative velocity deviation represents seismicvelocity less than mean value. An upper and lower zone is apparent.

tools to describe fracture flow properties.The upper fracture zone in each well was

isolated with pneumatic packers during bothtracer tests. A minimum of two packerswere used in each well with a total of 22packers in 9 wells. Twenty-nine (29)transducers were located within and aroundpacked-off zones throughout the well field.For the radially convergent test, well 0-0was first pumped at a constant rate for a fewdays prior to the tracer injection to establisha quasi-steady flow field. A three tracermixture of deuterium, fluoresceine, andmicrospheres was subsequently injected at

the upper fracture zone in well SW-3. Amixture of bromide and fluoride was injectedin the same zone two and a half hours later.The pumping continued for about one week,during which time the pressure in all 29zones and the flow rate at the discharge linewere continuously monitored and recorded.Also monitored in real-time were thefluoresceine and bromide concentration inthe discharge line using an in-linefluorometer and ion specific electrode asdescribed in section 1.3. Sample water wastaken from the discharge line every tenminutes using the automated sampling table

deducing subsurface fracture connectivityand structure. For example, a transmissivefracture identified by integrating boreholeflowmeter and geophysical data wasmeasured using the acoustic televiewerlog, and a plane representing the azimuthand dip of that fracture at its proper locationin a well could be displayed. This plane wasrepresented as a disk, and was used to seewhere that fracture would theoreticallyintersect other wells. The logs of otherwells were inspected to see if such a featurewas present at and around the extrapolatedlocation, and in this way quickly determinedwhich features extended between wells.Extrapolation also considered non-planarfractures and fracture measurement error.A complex 3-D model of the site wasdeveloped via this hypothesis testing anddata synthesis approach.

2.9 Cross-Hole SeismicImaging

Cross-borehole seismic surveys wereperformed to deduce the location of fracturesand fracture zones between wells. Themethod consists of equipping a boreholewith a piezoelectric source transducer, andan adjacent borehole with a string ofhydrophones. The transducer creates apressure pulse which is measured by thehydrophones. After the transducer is pulsedat a particular depth and measurementsare made, it is lowered and pulsed again.This procedure is repeated along the depthof the borehole. The seismic data fromeach receiver during each depthtransmission is analyzed to determine thetravel time and amplitude of each receivedseismic pulse. A numerical matrix thatincorporates the positions of each ray pathand its respective travel time and/oramplitude is inverted and the velocity and/or seismic attenuation of different pixelelements of a 2-D grid representing theplane between boreholes is determined.Figure 7 shows the concatenated seismicvelocity tomograms of well pairs SW3-SW2,SW2-00, 00-SE2, and SE2-SE3. The resultssupport the conclusions regarding thegeneral dual-layer, westwardly dippingfracture zones.

2.10 Interwell Tracer Tests

Radially convergent and two-well partialrecirculation tracer tests using bothconservative and reactive tracers wereconducted. The objectives of this test were:(1) to obtain an estimate of transportparameters; (2) to compare performance ofseveral different tracers; and (3) to comparetest methodologies and identify advantagesand shortcomings of these methods as

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Figure 8. Normalized breakthrough curves of three conservative tracers during radially convergenttest.

for later analysis in the laboratory.Figure 8 shows the breakthrough curves

of fluoresceine, bromide, and deuteriumnormalized to the injection concentration.Microspheres were not detected in thepumped water. It is possible that themicrospheres became negatively charged,coalesced and, hence, became stuck withinthe fracture zone. The backgroundconcentration of fluoride was too high forthe fluoride concentration result to bemeaningful.

The first tracer arrival occurred at about10 hours after injection. The tracerbreakthrough curves did not lie on top ofeach other, even through the three tracersare presumably conservative (Figure 8).The evidence suggests that the mostsignificant factor responsible for thisbehavior was that effective injection tracerconcentrations were different than predictedbecause of borehole mixing and storageeffects. After the mixture of tracers wasinjected, the tracers may have separatedwithin the injection zone, which was roughlyten times the volume of the injected fluid.Due to its higher density, bromide may havecaused the tracer mixture plume to sink tothe bottom of the injection zone. The massof fluoresceine that arrived at the pumpingwell was calculated to be about 15% of thetotal mass injected. Almost all of theremaining mass was recovered by pumping

transport equation was used to determinetransport parameters from field data. Alinear relationship was assumed to existbetween fluid concentration and adsorbedphase, with instantaneous chemicalequilibrium. Transport is modeled along a1-D stream tube between the injection andwithdrawal well. This is a reasonableapproach for this test since the ratio of fluidinjection to discharge is small. A dispersionlength of 32 meters was calculated from thebest-fit model. As described subsequently,the relevance of this parameter is veryquestionable.

3 Conclusions andRecommendations

3.1 Surface FractureCharacterization

Observation and measurement of surfacegeologic features is a standard and usuallyinitial investigative phase in any subsurfacecharacterization effort. A non-intrusive,relatively low cost surface study combinedwith previously documented geologic studiescan provide a means to determine thedominant types and orientations of fracturesets present, as well as their physicalproperties, spatial distribution, and probablemodes of genesis. The relative ease andlow cost of performing surfacecharacterization and the pertinentinformation that it reveals makes it a practicalnecessity, but it is not a substitute forborehole studies. For example, most of thefractures observable at the surface may notbe hydrologically significant. Dominantconductive fractures in the subsurface arenot typically observable on the surface, oronly limited fractures may be observable atthe surface, simply because few bedrockoutcrops are present.

3.2 Well Drilling

Air drilling is the most appropriate methodfor the installation of wells in crystallinebedrock in terms of its relative cost,penetration rate, and potential to yieldrelevant hydrogeologic information.Observation of drill cuttings, drilling rate,and flow out of the borehole during drillingmay be used to infer lithology and thelocation of transmissive fractures, forexample. The rate of penetration offers anobvious cost advantage over other methods,but a greater savings is probably due to theincreased information gained concerningfluid-bearing zones. Careful observationduring drilling and a descriptive log canprovide valuable hydrogeologic information.Of great significance is the fact that thedepth of transmissive fractures may be

out from the injection well. Other contributingfactors may include differences in the degreeof reaction (or non-reaction) between thetracers. Fluoresceine has been reportedelsewhere to react (fluoresce more) withcertain minerals in the rock giving increasesin apparent mass.

A partial-recirculation injection andrecovery tracer test was conducted usingboth reactive and conservative tracers. Theextraction and injection wells were the sameas in the previous test. The extraction andreinjection rates were 7.6 and 0.7 L/min,respectively. Bromide and fluoresceinewere used as conservative tracers andlithium was used as a sorbing tracer. Aspecially designed low volume tracerinjection system was used to minimize thewell bore effects observed in the previoustest (Figure 9). The test was halted after 22days, when the fluoresceine concentrationdropped to approximately one tenth of thepeak arrival concentration.

There was a significant differencebetween the recovery of bromide and lithiumduring the 22 days in which the test wasconducted. The percentage of bromiderecovered was 80% of the injected mass,and less than 30% of the lithium wasrecovered. In addition, the breakthroughcurve for bromide was much steeper with apeak arrival two days before the peak forlithium. A simple convective-dispersive

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Figure 9. Equipment layout for two well tracer tests with partial recirculation. Tracer is injected intosmall annulus between borehole wall and injection casing between packers. Tracer exitsthrough several feed-through valves evenly distributed over injection interval. Not to scale.

can mask the effects of importantheterogeneities, and semilog analysis ofdrawdown in the pump well will lead tounderestimates of transmissivity. Localtransmissivity heterogeneities can produceapparent anisotropic behavior in thedrawdown responses in observation wells.The installation of wells can connectpreviously unconnected fractures and createshort-circuit pathways. Pumping of wellscan further initiate transport of contaminantsto previously uncontaminated areas. Inorder to maximize the information necessaryfor characterization and minimize cost andthe potentially negative effects of pumping,pumping tests should not be conducted astests in and of themselves. Other hydrologictests which are necessary in characterizinga fractured formation yield more valuableinformation, and can simultaneously providepressure transients amenable to standardpumping test analysis. For example,impeller-flow meter profiling allows one todetermine which fractures are conductive,and pressure transducers can be installedin wells during the test to measure pressuretransients. A drilling method that enablesobservation of fluid emanating from the

observable, because water will be broughtto the surface with the cuttings when suchfractures are encountered. Flow can bemeasured with a bucket at the surface, andchanges in flow with depth may indicatethat the drill bit encountered anothertransmissive fracture or fracture zone. Inaddition, the flow measured at the totaldepth may be used as a relative measure ofwell yield. This information can be used toassess which wells are relatively highyielding and, therefore, are the bestcandidates for pumping tests or boreholeflow profiling tests, for example.

3.3 Pumping Tests

Drawdown transients in different types offractured formations can behave in a mannerdescribable by the Theis or Cooper-Jacobsolutions, but this behavior does notnecessarily indicate the aquifer can beconsidered an equivalent porous medium.Irrelevant parameter values can be derivedfrom analysis of drawdown transients, andanalysis of late-time drawdown may beessentially unrelated to the region of theaquifer that is of interest. Well bore storage

borehole can provide information regardingwhich well(s) are best suited for pumping.Pumping of wells during any type of testshould be minimized in duration, and effortsshould be made to analyze the entirepressure transient rather than resort only tolate-time data analysis.

3.4 Detection and Measurementof Subsurface Fractures

The identification and measurement offractures intersecting boreholes is anabsolute necessity. Besides its use indetermining the general fracture trends orsets, a database of fracture locations andorientations in boreholes enables one todetermine fracture-specific geophysicalproperties through comparison withstandard geophysical and flow logs. Thecombination of a flowmeter log and anacoustic televiewer log may enable thedetermination of particular hydraulicallyconductive fractures. The measuredorientation and dip may be used to associatethese fractures with fracture sets, and projectto intersections with other borehole wells.

Although the digital borehole scannerprovides the highest resolution data, it isrelatively expensive. However, given thecapabilities of the tool and imminent reduc-tions in its cost, it will be the best tool for usein the near future. At present, ATV andtelevision logs used together provide veryuseful fracture data, especially whencoupled with standard geophysical logs.Fractures can be easily mapped from theATV logs, and television logs can be used tocheck if particular fractures are altered and/or infilled. The ATV was most useful foridentifying the location of fractures anddetermining their orientation. Measure-ments of individual fracture properties, suchas aperture or roughness are not reliable.The determination of orientation and dip ofindividual fractures was very useful for lateruse, when the data was integrated withvisualization software and measured frac-tures were extrapolated to see where theymight intersect other boreholes. Most ofthe information gained from coring is ob-tainable through the combined data fromthe ATV, television, and standard geophysi-cal logs, which are more economical andtimely.

The large data set resulting from thefracture measurements makes a statisticalanalysis of spatial properties very tempting.However, because of the effects of drillingon the appearance of individual fractures,and on the fact relatively few of the manyfractures detected conduct fluid, statisticaldescriptions of fracture aperture and spacing(based on ATV data alone) should not be

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used to estimate general hydrologicproperties.

3.5 Borehole Flow Logging

Flow logging is a critical necessity in thecharacterization study. It provides a meansto identify and quantify the transmissivitiesof only the relatively few fractures or fracturezones which are in fact conductive. Thisinformation provides knowledge of thegeneral structure of the aquifer, from whichall future remedial planning emanates.Results from flow logging are best whenintegrated with other geophysical logs.

A necessity of all flowmeters is thatpumping or injection of the well-bore isrequired. Therefore, a flow field may beestablished in the contaminated field, andcontaminants can migrate to previouslyuncontaminated areas. In order to minimizethese effects, profiling should first beconducted in wells under natural conditions.In theory, either the impeller or thermal-pulse flowmeter could be used at this stage,although the lower sensitivity of the latter isprobably the better choice since flows arelikely to be very low. Vertical borehole flowwill probably be greatest soon after drillingwhen fractures are being connected by theinstallation of the well and the system is ina highly transient state. After this initialprofiling, the method of profiling multiplewells during the pumping of a single wellshould be implemented. The highest yieldingwell should be used as the pumping well,and driller’s logs can provide this information.Profiling in the pump well with the impellerflowmeter allows determination oftransmissivities of intervals adjacent to thewell, and by installing pressure transducersin neighboring wells, a multi-well pumpingtest is effectively conducted. This eliminatesthe need to perform a separate pumpingtest. Alternatively, one well can be used forpumping and other wells profiled with aheat-pulse flowmeter, for example. Theinformation gained is somewhat different,but perhaps more informative since onecan determine which fractures aretransmissive in other wells, although notquantitatively. If the impeller or thermal-pulse is used, it is recommended that adownhole inflatable packer be used aroundthe flow casing to increase the sensitivityand avoid borehole variation and turbulenceeffects and to reduce the number ofcalibrations needed.

The use of packer injection tests is notrecommended under most situations. Themethod is expensive and time consumingand creates the greatest non-equilibriumcondition in the aquifer. A method like thefluid conductivity logging yields preciselocations of transmissive fractures and can

be analyzed to determine thetransmissivities of these fractures. Thepacker-injection tests are probably notappropriate as a means to detect flowingfractures but perhaps as a later investigativephase once flowing fractures are found andsome quantitative assessment is sought.

3.6 Integration of Geophysicaland Hydrologic Logs

Conventional geophysical logs were usedin conjunction with flowmeter logs in orderto identify the particular hydraulicallyconductive fractures and/or fractured zonesintersecting the boreholes. It was foundthat by integrating the flowmeter resultswith the acoustic televiewer, television, 16-inch normal resistivity, caliper, and gammalogs, the particular conductive fracture orfractured zone could be determined. Afterthe conductive fractures were identified,interpolation of properties between wellswas made based on similarities in variousfracture geophysical properties, and thegeneral hydrologic structure of the aquiferwas deduced. The integration of variousgeophysical logs is an essential componentin any characterization effort. All of theconventional geophysical logs collected maybe potentially significant for use at othersites. Reference is made in the full report toother works where different tools have beenused.

3.7 Computer Visualization

Visualization software is now an affordablereality, and allows one to deducecomplexities not possible from traditional 2-D plots of borehole data. One very usefulfeature of some visualization softwareroutines is their ability to represent spatiallydistributed data in a true three-dimensionalperspective, and which allow real-timemanipulation of the viewing perspective.Hypothesis testing via interpolation offracture properties between wells isextremely beneficial. Visualization shouldbe a commonplace tool for subsurfacecharacterization.

3.8 Interwell Tracer Tests

In general, tracer tests were the mostdifficult field test to construct and operate,even with the highly sophisticated andexpensive equipment used at the RaymondField Site. Well bore storage and mixingeffects in the injection zone severely alteredthe intended test configuration and renderedthe tracer breakthrough curves amenableto arrival time analysis only. Even with theimplementation of the special injectionsystem that minimized borehole effects,

the usefulness of the test results is veryquestionable. The theoretical mixing lengthsthat yielded the best model fit to the datawere larger than the well spacing betweenthe injection and withdrawal wells. Thisresult generally points out the shortcomingsof using the convection-dispersion equationto model transport in highly heterogeneousfracture rock. Based on the field experienceat this site, tracer tests for characterizingfracture formations as part of a remediationprogram are problematic. Highly controlledand sophisticated and expensive equipmentis needed, and even in the best ofcircumstances there will most likely beconsiderable ambiguity associated with thederived parameters.

Notice:

The information in this project summaryand in the original report is the result of aresearch project funded jointly by the U.S.Environmental Protection Agency and theU.S. Department of Energy. Neither theoriginal report nor this project summaryshould be interpreted as official EPAguidance for site characterization at siteslocated in fractured rock formations.

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Andrew J. B. Cohen, Kenzi Karasaki, Sally Benson, Gudmundur Bodvarsson, Barry Freifeld, Pascual Benito, Paul Cook, John Clyde, KennethGrossenbacher, John Peterson, Ray Solbau, Bhaskar Thapa, Don Vasco, and Peter Zawislanski are with the Earth Sciences Division, ErnestOrlando Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720

Stephen R. Kraemer was the EPA project officer (see below).

The complete report is entitled "Hydrogeologic Characterization of Fractured Rock Formations: A Guide for Groundwater Remediators" and isreferenced as LBL-38142/UC-800 (144 pages, color figures). A limited number of copies of the full report are available from Subsurface RemediationInformation Center, P.O. Box 1198, Ada, OK 74821. A digital copy is available on the home page http://www.epa.gov/ada/kerrlab.html

The full report is available to DOE and DOE Contractors from the:Office of Scientific and Technical InformationP.O. Box 62Oak Ridge, TN 37831(615)576-8401

The full report is available from the:National Technical Information ServiceU. S. Dept. of Commerce5285 Port Royal Rd.Springfield, VA 22161

The EPA Project Officer can be contacted at:U.S. Environmental Protection AgencyNational Risk Management Research LaboratorySubsurface Protection and Remediation DivisionP.O. Box 1198Ada, OK 74821-1198