United States        Office of Water &       sw-729
             Environmental Protection    Waste Management       December 1978

             Agency          Washington D.C. 20460

             Solid Waste
vvEPA     Electrical Resistivity
             Evaluations at Solid Waste
             Disposal Facilities

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           ELECTRICAL RESISTIVITY EVALUATIONS
           AT SOLID WASTE DISPOSAL FACILITIES
            This report  (SW-729)was prepared
under the direction of Burnell Vincent, Project  Officer,
      by Paul H. Roux of Geraghty and Milter,  Inc.
         under purchase  order no. WA-6-99-2794-A
              for the Office of Solid Waste
           U.S.  ENVIRONMENTAL PROTECTION AGENCY
                          1978
        US. Envircn:r-,.;n*c:!
        Region V, Library
        ?'.'•• South Dearborn Street

noo
                     io  C0604

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Publication of this report does  not signify that the contents
necessarily reflect the views  and  policies of the U.S.
Environmental Protection Agency, nor does  mention of commercial
products constitute endorsement  by the U.S. Government.

An environmental protection publication (SW-729) in the solid
waste management series.
                 U,S. Environmental Protection Agency

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                             Preface







     The protection of ground-water resources is of vital



concern to health and environmental quality.  Thus, prompt



detection of potentially harmful pollutant emissions which



may be migrating from solid waste disposal facilities is



highly desirable and useful.



     Various technologies are available to detect contami-



nant presence and migration in ground-water systems, for



example, placement of monitoring wells or the several



geophysical methods.  This report deals with one such



geophysical method—electrical earth resistivity evalua-



tions.  The procedure is based on transmission of an electric



current into the subsurface materials and measurement of



the materials' resistance to the flow of that current.



Lower resistivity values indicate the presence of a greater



concentration of free or mobile ions, as may be found in



contaminated ground water.  Variations in resistivity values



also reflect the presence of varying geologic strata, but



they do not distinguish the cause of the differences in the



resistivity readings.  Thus, the investigator should have a



good knowledge of the geology of the study area in order to



differentiate between results indicating possible contamination



or simply the presence of earth materials with low resistance
                             111

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to an electric current.



     The use of electrical resistivity surveys in conjunc-



tion with placement of monitoring wells is stressed.  Remote



sensing, including resistivity evaluations, is not suggested



as a replacement for first-hand data which direct analysis



of a water sample can provide.  Instead, the report describes



how the resistivity method can be used to determine appro-



priate locations for monitoring wells and, under favorable



hydrogeologic conditions, can minimize the number of monitoring



wells needed to trace a contaminant plume.



     A description of the technique and specific examples of



field applications and case studies (under both favorable



and unfavorable hydrogeologic situations) are presented.



Basic concepts in data interpretation are also included.
                           IV

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                           CONTENTS

                                                             Page
INTRODUCTION	    1

EQUIPMENT 	    6

EXISTING DATA	   11

SITE RECONNAISSANCE	   13

     Sites Where Resistivity Will Work Well	   14
     Sites Where Resistivity Will Probably Not Work ....   18
     Sites Where Resistivity May Work, But With Some
          Difficulty	   23

RUNNING THE SURVEY	   34

     Manpower	   34
     Establishing Control 	   35
     Vertical Soundings and Single Depth Profiles 	   40
     Preliminary Resistivity Surveys	   43
     Detailed Resistivity Surveys 	   45
     Resistivity as a Monitoring Tool	   48

DATA REDUCTION AND INTERPRETATION 	   52

     Vertical Soundings 	   52
     Single Depth Profiles	   55

SUMMARY OF KEY POINTS	   58

CASE STUDIES	   59

     Preliminary Surveys	   59
     Detailed Surveys 	   78

REFERENCES	   93


                           FIGURES

                                                             Page

1.  Schematic Cross Section Showing Principals of
    Electrical Earth Resistivity Measurements 	    3
2.  Alternative Stake Design

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                       FIGURES

                                                               Page

 3.   Resistivity Unit Rain Shelter	    9

 4.   Alternative Reel Set-Up for One Man Operation	   10

 5.   Hypothetical Ground-Water Contamination Source
     Setting No. 1	   15

 6.   Hypothetical Ground-Water Contamination Source
     Setting No. 2.  .	   17

 7.   Hypothetical Ground-Water Contamination Source
     Setting No. 3	   19

 8.   Hypothetical Ground-Water Contamination Source
     Setting No. 4	   20

 9.   Hypothetical Ground-Water Contamination Source
     Setting No. 5	   22

10.   Hypothetical Ground-Water Contamination Source
     Setting No. 6	   24

11.   Hypothetical Ground-Water Contamination Source
     Setting No. 7	   26

12.   Hypothetical Ground-Water Contamination Source
     Setting No. 8	   28

13.   Hypothetical Ground-Water Contamination Source
     Setting No. 9	   31

14.   A Problem Related to Sites with Steep Topography ....   33

15.   Hand Auger for Shallow Soil Sampling and Probe for
     Extraction of Ground-Water Sample	   38

16.   A Method for determination of Optimum A-Spacing	   42

17.   A Method for Establishing Resistivity Measuring
     Point Locations	   47

18.   Monitoring Changes in Plume Configuration
     	  50

19.   Interpretation of Resistivity Delta Using the
     Empirical Cumulative Method	   54

                            vi

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                      FIGURES

                                                            Page

20.  Natural Scatter Effects of Geologic Conditions. ...   56

21.  Case Study 1 Location Map	   61

22.  Apparent Resistivity and Cumulative Resistivity
     Values for Vertical Resistivity Sounding Rl, Case
     Study 1	   63

23.  Case Study 2 Location Map	   67

24.  Case Study 3 Location Map	   70

25.  Apparent Resistivity and Cumulative Resistivity
     Values for Vertical Resistivity Soundings Rl and
     R4, Case Study 3	   72

26.  Case Study 4 Location Map	   74

27.  Trend of Resistivity Soundings Made at
     Calibration Point, Case Study 5  	   81

28.  Representative Resistivity Curves for Three
     Ground-Water Zones at Case Study 5	   83

29.  Extent of Contamination as Defined by Interpreta-
     tion of Resistivity Results, Case Study 5	   85

30.  Location Map and Extent of Contaminated Ground-
     Water Plume, 1972, Case Study 6	   86

31.  Location Map and Extent of Contaminated Ground-
     Water Plume, 1975, Case Study 6	   89

32.  Three Major Ground-Water Environments as Delineated
     by Interpretation of Resistivity Data, Case Study 7 .   91


                           TABLES

 1.  Results of Resistivity Survey, Case Study 1	    62

 2.  Results of Resistivity Survey, Case Study 3	    71

 3.  Results of Resistivity Survey, Case Study 4	    76
                             VII

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                                                             -1-
                          INTRODUCTION








     A necessary first step in any ground-water pollution inves-



tigation is to define the extent of the contaminated body of



ground water.  This is normally done by drilling and sampling a



number of observation wells.  In many cases, however, the elec-



trical earth resistivity method has proven effective in reducing



the number of observation wells needed, in allowing the wells to



be placed in the most useful locations and in generally expand-



ing the information obtained from the sampling program while re-



ducing the overall cost.  On the other hand, there are situations



where the resistivity method will provide little or no useful



information with regard to the contamination problem.  The in-



formation and advice presented in this report is based on the



experience gained from using the resistivity method, both suc-



cessfully and unsuccessfully, in a variety of ground-water



pollution investigations.





     A body of fluid introduced into an aquifer will tend to



remain intact rather than mix with, or be diluted by, the natural



ground water.  In many cases of ground-water pollution, the con-



taminating fluid is discharged into the aquifer as a continuous



or nearly continuous flow.  Thus, the contaminated ground water



will often be in the form of a plume, extending from the source



of contamination toward its natural discharge point.  The bound-



aries of the plume, usually quite distinct, will be established

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                                                                  -2-
by the size of the source area, the natural ground-water flow



pattern, and changes in the natural ground-water flow pattern



caused by pumping wells.  Bodies of contaminated water which



contain a high concentration of conductive substances will have



lower resistivity values than the surrounding natural ground wa-



ter because measured electrical earth resistivity is inversely



proportional to the electrical conductivity of ground water.



By measuring the resistivity at a number of locations in an area,



assuming a substantial conductivity contrast, it is often possible



to define the boundaries of a contaminated ground-water plume.



However, it should be kept in mind that some very serious types



of contaminants will not significantly raise the conductivity of



the ground water.





     A schematic diagram of the principal of earth resistivity



measurements (using the Wenner electrode configuration) is shown



in Figure 1A.  Essentially, four electrodes are placed in the



soil along a straight line so that the distances between elec-



trodes are equal.  The distance between electrodes is the A



spacing, which is approximately equal to the depth of the resis-



tivity measurement.  An electric current is passed through the



outer two electrodes and the resulting drop in potential between



the inner two electrodes is measured.  The Lee modification, which



allows a distinction to be made between horizontal and vertical



variations in the subsurface, is shown in Figure IB  (also see



page  43).

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                                                               -3-
   A.
  GROUND
     SURFACE
   B.
   GROUND
     SURFACE
                                                               OF

                                                       CURRENT  FLOW
 -LEGEND —


{ — ELECTRODE

A — DISTANCE  BETWEEN ELECTRODES

(WENNER CONFIGURATION IS SHOW)
  Figure  1.   Schematic cross  section showing principals of
                electrical earth resistivity measurements.

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                                                                -4-
     Apparent earth resistivity is controlled primarily by the



type of sediments in the subsurface, the degree of saturation of



those sediments, and the concentration of conducting ions within



the ground water.  With regard to sediment types, clean sand and



gravel has a high resistivity, silty or clayey sand has a lower



resistivity, and clay has a very low resistivity.  Within an area



of uniform sediment type, resistivity will be influenced largely



by the quality  (ion concentration) of the ground water.  It is



this property which allows delineation of subsurface plumes of



contaminated (highly conductive) ground water .





     Resistivity, as a tool for ground-water contamination in-



vestigations, can be used in several ways.  For example: to



detect polluted ground water in a particular area, to define



the extent of a polluted ground-water body, or to monitor the



movement of polluted ground water over a period of time.  To



illustrate the various uses of the resistivity method, the



following seven step ground-water contamination investigation



program is presented.





     1.  Assemble and analyze existing data.



     2.  Conduct a site reconnaissance.



     3.  Conduct a preliminary resistivity survey.



     4.  Install and sample preliminary observation wells.



     5.  Conduct a detailed resistivity survey.



     6.  Install and sample additional observation wells.



     7.  Establish a monitoring program incorporating resistivity.






Depending on specific requirements and conditions, individual

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                                                                -5-
steps in this program can be omitted.  Detailed discussion of



these seven steps and case studies which illustrate at least



some of them are included in subsequent sections.





     As mentioned previously, the resistivity method does not



always provide the desired answers.  Natural conditions which



can adversely affect the results of a resistivity survey by



masking resistivity contrasts between polluted and unpolluted



ground water include:  brackish (i.e. naturally conductive)



ground water; areally extensive, thick clay layers; horizontal



changes in geology or complex interbedding; deep water table;



and radical changes in topography within the study area.  In



addition to these natural conditions, a number of man-made ob-



stacles to successful resistivity surveys are frequently en-



countered.  These include:  buried electrical conductors, such



as pipelines and wires; metal fences; paved areas; and overhead



power lines.  It is essential that the resistivity equipment op-



erator be able to recognize, and if possible, to overcome or



avoid these pitfalls.





     No attempt is made in this report to include a theoretical



discussion of electrical earth resistivity or of ground-water



movement.  Treatments of these topics can be found in the list



of references.  Operators' manuals supplied by equipment manu-



facturers, in addition to operating and maintenance instructions,



usually contain some theoretical discussion.  An understanding



of the theoretical principles of electrical earth resistivity

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                                                              -6-
will prove essential in cases where complex interpretation is



necessary.  In addition, there are a number of measurement and



interpretative methods which are not discussed here but can be



found in the references listed on Page 93.






                           EQUIPMENT






     The basic equipment necessary to carry out a resistivity



survey includes the resistivity measuring unit, five electrodes



(metal stakes), four reels of wire, several (four-pound) hammers,



two 100-foot cloth (non-conductive) measuring tapes (200 feet is



better but more difficult to obtain), pencil and paper.  A variety



of resistivity unit, stake and reel designs are available from



the different manufacturers.  In addition to the basic equipment,



a number of modifications and additions have been found useful.



These include:





     - Modified electrode stakes.  Some manufacturers supply



       electrode stakes with welded cross members, which have



       been found to break at the weld when hammered.  Several



       alternatives are illustrated in Figure 2.  Always carry



       spare stakes.  To avoid damaging wire reels, never drive



       stakes with reels attached.






     - Pre-cut lengths of wires.  These are useful for extended



       single depth profiling.  Simply cut the four wires to the



       necessary lengths and eliminate the reels and measuring



       tapes.  Clip the wires to the stakes as in Figure 2-C.

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                                                                                -7-
    A.
             FOR  HARD  GROUND
          3/4" STEE L ROD
    24"
        /^"ALUMINIUM
              ROD
       TV  l' LONG
                UNION  BLOCK
                                COTTER  PIN
                                '/2'1 DIA. HOLE
                               B.
                                                         FOR  SOFT  6ROUND
                                                                      STEEL ROD WITH
                                                                      90°  BEND
              I  ALUMINIUM SQUARE  STOCK
c.
   OPTIONAL  STEEL DRIVE
  'CAP TO FIT OVER STAKE
    %"  STEEL ROD
        18"
                         WIRE CLIP
                                            A.   Stake  with  removable cross member.
                                                When  reel  is  resting on cross mem-
                                                ber,  friction  holds the union block
                                                up.   Remove the cross member and
                                                reel when  driving  the stake.
FOR FIXED LENGTH WIRES
OR PACK BOARD MOUNTED
       REELS
                       B.   Push stake  into ground and install
                           reel or clip.  Can also be hammered
                           i f necessary.
                                            C.   Use  with  clip on end of wire.
                      Figure  2.  Alternative  stake  design.

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                                                         -8-
   A shelter for the resistivity unit for use in rainy



   weather, as shown in Figure 3.  The shelter also serves



   as a carrying case.






   For work in wet, muddy areas, an empty wooden box or



   similar object for use as a platform for the resistivity



   unit.






   Equipment carrying cases.  Rigid suitcases of appro-



   priate sizes (three are good for weight distribution)



   with cutout foam inserts.  These work well for shipping



   and hand carrying.





   Backpack mounting of reels and stakes for easier one-



   man operation (See Figure 4.)






-  Emergency repair kit.  Fits into one of the carrying



   cases.  Include standard tools, electrical tape, extra



   clips and plugs for wire and spare batteries in moisture



   proof wrapper.





   Standardized data recording forms, and arithmetic graph



   paper in covered, rigid clipboard.






-  Slide rule or calculator for rapid data reduction.






   Safety glasses.  Steel splinters occasionally fly off



   the ends of the stakes when hammering them.

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                                                             -9-
           PLYWOOD  BOX  WITH  WATERPROOF  PAINT
           OR  POLYURETHANE
                                           PIANO  HINGE
                                                   FLAP  KEEPS HANDS
                                                   AND NOTEBOOK DRY
                                         FOAM  LINER
                                        (SOFT FOR SHOCKPROOFING )
         FEET IN FRONT ONLY SO TOP
         TILTS BACK FOR DRAINAGE
SET-UP USING WOODEN BOX BASE FOR
FOR EASIER  EQUIPMENT ACCESS
SHELTER  CLOSED  FOR
TRANSPORTING THE
   INSTRUMENT
                                           (WEATHER STRIPING AROUND
                                            FLAP FOR  WATERPROOFING
                                            CLOSED CASE)
 Figure 3.   Resistivity  unit  rain  shelter.

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                                                                   -10-
     SHOLDER STRAPS,
               I
        REEL  FOR WIRE
                                                  PLYWOOD BOARD
                                                        ,CLIP TO HOLD
                                                           STAKES
                                                        ELECTRODE
                                                          STAKES
PLUG IN CONNECTOR
                                               UNIT TO REEL CONNECTORS
Figure  4.   Alternative reel set-up for one  man operation.

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                                                                -11-





                         EXISTING DATA







      Interpretation of a resistivity measurement is reliable



 only if the results can be correlated with directly obtained



 data.  When using resistivity to define ground-water contamina-



 tion problems,  the more information regarding the subsurface



 geology,  hydrology and water quality that is available,  the more



 reliable the interpretation will be.  The best method of obtain-



 ing control information is to drill wells at the site, log the



 materials penetrated,  and collect and analyze water samples.  If



a preliminary survey is being run (to determine the best locations



 for these wells)  or if wells cannot be drilled,  existing sources



 of control must be sought.  In addition to subsurface information,



 aerial photographs and/or accurate maps are important to precisely



 locate points of resistivity measurement, and should be obtained



 prior to running a survey.





      If wells or borings are already present in the vicinity of



 the landfill, useful information to be obtained would include



 geologic logs,  geophysical logs, water-quality data (particularly



 specific conductance or total dissolved solids), and static water



 levels (to establish flow directions).  Well or boring data may



 be available from public agencies,  such as the U.S. Geological



 Survey, the State Geological Survey or Department of Environmental



 Protection; well drilling companies; construction and engineering



 firms.

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                                                               -12-
     If direct subsurface data is unavailable,  indirect informa-



tion may be useful.   An overview of geology and topography (bed-



rock outcrops, wetlands, roadcuts, etc.),  drainage,  ground-water



discharge patterns and surface-water quality,  can provide a



preliminary picture of subsurface conditions (this procedure is



outlined in the next section).  Published regional geologic maps,



both surficial and bedrock, may also be helpful.





     Aerial photographs of the study area are useful for several



purposes.  Prior to conducting the field work,  initial measuring



points can be selected based on inspection of aerial photos.



During the survey, points measured can be accurately located on



the photo for transfer to a base map.  Air photos also provide



a general overview of the area which cannot be obtained from the



ground, especially in wooded or marshy areas.

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                                                               -13-
                        SITE RECONNAISSANCE






     After available data has been examined, and prior to start-



ing resistivity measurements, an inspection of the study area



should be conducted.  The purposes of this inspection are:





        to establish if the resistivity method is applicable



        at this particular site (includes interference factors)






     -  to determine the extent of the preliminary resistivity



        survey needed





        to locate areas of interest for the preliminary survey





        to obtain as much data as possible for control





The important features to be considered include:  surface drain-



age, topography, surficial geology, condition of surface-water



bodies and stressed trees and other vegetation.  Features ob-



served should be noted on a map of the site.  Aerial photography



is especially useful at this stage of the investigation.





     Following are several hypothetical ground-water pollution



source (e.g. landfill, waste lagoon, chemical stockpile) settings.



At some of these hypothetical sites resistivity measurements will



provide a good definition of ground-water contamination with



little difficulty. At some, obtaining useful resistivity results



will be quite difficult and at others the resistivity method will

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                                                               -14-






provide a little useful information.  Recognizing into which



category a particular site falls will help prevent wasting time



running useless resistivity surveys.  It must be kept in mind



that the example cases are intended to illustrate individual



points and are therefore necessarily simplified.  In reality,



sites will generally be characterized by several overlapping



problems of varying degree.





Sites Where. Resistivity Will Work Well





Setting 1 -






     The contamination source is situated on a recharge area about



1000 feet from a small river, as shown in Figure 5.  The nature of



the contaminated ground water is such that it has a conductivity



ranging from five to ten times that of the natural ground water



in the area.  The site is characterized by a uniform unconsolidated



aquifer overlying crystalline bedrock.  The water table is shallow



and ground water is recharging the river.  At this site a resis-



tivity survey can  establish:





     1.  The lateral extent of the contaminated ground-water plume.



     2.  The distance the plume has traveled toward the discharge



         area.



     3.  Migration of the plume beyond the discharge area.  This



         might occur under the influence of pumping or deep mi-



         gration of the  leachate within the natural ground-water



         system.

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                                                                  -15-
              ^s^f^^i^J^^pp^^^^sySs^^
                  CONTAMINATION

                     SOURCE
             500
              l
 1000 FEET
	I
Figure  5.   Hypothetical ground-water contamination source
                       setting No. 1.

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                                                               -16-
     4. Depth to the top of the contaminated plume and possibly



        its thickness.





Setting 2 -






     The contamination  source is situated in a discharge area, in



this case a fresh-water marsh which is drained by several small



streams  (Figure 6).   The plume of contaminated ground water is



spread out over a larger area than in Setting 1,  its movement



slower and attenuation  of contaminants is greater due to the marsh



deposits.  The contaminated ground water has a conductivity several



times that of the natural ground and surface water in the marsh.



At this site the resistivity method can establish:





     1. The areal extent of significantly polluted water.  Unlike



        Setting 1 where the edge of the plume was well defined,



        in this case there will be a concentration gradient from



        highly polluted to natural ground water because of attenu-



        ation.





     2. If the contamination is spreading evenly, as shown in



        the sketch,  or is being channeled by preferential flow



        through more permeable zones.





     3. Possibly, the depth to which the contamination has migrated.






Setting 3 -





     The contamination source is situated over a two-aquifer sys-

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                                                                         -17-
    LEGEND

CONTAMINATION  LEVEL

 ^M— HIGH
 EZ3— MODERATE
 C~J — LOW
    1- UNCONTAMINATED
                                                      NORTH
                                        CONTAMINATION

                                           SOURCE
                                                          DIRECTIONS  OF
                                                          GROUND- WATER
                                                               FLOW
GRADATION  IN POLLUTION
   CONCENTRATION
    Figure 6.  Hypothetical  qround-water contamination source
                             setting No.  2.

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                                                               -18-
tem where the deeper aquifer is used for water supply, and sep-



arated from the upper aquifer by a leaky confining bed (Figure 7).



Ground-water flow in the shallow aquifer is toward the stream.



However, some contaminated water also moves into the lower aquifer



under the influence of the pumping gradient and toward the well.



In order to be able to define the deeper plume, the shallow aquifer



and confining bed must be fairly thin and a large conductivity



contrast must exist between natural and contaminated ground water.



In this case the resistivity survey can establish:





     1. The lateral extent of the contaminated ground-water plume.






     2. The distance the plume has traveled toward the natural



        discharge area.





     3. The presence and lateral extent of the plume between the



        contamination source and the pumping well.





     4. Possibly the thickness of the shallow aquifer and confining



        layer at various points around the site.






Sites Where Resistivity Will Probably Not Work





Setting 4 -






     The contamination source is situated in a wetland (discharge



area) adjacent to a major stream, as shown in Figure  8.   In this




case natural ground-water flow will be from the high area to the



south toward the river on the north.  As a result of increased

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                                                                           -19-
                                                                     N
                                                                    WATER TABLE


                                                                    UPPER AQUIFER

                                                 —^-:^^r-_-r-LJT.^--jr-_= -«	CONFINING LAYEf

                                                                    LOWER AQUIFER
                            J CONTAMINATED
                            GROUND WATER
    VERTICAL EXAGGERATION 5 TIMES
Figure  7.   Hypothetical ground-water  contamination source
                            setting  No.  3.

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                                                                  -20-
                                                       NORTH
                                              CONTAMINATED
                                               GROUND WATER
             CONTAMINATION
                                    CONTAMINATED
                                    GROUND WATER
                                —~— DEPOSITS — ^=~—
                                                          A'
GROUND WATER  ^-i
    FLOW
                                      NOTE: VERTICAL EXAGGERATION
                                            10 TIMES
Figure  8.   Hypothetical ground-water contamination source
                          setting  No.  4.

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                                                                -21-
recharge, a mound of ground water will develop under the source



and force the contamination downward.  While there may be sub-



stantial downward movement of water from the contaminant source,



the principal ground-water flow will be horizontal or upward to-



ward the river.  Thus, at this setting, the plume is  confined to



the area beneath the source and discharges directly to the river.



In this case the resistivity method will be of little or no val-



ue in an investigation of ground-water contamination.





Setting 5 -



     In this case the contamination source is situated in or at



the edge of a salt marsh or immediately adjacent to a salt-water



body (see Figure 9).  Since the naturally brackish water is high-



ly conductive, resistivity measurements in this environment will



be extremely low (on the order of 10 ohm-ft).  With the contami-



nated water flowing into this brackish environment, and mixing



with it to a certain extent, no resistivity contrast between con-



taminated and naturally occurring water will exist.





     If the setting is slightly brackish rather than salty, use



of the resistivity method may be feasible.  In this case, com-



parison of the specific conductances of the contaminated and



natural ground water will be helpful in determining if the re-



sistivity method is worth a try.  With a contrast in specific



conductance of about one order of magnitude, it is fairly safe



to assume that sufficient resistivity contrast exists, although



a contrast considerably less than this will be sufficient if



other conditions (e.g. depth to water, geology) are favorable.

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                                                                   -22-
                                                                      N
                                           SALT  WATER

                                               BODY
                                                             CONTAMINATED
                                                             GROUND WATER
     FRESH
     WATER
        DIRECTION OF
      GROUND WATER FLOW
        0
        I
250
 i
           500 FT.
     VERTICAL EXAGGERATION  4 TIMES
                                                                        A1
                                                                     SALT WATER
                                                                      BODY
                                                                 SALTY GROUND
                                                                   WATER
Figure 9.  Hypothetical ground-water  contamintion  source
                           setting No.  5.

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                                                             -23-
Setting 6 -





     The contamination source is located in a heavily developed



area as shown in Figure 10.  This situation occurs primarily



when investigating industrial contamination cases.  The large



paved areas and stockpiles interfere with resistivity work be-



cause stakes cannot be driven into the ground.  Buried pipes



and wires, overhead high-tension wires, and fences with steel



posts will adversely affect resistivity results.  The proximity



of secondary pollution sources  (including accidental spills and



airborne pollution) will further confuse the investigation.



Given these factors, a resistivity survey at this site is not



feasible.





     An important part of the initial site inspection is to



establish how much interference to a resistivity survey exists



at a site.  Objects and features of the site that will affect



resistivity results should be carefully mapped.  Based on this



map the areas open to resistivity measurements can be evaluated



and, if sufficient, a grid of measuring point locations can be



established.





Sites Where Resistivity May Work, But With Some Difficulty






Setting 7 -





     The ground-water contamination source is situated on the

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                                                                     -24-
                           HIGH TENSION
                              WIRES
                   CONTAMINATION

                      SOURCE
                                          CONTAMINATED
                                             WELL
                                  UNDERGROUND
                                     PIPELINE
     AREAS NOT SUITABLE
     FOR RESISTIVITY MEASUREMENTS
Figure  10.  Hypothetical ground-water  contamination source
                             setting No.  6.

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                                                               -25-
boundary between two very different geologic units.  In the case
illustrated in Figure 11, the source is on the edge of a valley
which is underlain by a clay layer and drained by a medium-sized
stream.  The clay pinches out in a sand layer about 500 feet
from the stream as shown in the cross section.  A characteristic
of this site is that contaminated water seeps are evident along
the northern edge of the source area.  Much of the contaminated
water is discharged from these seeps and flows over the surface
to the stream.  An important question here is whether or not a
second component of contaminated water flow exists in or beneath
the clay.

     One difficulty encountered at this site is that the clay
layer will serve to mask any resistivity contrast due to the
contamination by giving uniformly low readings over the site.
Since resistivity measurements made upgradient of the source
are expected to be much higher than those made downgradient
simply because of the geology change, (the resistivity of sand
is higher than that of clay) it may be difficult to find a con-
trast which can be attributed to contamination.

     If possible, a series of resistivity profiles (single depth
or fixed electrode spacings) should be run along the stream bank to
see if a contrast exists between the area downgradient of the
contamination source and other areas not likely to be affected.
However, if a contrast is found, the shallow contaminated clay

-------
                                                                     -26-
                                                      NORTH
  -CLAY-
                                     SURFACE LEACHATE
                                        DISCHARGE
      DIRECTION OF
      GROUND WATER
         FLOW
  -SAND-
c
CONTAIMNATfQN
  SOURCE
      - SAND -
                           400
                                     800 FT.
                                     	I
     \SOURCE
WATER TABLE


    SAND
                                            GROUND WATER
                                                 FLOW
   Figure 11.   Hypothetical ground-water  contamination source
                               setting No.  7.

-------
                                                              -27-
 layers may be responsible for it and the more important question is



 whether or not there is deep migration.  One possibility for re-



solving this question is to run profiles on the northern side of the



stream.  If contaminated ground water ( a low-resistivity zone) can



be identified here, it is likely that a significant component of



deep ground-water flow exists.  A thick clay layer can mask a zone



of contamination beneath it,  however, and the method may not work.





       In this type of setting, extensive direct control data will



 probably be necessary to define the problem with  any confidence.



 Installation of several wells to establish the needed control,



 however, may make resistivity measurements unnecessary.  This



 type  of situation then becomes very site specific and cannot be



 dealt with in general.  The  investigator should be able to rec-



 ognize, by a simple inspection, the type of difficulties that



 a setting such as this will  present with regard to interpretation



 of resistivity results.





 Setting 8 -





       In this case the ground-water  contamination  source is situ-



 ated  such that the water table is relatively  deep and the unsatu-



 rated material is anisotropic, as shown in Figure 12.  The con-



 taminated water will move primarily downward  through the unsatu-



 rated zone and thus will remain directly under the contamination



 source.  Once the polluted water reaches the  water table, however,

-------
                                                                   -28-
                    A1
            -45 ;
              • + 10-
         •+20-
              •+30-
        -+40-
        -+60-

        •+80-
•+50-
•+70-
        • + 90
                                                                NORTH
                                              DIRECTION OF
                                              GROUND-WATER
                                                 FLOW
                                                       2000 FT.
                                                  LEGEND

                                            — +90  ELEVATION  CONTOUR
                                                   IN FEET ABOVE MEAN
                                                   SEA LEVEL
                                                           -A
VERTICAL EXAGGERATION  10
                                  GROUND WATER FLOW DIRECTION
 Figure 12.   Hypothetical ground-water  contamination source
                             setting No. 8.

-------
                                                             -29-
it will begin to move in the direction of ground-water flow.



It is the direction and extent of this movement that the resis-



tivity survey is designed to establish.






     In this setting  (Figure 12), the water table lies 80 feet



below land surface.  This means that 80 feet of unsaturated ma-



terial must be penetrated by the electric current before any



effect caused by the pollution is encountered.  If this over-



burden is uniform from place to place, then the resistivity pro-



files may indicate a significant contrast between natural and



contaminated ground water.  If, as illustrated in Figure 12,



the unsaturated materials are not uniform, contrasts in resis-



tivity value probably cannot be attributed to contaminated wa-



ter with any certainty.





     The information needed to identify this setting is not



apparent on the surface.  If the investigator  has reason to



suspect, based on surface observation or general familiarity with



the region, that deep water table and complex geology are the



case, control points will be necessary to evaluate the usefulness



of the resistivity method.  Adequate control, in this case, would



include well logs and water quality data from at least one well



in the contaminated plume and at least one in a background area.



A vertical sounding (multiple electrode spacing survey)  would then



be run adjacent to each well and the interpretation of the measure-



ments correlated with the well logs.  Once control has been

-------
                                                             -30-
established, a number of vertical soundings should be run in a



grid pattern to extend this control over the entire study area.



Horizontal profiling (a fixed electrode spacing survey),  if used



at all, should probably be limited to filling in between vertical



soundings to refine the accuracy of plume definition.






     The resistivity operator should be aware that in this par-



ticular setting he may be easily misled into interpreting geol-



ogy changes as water quality changes.  Because of this possi-



bility, this type of setting, more than any other, requires ex-



tensive direct control.  Two precautions that may improve the



chances of success in some instances are:  first, run the sound-



ings as deep as possible  (to instrument limitation)  to include



the thickest possible section of saturated material; and second,



conduct the survey after a prolonged dry spell when  the unsatu-



rated layers have the highest resistivity and also to minimize



the interference effects of percolating rainwater.





Setting 9 -





     In this final hypothetical case, the ground-water contamina-



tion source is situated in an area of extreme topographic varia-



tion  (see Figure 13).  Several factors which can cause problems



in interpreting resistivity data come into play in this type of



setting.  One is the variation in depth to water which may  cause



large resistivity contrasts regardless of changes in water  quality.

-------
                                                                             -3]-
    *
    o
    c!
                                                                    NORTH
              500

              	I
 1000 FT.

  I
               250
 500 FT


_J
                                          DIRECTION OF GROUND WATER  FLOW
Figure 13 -  Hypothetical  ground-water  contamination  source Setting No. 9.

-------
                                                             -32-
Another is the lateral variation in geology that is likely to



occur in this particular setting, as shown in Figure 13.  By



using a theoretical interpretation (curve matching) of the data



which isolates the various layers and concentrates only on the



layer of interest (upper zone of saturation), the problems



caused by variations in geology and depth to water may be re-



solved.  A knowledge of the subsurface geology will be helpful



in this interpretation.  For a discussion of the theoretical



curve matching method, see Orellana and Mooney, 1966.








     A third factor is that if a profile is run along the base



of a cliff or steep hill, the result may be significantly diff-



erent than if the profile is run on the top of the cliff or hill,



even though the material being measured is identical.   This is



simply because the section of material included in the measure-



ment will be drastically different (see Figure 14).  This is a



minor difficulty and should only be a problem when:





     1.  The topography changes are extreme.



     2.  The resistivity contrast between contaminated and



         natural ground water is small.

-------
                                                                         -33-
                               S/TE  2
                                                         SITE 3
       — L EGEN D —
            SECTION  OF  EARTH
            BEING MEASURED
                                                                     GROUND SURFACE
GROUND SURFACE
                    SITE I
                                                UNIFORM  MATERIAL
                                              ASSUMING ISOTROPIC MATERIAL ;

                                               RESISTIVITY OF SITE I
                                              RESISTIVITY OF SITE  3
                                             RESISTIVITY OF SITE 2
     Figure 14.   A problem related to  sites with steep topography.

-------
                                                             -34-
                        RUNNING THE SURVEY








Manpower






     Before trying to determine the time and manpower necessary



(i. e. cost) for a resistivity survey, the size and complexity



of the site, as well as the objectives of the survey must be de-



fined.  These objectives are determined both by the needs of



the investigator and by the available control. For example, a



full definition of-the extent of contaminated ground water at a



landfill site may be desirable; however, due to lack of direct



subsurface data, this may not be possible using resistivity



alone.  In such a case resistivity may be useful as a prelimin-



ary tool to aid in the location of observation wells which



would then be used to establish control for a more detailed sur-



vey.  The initial survey in this case would be used to locate



probable contaminated and uncontaminated sites in areas where



access for a drill rig could be gained.  Besides the specifics



of the site, the purpose of the investigation controls the mag-



nitude of the necessary effort.





     Normally a resistivity crew should consist of 3 to 5 persons,



5 being the optimum for vertical soundings to depths greater than



about 100 feet.  For very limited work, two people may be best.



In the case of a three-person crew there would be one person to



operate the unit and record the results, one person to move and



set the two right stakes, and one for the two left stakes.  A

-------
                                                             -35-
five-person crew consists of the operator and one person to move



and set each stake.  Normally, the operator can record and plot



the data while the stakes are being moved.  If more speed is de-



sired, each stake mover can have 2 stakes; while one is being



used the other is being driven, and only the reel or clip has



to be shifted.  In this case, a sixth person to record and pl6t



the data for the operator may improve efficiency.   (Note:  it is



important to make preliminary data plots in the field to help



select other points for measurement as well as uncover measure-



ment errors.)





Establishing Control





     The following relationships have a distinct influence on



the success of a resistivity survey to define a contamination



problem.






     - Contrast between the conductivities of the contaminated



       and natural ground water.



     - Depth  (below land surface)  to the top of the contaminated



       ground-water body.



     - Thickness of the contaminated ground-water body.



     - Lateral variations in geology.





     If the contaminant does not have a significantly greater con-



ductivity than the natural ground water, if the ground water is



naturally highly conductive itself, or if the depth to water is



great, a large enough resistivity contrast may not exist and the



method may not work.  If the contaminated body is very thin rel-

-------
                                                             -36-
ative to the overlying material, it may not have a significant



influence on the resistivity readings.  Also, if the geologic



environment is overly complex, comparison of resistivity values



may not be possible.  Any of these conditions, if sufficiently



adverse, could preclude the success of a resistivity survey.



However, it is typically the combination of several of these con-



ditions which makes a site unsuitable for the resistivity method.





     The direct control required for a successful resistivity



survey increases as the above-mentioned conditions become more



adverse.  In the simplest situations a series of single depth re-



sistivity measurements is sufficient to define the areal extent



of a leachate plume.  More often, some degree of control is neces-



sary to achieve success.





     The resistivity operator should be able to determine the



suitability of the resistivity method for detecting contaminated



ground water at a particular site after the first few readings.



To do this he must establish whether the resistivity contrast be-



tween natural and contaminated ground water is sufficient to over-



come naturally caused resistivity fluctuations, or "natural



scatter."  Figure 20  (Page 56) illustrates this effect.  As



explained by Kiefs tad, if the contamincition level is low, the



difference 'Y' will be small and not detectable.  If the scatter



'X1 is large, the difference  '¥' will be masked.  Thus, resistiv-



ity measurements must be made in contaminated and uncontaminated

-------
                                                             -37-
areas within similar geologic environments, as well as in all



the geologic environments at the site.  The degree to which this



can be actually accomplished is dependent on the available control.






Geologic Control - Direct observations of subsurface geology by



means of drilling and sampling a sufficient number of boreholes



over the landfill site is the best method of establishing geo-



logic control.  It may, however, be desirable to use the resis-



tivity method to help locate the best sites for wells or, for



limited investigation, resistivity may be used without a drilling



program.  Assuming then that a resistivity survey is to be con-



ducted without the benefit of data from drilling, other sources



of geologic control data must be used.  Such sources would include:





     - Geologic logs from existing wells or boreholes at the site



     - Interpolation between geologic logs from wells or bore-



       holes located near the site



     - Gamma-ray or electric logging of existing wells or bore-



       holes for which no logs are available



     - Seismic surveying



     - Hand augering for shallow geologic evaluations  (See



       Figure ISA )



     - Estimates of subsurface conditions based on a general




       knowledge of the geology of the area and on surface obser-



       vations

-------
                                                                -38-
HAND AUGER FOR SHALLOW
  GEOLOGIC  EXPLORATION
      PROBE FOR SHALLOW
    GROUND-WATER STUDIES
A.
                    -HANDLE
                    •COUPLING
                    •PIPE
B.
SMALL HAND PUMP
 ( MARINE STORE )
                                       PLASTIC
                                       TUBING •
                     BIT
                                                   SAMPLE _/  ^—A
                                                   BOTTLE
               COPPER OR ALUMINUM
               TUBING ( HARDWARE STORE )
                                                     SMALL HOLES DRILLED
                                                     IN END
                                                    END HAMMERED FLAT
 Figure 15.  Hand auger  for  shallow soil  sampling and probe  for
                      extraction of ground-water sample.

-------
                                                              -39-
     Resistivity profiles should be run in areas of contrasting



geology, as established by the best available control to determine



the natural scatter.  In cases where major changes in geology



occur but cannot be defined by the available control, resistivity



may have to be ruled out as a method for detecting contamination.



If, however, control is sufficient to identify areas of major



geologic change, it is often possible to make adjustments in data



reduction to eliminate this effect.  (See section describing re-



duction of single depth profile data, Page 55 .)





Water Quality Control - Since electrical earth resistivity is in-



versely proportional to the specific conductance of the ground



water in an area, measurement of specific conductance is very



useful in the interpretation of the resistivity data.  When wells



are available in both contaminated and uncontaminated areas, the



specific conductance contrast of ground water from these wells



can be determined.  Where the water table is shallow, or the



overlying sediments are very soft, ground-water samples can often



be obtained with a probe of the design shown on Figure 15B.  If



ground-water samples cannot be obtained, measurement of the spe-



cific conductance of surface water in discharge areas may be



helpful in simple hydrologic situations.  In the event water quality



control cannot be established, resistivity measurements must be



made near the downgradient toe of the landfill, and also in an



area upgradient of the landfill but with a similar geology.

-------
                                                             -40-
These can then be considered contaminated and uncontaminated



readings respectively, provided of course, the operator can



establish which direction is downgradient and how far upgradient



the contamination might have spread.  The contrast in resistivity



between these two areas is then compared to the natural scatter



caused by geologic changes in the area.  If the contrast caused



by contamination is significantly greater than the contrast



caused by geologic changes, the control is adequate and the sur-



vey can proceed.  If the contamination cannot be distinguished



from the geology, resistivity will not be useful without addi-



tional control.





Vertical Soundings and Single Depth Profiles





     Vertical soundings are used to define the thickness of layers



of contrasting resistivity in the subsurface.  In ground-water



contamination work, the principal value of vertical sounding is



in establishing control.  While it may be. theoretically possible



to define the thickness of a leachate plume, experience indicates



that this usually cannot be done.





     Single depth profiles are simply resistivity measurements



made with a fixed electrode  (A) spacing.  The electrode spacing,




roughly equivalent to the thickness of the section being measured,



is established by direct control or by interpretation of vertical



soundings.  The principal value of single depth profiles is that



they are much  faster to run than vertical soundings.

-------
                                                             -41-
     By running vertical souridings in contaminated areas, the
depth, or A-Spacing, at which the effect of the contaminated wa-
ter has most influence on resistivity can be determined.  For
example, in Figure 16 a plot of apparent resistivity 'vs. depth
below land surface is shown.  In this hypothetical case, contami-
nated ground water is present beneath a layer of clean sand and
on top of impermeable crystalline rock.  If the A-Spacings are
too shallow or too deep, the contamination will either not be
detected or will be masked.  The cumulative plot (see section on
data reduction for description)  of the vertical sounding results
shows the correct A-Spacing to be about 50 feet.  At this depth
the apparent resistivity values  are near the lowest point, and
almost all of the contaminated section is included.  If a signif-
 icant resistivity contrast cannot be found at any depth using
vertical soundings,  resistivity  will not be useful at that site.
In some cases, such as in areas  of steep topography, A-Spacings
may have to be adjusted from one portion of the site to the next.
This is established by comparing the results of a series of
vertical soundings run at various elevations around the site.
Once the A-Spacings have been selected, the definition of the
areal extent of the plume or slug of contamination is undertaken
by running a series of single depth profiles.

     Another important use of vertical soundings is to define
contaminated plumes in geologically and hydrologically complex

-------
                                                                           -42-
Note:   Selected A - spacing, 50 feet,  is near  the  bottom of the contaminated
       zone and near the lowest apparent resistivity  measurement.
OT
CO
tti
cc
w
a:
a.
a.
     10000
    8000
 i
 E
~   6000
4000
     2000
                                            CUMULATIVE

                                             RESISTIVITY
                                    APPARENT  RESISTIVITY
                               X
                                         OPTIMUM  A-SPACING

                                         ""   ( 50  FEET )
                                     \  i'
                                      \

                                      ^._^-*'
20          40          60

       ELECTRODE SPACING  (FEET)
    (or approximate depth below (and surface)
                                                          80
                                                                    20000
                                                                    16000  _
                                                                           «*-
                                                                           i
                                                                           E
                                                                         12000
                                                                           t-
                                                                           >
                                                                           P
                                                                           CO

                                                                           CO
                                                                           Hi
                                                                           a:
                                                                    8000
                                                                         4000
                                                                  100
  Figure 16.   A  method for determination of  optimum A - spacing,

-------
                                                             -43-
areas.  In such settings, detailed interpretation at each meas-


uring point may be necessary to distinguish water quality con-


trasts from geologic contrasts.  Such interpretation can only be


done with vertical sounding data.



     A technique useful in complex areas is the Lee modification


of the Wenner electrode spread (shown on Figure IB).  With this


method, the resistivity is measured first between P-j_ and ?2r
                                                     *

giving the "Full" value.  Resistivity is then measured between


P]_ and Pg and between PQ and ?2,  giving the "Lee Left" and "Lee


Right" readings respectively.  Lateral .variation in geology should


be detected by a large difference between the Lee Left and Lee


Right readings.



Preliminary Resistivity Surveys



     A preliminary resistivity survey is defined here as a brief


survey conducted at a site where sufficient control to warrant


a detailed survey is unavailable.  A preliminary survey is typi-


cally brief and thus is relatively inexpensive.  One or two days


in the field and one day to analyze the data is usually sufficient


for even a large and complex site.  If a good deal of direct con-


trol is available at a site, the preliminary survey can be skipped


and a detailed survey run directly.



     The primary purposes of a preliminary resistivity survey are:


1)  to establish the validity of the resistivity method at that

-------
                                                             -44-
particular site, 2) to establish the presence of pollution in the



ground-water system, and 3) to establish the best locations for



the installation of observation wells.





     As a first step in the preliminary survey the operator should



become familiar with the site to be investigated.  The map and



notes from the site reconnaissance, along with whatever other



data is available regarding geology, hydrology, and water quality,



provide the background information.  Based on this background



data, several  (up to 5 or 6) key areas at the site are selected.



These areas should represent natural and contaminated ground wa-



ter zones and any major geologic or topographic changes that are



known to exist.  A vertical sounding should be run for each key



area.  If direct control is available, the sounding should be



run as close as practical to the control point.  After each ver-



tical sounding has been completed and the data reduced, a few



single depth profiles should be made in a line starting at the



vertical sounding and running across the key area.  This will



help establish the areal extent of the subsurface conditions de-



fined by the vertical sounding.  The location of resistivity



measurements should be carefully mapped relative to each other



and to the available control points.  Any other pertinent infor-



mation, such as the location of discharge area, seeps, dead trees,



etc., should be noted on the map.  The field work should take one



or two days depending on the size, complexity and ease of access



of the site.

-------
                                                             -45-
     After the field work has been completed, the data is further



reduced if necessary (curve matching interpretation, for example,



would be done now), and compared to the direct control data or,



if this is not available, to what is assumed to be the condi-



tions in the key areas.  At this time the usefulness of a more



complete resistivity survey will be apparent and the type and



amount of additional control data needed can be established



along with the best location for more resistivity measurements or



control points.






Detailed Resistivity Surveys





     A detailed resistivity survey is defined here as a survey



which provides adequate data to fully define the areal extent of



ground-water contamination and, if possible, to determine the



depth to and thickness of the contaminated ground-water body.



The detailed resistivity survey will typically require from a



few days to a few weeks, depending on the complexity of the site,



the number of vertical soundings required to establish suffi-



cient control, the ease of access to the site, and available man-



power.  The detailed survey layout is based at best on a formal



preliminary survey and at least on a few initial vertical sound-



ings made near direct control.






     Direct control is the key factor in the success of the de-



tailed survey.  If this control is not available, it is usually



necessary to first run a preliminary survey and then drill borings

-------
                                                               -46-
or wells to establish control points.   If this is not done,  the op-



erator may find himself in the unfortunate position of having



spent six or eight man-weeks and then not be able to interpret



the data with any confidence.






     In setting up a plan for a detailed resistivity survey, all



previously gathered data, including preliminary inspection results,



available background data, results of preliminary surveys, well



drilling or soil boring logs and water quality data, are taken into



account to develop a theoretical picture of the contaminated ground-



water plume.  With this picture in mind, the detailed survey plan



can be constructed to confirm  (or refute) the theoretical plume



and refine the mapping of its dimensions to the desired degree.  If



the available direct control is not sufficient to allow a confident



appraisal of the subsurface conditions of the entire site, the



number of vertical soundings should be increased in order to allow



a more confident interpretation of subsurface conditions.





     At this point, the planning of the detailed survey should be



obvious to the operator who is now quite familiar with the site



and knows what his objectives are.  In the -event further guidance



is neeided, the following plan can be followed:






     1.  Place a uniform grid system over a map of the site to be



         surveyed  (an example is shown on Figure 17).  Generally,



         the area of the grid squares should be about 1/4 the size



         of the source  (or less).

-------
                                                                    -47-
                               (DISCHARGE  AREA)
                   O    FIRST ROUND MEASURING POINT (NATURAL)
                   0    FIRST ROUND MEASURING POINT (CONTAMINATED)
                   A    SECOND ROUND MEASURING POINT (NATURAL)
                   A    SECOND ROUND MEASURING POINT ( CONTAMINATED )
                  	  EXTENT OF THEORETICAL PLUME
                        EXTENT OF ACTUAL PLUME
Figure  17.   A method for  establishing resistivity measuring
                          point locations.

-------
                                                               -48-
     2.   Outline the perimeter of the contamination source and the



         discharge area.



     3.   Based on all available data, outline where the plume should



         be (theoretical plume).



     4.   Mark each grid point necessary to confirm the location of



         the theoretical plume shown as circles on Figure 17.



     5.   As the survey is being run, add measuring points to the



         grid system as necessary to define the actual plume (shown



         as triangles on Figure 17).





     This plan is somewhat oversimplified in order to fit most all



general cases and is not meant to be followed exactly, but rather



to illustrate the principles involved in planning the survey.  Re-



member that to define the contaminated plume, a resistivity contrast



which can be attributed to water quality must be found.  When a



number of contrasting points have been located, the results are



contoured and the areal definition portion of the survey is com-



plete.  To establish the depth to and possibly the thickness of the



contaminated plume, a series of vertical soundings are run in the



contaminated area.





Resistivity as a Monitoring Tool






     As defined here, monitoring is the repeated measurement of the



same parameter at the same location over a period of time.  In this



case, the purpose of monitoring is to detect changes in ground-



water quality with time and to relate these to modification of the

-------
                                                             -49-
contamination source or other occurrences.  Typically ground-



water monitoring is accomplished by installing and periodically



sampling a series of wells.  Resistivity as a monitoring tool



should not be used to replace monitoring wells, but rather to



expand the monitoring system while reducing the number of wells



needed.  Once a detailed resistivity survey has defined a plume



of contamination and the source has been modified to reduce dis-



charge of pollution, a monitoring program is needed to establish



the effectiveness of the modifications made to minimize the



effects of contamination.  This must accomplish two goals:



1) keep track of the lateral extent of the plume, and 2) determine



water quality changes within the plume.  To effectively monitor



the latter, 2 or 3 wells are installed in the center of the



plume along a line parallel to the direction of ground-water



movement.  However, to keep track of the boundaries of the plume



will, in this case, require an additional 11 wells, as shown on



Figure ISA.  To be useful, this type of monitoring well must be



located just outside the contaminated plume.  If the plume en-



larges, more wells must be drilled.  On the other hand, if the



plume contracts, as is hoped, the stationary monitoring wells



cannot detect this movement.  To effectively monitor an enlarge-



ment and then a contraction of the plume, 19 more wells would be re-



quired in this case (see Figure 18B), a relatively expensive



proposition.



      An  alterpativp to  the monitoring  svstem  rtescriheri  above

-------
                                                                                      -50-
(A.)

IN THIS HYPOTHETICAL
SITUATION, ELEVEN
MONITORING WELLS
ARE  INITIALLY
INSTALLED.
CONTAMINATION
    SOURCE
.-ORIGINAL ': •'  :.;  '.

 ••-.:•' CONTAMINATED ,

'  • .. '•  ' PLUM  '.-. ;'
 (B.)

 NINETEEN MORE
 WELLS ARE
 INSTALLED
 AFTER TWO
 CHANGES IN   •
 PLUME
 CONFIGURATION.
 (C.)
 AN ALTERNATIVE TO
 A and B  ABOVE ;    <*

 INITIALLY, THREE
 MONITORING WELL
 PLUS RESISTIVITY.

 AFTER TWO
 CHANGES IN
 PLUME CON-
 FIGURATION, TEN
 MONITORING
 WELLS  PLUS   \
 RESISTIVITY.
 CONTAMINATION
     SOURCE
CONTAMINATION
     SOURCE
     FIRST  OROUP OF
     MONITORING WELLS
    SECOND GROUP OF
    MONITORING WELLS
    THIRD GROUP OF
    MONITORING WELLS
RESISTIVITY
MEASURING POINT
   Figure 18.    Monitoring changes  in plume configuration.

-------
                                                             -51-
would be to install three wells to monitor the plume size and



use resistivity measurements to provide data on location of



the plume between and beyond the wells (see Figure 18C). With



this monitoring system, as the plume expanded or contracted,



resistivity measuring points could be moved accordingly with



no particular problem.  All resistivity measuring points should



be carefully marked and numbered, just as if they were well



locations.  Each time the plume expands or contracts, only 2 or



3 new wells need be added to the system and the monitoring costs



are greatly reduced.  This is especially true since the cost of



analyses of water samples is a major part of the monitoring cost.





     One might be tempted to eliminate the "out-of-plume" moni-



toring wells altogether and rely on resistivity alone.  This is



not recommended.  Resistivity is affected by many factors and



the operator will not be certain as to what to attribute shifts



in resistivity without some direct control.  For example, a



change in soil moisture, addition of fertilizer to an area, ma-



jor variation in an airborne contamination load in an industrial



area or an unnoticed calibration shift in the resistivity equip-



ment itself might be interpreted as a plume shift. For this rea-



son, monitoring data, as all resistivity data, should be sub-



stantiated by direct sampling.

-------
                                                             -52-
                DATA REDUCTION AND INTERPRETATION





Vertical Soundings






     The procedures developed to interpret resistivity data are



grouped into two basic types: theoretical and empirical.  In



using the theoretical method, the field data are plotted, de-



scribing a curve which is compared with sets of master curves



developed for numbers of resistivity layers with definite ratios



of resistivity and thickness.  The values of resistivity for



each geologic unit as well as its thickness and depth below



land surface can then be determined.





     With most of the empirical methods only depths and thick-



nesses can be calculated.  Resistivity values can be compared



from station to station to find contrasts, but actual resistivity



cannot be determined.  The cumulative resistivity plot is one



empirical method which is easy to use and has given good results



at many sites.  With this method both the apparent resistivity



and the accumulated resistivity values are plotted with respect



to the electrode spacing.  The apparent resistivity points are



connected by a smooth curve and the cumulative point by a series



of straight lines.  The slope changes (breaks) in cumulative curves



generally indicate the depth to the underlying unit, and direction of



slope change indicates the relative resistivity values of the



subsurface materials.  That is, if the slope increases, the



underlying formation is assumed to have a higher relative resis-

-------
                                                             -53-
tivity and if the slope decreases, the underlying formation is

assumed to have a lower relative resistivity.



     Figure 19 shows an example of the cumulative resistivity

interpretation of a vertical sounding run directly on the top

surface of a landfill.  The sounding was run adjacent to an ob-


servation well that penetrated the landfill and underlying un-


consolidated sediments.  From the apparent resistivity curve

it can be seen that the area at the base of the landfill (35 feet


below land surface), where leachate should be the most concen-
                                                            »
trated, has the lowest resistivity.  From there, resistivity

increases steadily to 100 feet and then drops at 110 feet.   In-


terpretation of the cumulative resistivity plot is shown in the

column adjacent to the well log.  Four slope changes are appar-

ent in the cumulative curve.  The first break downward represents,


as would be expected, the contaminated water table.  The next

two upward breaks represent zones of higher resistivity and clean-

er water.  In this case, leachate is apparently restricted in its

downward migration by the very fine sand layer,and the lower

leachate concentration below this point accounts for the upward

break.  The change to a coarser sand containing relatively clean

water is picked up by the third break.  The final break occurs

near the depth where rock was encountered by the drill rig.



     This type of interpretation has been found to be useful in

ground-water contamination studies.  Theoretical curve matching,

-------
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-------
                                                             -55-
a more difficult procedure, has not been found to produce better
results for most work of this type.  If theoretical curve match-
ing interpretation is desired, it will be necessary to obtain a
set of curves, which come complete with detailed instructions.
(See references.)
Single Depth  (Horizontal) Profiles

     Normally, the only interpretation possible for single depth
profiles is to establish whether an area is contaminated or un-
contaminated.  This procedure may be quite precise or completely
arbitrary depending on the available control data.  In cases
where no direct control is available, monitoring wells would be
installed at locations of high and low resistivity values to de-
termine actual water quality and the relative degree of ground-
water contamination.  Information obtained from these wells re-
garding geology and water quality can then be expanded to other
portions of the site by inference from additional resistivity
work.

     In areas where geologic changes cause major resistivity
shifts, adjustments in measured resistivity values may have to
be made.  As discussed before, the change in resistivity values
caused by changes in geologic conditions is termed "natural
scatter" and has the value 'X1 in Figure 20A.  The change in re-
sistivity values caused by changes in water quality is given the
value 'Y'.   If 'X' is large with respect to 'Y1, it becomes

-------
                                                                         -56-
  A.
                           f
                         PROFILE
                                             -CHANGE DUE TO

                                             CONTAMINATION
                            NATURAL SCATTER—>• X

                                        T
                        SAND



                     ( NATURAL SROUND WATER )
            NATURAL  SCATTER AND CONTAMINATION EFFECTS < a«.r Ki.f.t«d. *.,«,..>
B.


( 1090 ) ^
-e-

CONTAMINATION
SOURCE



(1050) (1000)
/~^ Q


4$
/O
^/* V

DIRECTION OF
GROUND WATER
FLOW

X f^\
/ V_x
^i&w'i&P
/^ < fsT /
• '_' -';_ ;• /
"• • */" = /
'•" ' "" f
- 'C^iSCSI -->^' '
^"- ,; ^-" tt$&) ( soo ) (4zo
.•^ ^?";-%**"fcX CONTAMINATED
'.;-, ^-" -;''' GROUND WATER
c.
DIRECTION OF
GROUND WATER
FLOW
(0)
^ / A°'
/ -&
CONTAMINATION *£jlOj&/,&f /
SOURCE C^tbft&iifjP^ /
'*•'... \. -,. . /
';•*-."*/

(-40) (-90) H^»-/
/^\_ /r\ • ,,jO,v ' •*.*£%* r\- _zrx
~\_^ t 	 ^ "^jr^ j» v -^.jr v^/ \^r
Jf," t^m
- -> - -*"**0' <-|0) <-•>
S? *^ ' •
•?5"^
.O/ ^P ., — J^ ; , 	 CONTAMINATED
•?/<£• , ' -V. , pi*'"";- GROUND WATER
    UNCORRECTED  RESISTIVITY DATA



LEGEND

(302) RESISTIVITY VALUE (ohm -ft )
 -0- RESISTIVITY MEASURING POINT LOCATION
    (line through circle indicates electrode orientation)
RESISTIVITY DATA CORRECTED BY

THE NEGATIVE  DEPARTURE METHOD
 Figure 20.   Natural  scatter effects of geologic conditions

-------
                                                             -57-
difficult to interpret the data.  It may be possible to eliminate



or reduce the 'X' value if geologically similar environments can



be found both inside and outside the potential contamination



areas, and resistivity measurements are made in each area.  As



shown in Figure 2OB, the uncontaminated sand and gravel area has



a resistivity of about 1,000 ohm-ft, and any significant depart-



ure from this, e.g., the 420 ohm-ft reading, indicates pollution.



The silty sand area, however, has a natural  resistivity of



about 500 ohm-ft and a 420 ohm-ft reading does not represent



polluted water.   Cartwright and Sherman (1972) describe the nega-



tive resistivity departure method to allow contouring of this



type of data.  By setting the apparent resistivity of uncontami-



nated portions of each major geologic environment at zero and



plotting the differences between these values and the values ob-



tained in the suspected contaminated areas, a resistivity residual



map is prepared.  Thus, by subtracting 1090 ohm-ft from measure-



ments made in the sand and gravel area and 510 ohm-ft from the



silty sand aaea readings, the map shown in Figure 2OB would be-



come the map shown in Figure 20C.  The higher the negative value,



the greater the influence of contamination.

-------
                                                             -58-
                        SUMMARY OF KEY POINTS






1.   Do not place too great a reliance on the resistivity method



    in a pollution investigation. Resistivity is one investiga-



    tive tool, the usefulness of which varies from site to site.



    In addition, resistivity is an indirect method, and as such



    must be backed up by some direct data.





2.   Establish reliable control. Run a few vertical soundings



    right next to wells where geologic logs are available.



    Make sure the interpretation of the resistivity results



    agrees with the known subsurface conditions to a reasonable



    degree.





3.   Do an easy site first. Success at an easier site will provide



    the confidence and intuition needed for success at more



    difficult sites.





4.   Reduce the data immediately. This accomplishes two purposes:



    a) Prevents the running of obviously unnecessary profiles;



    b) Saves wasted time going back to the site to run obviously



       needed profiles.



5.   Look for resistivity contrasts. Since resistivity is  an in-



    direct method it relies on comparison of measurements rather



    than absolute numbers.  In addition, the contrasts found



    must be attributable to water quality, which requires good



    control.

-------
                                                              -59-





                           CASE STUDIES





     The following case studies include a series of preliminary



resistivity surveys conducted at three landfills and one liquid



waste disposal site in southern New Jersey, and three detailed



investigations conducted at major industrial and municipal waste



disposal sites in the northeast.






Preliminary Surveys





     A preliminary resistivity survey is essentially a quick and



inexpensive effort conducted with a minimum of geologic or water-



quality control data.  The principal purpose of the preliminary



survey is to obtain information which can be used to design a



well drilling program or a detailed resistivity program in con-



junction with drilling and direct sampling.





Case Study 1 -





     Case study 1 involves a small municipal and industrial waste



landfill of approximately 3 acres and about 20 feet thick.  It



accepts primarily non-chemical, inert, industrial waste products,



brush and leaves.  At the time of this survey, a large amount



(several hundred bushels) of a white powder in plastic bags was



found on the landfill.  This powder turned out to be composed



largely of highly soluble inorganic salts, which, once in the



ground-water system, would be expected to greatly increase its



conductance.  A water sample taken from a well located near the

-------
                                                             -60-
landfill had a specific conductance of 2,390 jomhos per cm, while



the specific conductance of natural ground water in this area



is about 100 jumhos/cm.  Thus, the contrast in conductance between



contaminated and uncontaminated ground water at the site is favor-



able for resistivity work.






     This landfill is situated directly on the outcrop of the



Cohansey Sand, a major aquifer in southern New Jersey, and is a



recharge area, with the nearest discharge point located about a



mile away.  Ground water at the site occurs at about 10 feet be-



low land surface under unconfined conditions and moves east to



west.  The Cohansey Sand, which is a medium-to-coarse grained



quartz sand, has a high resistivity under natural water quality



conditions.






     One vertical resistivity sounding and six single depth re-



sistivity profiles were run at this site.  The location of these



measuring points in relation to the landfill and the well are



shown on Figure 21.  Also shown on this figure are the resis-



tivity values for the single depth profiles and the value at 35



feet below land surface for the vertical sounding.  The complete



data sheet for this survey is given in Table 1.






     Figure 22 is a graph of the apparent resistivity and cumula-



tive resistivity values for R~l.  The cumulative curve shows one



break at 13.5 feet which is clearlv the water table  (the expected



error with this method is  up  to 20 percent).  Under natural condi-



tions, this break would have been less severe, but since the water

-------
                                                                     -61-
                                         R3
                                       ^(7800)
                            (302)

                        f*\- LANDFILL
                                              R2
                                              (5560)
      LEGEND
(302)   RESISTIVITY VALUE ( ohm -ft)
 &    RESISTIVITY MEASURING POINT LOCATION (WITH ELECTRODE ORIENTATION)
  •    WELL LOCATION
           Figure  21.   Case  Study 1  location  map.

-------
       Table  1 - Results of Resistivity Survey,  Case Study 1.
                                                                                             -62-
ELECTRODE
SPACING
(Ft )
DIAL READING * SCALE MULT (Ohms)

FULL
LEFT

RIGHT
APPARENT RESISTIVITY

FULL

LEFT
(Ohm-Ft )

RIGHT
CUMULATIVE
RESISTIVITY
(Ohm- Ft )
SITE 1
5
10
15
20
25
30
35
171
71.5
40.4
20.9
11.8
10.8
8.9
108
50.4
26.8
11.3
6.3
4.1
-
59
20.4
13
9.1
6.0
6.2
-
855
715
605
418
295
324
313
540
504
402
226
157
123
-
295
204
195
182
150
186
-
855
1570
2175
2593
2888
3212
3525
SITE 2
40
139
60.7
79.8
5560
2428
3192

SITE 3
50
156
63.0
82.0
7800
3150
4100
-
SITE 4
40
7.56
_
-.
302
_
_
_
SITE 5
50
27.1
6.35
18.0
1355
318
900
_
SITE 6
50
23.2
5.60
13.0
1160
280
650
-
SITE 7
50






40.9






10.3






22.1






2045






515






1105






_







-------
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                                                                          -63-
                                                                        m
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-------
                                                             -64-
here is contaminated, the shift in resistivity values is en-



hanced and the change in slope is greater.






     The left and right apparent resistivity values  (Lee configu-



ration) shown on Table 1 for R-l differ substantially in the un-



saturated zone and then become more similar below the water table.



The reason for this was found after a brief inspection of the



ground surface at the R-l location.  The surface was of relatively



fine material which had been compacted by landfill equipment and



thus had a fairly low permeability.  The shape of the surface



was such that the right electrodes were in a depression and the



left electrodes on a high area  (about a foot or two difference



in elevation).  The soil in the depression was damp while the



soil on the high area was dry at the surface.  Apparently, a



previous rain had caused runoff from the immediate area to pond



in the depression and this water was still percolating to the



water table at the time of the resistivity measurements.  Con-



sequently, the soil beneath the right side of the resistivity



line was wetter than under the  left side and the resistivity



readings were correspondingly lower.  Usually, the reason for a



left and right side discrepancy is not apparent at the surface,



but rather reflects subsurface horizontal variations in geology.






     The depths selected for additional horizontal profiles were



based on changes in land surface elevation, in an attempt to keep



the readings roughly consistent with the  35-foot reading at Rl.



For example, if the elevation of ground surface at the point

-------
                                                             -65-
where R2 was run is 5 feet higher than at Rl, the reading would




be made at 40 feet.  This, of course, is only a partial correc-




tion since the thickness of the unsaturated zone is different




for the two measurements.  To fully compensate for this differ-



ence, vertical soundings would be required at both sites and




theoretical curve-matching interpretation techniques employed.




This would only be warranted under much more complex situations



than are present at this site.






     As shown on Figure 21f the two resistivity measurements in




the predicted path of leachate flow, Rl and R4 have much lower



resistivities than do the other five profiles.  Profiles R5,R6,




and R7, while not in a direct plume of contaminated ground water,




are close enough to the landfill and a nearby private industrial




waste disposal site to receive some mineralization from these



sources during periods of heavy precipitation.  The difference




between the readings at R2 and R3, both in background areas,




might be that R2 is in a cultivated field which probably has been



fertilized, and R3 is in a fallow field.  This difference in re-



sistivity between cultivated and fallow fields has also been




noted at other sites.






     In summary, the hydrogeology of the Case Study 1 landfill




is relatively simple, and a brief resistivity study (completed in




half a day), without the benefit of geologic or water quality con-




trol (obtained later), was sufficient to establish the existence

-------
                                                             -66-
of a distinct leachate plume and give some indication of the



location of the plume.  In this case, the plume flows under a



larger industrial waste disposal area where it becomes mixed



with other contamination.   With the addition of another 6 or 7



profiles, the entire zone of ground water contaminated by land-



fill leachate could probably be defined and decisions regarding



abatement procedures could then be based on this data.





Case Study 2 -





     This landfill is no longer in operation and the owner is



currently in litigation stemming from alleged leachate damage to



a nearby lake.  The waste disposal area is presently well covered



with soil and grass, and while its actual size during operation



is not known, it is estimated that the site was approximately 10



acres.  The types and amounts of wastes accepted at the landfill



are not known; however, it is reported that large amounts of in-



dustrial chemical waste and probably liquid chemical waste were



accepted.  A site map is shown in Figure 23.





     The landfill directly overlies the Cohansey aquifer, which is



described in Case Study 1.  Ground-water flow is from southwest



to northeast under the landfill and toward a nearby stream.



Several areally extensive leachate seeps which flow directly



into the creek are apparent along the northern toe of the land-



fill.  From the results of the resistivity measurement, ground

-------
                                                                -67-
       LEGEND
(696)   RESISTIVITY VALUE (ohm-ft)

 ~    RESISTIVITY MEASURING POINT
 •**    LOCATION
        500 FEET
        Figure 23.   Case Study 2  location map.

-------
                                                             -68-
water of extremely low resistivity is apparent on both sides



of the creek near the northern toe of the landfill  (Figure 23).



Areas of considerably higher resistivity are apparent along the



stream both to the north and south.  It was not certain how



much effect geology had on these readings due to the lack of



geologic controls for the area.






     Subsequent to the resistivity measurements, three observa-



tion wells were installed in the vicinity of the landfill.  The



locations of these wells,- designed LI, L2, and L3 are shown in



Figure 23.  Results of chemical analyses of water -samples from



the wells support the conclusions drawn from the resistivity re-



sults that leachate has migrated through the subsurface to and



beyond the stream.





     A sewer line runs parallel to the stream opposite the land-



fill as shown on Figure 23.  It is unknown how much, if any, of



the contamination apparent in LI has been contributed by the



sewer line.  Additional investigation would be necessary to es-



tablish this.  It is clear, however, that contaminated ground



water is present in the subsurface across the stream from the



landfill.





Case Study 3 -





     This case study, a large privately-owned landfill, is an



example of a situation where electrical earth resistivity is not

-------
                                                             -69-
useful in defining areas of ground-water pollution.  The in-



effectiveness of the resistivity method at this site is due



primarily to the geologic setting.  The landfill is underlain



by a thick, areally extensive clay layer which masks any varia-



tion in ground-water quality which might be reflected by re-



sistivity measurements.  The presence of the clay will also



restrict the downward movement of leachate from the landfill,



and leachate is probably discharged at or near the surface and



directly .into the adjacent river and wetlands.  Therefore,



there is not likely to be a subsurface plume of contaminated



ground water to be detected by a resistivity survey at this site.





     In an effort to test the applicability of the resistivity



method, two vertical soundings and five horizontal profiles were



run at the site.  The locations of these measurements with re-



spect to the landfill and creek are shown on Figure 24.  The



results of the horizontal profiles and the deepest measurements



of the vertical soundings, also on Figure 24, show no pattern of



contrasts that can be attributed to the landfill.  The completed



data sheet for this survey is given in Table 2.  The cumulative



plots of the two vertical soundings are shown on Figure 25.



While the depth to water is apparent on both cumulative plots,



no other breaks are present, indicating uniform water quality



or uniform geology with depth.






     Subsequent to the resistivity measurements, two test borings

-------
            LEGEND
  (103)  RESISTIVITY  VALUE ( ohm - ft )

   0

   •   TEST BORING LOCATION
RESISTIVITY MEASURING POINT
LOCATION
                                                                          4,000 FT.
NOTE- Resistivity values shown are at
     different A-Spacings (Table 2)
     to correct for variation in land
     surface elevation.
     R6
     (Z35)
                                                       0	500
                                                                1000 FT.
              Figure  24.   Case Study  3  location  map.

-------
Table 2 - Results of Resistivity Survey, Case Study 3.
                                                                                      -71-
ELECTRODE
SPACING
(Ft )
DIAL READ
FULL
NG < SCALE ML)
LEFT
LT (Ohms)
RIGHT
APPARENT R
FULL
ESISTIVITY
LEFT
(Ohm-Ft )
RIGHT
CUMULATIVE
RESISTIVITY
(Ohm- Ft )
SITE 1
10
20
30
40
50
60
70
'77.0
10.7
4.18
2.25
1.42
1.04
0.77
33.3
5.20
2.06
0.56
0.42
0.81
0.39
42.2
5.00
2.06
1.65
0.96
0.17
0.38
770
214
125
90
71
62
54
333
104
61
22
21
49
27
422
100
61
66
48
10
27
770
984
1109
1199
1270
1332
1386
SITE 2
50
70
2.41
1.17
1.19
0.60
1.21
0.52
120
82
59
42
60
36
_
_
SITE 3
70
1.47
SITE 4
10
20
30
40
292
19.4
3.55
2.20
1.04

116
6.5
1.53
1.48
0.46
103
73
32


160
12.3
1.61
0.56
2920
582
107
88
1160
130
46
59
1600
246
48
22
2920
3602
3709
3797
SITE 5
50
1.05
0.27
0.59
52
14
30
_
SITE 6
70
3.35
1.58
1.67
235
111
117

SITE 7
50


1.13


0.58


0.51


57


29


26






-------
                                                                                               -72-
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-------
                                                             -73-
were drilled in the vicinity of this landfill.   The locations



of these borings, designated Fl and F2, are shown on Figure 24.



Both borings encountered a layer of dense clay  over their en-



tire depth, which was 80 feet for Fl and F2.   In addition, wa-



ter samples taken from these two borings indicate a relatively



highly mineralized water with specific conductances of 290 and



430 umhos/cm respectively.  Thus, the geologic  and natural wa-



ter quality conditions would apparently account for the low re-



sistivity readings encountered at almost all  points near this



site.





     Electrical resistivity cannot be successfully used at this



site to define ground-water contamination.   Installation of



deep monitoring wells and the collection and analysis of water



samples would be necessary here.





Case Study 4 -





     Case Study 4 is an industrial liquid waste disposal site



(see Figure 26).  The site, currently abandoned, includes sev-



eral lagoons, one still half-filled with a greenish-yellow



liquid chemical waste.  The other lagoons are partially or



totally filled by sand.  In addition, traces  of chemical powders



and discarded drums of chemical waste are scattered and buried



around various portions of the site, which occupies approximately



seven acres.

-------
                                                                      -74-
                                                               300 FT.
( 19)
RESISTIVITY  MEASURING  POINT
LOCATION

WELL  LOCATION


RESISTIVITY  VALUE (ohm-ft)
                                           NOTE: RZ AND R3 ARE AT THE SAME
                                                POINT, WITH THE LINE OF
                                                ELECTRODES ROTATED 90°
        Figure 26.   Case Study  4  location  map.

-------
                                                             -75-
     This waste disposal site is situated directly on the



Cohansey aquifer.  Highly visible evidence of damage caused by



the operation is  in the form of large numbers of dead trees



(see Figure 26) around the perimeter of the site and extending



for hundreds of feet beyond in several areas.  Ground-water



movement in the area is in a southwesterly direction beneath



the site toward a river located several hundred yards away.





     At the time of this survey, there were no wells or surface



discharges with which to establish ground-water quality at this



site.  In an attempt to determine the presence of the contamina-



ted body of ground water beneath the site, several resistivity



measurements were made.  The locations of these resistivity



measurements relative to the lagoons, and the resistivity values



of the single depth profiles are shown on Figure 26.  The appar-



ent resistivity values for the vertical soundings and single



depth profiles are given on Table 3.





     The results of the single depth measurements indicate a



very highly conductive body of ground water directly under the



site, and a migration of the body, with some decrease in con-



ductivity, to the west.  To the east of the site,  is an area of



less mineralized water.  Additional resistivity measurements



along line A-A' as shown on Figure 26 should identify the width



of the plume.  Once the width of the plume has been defined,



additional measurements should be made to trace it as far as



possible toward its discharge point.

-------
                                                                                           -76-
         Table 3 - Results of Resistivity Survey, Case Study 4.
ELECTRODE
SPACING
(Ft )
DIAL READI
FULL
NG » SCALE MU
LEFT
LT (Ohms)
RIGHT
APPARENT R
FULL
ESISTIVITY
LEFT
(Ohm-Ft )
RIGHT
CUMULATIVE
RESISTIVITY
(Ohm- Ft )
SITE 1
10
20
30
9.55
2.17
0.65
2.17
0.90
0.19
7.44
1.28
0.45
95.5
43.4
19.5
21.7
18
5.4
74.4
26
13.5
95
139
159
SITE 2
30
31.4
75?
0
942 •
2250?
0?
-
SITES
5
10
15
20
25
" 30
35
9630
1930
585
250
87.9
40.9
33.3
3570
920
200
96
37
87
70
3760
710
225
77
21
0
0
48,. 150
19,300
8775
5000
2198
1227
1165
17,850
9200
3000
1920
925
2610?
2450?
18,800
7100
3375
1540
525
0?
0?
48, 150
67,450
76,225
81,225
83,423
84,650
85,815
SITE 4
   20
9.4
5.7
3.6
188
114
72

-------
                                                             -77-
     Analyses of the vertical sounding measurements made at



site R3 indicate that this is not, in fact, a background area.



While the upper layers have a very high resistivity, there is



a sharp downward break in the cumulative plot at around 6 feet



below land surface at about the water table.  Thus, the ground-



water quality at R3 is not natural but rather the unsaturated



sand is clean (very high resistivity),and the ground water is



contaminated.  The high resistivity upper layer masks the con-



taminated water, while at the resistivity measurement sites,



chemicals spilled on the ground have contaminated everything



from ground surface on down and the readings are correspond-



ingly lower.   A series of single depth measurements along line



B-B1 on Figure  26 should indentify the extent of this upgradi-



ent pollution migration.






     Subsequent to the resistivity measurements, three test wells



were drilled at the site.  The locations of these wells are



shown on Figure 26.  Well 3, some 700 feet northeast of the site,



is intended as a background well.  Geologic samples taken from



the wells during drilling revealed the generally sandy nature



of the area although coarser material is found at depth.  Con-



siderable silt is present in the upper layers.  Clay layers, if



present, are too thin to be defined by the method of drilling



used at this site  (augering).  Analyses of water samples from



the four wells indicate highly polluted water in the shallower



layers  (20 feet below land surface at Well 2) and slightly con-

-------
                                                             -78-
taminated water in the deeper zone (60 feet below land surface



at Well 1) .   Well 3 was drilled in an cirea of natural quality



water to a depth of 20 feet.  The TDS (total dissolved solids)



concentration of Wells 1, 2, and 3 are 142, 54,000 and 49 mg/1,



respectively.  All else being equal,  the higher the TDS, the



lower the resistivity.  A resistivity measurement in the vicinity




of Well 3 should have been made to establish a background



resistivity value.





     Based on the now available data, this site seems ideally



suited for a detailed resistivity survey to identify the areal



extent of contamination.






     A series of single depth profiles between the site and



the river would probably define the areal extent of the plume.



The depth to the top of the plume should be easy to determine



along its path.  The extreme conductivity of the contaminated



ground water, however, would probably riot permit the determina-



tion of the thickness of the contaminated body.





Detailed Surveys





Case Study 5 -





     Contamination at this site was caused by disposal of in-



dustrial process water containing high concentrations of sodium



chromate and sodium hydroxide into an unlined lagoon in perm-



eable Atlantic Coastal Plain sediments..  Before this method of

-------
                                                             -79-
waste-water disposal was discontinued, traces of the process



water occurred in two nearby wells.





     Topographically, the site is relatively flat with ground



surface elevations ranging from approximately 90 feet  (27.5 m)



to 110 feet (33.6 m) above MSL.  Ground water occurs at shallow



depths of six to twelve feet below the ground surface under un-



confined conditions.






     The unlined lagoon was dug into the sediments of the sur-



ficial Bridgeton Formation and the underlying Cohansey Sand,



which is a yellow brown to red brown, medium-to-coarse grained



quartz sand.  The Cohansey is 120 to 140 feet thick in the site



area and occurs 0 to 12 feet below the ground surface.  A



brownish black clay unit up to 30 feet thick separates the



Cohansey Sand from the underlying sands of the Kirkwood Formation.



The Cohansey Sand is the major aquifer in this region. Composed



of highly permeable sands and gravels, this aquifer is able to



transmit and store large quantities of water.  The Bridgeton and



Kirkwood Formations are developed locally for domestic and agri-



cultural use.





     In order to test the applicability of the electrical resis-



tivity method, several single depth profiles were made at the




plant site using the Wenner electrode configuration.  Interpre-



tation of these preliminary data indicated that the method



could delineate the contaminated zone because significant varia-



tions in measured resistivity values appeared attributable to



variations in contaminant concentrations in the subsurface.

-------
                                                             -80-
     In order to properly evaluate the significance of the


measured resistivity values, a calibration point was estab-


lished in a field north of the plant site and upgradient  (in


terms of ground-water flow)  from the source of the contamina-


tion.  The aquifer at this location, based on available in-


formation, was uncontaminated. Periodic resistivity measure-


ments were made at this calibration point at intervals of 10


feet to a total depth of 100 feet to develop a pattern of re-


sistivity values for uncontaminated subsurface conditions, and


to determine any variation in measured values with respect to


time, precipitation, or other factors which might affect the


resistivity measurement.



     Evaluation of the trend of resistivity measurements made


at the calibration point  (Figure 27) indicates a relatively


narrow range of variation in the resistivity values, except

                         +
for those in the upper 20- which would be most affected by


climatic conditions  (intensity and frequency of precipitation).


For example, it was noted that resistivity values for a depth


of 10 ft showed marked decreases in resistivity after a rainfall,


whereas values for depths greater than 20 ft did not appear to


be similarly affected.



     Analysis of the preliminary resistivity measurements and


information regarding the contamination of wells on adjacent


properties indicated that the contamination had extended beyond

-------
                                                                                            -81-
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8
8
8
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                                                                                    *•—*

                                                                               2    o
                                                                                    z
                                                                                    u

                                                                                    (O
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                                                                                    o
                                                                                    o
                    Lg
                                                                                    o
                                                                                    Ul
                                                                                    _l
                                                                                    UJ
                                                                              _0
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                                                                        X!
                                                                        -P

                                                                        -P
                                                                         (0

                                                                         QJ
                                                                                              in
                                                                         g  d

                                                                         W C/3
                                                                         Cn

                                                                        •H  CO

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                                                                         O  *
                                                                         CO -P
                                                                            C

                                                                        -P  O
                                                                        •H  fli

                                                                        -H  C
                                                                        -P  O
                                                                         CO -H
                                                                        -H -P
                                                                         co re)
                                                                         CU M
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                                                                                            cu
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                                                                                           EM
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                    (Idl-NHO)  JkllAllSIS3y

-------
                                                             -82-
the limits of the plant property in a general southwesterly



direction.  Available data also indicated that a plume of con-



taminated ground water extended in a northwesterly direction,



presumably in response to the pumping of wells in this area.





     Utilizing these preliminary data, additional resistivity



measurements were made at various loceitions in the vicinity of



the plant site.  At each location resistivity measurements were



made at intervals of 10 ft in depth to a total of 100 ft.  Rep-



resentative resistivity curves are presented in Figure 28.  An



interpretation as to the general degree of aquifer contamination



was inferred by classifying the resistivity data as follows:



0 to 500 ohm-ft, significant contamincition of the ground water;



500 to 1,000 ohm-ft, intermediate or partial degree of contami-



nation; and greater than 1,000 ohm-ft, uncontaminated conditions.



The 1,000 ohm-ft isoresistivity contour was interpreted as in-



dicating the approximate leading edge of the contaminated zone.



The choice of limiting values for each of these zones was arbi-



trary but based in part on data obtained from the calibration



point, existing wells, and the preliminary investigation in



addition to more recent observations.  These inferred zones of



contamination were later supported by data from chemical anal-



yses of ground-water samples obtained in test wells drilled at



the site.





     An interpretation of the general extent of the contamination



at various depths below the ground surface, based on the analysis

-------
                                                                            -83-
           o  O  >
 6606
 o         o         o        o
 8         8         ?        8
(Id-WHO) AllAUSIS3d  lN3HVddV
                                                             -8
                                                                  Ul
                                                                  C9
                                                                  O
                                                                  Q.
                                                                  V)
                                                                  Ul
                                                                  O
                                                                  O
                                                                  oc

                                                                  o
                                                                  Ul
                                                                  _l
                                                                  Ul
                                                             _o
                                                                            OJ
                                                                            0)
                                                                            S-l
   m
 M
 O >i
m T3
   3
 tn -P
 0) co
 >
 ^ QJ
 d en
 O (0
   u
 >i
•P -P
•H (0
 >
•H cn
-P 0)
 cn c
•H O
 in N
 0)
                                                                               (0
                                                                               I
 >
•H
-P
 (0
•P C
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 0) O
 U) S-l
 0) tn
 S-i
 a
 OJ
CX)
CN

 0)
 >-l
 r»
 Cn

-------
                                                             -84-
and evaluation of the resistivity data is presented in Figure



29.  These figures indicate zones of treasured resistivities



believed to reflect variations in contaminant concentrations



in the subsurface.  These figures indicate a low resistivity,



0 - 500 ohm-ft, zone extends approximately 2,800 ft downgradient



from the location of the former unlined holding pond at the



plant site, and that the contaminated water/fresh interface



occurs about 3,000 ft downgradient.  This interpreted location



of the leading edge of the contamination was later corroborated



by analysis of pumping test data from test wells drilled at the



site.






Case Study 6 -






     This study was carried out at an industrial plant site where



various types of concentrated liquid weiste is kept in holding



ponds.  Figure 30 shows the location of the resistivity measure-



ments, the sources of contamination, arid the natural discharge



area.  Liquid waste had percolated into and contaminated the



underlying aquifer to the extent that contamination was detected



in wells on the. plant property.  A resistivity survey was em-



ployed to define the limits of the contaminated ground water so



that an efficient abatement system could be designed.  An exten-



sive test drilling and water sampling program was conducted



following the resistivity survey.

-------
                                                                           -85-


      INTERPRETATION OF THE EXTENT OF
    CONTAMINATION AT A DEPTH OF  20 FEET
  INTERPRETATION OF THE EXTENT OF
CONTAMINATION AT A DEPTH OF 40 FEET
                     T
       INTERPRETATION OF THE EXTENT OF
     CONTAMINATION AT A DEPTH OF 60 FEET
       INTERPRETATION OF THE EXTENT OF
     CONTAMINATION AT A DEPTH OF 100 FEET
  INTERPRETATION OF THE EXTENT OF
 CONTAMINATION AT A DEPTH OF 80 FEET
                                                    LEGEND
                                                               RESISTIVITY STATION
                                                               0-500 OHM-FT

                                                               500- 1000 OHM-FT
                                                         SCALE
                                                               0   500  1000
                                                                  FEET
                                                        (  After: Berk and Yore )
Figure 29.   Extent of  contamination as  defined  by interpretation
                     of resistivity  results,  Case  Study  5.

-------
          1284
          A
                                                                              -86-
  4012
  A
           8920
SOURCES  OF CONTAMINATION
«I-*\J



>
^^


144
A
i
\
526





ir
«4
I
1
1
1
1
|
1
£27)
AJ
i
i
i
i
\
t
A V / '
44
,22 """I6
A ** 366
A




i
                                          i     A     L
                                          !M    411 462 /
                                                     V      892 ?"
                                                      >     A^A
                                                  378 495 951 fma»»
                                                   A  AAA*920
                                                                   LATERAL EXTENT OF
                                                                CONTAMINATED GROUND WATER
                                                               BODY ACCOROIN6 TO RESULTS OF
                                                                      RESISTIVITY
                                                                              1219
                                                                 T
                                                                             1187
                                                                             A
                                                                                   I6O5. I6OS
                                                               A   S  ^\

                                                            I    / A     X
                                                                305
                                                                A
                               DISCHARGE  AREA
         LEGEND
    920-APPARENT RESISTIVITY IN OHM-FEET
    A -RESISTIVITY  MEASURMENT POINT

       SCALE: i"=ioo'
         Figure 30.   Location map  and extent of contaminated ground-water
                                 plume,  1972,  Case Study 6.

-------
                                                             -87-
     The study area is apparently an old flood plain.  The



upper deposits grade from predominantly clay in the western



portion of the property to mostly sand in the eastern portion.



These shallow deposits grade downward into a nearly continuous



clay zone from 20 to 40 feet below land surface.






     Analyses of ground-water samples indicate specific conduc-



tance ranges from 150 to 400 mhos/cm (micromhos per centimeter)



for natural ground water and from 2,000 to 6,000 mhos/cm for



contaminated ground water.  Depth to the water table, or top of



the plume, ranged from 1 to 16 feet below land surface.  The con-



taminated plume was generally between 10 and 20 feet thick.



There were numerous physical obstructions present at the site;



however, interference with the resistivity data was avoided by



carefully locating the resistivity measurement points.





     Nineteen vertical electrical soundings were conducted at



the study site using the Wenner electrode configuration with



readings made at 3-foot intervals from 3 to 51 feet below land



surface.  Interpretation of these data with the cumulative



method enabled the geologic and hydrologic information obtained



from the initial test wells to be projected throughout the area



of investigation.  Based on the results obtained from the ver-



tical electrical soundings, an A-spacing of 51 feet was selected



for the single depth measurements, of which a total of 79 were



made.  The apparent resistivity value of each measurement is

-------
                                                             -88-
shown on Figure 30.





     After comparing the resistivity values and chemical anal-



yses of the ground-water samples,  500 ohm-ft at an A-spacing



(depth) of 51 feet was selected as the maximum resistivity of



the contaminated slug.  The value of 500 ohm-ft was used to



define the approximate limit of the contaminated ground-water



body (Figure  30)•  The results of the chemical analyses of



approximately 120 water samples from 40 wells confirmed the



accuracy of the resistivity results.





     Approximately three years after the completion of this



resistivity survey and 2 years after abatement procedures began,



a second survey was conducted to determine if the boundaries



of the slug of contaminated grourid water had moved during this



period.  The methods used in the second survey were identical



to the first to allow direct comparison of the data.  The re-



sults of the new survey are shown in Figure 31.  Based on these



results, it is clear that most of the slug boundary has remained



in essentially the same location, with the exception of areas



A and B where slight spreading had occurred.  Analyses of water



samples from wells in these two areas confirmed this interpre-



tation.  The detected spreading of the plume was attributed



partially to the destruction of one of the abatement wells and



partly to an inadequate surface drainage system.  This case then



represents the successful use of resistivity as a monitoring



method, the results of which is the basis for improved abatement

-------
   • •
590
     610
                                                                              -89-
                                N
                                                          706
                  590
                                                              755
                675
                                                                   633
                                                                             860
                      555
                                                                        700
                  560
SOURCES  OF CONTAMINATION
                                                               I97S LATERAL EXTENT OF
                                                               CONTAMINATED GROUND WATER
                                                               BODY ACCORDING TO RESULTS
                                                               OF RESISTIVITY.
    DRAINAGE
    DITCH
                                DISCHARGE  AREA
          L EGEND
   155.
         ...APPARENT  RESISTIVITY IN OHM-FEET
         ...RESISTIVITY MEASURING POINT

          AREA  OF PLUME SPREADING BETWEEN
          1972 AND 1975
Figure 3'  ~  Location  map  and  extent
            of  contaminated ground-
            water  plume,  1975,  Case
            Study  6.
                    100  FEET

-------
                                                             -90-
procedures.





Case Study 7 -






     This investigation took place at a municipal and industrial



solid waste landfill covering an area of approximately 50 acres



as shown on Figure 32.  The resistivity survey was part of a



general hydrologic investigation of the landfill site.  The



presence of a brackish ground-water zone southeast of the land-



fill precluded the definition of the contaminated plume by the



resistivity method in this one area.





     Thirty-six wells were installed at the site for the purpose



of defining the subsurface geology, shape of the water table,



and water quality.  In an attempt to further extend this informa-



tion over the landfill site and surrounding area, 17 vertical



electrical soundings were conducted.  Figure 32 shows the loca-



tion of the resistivity points in relation to the landfill.





     Depth to the water table varies from approximately 2 to 28



feet and depth to bedrock from 5 to 90 feet below land surface.



The thickness of the contaminated plume ranged from approximately



20 to 60 feet.  Four basic ground-water environments were es-



tablished from analysis of water samples taken from the test



wells.  They are natural ground water, ground water contaminated



by landfill leachate, ground water contaminated by leachate from



a fly ash dump, and brackish ground water.  Typical specific con-



ductances were 140, 5990, 4610, and 36,300 umhos/cm,  respectively.

-------
                                                                              -91-
                                                          12240
                                                                           NORTH
                                                 Shallow
                                                 Bedrock
            NATURAL GROUND WATER ENVIRONMENT
                                                               SHREDDER PLANT
                       S   HIGHLY MINERALIZED GROUND WATER
                                      ENVIRONMENT
                                                                    TREATMENT
                                                                     PLANT
                            \ ( LANDFILL ft  FLY ASH LEACHATE

                              N
   LEG  END
      RESISTIVITY STATION WITH APPARENT
  ~w~  RESISTIVITY VALUE IN OHM - FEET.

= = = = = APPROXIMATE LIMIT OF ENVIRONMENT

	LIMIT OF LANDFILL      ^BBMm ROAD

NOTE A-Spocinos equal to approximately twice the depth to water.
                                                                 5OO
                                                                           IOOO FEET
Figure  32.   Three major  ground-water  environments  as delineated  by
                interpretation of resistivity  data, Case Study  7.

-------
                                                             -92-
     The resistivity data,  as shown on Figure 32, defined three



distinct ground-water quality zones.  The natural and brackish



zones are separated from the zone containing landfill and fly



ash leachate.   A zone of natural ground-water quality, where



depth to bedrock was very shallow (shown on the top of Figure



32), displayed much higher resistivity values.  Leachate from



the fly ash dump is also distinguished from that of the land-



fill by higher resistivity values which reflect the somewhat



lower conductivity of the fly ash leachate.  The accuracy of



the resistivity method in defining these zones was confirmed



by the test drilling and water-sample analyses.  Definition of



the plume boundaries to the east and west was not established



because of the difficulty of access  (flooded marshland) in these



areas.

-------
                                                             -93-
                         References
Case Studies

Berk, W. J., B. S. Yare.  An integrated approach to delineating
     contaminated ground water.   (In press.)

Cartwright, K., and M. R. McComas.  Geophysical surveys in the
     vicinity of sanitary landfills in Northeastern Illinois.
     Ground Water, 6(5):23-30, Sept.-Oct. 1968.

Cartwright, K. , and F. B. Sherman, Jr. Electrical earth
     resistivity surveying in landfill investigations.  Reprinted
     from Proceedings of the 10th Annual Engineering and Soils
     Engineering Symposium.  Moscow, Idaho, 1972. 16p.

Hackbarth, D. A. Field study of subsurface spent sulfite liquor
     movement using earth resistivity measurements.  Ground Water,
     9(3):11-16, May-June, 1971.

Kelly, W. E.  Geoelectric sounding for delineating groundwater
     contamination.  Ground Water, 14(1):6-10, Jan.-Feb. 1976.

Klefstad, G., L. V. A. Sendlein, and R. C. Palmquist.  Limitations
     of the electrical resistivity method in landfill investiga-
     tions.  Ground Water, 13 (5):418-427, Sept.-Oct. 1975.

Stollar, R. L. , and P. H. Rous.  Earth resistivity surveys—a
     method for defining fround-water contamination.  Ground Water,
     13(2):145-150, Mar.-Apr. 1975.

Warner,  D. L.  Preliminary field studies using earth resistivity
     measurements for delineating zones of contaminated ground
     water.  Ground Water, 7(1):9-16, Jan.-Feb. 1969.

Theoretical Discussion and Interpretation

Griffiths, D. H., and R. F. King.   Applied geophysics for engin-
     eers and geologists.  New York, Pergamon Press, 1966. 223 p.
     (Elementary; one-quarter of book cm resistivity.)

Moore, R. W.  An empirical method of interpretation of earth-
     resistivity measurements.  American Institute of Mining
     and Metalurgical Engineers Technical Publication 1743.
     New York, 1944.  8 p.

-------
                                                        -94-
Orellana, E., and H. M. Mooney.   Master tables and curves for
     vertical electrical  sounding over layered structures.
     Costanilla de Los Angeles  15,  Madrid,  Interciencia, 1966.
     [34 p.]  (Specify Wenner or Schlumberger master curves.)

Van Nostrand, R. G., and  K.  L.  Cook..   Interpretation of
     resistivity data.  U.S. Geological Survey Professional
     Paper No. 499.  Washington, U.S.  Government Printing Office,
     1966.   310 p.
                                                        ya!752
                                                        SW-729
                                      U. S. GOVERNMENT PRINTING OFFICE ; 1979 O - 284-197

-------