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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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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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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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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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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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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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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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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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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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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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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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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^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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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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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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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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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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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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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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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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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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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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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
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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.
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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,
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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.
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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.
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-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,
-------
z
0
tt
E w
> a:
i- S;
2£
§
cu
•
\
S
8
9
ts
si
ll
o -°
m -o
a «
M
a 8
d a
a,
g
QJ
C
o
-r^
4J
1C
4-1
0)
i-t
OJ
4-J
C
0>
f—(
0)
X1IAUSIS3U !N3«VddV
-------
-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
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- -> - -*"**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
_
-------
(W-uiqo) AJLIAIlSISay 3AIJ.VinwnO
-63-
m
u
-H
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U-l T3
D
U) -P
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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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-------
-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-
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-------
-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.
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-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.
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1284
A
-86-
4012
A
8920
SOURCES OF CONTAMINATION
«I-*\J
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^^
144
A
i
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526
ir
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I
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1
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44
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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.
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-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
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-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
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-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.
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-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.
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-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.
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-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.
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-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
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