Terrain Conductivity Investigation of Groundwater Flow Near the Dam at Alpine Lake,

West Virginia

Senior Thesis By

Tom Darby II

April 30, 2003

Table of contents: Acknowledgements……………………………………………… 2

Abstract………………………………………………………….. 3

Introduction……………………………………………………… 4

Methods…………………………………………………………. 8

Discussion………………………………………………………. 14

Conclusion……………………………………………………… 25

References………………………………………………………. 27

List of Figures:

Figure 1: Digital orthophoto of the survey site………………… 5

Figure 2: Topographic map of the survey site…………………. 6

Figure 3: Discharge area……………………………………….. 7

Figure 4: Picture of the EM-31………………………………… 8

Figure 5: Detailed survey grids………………………………… 11

Figure 6: Survey grids on a digital orthophoto………………… 12

Figure 7: Standing water……………………………………….. 15

Figure 8: Location of standing water and discharge area………. 16

Figure 9: Contour map of terrain conductivity values………….. 17

Figure 10: Photograph of the dam………………………………. 20

Figure 11: Orthophoto with EM data superimposed.……………. 21

Figure 12: Topographic map of reclaimed mine land…………... 23

Figure 13: Topographic map with EM data superimposed……… 24

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Acknowledgements:

I would like to thank the Alpine Lake Resort for allowing me to conduct the survey

on the resort property. I would like to thank the NASA Space Grant Consortium for

helping to fund this project, and also the Eberly College of Arts and Sciences for helping to

fund my travel expenses when I presented my research at the Southeast/South Central Joint

Section Meeting of The Geologic Society of America in Memphis Tennessee. I would like

to thank Consol Coal Company for allowing me to use their EM-31 terrain conductivity

meter and magnetometer.

I would like to thank my two field assistants Jason Alexander and James Darby.

Most of all, I would expressly like to thank my research advisor Dr. Tom Wilson. His

guidance and support on this project helped me to better understand what I need to do and

how best to accomplish it. Without his help, none of this would have been possible.

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Abstract:

In this study, terrain conductivity surveys were undertaken over an area bordering

the dam at Alpine Lake in Terra Alta, West Virginia. The sudden development of a spring

some 600 yards from the dam raised local landowners’ concerns that the dam might be

leaking. Spring effluent was significant enough to wash out a large ditch and threatened to

undercut a local road just a few feet away.

Terrain conductivity surveys were undertaken near the spring using the Geonics

EM31 terrain conductivity meter. Initially, stations were occupied every 20 feet in a 300

foot by 80 foot grid centered up gradient of the spring. Terrain conductivity data suggests

two possible sources for this leak. A high conductivity zone associated with subsurface

drainage was mapped in the nearby area, but did not lead in the direction of the dam.

Eventually the survey grid was expanded to include 4 additional survey grids so that high

conductivity regions between the dam and spring could be accurately followed. Individual

survey grids ranged in size from 300 feet to 440 feet in length by 140 feet to 300 feet in

width. A magnetic survey was also conducted in the vicinity of the dam to evaluate the

possibility that terrain conductivity anomalies might be associated with buried metallic

sources. The magnetic survey confirmed that terrain conductivity anomalies were

associated with conductive groundwater, rather than from metallic debris.

Initially, there appeared to be two possible source areas for the spring. The first,

and most obvious source was the dam. The second possibility was associated with an old

stone quarry. The old quarry high-wall bordered the re-contoured mine spoil, which now

serves as a golf course. Contour maps of the terrain conductivity data indicate that the

newly developed spring can be traced beneath the golf course to the base of the old stone

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quarry high-wall. Inspection of the area confirmed that springs near the base of the high-

wall were the most likely source of the newly formed spring.

The combined use of terrain conductivity and magnetic surveys proved to be a low

cost and effective approach to identify the source of this newly developed spring.

Introduction:

Located 3 miles to the east of Terra Alta, West Virginia is the Alpine Lake Resort.

There are many attractions at the resort, but one of the biggest is Hulls Lake. Hulls Lake is

a 148 acre lake that is contained on the southwestern edge by an earthen dam that is

approximately 250 yards long and 50 feet high. Figure 1 shows a digital orthophoto of the

survey area. The dam is located at the top of the Figure 1. The reddish colors highlight the

golf course fairway. The surveys were conducted almost entirely on the golf course with

the exception of some data was collected on the earthen dam.

During the 1940’s and 50’s, the Mississippian age Greenbrier Limestone was being

quarried at this same location. Remains of the quarry can still be seen today. The quarry

high-wall is highlighted in Figure 1 by a green line. After the quarry operation was closed,

the land was reclaimed and the quarry pit was filled in. Figure 2 provides another view of

the area. This figure is a topographic map of the area that was revised in 1976. The

revisions to the topographic map were made after the quarry had become inactive and

before the golf course was constructed. The purple areas on Figure 2 show areas where

revisions were made to the topographic map. The revisions had to be made because of the

change in the topography that occurred during quarry reclamation. Once the quarry was

filled in, the golf course was constructed on top of the fill.

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Figure 1: This is a digital orthophoto of the survey area. Reddish areas highlight fairways of the present day golf course. The dam is at the top of the figure, and the green line represents the location of the high-wall making the boundary between mined and unmined

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Figure 2: The circled area on the figure shows the location of the old limestone quarry. This topographic map was revised in 1976.

In July of 2002 the Alpine Lake Utility Board encountered drainage problems

along the edge of the golf course. Ditches were being dug around the edge of the course

for the installation of new waterlines. During the ditch digging operation they opened a

near-surface spring from which excessive amounts of water began to discharge. The ditch

was washed out. The bank collapsed as discharge continued. The water was channeled

into a nearby culvert, but the erosion and subsidence continued to occur around the

discharge area. The discharge also began to erode the culvert. The Alpine Lake Utility

Board was fearful of this development since excessive erosion of the culvert would

eventually undercut the road and also cause it to collapse. Figure 3 shows the discharge

area. The subsidence can be seen in the center of the figure. This picture was taken from

the road looking back toward the dam. This picture was taken later in the summer after the

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problem was first noticed. At the time of this photo (Figure 3) discharge had diminished

considerably as a result of drought conditions during the summer. Taking into account the

conditions that summer, considerable water was sill being discharged.

Figure 3: This photograph shows the discharge area and associated erosion. Additional subsidence and erosion can be observed in the surrounding area.

Although drought conditions seemed to mitigate the problem, it was important for

Alpine Lake management to know and better understand the origin of this drainage. Road

management crews and local residents were concerned the water was coming from a leak

in the dam located approximately 600 yards to the north of the discharge area. If the water

were coming from the dam, the problem would likely worsen and require costly

remediation efforts.

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Initially, two locations were identified as possible sources for the water. The first

and most obvious location was the dam. The dam and surrounding recreation area is a

very important site for the people of Alpine Lake. If the dam was leaking, the location of

the leak had to be identified and repaired. The second possibility is associated with

discharge from springs located along the base of the limestone quarry high-wall. Spring

discharge might flow underground and then re-emerge as discharge along the edge of the

golf course. Both theories were taken into consideration as the survey was conducted.

Methods: For this investigation, two geophysical devices were used. The first, and most

important, was the Geonics EM-31 terrain conductivity meter. The EM-31 is a one-man

portable device. The instrument does not make contact with the ground during operation,

but is carried by the operator with the aid of a shoulder strap. Figure 4 shows the EM-31.

Figure 4: This figure shows the EM-31 terrain conductivity meter. The design of the EM-31 allows for quick terrain conductivity surveys to be made.

The EM-31 can be used for many different applications. Some of the uses are:

locating or mapping contamination plumes, detecting voids and cavities, mapping the

subsurface bedrock topography, extending known gravel deposits and finding new ones,

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locating buried metallic-type conductors, and exploring archaeological sites (McNeil

1980). At Alpine Lake the EM-31 was used to detect the difference in conductivity

between saturated and unsaturated soils. Saturated soils have a higher conductivity because

of the dissolved ions in the groundwater.

The EM-31 has a simple design. There are 3 main components that make up the

device. The first component is the transmitter coil. The transmitter coil is powered by an

alternating current at an audio frequency of 6500 Hz (McNeil 1980). The alternating

current in the transmitter coil produces a magnetic field (the primary magnetic field). This

alternating electromagnetic field induces small currents beneath the earth’s surface. These

earth currents generate secondary magnetic fields within the earth. Together, the primary

and secondary electromagnetic fields induce alternating electromagnetic fields in the

receiver coil. The receiver coil for the EM-31 is located 12 feet away from the transmitter

coil. The ratio of the primary magnetic field to the secondary magnetic field is linearly

proportional to the terrain conductivity. This relationship allows recorded magnetic field

intensities to be converted directly into terrain conductivities. Terrain conductivity is

displayed on a meter (the 3rd component). The meter is located between the receiver coil

and the transmitter coil and is easily read by the operator. The conductivity read from the

meter is the apparent conductivity inferred from the secondary magnetic field at the

receiver (McNeil 1980). The units for conductivity that were used for this survey are

millimhos per meter (mmho/m).

The EM-31 is used for near surface geophysical work. Maps of EM-31

observations help locate area where additional investigations can be undertaken using the

EM-34 terrain conductivity meter. The EM-34 has a much great depth of investigation.

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Depending on the range of intercoil spacing employed in the EM-34 survey, information

from depths of 60 meters or so can be obtained. The depth of penetration achieved by the

EM-31 is relatively shallow due to its small intercoil spacing and relatively high operating

frequency. The effective depth of penetration for this intercoil spacing is about 6 meters.

This depth will vary depending on the type of material that is present in the subsurface.

Highly conductive materials buried at depths greater than 6 meters are often detectable.

During the Alpine lake survey, the design layout allowed for easy transitions from

line-to-line. There were five different grids surveyed across the area. The length of survey

lines within the grids ranged from 300 feet to 440 feet. The distance between individual

survey lines varied from 20 feet to 100 feet. Figure 5 shows all of the points where the

data was collected, as well as how each grid is oriented in space. Figure 6 shows this same

grid, but in this figure, the gird is superimposed on the digital orthophoto. This allows the

survey grids to be seen with respect to the area that was surveyed.

The instrument reads continuously while it is on. The simplicity of device allows

for measurements to be made quickly and accurately even when walking from station to

station. For this survey, measurements were taken every 20 feet perpendicular to the

expected flow direction of the groundwater, but the meter was constantly monitored while

it was being moved within the survey area. If changes were noticed while walking

between stations, the distance and meter reading were recorded. This allowed for short

wavelength anomalies (less than 20 feet) to be identified and considered. This also permits

accurate identification of the onset of near surface anomalies.

The EM-31 can be operated in vertical and horizontal magnetic dipole modes. In

the normal operation position one makes vertical dipole measurements. The vertical dipole

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measurements yield greater depth of investigation than do horizontal dipole measurements.

The vertical dipole measurements yield information from depths of about 6 meters or so.

When the instrument is rotated 90° onto its side, the magnetic dipole field is oriented

horizontally. The depth of investigation provided by the horizontal dipole measurement is

generally less (3 meters or so).

4368800

4368900

4369000

4369100

4369200

4369300

4369400

4369500

628650 628750 628850 628950

Figure 5: This map shows 5 individual survey grids. Each point represents the location of a terrain conductivity measurement.

meters

meters

11

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Figure 6: Measurements were clustered in grids to identify trends in terrain conductivity anomalies. The initial detailed survey revealed fairly long wavelength anomalies that could be followed using a wider spacing between grid lines and between grids. The survey lines were oriented roughly perpendicular to the flow of the groundwater beneath the area.

The initial grid of data was collected north of the discharge area. Both horizontal

and vertical dipole measurements were made. After the data was collected a comparative

analysis of the data sets for both orientations was conducted. It was determined that the

horizontal orientation was not useful for mapping the deeper water saturated zones on the

site. Conductive features observed with the vertical dipole measurements were too deep to

be followed by the horizontal dipole observations. During the remainder of the survey,

only the vertical dipole measurements were collected. This saved time and allowed more

data to be collected over a larger area.

The EM-31 vertical dipole measurements proved to be very useful for following

water saturation zones throughout the area. There is very little cost associated with the

operation of the device. The simplicity of the operation and the ability of the surveyor to

move quickly across the area allow for a large area to be covered in a short amount of time.

Another advantage of the EM-31 is that the device never comes in contact with the ground.

Unlike the resistivity survey, direct injection of current using electrodes is not required.

The EM-31 never leaves the hands of the surveyor.

Once the survey is over, the task of analyzing the data continued. Measured values

of terrain conductivity were entered into a computer database. There, the data was

imported into Surfer 7.0. In Surfer, the data was gridded and contoured. Terrain

conductivity contours clearly reveal anomalous conductivity trends. Contour patterns were

easily interpreted. Different variations of these contour maps were used in the analysis.

By changing the contour interval and also the range of the color scale, the internal detail of

the anomalies could be revealed. It was important to view different maps with different

contour intervals and color scales because maps with large contour intervals and narrow

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color scale reveal only the longer wavelength anomalies. Decreasing the contour interval

revealed the details of smaller internal anomalies. The contour maps of the data that were

produced will be presented in a later section of this paper.

A magnetic survey was also conducted in the vicinity of the earthen dam. There

was some concern that terrain conductivity anomalies, if they were observed near the dam,

might be associated with buried metallic debris. Unlike the terrain conductivity meter, the

magnetometer responds only to metallic objects. Use of both data sets allows us to

differentiate between metallic and groundwater induced regions of high conductivity.

Contour maps of both the EM data and the magnetic data were produced in Surfer 7.0.

Comparison of the two data sets revealed that terrain conductivity anomalies were not

associated with magnetic anomalies but were associated most likely with regions of

relatively higher water saturation.

Discussion:

Field observations along with the terrain conductivity surveys were useful. Figure

7 shows a location on the survey site where standing water was observed. This photograph

was taken in late August 2002. Most of the summer of 2002 was a period of drought. In

late August 2002 a day and a half of rain showers interrupted the drought. Given the

drought conditions, the ground was very dry, and most of the water was quickly absorbed

into the ground. Two days following this period of rain a large amount of standing water

remained in a low spot near the quarry high-wall. Although there were showers in the days

before this picture was taken, the surrounding area was completely dry suggesting that

water accumulation in the area was sustained by discharge from local springs. Figure 7

shows a picture of this location. The quarry high-wall is located off to the upper right in

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Figure 7. Figure 8 shows the location of the standing water as well as the location of the

discharge area on the digital orthophoto.

The observation of the standing water revealed that the water was coming from

springs in the quarry high-wall. The EM-31 surveys were continued throughout this area.

The middle 3 grids (Figure 5) were surveyed after the observation was made. Terrain

conductivity anomalies can be followed from the small pond in Figure 7 to the discharge

area (see Figure 9).

Figure 7: This photograph was taken 2 days after a day and a half period of rain. The water in the picture is 6-8 inches deep. The quarry high-wall is located in the upper right of the photo.

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Discharge area

Standing water

Figure 8: The locations of the discharge point and the standing water are illustrated on the above orthophoto.

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Figure 9 is a contour map of the vertical conductivity measurements made

throughout the area. In the figure, three pronounced high conductivity anomalies are

observed. Individual anomalies are numbered on Figure 9.

Dis

Standing water

DisDis

Standing water

1

3

charge pointcharge pointcharge point

2

Figure 9: This is a contour map of the vertical terrain conductivity data. The main anomalies are numbered 1-3.

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Anomaly 1 defines a narrow zone of relatively high terrain conductivity. This

anomaly originates near the body of standing water (Figure 7). The anomaly follows the

high-wall of the old limestone quarry southward toward the discharge point. On its

southern end, the anomaly separates into two trends. One turns to the east and continues to

follow the high-wall. The other breaks off in a southwesterly direction and quickly

disappears.

As previously mentioned, the EM-31 has a limited depth of penetration

(approximately 6 meters). This restricted depth of penetration helps to explain the

disappearance of the anomaly. To the southwest of area 1 in Figure 9, terrain conductivity

drops to 1 mmho/m or less. This region of low conductivity coincides with an area that is

topographically higher than in the surrounding areas. It is likely that increased depth to

the old quarry pit floor and to any water draining beneath the surface makes it nearly

impossible to follow. Use of the EM-34 could help resolve the deeper conductivity

features beneath the area, and a more complete picture could be obtained.

Area 2 is the area located nearest to the discharge point. Very high conductivity

values were observed this area. The surface is much closer to bedrock, and standing water

was observed in places. In the area, there was a visual correlation of increased terrain

conductivity and increased water saturation as indicated by soggy ground and sometimes,

standing water. The terrain conductivity anomaly observed in area 1 can be followed

along the high-wall into this area. However, the discharge point was located uphill from

this area of standing water. The discharge point is located near the west end of area 2 and

must have a different source. The spur-like anomaly that breaks off of anomaly 1 and

heads southwest may be the source.

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The final major anomaly is located in area 3. This anomaly is present at the dam.

Some leakage from the dam is inferred from high conductivities observed in the northwest

corner of the survey area along the top of the dam. This anomaly is discontinuous. There

is no evidence that the anomaly extends from the dam out across the golf course to the

spring. The area separating the dam and golf course anomalies has a very low conductivity

and it is also topographically low and dry. The earthen dam is marked by relatively high

conductivities suggesting high water saturation within the dam itself. Some seepage is

common to any earthen dam. However, association of discharge from the dam with the

main anomaly observed along the high-wall is not supported by the observations. Figure

10 shows the section of the dam that was surveyed. Beyond the red line in the figure, the

topography starts to drop. This is a dry area marked by low conductivities. The gray stripe

above the red line is the road that passes across the middle of the dam. (Figure 10 can be

seen on the following page.)

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Figure 10: This is a photo of the earthen dam. The entire height of the dam cannot be seen. The foreground area between the dam and the trees is the area of low topography and the also the area of low conductivity. This area is highlighted with a red line.

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Figure 11: In this figure the contour map of the terrain conductivity data is superimposed on to the digital orthophoto. Contrasting colors highlight the anomalies noted on Figure 9.

Color contours of the terrain conductivity variations observed between the dam and

the discharge point are superimposed over a digital orthophoto of the area (Figure 11).

Contrast enhancement emphasizes the appearance of conductivity anomalies observed in

the area. The terrain conductivity data has been superimposed on the orthophoto to help

portray the geographic location of each of the anomalous features. The anomaly that is

present along the base of the quarry high-wall stands out very well against the darker

colors on the map. The high conductivity spur that breaks off the main anomaly along the

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base of the high-wall is circled in the figure. This was mentioned on Figure 9, but it can be

seen a lot better in this figure. The dark blue area to the south of the spur represents the

area of increases topographic relief.

In the northern part of Figure 11, the dam-anomaly is very clear. The low

conductivity region immediately south of the dam highlights the lack of continuity

between high conductivities along the dam and more to the southeast along the high-wall.

This information, as before mentioned, suggests the dam may be leaking, but the anomaly

is most likely associated with increased water saturation caused by seepage, and not an

anomaly associated with a major leak in the dam. This anomaly is not responsible for the

discharge observed to the south.

The survey suggests that high-wall springs are the source. We are still left to

wonder, “Why does the water discharge along the roadside?” To answer this question,

maps of the recontoured mine spoil were used. Figure 12 shows the area before the golf

course was installed. One thing that stands out on this map is the existence of a pond

located on the reclaimed land. This feature can be seen on Figure 12. The red circle on the

map is there for reference purposes and will be explained in the follow section.

The pond on Figure 12 is located in an area of low topography on the pre-existing

reclaimed mine surface. Contour lines reveal the old surface drops downhill for the high-

wall into the pond. Overflow from the pond would drain to the south, following the

topographic low. Terrain conductivity anomalies suggest that the old reclaimed mine

surface still controls the drainage in this area to some extent. The conductivity spur

suggests that effluent from high-wall springs is, in part, channeled into the area previously

occupied by the pond. This area is also most likely the source of water producing

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anomalous discharge along the roadside. Figure 13 also shows a map of the area before

the golf course was put in. In this figure the EM data has been superimposed over the

map.

Figure 12: This map shows the area before the golf course was built. Notice the pond that is present near the red circle. The circle is referenced for comparison to Figure 13.

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Figure 13: This figure shows the conductivity data superimposed on the map of the reclaimed mine spoil.

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A present-day map showing topographic contours on the golf course was not found

during the investigation. However, a visual comparison suggests this area has changed

significantly. The old pond is now buried beneath a few meters of fill. Although the pond

was filled, it is likely that the pre-existing surface retains relatively low permeability and

still serves to channel water flow into the old pond area. Resolution of topographic

features on the reclaimed mine surface shown in Figure 12 is too poor to rule out the

existence of a drainage path that would carry groundwater directly south of the pond to the

discharge point rather than to the southeast. A series of EM-34 terrain conductivity

soundings could be surveyed over this area to try and resolve deeper conductivity

structures and map out individual flow paths. Such a survey goes beyond the scope of this

study but is recommended in any future study of this area.

Conclusions:

Three major anomalous regions are present within the Alpine Lake survey. These

areas of increased terrain conductivity are associated with an increase in ground saturation.

Mapping of these zones across the area define probable migration pathways for

groundwater flow. The anomaly that extends along the base of the quarry high-wall has

been determined to be the source of the discharging water. However, this anomaly does

not lead directly to the discharge area. Through interpolation of a high conductivity spur

breaking off the main anomaly and examination of contour maps of the reclaimed quarry a

probable flow path was determined. Future EM-34 terrain conductivity surveys could be

conducted on the site to help resolve deeper conductivity structures and possible

groundwater flow paths.

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The survey and data analysis ruled out the possibility of significant leakage from

the dam. Increased conductivity along the dam is attributed to increased soil saturation

from seepage and not a major leak. Examination of the anomaly present on the dam shows

that the anomaly is discontinuous. There is an area between the dam and the anomaly at

the base of the quarry high-wall that has very low terrain conductivity values.

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References: McNeil, J. D., 1980, Electromagnetic terrain conductivity

measurement at low induction numbers: Technical Note TN-6, GEONICS Limited, Ontario, Canada, 15p.

Burger, Robert H., 1992, Exploration Geophysics of the Shallow Subsurface: Prentice Hall, 489 p.

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Methods: