Terrain Conductivity Investigation of Groundwater Flow Near the Dam at Alpine Lake, West Virginia. Senior Thesis By Tom Darby II

Transcription

1 Terrain Conductivity Investigation of Groundwater Flow Near the Dam at Alpine Lake, West Virginia Senior Thesis By Tom Darby II April 30, 2003

2 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 Figure 13: Topographic map with EM data superimposed 24 1

3 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. 2

4 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 3

5 quarry high-wall. Inspection of the area confirmed that springs near the base of the highwall 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 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. 4

6 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 5

7 Figure 2: The circled area on the figure shows the location of the old limestone quarry. This topographic map was revised in 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 6

8 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. 7

9 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, 8

10 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 3 rd 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. 9

11 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 10

12 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) meters Figure 5: This map shows 5 individual survey grids. Each point represents the location of a terrain conductivity measurement meters 11

13 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. 12

14 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 13

15 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 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 14

16 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. 15

17 Standing water Discharge area Figure 8: The locations of the discharge point and the standing water are illustrated on the above orthophoto. 16

18 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. Standing water 3 1 Discharge point 2 Figure 9: This is a contour map of the vertical terrain conductivity data. The main anomalies are numbered

19 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. 18

20 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.) 19

21 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. 20

22 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 21

23 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 highwall 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 22

24 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

25 Figure 13: This figure shows the conductivity data superimposed on the map of the reclaimed mine spoil. 24

26 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. 25

27 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. 26

28 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. 27