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Technical Memorandum on Surface Geophysical Surveys, Well 16 Source Characterization
Section 11 - Open Areas
A geophysical survey was conducted at all the "open" areas within a 2,000-foot radius of well 16 in order to possibly identify any unknown SWMUs. The "open" areas are defined as those areas relatively clear of vegetation such as trees and shrubs. This additional work was added at the request of CSSA as part of the Well 16 source characterization study. Geophysical surveys were performed at a total of six sites as part of this phase of the investigation. The sites were assigned the following names listed in order of discussion: North Pasture, South Pasture, Well 16 West, Well 16 East, Gate 6, and Salado Creek. The North and South Pasture sites are relatively large open areas northwest and southwest of well 16. The locations of these sites are shown on Figure 1.2.
An EM survey was performed at each of the open area sites mentioned above. Grid points were staked at 100-foot intervals with data transects conducted at 50-foot intervals in both north-south and east-west directions. Both apparent conductivity and in-phase EM readings were collected at two foot intervals along each transect. A datalogger was used to collect readings which were subsequently downloaded and contoured using Surfer contouring software.
The North Pasture area is located in the north pasture northwest of well 16 and north of well D. It is a large (1100 by 1100 feet) open area bounded by trees to the east and west and a fence line to the south. There is a north-south oriented creek running through the middle of the site (Figure 11.1). The creek is generally dry except for periods following a heavy rain.
11.1.2 Geophysical Survey Results
No EM anomalies were detected on either the apparent conductivity or the in-phase data in the North Pasture area (Figure 11.2, Figure 11.3, Figure 11.4, and Figure 11.5). The variations in conductivity data appear to reflect changes in thickness of soil cover overlying limestone bedrock. The variations in in-phase EM data for the east-west profile is due to instrument drift that occurred between the two in-phase data sets that were collected for this site. This was verified by the quality control profile that was conducted at the beginning and end of the survey.
The South Pasture is located in the inner cantonment directly south of the North Pasture and southwest of well 16 (Figure 1.2). Like the North Pasture, the South Pasture area is relatively large, extending 1100 feet by 900 feet. The site is bounded by a fence line and paved road to the north and east, and trees to the west. The north-south oriented creek present in the North Pasture continues onto the South Pasture flowing south toward nearby Salado Creek. The creek is usually dry except for periods following a heavy rain. Of significant importance is a 6-inch underground water supply line that runs through the northwestern portion of the site. This water line has apparently been capped. Other surface features in the South Pasture area include a pile of metal cones, an electric fenced-in area, and a corral to the northeast, power lines to the north and east, well 4 to the southwest, dirt mound ridges to the north, scattered trees, and occasional miscellaneous metal debris (Figure 11.6).
11.2.2 Geophysical Survey Results
One strong linear EM anomaly was detected in the northwest portion of the South Pasture on both the apparent conductivity and in-phase contour maps (Figure 11.7, Figure 11.8, Figure 11.9, and Figure 11.10). The anomaly trends northeast-southwest and corresponds to the underground water supply line discussed above. Conductivity and in-phase contour maps also reveal an isolated anomaly in the same area as the underground water line at coordinates 20,80. This anomaly directly corresponds to an overhead power line in the vicinity. There is an isolated anomaly located in the north central part of the site on the north-south profiles of the conductivity and in-phase data. This anomaly, located at coordinates 45,67.5 corresponds to a bermed burn area containing metal wire, wood, rock and ashes. The north-south and east-west conductivity contour maps reveal an EM anomaly present in the northeastern portion of the site. This anomaly trends northeast-southwest and is identified as A on Figure 11.11. A 1.5-inch diameter metal pipe is located on the southwestern end of anomaly A which may have affected the EM readings in that area. There is another isolated anomaly located in the northeastern portion of the site present only on the east-west oriented in-phase contour map (Figure 11.10). The anomaly is located at coordinates 90,60 and is associated with the pile of metal cones in the area (see Figure 11.6). In the central and southwestern portions of the site, specifically at coordinates 55,35, 10,0, and 30,0, there are some peculiar cyclic fluctuations in EM conductivity and in-phase readings. These appear to be related to some type of high frequency noise, possibly 60 Hz (Figure 11.11). These readings are not related to any overhead power lines or any other surface features in the area. Furthermore, the quality control profile that was performed at the end of the survey did not reveal the presence of this high frequency noise in the southwestern corner of the site (Figure 11.12).
In summary, most of the conductivity and in-phase anomalies observed at the South Pasture were associated with cultural features, such as underground piping, overhead power lines, and miscellaneous metal debris on the ground surface. One EM anomaly in the northeastern portion of the site, identified as A on Figure 11.11, was not related to any cultural features mentioned above. This anomaly may be associated with metal piping which was discovered in the area. Finally, there are some unexplained high frequency variations in conductivity and in-phase readings in the central and southwestern portions of the site. These variations appear to be related to some type of high frequency noise in the area. The location of this high frequency noise is shown on Figure 11.11. The source of this noise is not known at this time.
Well 16 West is located along a narrow strip of grass west of Well 16 and Well D. The site is bounded by a fence line to the north and a paved road to the south. The same dry creek found in the North and South pasture areas runs through the middle of this site. There is also a metal pipe coming out of the ground to the northwest near coordinates 175,50 (Figure 11.13). Given the small, narrow area, the EM survey at Well 16 West was performed at 25-foot intervals in the north-south and east-west orientations.
11.3.2 Geophysical Survey Results
Two EM anomalies were detected at Well 16 West on both the apparent conductivity in-phase contour maps (Figure 11.14, Figure 11.15, Figure 11.16, and Figure 11.17). These anomalies are identified as A and B on Figure 11.18. Anomaly A trends northwest-southeast and is located in the western portion of the site near coordinates 17.5,5. Anomaly B trends northeast-southwest and is located in the central potion of the site near coordinates 42.5,5. These tow anomalies do not appear to be related to any surface features and there are no known underground utility lines in the area. Conductivity and in-phase values generally increased to the north toward the fence line that separated this site from the North Pasture.
In summary, two anomalies (A and B) were observed at Well 16 West that do not correspond to any surface features in the area (Figure 11.18). It is not known whether any underground utility lines are present at the site.
Well 16 East is located along a narrow strip of grass directly east of well 16. Similar to Well 16 West, Well 16 East is bounded by a fence line to the north and a paved road to the south. In addition, there is a telephone pole line to the north. A mound of dirt is present to the east which may have been a parking area at one time (Figure 11.19). Given the small, narrow area, the EM transects at Well 16 East were preformed at 25-foot intervals in north-south and east-west directions.
11.4.2 Geophysical Survey Results
The conductivity and in-phase contour maps reveal an anomaly on the northwest corner of the site at coordinates 0,6 (Figure 11.20, Figure 11.21, Figure 11.22, and Figure 11.23). This anomaly, labeled as A on Figure 11.24, may be related to an overhead utility line in the area. The fence line is located approximately 30 feet north of anomaly A, not considered close enough to influence EM readings. The fence line gets closer to the surveyed area from west to east across the site. This is reflected in the EM contour maps.
The Gate 6 area is located in the north pasture northwest of gate 6. The site is bounded by a fence line to the south and by trees to the north and west. Also present on the site are a drainage ditch to the south, a power line and burn pit in the central portion of the site, and a dirt mound to the east (Figure 11.25).
11.5.2 Geophysical Survey Results
The conductivity and in-phase contour maps reveal an anomaly on the western portion of the site (Figure 11.26, Figure 11.27, Figure 11.28, and Figure 11.29). This anomaly trends northeast-southwest across the site and is associated with an abandoned underground 4-inch diameter metal pipe. Exposed portions of this metal pipe were observed during the field investigation. The pipe continues west toward well 16. The conductivity and in-phase contour maps from the north-south EM transect reveal an isolated anomaly at coordinates 25, 17.5. This anomaly, identified as A on Figure 11.30, is relatively subtle and not related to any known surface or subsurface features. A larger isolated anomaly exists on the east-west transects of the conductivity and in-phase contour maps at coordinates 52.5,15. This anomaly, identified as B on Figure 11.30, does not appear to be related to any surface features. The anomaly is similar in nature to the anomaly generated from the underground piping in the western portion of the site. Therefore, Anomaly B may be related to metal piping or fragments in the subsurface. An isolated anomaly was detected on the east-west transect of the conductivity contour map at x,y coordinates 35,15. This anomaly, identified as C on Figure 11.30, is associated with a burn pit located in the same area.
In summary, there were three anomalies (A, B, and C) identified in the Gate 6 area. Anomalies A and B do not correspond to any known surface features (Figure 11.30) and may be due to the presence of isolated buried metal debris in those areas. Anomaly C, on the other hand, is located in a burn pit area.
The Salado Creek site is located south of SWMUs B-28 and B-19 and west of SWMU B-4 (Figure 1.2). The site is bounded by trees and SWMU B-4 to the east, B-28 and B-19 to the north, and Salado Creek to the west. The site is relatively clear except for a few scattered trees and a wood pile (Figure 11.31).
11.6.2 Geophysical Survey Results
Several conductivity and in-phase anomalies were observed at Salado Creek (Figure 11.32, Figure 11.33, Figure 11.34, and Figure 11.35). These EM anomalies are labeled as A, B, C, and D on Figure 11.36. Anomaly A is located in the northwest portion of the site from coordinates 5,42.5 to 10, 42.5. This anomaly may be associated with the trash pile present in the same area. Anomaly B is located in the west-central portion of the site from coordinates 30,20 to 30,37.5. This anomaly is a long, linear feature oriented in the north-south direction. Anomaly B is adjacent to SWMU B-4A and therefore may indicate the presence of another trench in this area. Anomaly C is located in the southeast portion of the site from coordinates 22.5,0 to 22.5,20. Anomaly C is also a long, linear feature depicted in the east-west oriented conductivity and in-phase contour maps. Due to its dimensions and proximity to SWMU B-4, Anomaly C may be an additional trench in the area containing buried waste material. Anomaly D is located just west of Anomaly C at coordinates 15,0. This anomaly is evident on the east-west oriented EM contour maps. Anomaly D appears to extend south of the mapped area and therefore may correspond to another buried trench in the area.
In summary, there were four anomalies (A, B, C, and D) identified at Salado Creek (Figure 11.36). One of the anomalies (A) may be associated with a trash pile located in the same area. The other three anomalies (B, C, and D) may be indications of additional trenches in the area containing buried waste material. These three anomalies observed at Salado Creek are proximal to the southwestern portion of SWMU B-4 (Figure 1.2).