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Technical Memorandum on Surface Geophysical Surveys, Well 16 Source Characterization

Section 12 - Conclusions

12.1 - Summary of Geophysical Results

Surface geophysical surveys were performed at a total fifteen sites at CSSA to determine the presence and location of potential waste disposal areas. Seven of these sites were SWMUs and the remaining eight were site sites added by CSSA for completeness of a surface investigation of Well 16 source characterization. EM surveys were conducted at all fifteen sites, and GPR surveys were conducted at nine sites. Two GPR methods were employed, one using the GSSI SIR-3 system and one using the Pulse EKKO IV system.

Overall the EM surveys were successful in determining the presence or absence of disturbed areas potentially containing waste material (i.e, metal debris, trash) that might contribute to the groundwater contamination at Well 16. GPR had limited success in determining the existence of trenches and/or disturbed areas within selected areas. This was probably due to to interference from the surficial clay present at CSSA. With the exception of the Oxidation Pond, the Pulse EKKO GPR method generated better results than the GSSI SIR-3 GPR method with regard to the identification of buried trenches. Pulse EKKO data reinforced and at times added to the results of the EM surveys. For example, three inferred trenches were identified at SWMU B-4 during the Pulse EKKO GPR survey. The presence of these trenches at B-4 were not delineated as clearly from the EM survey results.

Both EM and GPR anomalies were detected at the following SWMUs: B-2, B-3, B-4, O-1 (oxidation pond), and B-28. No EM or GPR anomalies were detected at SWMUs B-1 and B-19. The anomalies detected at Well 16 and Southeast Well 16 during the EM and GPR surveys were due to cultural disturbances (i.e., fences, underground piping, telephone lines, etc.). EM anomalies were observed at most of the "open" area sites including the South Pasture, Well 16 West, Well 16 East, Gate 6, and Salado Creek. No EM anomalies were observed in the North Pasture. Table 12.1 below lists the total number of anomalies detected at each of the fifteen sites using EM and GPR methods.

Table 12.1 - Geophysical Anomalies

12.2 - Further Work

The geophysical anomalies detected at SWMUs B-2, B-3, B-4, O-1, and B-28 were used to locate a series of soil borings at each site in February and March 1995. In order to avoid drilling through potential buried waste material, the soil borings were placed adjacent to the geophysical anomalies. Soil borings were also drilled at SWMUs B-1 and B-19 even though there were no geophysical anomalies detected at these locations. No soil borings were drilled in the "open" areas. Soil and groundwater samples were collected from the soil borings as a continuing part of the source characterization study. The results of this study are incorporated in a separate report entitled "Technical Memorandum on Soil Boring Investigation, Well 16 Source Characterization," (June 1995).

Additional work may be necessary to verify the existence of high frequency noise (and possible source and extent) detected in the south pasture during the EM survey. Furthermore, the southern extent of the EM anomalies detected at Salado Creek and determination of potential trenches containing buried waste material at Salado Creek and SWMU B-4A require additional investigation. It is anticipated that these areas will be addressed in soil gas surveys to determine which, if any, of the geophysical anomalies are source(s) of well 16 contamination, followed by trenching to establish the subsurface cause of the anomalies and characterize wastes at the SWMUs for closure data.