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1 Cjeofo{gp�icalrlecFuurs 2nc Page 4 GeoProbe Installation Report Cardoza(Super Stop) Project No 662 2 June 12,2001 ' u A clay unit was encountered at depths ranging 15 — 20 feet bgs, and was a dark grey color Li The soils were moist starting at the first sampling interval of 5 feet bgs, and ' occasionally contained roots, and black organic materials • No field evidence of contamination was noted during the investigation • Groundwater was encountered during drilling at a depth of approximately 24 feet bgs, ' based on drill rod observations It is noted that the level of groundwater in well MW-2 was 21 feet below grade surface (bgs) the previous week • The acid preservative in the vials reacted strongly with the groundwater when sampling, ' especially in GP-5 and GP-6 The production of gas made it difficult to preserve a zero headspace condition in the VOAs See Appendix B, Bonng Logs for more information 4.0 CONCLUSIONS AND RECOMMENDATIONS Based on our interpretation of the data collected over the course of this subsurface investigation, GTI have reached several conclusions These conclusions are based on the premise that the data we considered, although incomplete, are representative of actual site conditions We acknowledge that there may be undiscovered conditions, which would upon their consideration, change our interpretation and thus our conclusions Our recommendations are based on our knowledge of site conditions, and on the state and limitations of subsurface investigative technology 4.1 Conclusions ' The field activities armed ap revious conclusion that MTBE soil contamination is limited to a small area in the vicinity of the former UST excavation The highest concentrations of MTBE in soil were detected in previous borings GP-1 @ 16' bgs and GP-2 @ 26' bgs I What is surprising is that no significant contamination was present in the water samples taken The MTBE present in wells MW-1 and 1\4W-3 would be expected to migrate in the predominant groundwater direction of east-northeast The absence of MTBE in GP5, GP-6 or GP-7 suggests various possibilities ' • The MTBE plume is of small extent and does not reach beyond the property eastern boundary • The MTBE is part of a submerged plume migrating below the water table and as ' such was not detected in shallow groundwater This situation is more probable • The soils at the site are preventing MTBE migration This situation is unlikely because clays are present at depths of 15 — 25 feet bgs, with sands occurring above ' and below the clays The MTBE present in soils at depths of 21', 31' and 36' in MW-3 indicates that vertical migration of MTBE is occurring near the source 4.2 Recommendations In order to more accurately define the groundwater plume, GTI makes the following recommendations