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1 Page 5,CAP ARCO rcuni man obtained during the vapor extraction test are consistent with the conclusion that the contaminated zone between 15 and 35 feet is highly permeable and enabled gasoline leaking ' from the broken coupling to migrate relatively freely Assuming that the unauthorized product line release in 1991 is the source of the contamination ' in the central part of the site, gasoline migration was downward through the vadose zone into the lower sand layer, contaminating groundwater at a depth of 60 to 70 feet The subsequent rise in groundwater levels into the lower silt layer since 1992 has coincided with a drop in contaminant concentrations This could be due to adsorption of contaminants onto the fine-grained soil particles of the silt layer, or it could indicate natural degradation of the contaminants over time Motor oil and diesel have each been detected one time in MW-3 near the diesel dispenser ' Diesel was not detected in vadose-zoite soil samplers from this well or in soil samples from either the tank excavation or piping, indicating that the diesel contamination is possibly due to a minor surface release, perhaps in the vicinity of the dispenser Diesel is less mobile than gasoline and ' does not appear to present a significant risk at this site Relative to many sites, the groundwater gradient and flow direction at this site have remained fairly stable since monitoring began Flow is eastward and the gradient is moderate Detection of gasoline in MW-2 has been consistent, suggesting that the downgradient limit of the dissolved gasoline plume may be beneath the sidewalk or Pacific Avenue ' 4.0 SOIL REMEDIATION ALTERNATIVES ' Three types of remedial options have been considered for contaminated soil at the site The first type encompasses techniques that require excavation of the contaminated soil The second and third types involve underground treatment using either hydrocarbon-degrading bacteria or vapor extraction 4.1 Excavation and Treatment or Removal At sites with shallow contamination, excavation of the contaminated soil is a relatively rapid and effective method of eliminating the contamination and reducing the risk of future ormindwatpr contamination Excavation can be accomplished quickly if adequate space is available and the affected soil is accessible to excavation equipment At this site, however, excavation is ' impractical for several reasons Contamination extends to a depth of at least 50 feet in some areas, and is beyond the reach of conventional excavation equipment 1