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some extreme cases a"curtain"of treatment wells is installed downgradient of the plume to prevent it from migrating. However,this technology has one certain and one possible advantage over pump and treat. Its major advantage is that it requires no disposal of treated groundwater, which results in a great reduction in sampling, laboratory, and disposal costs. This is demonstrated by the quantitative cost comparisons that follow. The other possible advantage of the method is that it is purported to work at a faster rate than pump and treat,thereby reducing the total time needed for site remediation. However,as is the case for pump and treat,this depends largely on site conditions. LIMITATIONS AND DISADVANTAGES OF THE METHODS As noted above,the effectiveness of either method is subject to the specific characteristics of the site. In general,hydrogeologic conditions are the principal factor in determining whether the method will be effective. Specifically,both methods operate by drawing contaminated groundwater toward the pumping well, and the ability of the pumping well to achieve this depends on the permeability(or hydraulic conductivity)of the water-bearing sediment. If the contaminated groundwater is present in a permeable zone, such as a well-sorted sand or gravel-layer, groundwater flow is relatively rapid and moderate to high pumping rates can be achieved. If the contaminated zone consists of fine-grained, low-permeability sediment such as clay or clayey sand,then flow is reduced and more pumping wells will be needed to affect the contaminant plume. Fortunately,the zone where hydrocarbons have been detected at this site consists of fine- to-medium-grained sand and should have sufficient permeability for either method to be effective. Based on examination of the data,Upgradient Environmental estimates that two wells will be sufficient for either method. The other main limitation of groundwater extraction is that the low concentrations of contaminants means that large volumes of water must be pumped to remove relatively little contamination. This large volume of pumped and treated groundwater must then be disposed of,which can become expensive. This is especially true if the receiving party (either the City Sanitation Department or the State storm'water system)has restrictive requirements about the types and concentrations of contaminants that will be received. In the case of Valley Motors,the City of Stockton Sanitation Department does not accept certain hydrocarbon solvents or gasoline oxygenates,and requires that these be reduced to non-detectable concentrations prior to disposal. Hence, it would be.necessary to sample the treated water to demonstrate that these chemicals are,not present in the disposal stream. The City of Stockton requires weekly sampling of treated groundwater discharged to the sanitary sewer,which is a significant disadvantage of pump and treat due to the increased sampling and laboratory cost. The laboratory cost would depend on the required analyses, and because the City requires analyses of chemicals that have not been routinely sampled 3