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I05 May 1997 AGE-NC Project No 95-0173 Page 12 of 29 ' of the site poses concern based on the downgradient direction from the site and the close proximity Based on the data collected from downgradient wells MW-5 and MW-7, the plume does not appear to have migrated beyond these wells Discussions with representatives from Cal Water indicated that analysis of water samples collected from Station 4 have not detected volatile organic compounds or petroleum hydrocarbon constituents ' 7.0. SOIL REMEDIATION ALTERNATIVES 7 1 SOIL VAPOR EXTRACTION Soil vapor extraction (SVE) is a well accepted method of treatment for hydrocarbon-impacted soil, ' and has been used at numerous sites with an excellent rate of success The method is most effective on volatile hydrocarbons in permeable soil 711 Principles For most SVE systems, a vacuum blower is used to volatilize hydrocarbons in the vadose (unsaturated) zone and draw the vapor into extraction wells screened within the contaminant plume ' The hydrocarbon vapor is drawn from the wells and"treated" in one of several possible treatment units (carbon canisters, internal combustion engines, thermal oxidizers and catalytic oxidizers) Air in3ection wells may be installed outside the contaminant plume to increase air flow to the extraction ' unit An internal-combustion (IC) system draws the vapor stream into the carburetor system of the engine and mixes the vapor with air prior to combustion Exhaust gasses (oxides of carbon and hydrogen) are further broken down in a catalytic converter before being emitted to the atmosphere These systems require supplemental fuel, such as propane or natural gas, which can greatly increase 1 expenses as vapor concentrations decrease below levels normally required for combustion Typically, internal combustion engines work effectively where in-situ hydrocarbon concentrations approach the 40,000 parts per million - volume (ppmv) required for peak performance of the engine As concentrations decrease belmir 20,00n nnmv Qi1nn1PMPnta1 fi1PI rPgvirementS, and therefore rr... , rr......_.__. expenses,become burdensome Another disadvantage of these systems is the noise generated by the engine, however, this can be mitigated by placing the unit inside an enclosure, such as a building 1 A c.rwOn-canister -sy�.s,t.e..m... nnPrntPC by rniitinaa hvArnenrhnn vannr thMli9h activated carbon filters r,_.»._- -.1 -v r - o- where adsorption of hydrocarbons onto carbon takes place, removing the hydrocarbons from the vapor stream This type of system works best for low concentrations of hydrocarbons, but becomes expensive when high concentrations of hydrocarbon vapor necessitate frequent canister replacement Advanced GeaEmironmental,Inc