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4 F 3 o f fry E,• '� .� k. '# p I 2. 11)I?N'€'II IAIILE IZEiNli:1)IAI,A1,TI:IZNA'I'IVES As discussed in the previous section, boil) soil and groundwater have been impacted by a release(s)of petrolcum hydrocarbons at former Exxon RS 7-3942. Because the groundwater level in Stockton has fluctuated dramatically,soils above and below the present water table now contain petroleum hydrocarbons. Remedial alternatives for both soil and groundwater need to be addressed. 2.1 VADOSE%UNI: CONTROL OR REIIIOVAL TECIINIQUCS Below is a list of vadose zone control or removal techniques employed at remediation sites: I. Volatiliz_rtion or vapor extraction is the process by which volatile hydrocarbons are renlovcd from the soil matrix by extracting;vapors from wells that have been in- stalled in the soils containing hydrocarbons. 'Phis technique is well Suited to rela- tively permeable soils that contain volatile hydrocarbons such as gasoline. The rate of hydrocarbon removal is much grcatcr than any other in-situ method,and the cost to install and operate a soil vapor extraction system is low relative:to other remedial options. This method,when used with a vapor treatment system that oxidizes the hydrocarbons to carbon dioxide and water, cliluinates the need to transport contami- nated soils from the remediation site. Testing has demonstrated that in-situ soil venting is a viable nlcillod of remediation at former Exxon RS 7-3942. 2. In-situ stead) struggling is a vohuili�,ation technique. In-situ steam stripping is a process that consists of a mobile system that injects steam into the soil through Eargc dian)etcr:angers,captures tile vapors emitted from the soil,and processes these vapors using condensation followed by carbon :adsorption. The advant.)ge gained by ))sing steaul is tllat C'ompoltnds are more volatile at higher temperatures. The conlponcnls of.;asoline :are, however, highly volatile and do not require the higher temperature or added cost elf steann to be removed. 3. Vi riFication is a pro(:ess userl to intnlobilize contaminants in a glass by electrically heating; the soil 10 tcmpCratures (1,600 to 2,000 C) at which the soil melts. This technique is mainly applicable to soils containing; metals rather than volatile hydro- carbons. 4. Capping r'cfers to installing;a nearly irltpermeable barrier on the surface of the site. The cap is designed to prevent rainfall from penetrating the vadose zone and leach- ing the hydrocarbons into the aquifc-or groundwater. This process does nothing to treat or remove the hydrocarbons from the soil. It is preferable to remove the source of hydrocarbons that are or may impact groundwater rather than. leave it in place. 5. 5nil washing is a tecluliquC that uses compounds that will solubilize hydrocarbons in water, such as surfactants (to wash hydrocarbons From the soils). Because this process requires that surfactatnts be introduced into the soils ant! ultimately ground- water,a permit will be difficult to obtain frons the Regional Wafter Quality Board. G. Ire-situ W0,101-licttl I1•ea1111Clll uses n.Uurally occurring microorganisms to decompose hydrocarbons. The use of this Fornl of treatment is highly site-dependent. Popula- tions of microor-1a11lisrlls are e•)tiler enc0-lrag,ed to grow 01'lntrodmCe(I along with the Mi'TiM`1.• �t WAR 11201:10