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' 2.1 Stoichiometry for Target Compounds and Intermediate Products The principal constituents of concern at the French Camp Grain Elevator site are carbon ' tetrachloride (CC14) and chloroform (CHC13). Chloroform is a daughter product formed from the partial molecular breakdown of carbon tetrachloride. Chloroform is further broken down anaerobically to methylene chloride (CH2C12), which — in tum — releases the final breakdown ' products, carbon dioxide, water and chloride iron. The presence of chloroform suggests that the groundwater contamination is degrading naturally, although the rate of natural attenuation has not been quantified and appears to be limited. The following reactions generally represent a dehalogenation process in the presence of reduced iron metal and water(Gilham and O'Hannesin, 1994): Considering only the presence of water, zero-valent iron and a chlorinated organic compound X-C 1, the following equations est ed: 2Fe° He 2++4e 3H2O 3H++ 30H- 211++2e H2 X-Cl +H++2e—> X-H+ Cl- 1 2Fe°+3H2O +X-C 1 --> 2Fe2++30H-+H2+X-H+Cl- . . . . (1) In this case, for each mole of Cl- released to the solution p ase, 2 moles of Fee+ should also appear in solution. On the other hand, if the dissociation of water is ' not included, the overall reaction can be written: Fe°+X-C 1 +H2O —* Fe2++OH"+X-H+Cl" (2) tIn this case, only one mole of Fe 2+ would be released to the solution phase for each mole of Ct. Thus, the complete dechlorination of 1.0 mg/1 of CT would I result in a chloride concentration of 0.92 mg/l, and would contribute 2.8 or 1.4 mg/l of iron,based on equation(1) or(2),respectively. Depending on pH and Eh conditions, iron may occur as ferrous hydroxide Fe(OH)2 or as FeCO3. Chloride concentrations and iron concentrations may increase as a result of the degradation process. These constituents (chloride and iron) are included in the proposed monitoring of the pilot scale study. RACONTGRAMCCACargiIDFrench Camp\Waste Discharge Application\supp info 6-29-04.doc 5