###### Abstract  
Groundwater at trichloroethylene (TCE)-contaminated sites lacks electron donors, which prolongs TCE's natural attenuation process and delays treatment. Although adding electron donors, such as emulsified oil, accelerates TCE degradation, it also causes the accumulation of hazardous metabolites such as dichloroethylene (DCE) and vinyl chloride (VC). This study combined in situ chemical reduction using organo-iron compounds with enhanced in situ bioremediation using emulsified oil to accelerate TCE removal and minimize the accumulation of DCE and VC in groundwater. A self-made soybean oil emulsion (SOE) was used as the electron donor and was added to liquid ferrous lactate (FL), the chemical reductant. The combined in situ chemical reduction and enhanced in situ bioremediation achieved favorable results in a laboratory microcosm test and in an in situ biological field pilot test. Both tests revealed that SOE+FL accelerated TCE degradation and minimized the accumulation of DCE and VC to a greater extent than SOE alone after 160 days of observation. When FL was added in the microcosm test, the pH value decreased from 6.0 to 5.5; however, during the in situ biological pilot test, the on-site groundwater pH value did not exhibit obvious changes. Given the geology of the in situ pilot test site, the SOE+FL solution that was injected underground continued to be released for at least 90 days, suggesting that the solution's radius of influence was at least 5 m.  
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therefore, large amounts of groundwater must be drawn and replaced continuously to enable the oil to diffuse into soil pores [7]. Emulsified edible oil (emulsified vegetable oil) has fine and evenly distributed droplets, and can thus more easily diffuse into the pores of different types of soil [8; 9; 10]. Emulsified edible oil releases carbon sources and fatty acid, which can stimulate anaerobic reductive dechlorination. In addition, microbes can be applied to facilitate the complete removal of chlorine to form nontoxic chlorinated ethenes (CEs) [11; 12; 13]. Previous studies have injected emulsified oil into contaminated aquifers to form a bioreactor system. Through the slow release of a CEVO, microbial activity in the aquifer is stimulated over a long period of time to achieve complete dichlorination [14] and remove heavy metal and organic pollutants in groundwater [15; 16]. Although CEVOs are favorable hydrogen-releasing substrates [17], in situ bioremediation using CEVOs to stimulate anaerobic reductive dechlorination of contaminated groundwater has a few defects. Biodegradation of a CEVO generates organic acid, which decreases the environment's pH value and causes soil acidification problems. In anaerobic environments, sulfates may form hydrogen sulfide and generate odors. Moreover, when vinyl chloride (VC), the metabolite of reductive dechlorination of tetrachloroethylene (PCE) and TCE, accumulates to a certain level, microbial activity may decline [18] or the growth of microbes that can degrade VC may be partially inhibited [19]. Subsequently, the VC degradation rate decreases and prolongs the time required for bioremediation of the contaminated site. Adding zerovalent iron can effectively overcome these defects. Zerovalent iron has three main mechanisms in water: (1) in anaerobic environments, it releases electrons from its surface to facilitate the reductive dechlorination of chlorinated organic compounds (RCI); (2) Fe\({}^{2+}\) is oxidized into Fe\({}^{3+}\) and releases electrons to facilitate the reductive dechlorination of RCI; and (3) zerovalent iron is oxidized in aqueous solutions to reduce water to H\({}_{2}\) and OH\({}^{-}\), and H\({}_{2}\) can be used as the electron donor to facilitate catalyzed hydrogenolysis with RCI, thereby achieving the goal of dechlorination [20].