Vadose Zone Processes and Chemical Transport

soils. A natural obstacle to this practice is the accelerated aging of iron and loss of reactivity resulting from O2 Permeable zerovalent iron (Fe0 ) barriers have become an estabin the soil atmosphere. Despite this apparent limitation, lished technology for remediating contaminated ground water. This same technology may be applicable for treating pesticides amenable situations could arise where soil and site characteristics to dehalogenation as they move downward in the vadose zone. By (i.e., depth to ground water) would allow horizontal conducting miscible displacement experiments in the laboratory with placement of a PRB below the source of contamination. metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1One potential application for this approach is the numethylethyl)acetamide; a chloroacetanilide herbicide] under unsatumerous point sources of contamination caused by inadrated flow, we provide proof-of-concept for such an approach. Transvertent or deliberate pesticide spills. In practice, conport experiments were conducted in repacked, unsaturated soil coltaminated soils would be excavated, a permeable iron umns attached to vacuum chambers and run under constant matrix barrier placed in the pit, and the soil replaced. Theoretipotential ( 30 kPa) and Darcy flux (approximately 2 cm d 1 ). Treatcally, as the pesticide desorbs from the soil matrix and ments included soil columns equipped with and without a permeable migrates through the iron barrier it would be transreactive barrier (PRB) consisting of a Fe0–sand (50:50) mixture supplemented with Al2(SO4 )3. A continuous pulse of 14C-labeled metolachlor formed and further degraded in the subsoil. (1.45 mM ) and tritiated water (H2O) was applied to top of the columns Pesticide spills and inadvertent discharges of agrifor 10 d. Results indicated complete (100%) metolachlor destruction, chemicals are common occurrences on farmstead and with the dehalogenated product observed as the primary degradate agricultural cooperatives. In 1990, it was estimated that in the leachate. Similar results were obtained with a 25:75 Fe0–sand there were more than 14 000 agrichemical facilities in barrier but metolachlor destruction was not as efficient when unanthe USA that store, sell, mix, or apply pesticides and nealed iron was used or Al2(SO4 )3 was omitted from the barrier. A fertilizers (Norwood and Randolph, 1990, p. 7–15). Alsecond set of transport experiments used metolachlor-contaminated though numerous advances have been made in the consoil in lieu of a 14C-metolachlor pulse. Under these conditions, the struction of pesticide containment facilities, recent suriron barrier decreased metolachlor concentration in the leachate by veys of pesticide distributors indicate prevalent soil approximately 50%. These results provide initial evidence that permeable iron barriers can effectively reduce metolachlor leaching under contamination (Minnesota Department of Agriculture, unsaturated flow. 1997). To combat this problem, researchers have attempted to devise remedial treatments for pesticidecontaminated water and soil. Numerous researchers have proposed the use of advanced oxidation processes Z iron (Fe0 ) barriers have become an esfor destroying pesticides in rinse water and soil (Tyre tablished technology for remediating ground water et al., 1991; Sun and Pignatello, 1992; Pignatello and contaminated with halogenated hydrocarbons (Wilson, Baehr, 1994). Few examples exist, however, where iron 1995). Metallic iron is an avid electron donor and its has been used as an abiotic reduction treatment for oxidation (Eh 0.409 V) can drive the reduction of xenobiotic-contaminated soil (Singh et al., 1998a,b). many redox-sensitive contaminants. While oxygen is the This study was conducted in conjunction with a larger normal electron acceptor during iron corrosion in aerofield-scale demonstration project where contaminated bic environments, under anaerobic conditions, such as soil from a metolachlor spill site was treated with zerovathose encountered in ground water, waterlogged soils, lent iron in large soil windrows (Comfort et al., 2001). or artificial impoundments (e.g., runoff ponds), electron Results from this field trial showed that metolachlor transfer mediated reactions of many organic contamiconcentrations were decreased by 72 to 99% within 90 d nants are favored. Permeable reactive barriers (PRB) following treatment with various combinations of Fe0, are particularly attractive for in situ remediation beacetic acid, and Al2(SO4 )3. Although the metolachlor cause they provide long-term solutions with low opconcentrations were dramatically reduced, the potential erating costs and are less expensive than conventional for leaching from the treated soil remained, especially cleanup methods (O’Hannesin and Gillham, 1998). Alif the soil was returned to its original location (runoff though in situ PRBs have been successfully used to pit). To counteract this potential problem, it was proremediate chlorinated solvents in ground water, less posed that a permeable iron barrier be placed in the emphasis has been placed on PBR use in unsaturated bottom of the excavated pit before returning the treated soil. To evaluate whether this approach would be feasiSchool of Natural Resource Sciences, Univ. of Nebraska, Lincoln, NE 68583-0915. Received 14 May 2001. *Corresponding author ([email protected]). Abbreviations: BTC, breakthrough curve; HPLC, high performance liquid chromatography; PRB, permeable reactive barrier. Published in J. Environ. Qual. 31:962–969 (2002).