Clarifying phytoremediation dataReferences to research in OriSchipper’s article “Phytoremediationreleases TCE to the atmosphere” re-quire some clarification (1). Knowingthat phytoremediation uses complexliving organisms to deal with environ-mental problems, any one or combi-nation of factors at a site or in anexperimental arrangement may great-ly alter results.

In evaluating a remediation tech-nology, distinctive features of the pro-posed site or experimental designmust be considered. Determininghow unique factors may affect theobserved results should be the prima-ry goal rather than an attempt togeneralize the situation and the com-plications posed by these factors. Inbiological treatment, understandinghow organisms react to environmen-tal conditions, particularly thoseconditions unique to that site orexperimental design, is of prime im-portance. For living systems, thesereactions are not always predictable.Phytoremediation systems are ex-tremely complex, including plant–microbe interactions and plants withdifferentiated tissues and vasculartransport systems. Media considera-tions add more complications asplants cross multiple media bound-aries, including groundwater (saturat-ed and unsaturated), soil, soil vapor,and open atmosphere. In consideringdifferent contaminants, one mediummay play a dominant role (e.g., soilfor PCBs and PAHs), or contaminantsmay readily exist and transfer be-tween media. In such cases, slightdifferences in conditions can leadto great differences in observations.

Newman et al. showed that plantsremoved the majority of the trichloro-

ethene (TCE) at a field site understudy; plant pathways examined in-cluded transpiration through theleaves, metabolism within the plant,degradation within the artificialaquifer (production of anaerobicbreakdown products), and mineral-ization within the plant system (2).Their studies of microbial activityshowed that TCE loss could not beaccounted for through that pathway.As in any field system, a total massbalance was not obtained; however,the researchers accounted for approx-imately 70% of the dosed TCE.

Ma and Burken recently publisheda paper that revealed another path-way for TCE in phytoremediation:diffusion from stems or trunks dur-ing transport (3). Plant stems containspecialized cells that aid gas ex-change with the atmosphere, so itwould seem apparent that TCE couldtransport through these cells as well.When applying phytoremediation toa field site, any one or combinationof these pathways may play a domi-nant role with different field sitesproducing different results. Thesetwo papers indicate there are variouspathways whereby TCE (and presum-ably other VOCs) can be removedfrom a contaminated aquifer; whatbecomes necessary is to determinewhat dominates removal for that site.For example, the microbial actionthat was minimal in the 1999 studymay prove to have a much largerrole at some sites, as was seen in aphytoremediation plot at CarswellAirbase (4). To better elucidate plantinteractions with groundwater contam-inants such as TCE, we are planninga joint study over the next year to si-multaneously look at the mecha-nisms described in both previousstudies.

References(1) Schipper, O. Environ. Sci. Technol. 2003,

11, 206 A.(2) Newman, L. A; et al. Environ. Sci. Technol.

1999, 33, 2257–2265.(3) Ma, X.; Burken, J. G. Environ. Sci. Technol.

2003, 37, 2534–2539.(4) Godsy, E. M.; Warren, E.; Pagnelli, E. E.

Int. J. Phyto. 2003, 5, 73–87.

MILTON GORDONProfessor

University of WashingtonDepartment of Biochemistry

Seattle, Wash.

JOEL BURKENAssociate professor

University of Missouri, RollaDepartment of Civil, Architectural

and Environmental EngineeringRolla, Mont.

LEE NEWMANAssistant professor

University of South CarolinaArnold School of Public Health

Columbia, S.C.

Letter▼

310 A ■ ENVIRONMENTAL SCIENCE & TECHNOLOGY / SEPTEMBER 1, 2003 © 2003 American Chemical Society

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