Laboratory and field tests have shown that, unlike bacteria, poplar trees are able to carry out complete degradation of fully chlorinated hydrocarbons to carbon dioxide and chloride. CO2 was produced from the degradation of trichloroethylene (TCE), carbon tetrachloride (CT), and perchloroethylene (PCE) by axenic tissue cultures of poplar cells. Chloride ion accumulated in the media when poplar plants growing hydroponically were exposed to TCE or 1,1,1-trichloroethane (TCA). Pilot scale experiments in the field with TCE-exposed poplar demonstrated mass removal of TCE and CT from an artificial groundwater exceeding 95 %. There was no enhancement of TCE and CT degradation in the rhizosphere soils and air emissions from leaves were less than 5 % of the total removal. Chloride ion accumulated in the soils, suggesting that significant dechlorination occurred. Poplars have the potential for destructive removal of TCE and CT without harmful air emissions or accumulation of a hazardous solid waste.

Axenic tumor cultures of poplar cells, clone H11-11, were grown in the presence of [14C]-trichloroethylene (TCE) (uniformly labeled). The cells were capable of metabolizing TCE to produce trichloroethanol, di- and trichloroacetic acid. Some of the carbon from TCE was found in insoluble, nonextractable cell residue, and small amounts were mineralized to [14C]CO2. Poplar cuttings grown in soil and exposed to TCE produced the same metabolites. In field trials, trees were planted in soil in test cells and exposed to TCE via underground water injection during the growing season. During the growing season, at least 95% of the TCE was removed from the influent water stream in cells containing trees. Mass balance studies conducted in the laboratory indicated that 70 to 90% of the TCE was transpired; however, greenhouse and field study results showed that less than 5% of the total TCE taken up by the plants is transpired. These results show that significant TCE uptake and degradation occur in poplars. Poplars appear to be useful for in situ remediation of TCE-contaminated sites under proper conditions.

Poplar trees were found to be capable of taking up trichloroethylene (TCE) and degrading it to several known metabolic products: trichloroethanol, trichloroacetic acid, and dichloracetic acid. Poplars were also shown to transpire TCE in measurable amounts. To eliminate the bi possibility that the degradation we observed was produced solely by rhizosphere organisms, axenic poplar tumor cell cultures were tested; the cultures produced the same intermediate metabolic products. When dosed with [C-14]TCE, cell cultures also produced low levels of radiolabeled carbon dioxide and a labeled insoluble residue. These results show that significant TCE uptake and biotransformation occurs in poplar, which demonstrates the potential for the use of poplars for in situ remediation of TCE.

Poplars are deep rooted, fast growing trees with high transpiration rates and are well suited for removal of pollutants from shallow and moderately deep aquifers. Laboratory experiments have shown that poplars can remove trichloroethylene (TCE) from soil and oxidize it to carbon dioxide. Seedlings were placed in bioreactors with the root zone separated from the shoot zone (headspace). {sup 14}C-labeled carbon dioxide was recovered from the headspace after the soil zone was dosed with radiolabeled TCE. Additional products of TCE oxidation were found in plant tissue and in axenic plant tissue cultures: trichloroethanol and di- and trichloroacetic acid. A model is presented showing how a poplar plantation planted down-gradient of a contaminated site could intercept a TCE plume from a dense nonaqueous phase liquid source. Costs for plant bioremediation are estimated to be about 20% of that for a conventional pump and treat system. Planned field experiments with poplars for TCE removal and degradation are described.