The decontamination of perchlorate-contaminated water by woody plants was investigated in sand and hydroponic bioreactors. Willow trees were found to be the most favorable woody plants with phraetophytic characteristics in comparative screen tests with eastern cottonwoods and Eucalyptus cineria. Willows decontaminated aqueous solutions dosed with 10-100 mg/L of perchlorate to below the method detection limit of 2 µg/L. Two phytoprocesses were identified as important in the remediation of perchlorate-contaminated water: (a) uptake and phytodegradation of perchlorate in the tree branches and leaves and (b) rhizodegradation. Exposure of rooted willow trees to perchlorate-dosed media stimulated rhizodegradation. Homogeneous degradation studies using media from the root zone of dosed willow trees confirmed that rhizosphere-associated microorganisms mediated the degradation of perchlorate to chloride. Experiments conducted with varying ranges of nitrate concentrations clearly indicated that high nitrate concentrations interfered with rhizodegradation of perchlorate. This study provides evidence that the efficacy of phytoremediation of perchloratecontaminated environments may depend on the concentration of competing terminal electron acceptors, such as nitrate, and the nitrogen source of the nutrient solution. Since perchlorate does not volatilize from water readily, a perchlorate remediation scheme may involve an intensively cultivated plantation of trees with phraetophytic characteristics and irrigation with the contaminated water.

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