MONITORING REMEDIATION WITH TREMBLING LEAVES: ASSESSING THE EFFECTIVENESS OF A FULL-SCALE PHYTOREMEDIATION SYSTEM

摘要

A full-scale phytoremediation system was installed to treat soil and groundwater contaminated with chlorinated solvents and petroleum hydrocarbons at a site in Milwaukee, Wisconsin. The remedial objectives for the site were threefold: remove residual contamination from soil in the source area; reduce recharge through the soil in the source area; reduce the discharge of affected groundwater to a downgradient river. The phytoremediation system consisted of 485 deep-rooted hybrid poplar trees. Remediation progress monitoring at the site included: chemical analysis of soil samples to document decreases of contaminant concentrations in unsaturated soil; soil moisture monitoring to estimate the changes in recharge as roots develop; and water level measurements to assess uptake of groundwater by the trees. Monitoring was also necessary to assess the health of the trees. Root growth was monitored in situ via a downhole camera and subsurface mini-rhizotron tubes. Pore gas chemistry was monitored to assess the effectiveness of a root aeration system. Leaf tissue samples were collected to assess nutritional needs. First-year soil moisture monitoring results indicated that recharge was not yet significantly affected by the presence of young trees on-site. Water levels in on-site wells did not yet show evidence of diurnal or seasonal changes in water levels associated with transpiration. Tree roots extended up to a meter laterally from the trees, and were present at depths of up to 3 meters after the first growing season. The trees increased in height by 1 to 2 meters in one summer. The soil aeration system increased oxygen and decreased carbon dioxide concentrations in pore gas. Tree loss due to late planting and phytotoxicity was less than 15%. Leaf tissue analysis indicated that the trees were in good nutritional health. Environmental monitoring at phytoremediation sites should be tailored to site conditions and the remedial objectives. The monitoring must document that the remedial objectives are being met and also that the vegetation remains healthy.