Changes in ground-water levels and contaminant geochemistry were observed in a shallow aquifer contaminated with coal-tar near Charleston, South Carolina, before and after the installation of a hybrid poplar tree grove. The grove was planted in the fall of 1998 as part of site remediation activities and consisted of about 600 6-ft rooted hybrid poplar cuttings. Monitoring of ground water, tree tissue samples, and sap flow was performed to evaluate performance. Ground-water levels decreased about 3.5 ft across the planted and unplanted areas of the site between 1998 and late 2000 due to regional drought conditions in the Southeast. After 2001, decreases in ground-water levels in the planted area were about 1 ft deeper than measured in unplanted areas. Estimates of water flow through the 7-year old trees using sap-flow instruments indicate that the poplar trees are transpiring nearly 5 gallons of water per day per tree. Long-term monitoring activities show that the hybrid poplars also have decreased ground-water contaminant concentrations in planted areas to a larger extent than observed in unplanted areas. The magnitude of the observed decrease in ground-water contaminant concentrations in the planted area is dependent upon the presence or absence of pockets of coal-tar as a dense nonaqueous-phase liquid (DNAPL) beneath the trees. In the planted areas not characterized by DNAPL, milligram per liter concentrations of benzene, toluene, and naphthalene in monitoring wells have decreased since 2000 by an average of 85, 83, and 82%, respectively. These areas exhibited a seasonal variation in dissolved-phase contaminant concentrations, with the lowest concentrations observed during spring and summer, when transpiration rates and ground-water uptake by the hybrid poplar trees were measured to be at maximum. Benzene, toluene, and naphthalene, as well as other coal-tar related polycyclic aromatic hydrocarbons, such as styrene, were detected in various tree tissues sampled during the summers, when the concentrations in ground water were lowest. In areas where DNAPL existed beneath the trees, concentrations of benzene, toluene, and naphthalene have decreased only an average of 32, 55, and 49%, respectively. A long-term trend is noted in decreasing dissolved-phase contaminant concentrations in ground water beneath the trees in areas with no DNAPL present.