Analysis of Hydrologic Data to Evaluate Phytoremediation System Performance
A long-term environmental monitoring program is providing hydrologic data that demonstrates poplar trees are influencing groundwater flow at the J-Field site, Aberdeen Proving Ground, Maryland. Advanced data collection and analyses techniques have enabled researchers to construct a water budget for the study area and to estimate the amount of groundwater the trees are transpiring on a daily and annual basis. During the growing season, groundwater flow is influenced by the transpiration activities of the poplar trees, as evidenced by shifts in groundwater flow towards the center of the poplar grove and a measurable cone of depression on the water table. Continuous monitoring of the aquifer indicates that the trees transpire enough groundwater to produce daily fluctuations of the shallow water table. Estimates of peak summer groundwater transpiration rates for the poplar grove range from 12 to 21 gallons per day per tree (gal/day–1/tree-1) (45 to 80 L/day–1/tree-1) based on sap flow measurements and later substantiated by meteorological data and groundwater modeling analyses. The poplar trees induce upward gradients towards the tree roots. During the summer, gradient shifts were observed to a maximum depth of 25 ft (7.6 m). Hydrologic analyses demonstrate that the trees are intercepting a significant component of groundwater flow during the summer and early fall and are thereby limiting the discharge of contaminated groundwater to the marsh during this period. Trees tissue and transpiration gas sampling confirm the poplar trees are withdrawing contaminant mass from the aquifer.
These hydrologic and geochemical data demonstrate the J-Field site provides ideal hydrogeologic conditions for the successful application of phytoremediation. J-Field is underlain by a low permeability, shallow water table aquifer that discharges to a biologically active freshwater marsh. Based on the favorable results of the pilot study, 600 additional trees are being planted to improve the capacity of the phytoremediation system to provide hydraulic containment of a contaminant plume.