PARENT SESSION
Oral Session #67: Ecotoxicology and Applied Ecology.
Presiding: W. Snyder
Wednesday, August 7. 1:00 PM to 4:45 PM. Cochise Meeting Room, TCC.

An investigation of obligate versus facultative water use by Populus deltoides during phytoremediation of polluted groundwater.

Clinton, Barton^*,1, Vose, James¹, Vroblesky, Don², ¹ USDA Forest Service Southern Research Station, Otto, NC² US Geological Survey, Columbia, SC

ABSTRACT- Plants are increasingly used for the remediation of polluted groundwater, and knowledge of water use patterns is critical for the success of phytoremediation approaches. We were interested in knowing if rainfall (e.g., irrigation) inputs reduce the effectiveness of Populus deltoides as a phytoremediation tool due to switching from groundwater as a primary source of transpirational water to upper soil water sources. We selected two mature eastern cottonwood trees located in north-central Texas growing over plumes of trichloroethene (TCE) in groundwater at depths of 2 to 3.4 m. For both pre- and post-irrigation, we measured sapflux using the Granier thermal dissipation approach, TCE concentrations in groundwater and xylem sap, and determined the isotopic signatures for ¹⁸O and D in groundwater, irrigation water, and xylem sap. The isotopic signature rankings for both isotopes examined were groundwater <xylem sap <irrigation water. Irrigation water was applied at the rate of 1600 l/hr (5 cm rainfall) on two consecutive mornings. Xylem sapflow rate increased following irrigation by 11% the first day, 61% on the second, and at the end of the experiment (day 3) sapflux remained high (+55%). This increase suggests that trees used a combination of surface and groundwater in transpiration. Furthermore, there was a shift following irrigation toward the heavier isotopes of ¹⁸O and D by 20% and 16%, respectively, indicating increased use of surface water. Overall, xylem sap [TCE] decreased by 21.5% following irrigation, and ranged along the stem from 15.1% to 34.1%. Changes in the xylem sap isotopic signature and TCE dilution following irrigation is evidence of facultative water use by Populus deltoides.

KEY WORDS: Phytoremediation, Populus deltoides, water use