DIURNAL CYCLES IN GROUND-WATER INDUCED BY MICROBIALLY-MEDIATED REDUCTION-OXIDATION CONDITIONS AT A PHYTOREMEDIATION DEMONSTRATION SITE IN FORT WORTH, TEXAS

In 1996, a field-scale demonstration project was initiated and managed by the U.S. Air Force at a site in western Fort Worth, Texas, using 1-year-old stems of eastern cottonwoods (whips), and 1-year-old eastern cottonwood seedlings (calipers). The primary objective of the demonstration project was to determine the effectiveness of eastern cottonwoods at reducing the mass of dissolved trichloroethene within an aquifer. The U.S. Geological Survey conducted a study, in cooperation with the U.S. Air Force, to determine if microbially-mediated reduction-oxidation (redox) conditions caused by the root systems of the cottonwoods were able to induce a diurnal cycle on the ground-water geochemical system. Phyto-induced diurnal cycles have been observed before under controlled laboratory conditions, but they had not been observed in a field demonstration.

To record geochemical conditions in ground water, monitoring instruments were installed in eight wells from August 14, 2004, through August 20, 2004, and again from October 3, 2004, through October 14, 2004, to continuously measure temperature, pH, specific conductance, and dissolved oxygen. Diurnal cycles were observed in all four parameters but were most common and most distinct for specific conductance. The likelihood and magnitude of diurnal cycles (if present) at a particular well seemed to be directly related to the predominant terminal electron-accepting process at that well. Diurnal cycles were not observed in wells with aerobic reduction, and their magnitude and likelihood increased as the terminal electron-accepting processes proceeded from nitrate reduction to iron (III) reduction to sulfate reduction to methanogenesis.