Hydrologic effects of cottonwood trees on a shallow aquifer containing trichloroethene
In April 1996, a field demonstration was begun to evaluate the use of cottonwood trees to help clean up a trichloroethene contaminated shallow aquifer (< 4 meters below land surface) at the Naval Air Station, Fort Worth, Texas. Transpiration, climate, soil-moisture, and tree-root data were collected and used to parameterize and validate the hydrologic model PROSPER, which was used to simulate evapotranspiration for a 12-year period and to determine the trees' uptake of contaminated water from the saturated zone (aquifer). Simulated annual evapotranspiration from the combined unsaturated and saturated zones ranges from 25 to 48 centimeters for the period when the tree plantations have achieved a closed canopy (year 12 and beyond), depending on assumptions regarding climatic conditions, soil-water availability, and root growth. Simulated annual uptake from the saturated zone for year 12 and beyond ranges from 12 to 28 centimeters. The ground water flow model MODFLOW was used to simulate the effects of this predicted transpiration on ground water flow in the aquifer. Although transpiration from the aquifer is predicted to reach between 50 and 90 percent of the initial volumetric flux of ground water through the demonstration site, the outflow of contaminated ground water from the site will likely be reduced by only 20 to 30 percent. The discrepancy between predictions for volume of water transpired from the aquifer and the reduction in volumetric outflow of contaminated ground water can be attributed to a predicted increase in ground water inflow to the site and the release of water from storage in the aquifer. It may be possible to achieve a greater amount of hydraulic control if additional trees are planted.