The U.S. Department of Energy is investigating the feasibility of establishing large plantings of native desert phreatophytes to enhance hydraulic control of shallow contaminated groundwater at a former uranium mill site. We designed a study to (1) evaluate methods to accelerate establishment of native phreatophytes above contaminated groundwater, (2) determine if irrigated transplants would transpire groundwater after irrigation ceased, (3) evaluate uptake and accumulation of groundwater contaminants in aboveground plant tissue, and (4) estimate phreatophyte transpiration rates for input to groundwater flow simulations. We transplanted and irrigated seedlings of Atriplex canescens (ATCA) and Sarcobatus vermiculatus (SAVE), grown in a greenhouse from locally-harvested seed, into four test plots overlying a shallow groundwater plume. Students annually measured plant growth in the test plots and in reference areas outside the plume. We analyzed stable isotopes of hydrogen and oxygen in plant tissue, soil, and groundwater to test hypotheses regarding sources of transpiration water. We sampled and analyzed levels of groundwater contaminants in the stems and leaves of ATCA and SAVE growing in test plots and in reference areas. Finally, we applied empirical relationships between fractional cover, air temperature, and satellite imagery to estimate transpiration rates for hypothetical large-scale plantings.
Results/Conclusions
Overall, 77% of irrigated seedlings grew to maturity, SAVE mortality was higher, ATCA grew larger than SAVE, and the largest plants were in river terrace plots. Two years after irrigation ceased, oxygen and hydrogen isotope data indicated that healthier plants growing on a river terrace were primarily transpiring shallow groundwater. Less healthy plants growing away from the river terrace were primarily using a combination of rainwater and residual irrigation water. Uranium in stems and leaves was statistically higher in test plots overlying the plume than in reference areas outside the plume; however, uranium, strontium, and selenium were all well below toxicity thresholds for grazing animals. Transpiration rates for ATCA and SAVE growing on the river terrace overlying the plume ranged from 258 to 352 mm yr-1, and transpiration discharge of groundwater (calculated as the difference between mean annual transpiration and precipitation) for a hypothetical 9.6 hectare planting ranged from 7603 to 16,707 m3 yr-1. Results suggest that a large-scale desert phreatophyte planting could transpire a significant volume of groundwater without creating unacceptable exposure to grazing animals. An ongoing study is refining the transpiration algorithm by combining field measurements of leaf area index and multispectral data from both high-resolution unmanned aircraft systems and low-resolution satellite imagery.