Trichloroethylene (TCE) is a carcinogenic, chlorinated volatile organic compound that was commonly used as a degreasing solvent for aircraft maintenance at many US Air Force bases. Past improper disposal of TCE has resulted in contaminated groundwater at many of these facilities. Phytoremediation, defined as the use of plants and their associated microorganisms to stabilize or remove contamination, has been implemented as part of a TCE groundwater cleanup at Travis Air Force base near Sacramento, CA and is being considered as a remediation option at other bases. Volatilization of TCE from leaves and the surface of the soil near the trees were shown to be the most important removal mechanisms at the Travis site. Past studies conducted on indigenous trees growing above TCE contaminated groundwater at several Hill Air Force Base (HAFB) locations have also shown that TCE is taken up and volatilized by the trees. However, phytoremediation has not been implemented, in part because of the difficulty in predicting the potential effectiveness of TCE removal over time. Flow through or recirculating chambers were used to quantify the amount of TCE removed by volatilization through leaf, trunk, and soil surfaces. Tenax™ sorbent tubes, used to collect TCE from the chambers, were analyzed by thermal desorption gas chromatography/mass spectrometry. Tree cores were collected using an incremental borer and analyzed by headspace GC/MS to quantify the TCE mass contained in the trees. Field measured transpiration stream concentrations (TSC) and groundwater data were used to calculate transpiration stream concentration factors (TSCF) for TCE. Comparing current and historical data, it was found that trees reach a steady state TSCF value of 0.26 after about 15 years. Using this information, it was predicted that a phytoremediation plot containing 40 poplar trees located in a seep area within HAFB OU2 would remove 4.82 kg of TCE annually. A larger plot covering the entire hillside above this seep (160 trees) could remove up to 19.28 kg of TCE annually, once trees reach a steady state TSCF.

Precipitation over coal fly ash surfaces raises concerns about potential downstream impacts of dissolved selenium (Se) on aquatic ecosystems. Detention ponds hold runoff until aqueous Se concentrations meet water quality regulations: within 10 days, 90% of runoff must be discharged and cannot exceed 11.8 μg Se L-1 (ppb). Constructed wetland (CW) phytoremediation is a potential treatment option to meet these regulations. This research was conducted to 1.) Assess the potential of native southeastern aquatic macrophytes for Se tolerance and removal efficiency compared to unplanted (UNP) detention ponds; 2.) Evaluate seasonal influence on Se removal in CWs over four, weekly flood-discharge cycles; and 3.) Determine the effect of temperature on aqueous Se removal. Monoculture CW microcosms (110 L) were flooded six days with 500 or 1000 ppb Se, as selenite (SeO32-) or selenate (SeO42-) evaluating five plant species for Se tolerance. Seasonal evaluations were conducted with cattail (Typha angustifolia L.; CAT) and duckweed (Lemna minor L.; DWD) over four weekly flood-discharge cycles at 16 or 32 ppb SeO42--Se. Cattail, DWD, or UNP volatilization chambers (VCs) flooded with a 3 L solution at 35 ppb SeO42--Se were evaluated under 12 h photoperiod at either 20 or 30°C. All experiments contained a zero Se control. Water, plant, and soil total Se concentrations were determined by inductively coupled plasma-mass spectroscopy (ICP-MS) and analyzed with PROC GLM (SAS EG 7.1) at α=0.05. Cattail and DWD were selected for further evaluation with SeO42--Se. The greatest aqueous Se removal was observed in the summer (73%), followed by the fall (42%) and spring (41%), compared to winter (18%). Temperature was strongly correlated with Se removal (0.65, P<0.0001). Except for summer, after two flood-discharge cycles, CAT and DWD improved aqueous Se removal compared to UNP controls. Volatilization chamber data indicated greater Se removal at 30 (69%) compared to 20°C (54%). At 30°C, DWD decreased aqueous Se concentrations below 11.8 ppb Se within 10 days. Given their efficacy and abundance, CAT and DWD are deemed suitable species for phytoremediation in CWs supplied with Se-impacted waters.