Phytoworx Where To Buy
The purpose of this report is to briefly summarize the current state ofphytoremediation technology, and then focus on the use of poplar trees(Populus sp.) to degrade trichloroethylene (TCE) in groundwater. Thesummary of phytoremediation will serve as an introduction to this technology.It will address some common concerns such as the cost and performance ofphytoremediation. The analyzation of TCE phytoremediation will begin withseparate discussions of TCE and poplars, followed by a detailed section on theuse of poplars to treat TCE contamination. The final section will present threecase studies detailing Department of Defense and Superfund sites where poplarshave been planted in order to treat TCE contamination in groundwater.
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Understanding TCE volatilization rates in poplar trees is critical for thistechnology to gain widespread acceptance amongst hazardous waste site managersand regulators. There may be concern if the trees are transpiring highconcentrations of TCE into the atmosphere, where the pollutant becomes an airquality concern. Proponents of phytoremediation argue that VOCs will volatilizefrom the groundwater, through the soil, and into the air in the absence oftrees. For example, plants will invade sites that are left unattended forextended periods of time, and invasive plants may evapotranspirate thecontaminant. That being the case, there would be some evapotranspiration in theabsence of a treatment strategy. Phytoremediation schemes would only acceleratethe process of volatilization that occurs naturally. Still, volatilizationconcentrations are decreased by a number of factors, such as exclusion ofnonpolar compounds at the roots. According to Davis et al. (1996), “Veryfew contaminants are sufficiently water soluble, non-toxic to plants, andvolatile enough to reach atmospheric concentrations that would be of concern by[evapotranspiration].”
Investigations were conducted to follow up the work of Newman et al. (1997a)by conducting field trials of TCE phytoremediation in the state of Washington(Gordon et al. 1997, Newman et al. 1997b). These field experiments wereconducted by Occidental Chemical Corporation along with researchers from theUniversity of Washington and Washington State University. The state ofWashington approved a two year field experiment where TCE was added to 3.7 x6.1 meter cells that were 1 meters deep and double lined withpolyethylene. Hybrid poplars (Populus tricocarpa x deltoides, H-11-11)were planted in the experimental cells and soil was only added to the controls.TCE was added to the cells at a concentration of 50 ppm in the water. Theinvestigators found that over 95% of the TCE was removed from the stream waterin the plots with trees. During the first year of the trial, however, 65% ofthe added TCE was removed in the control plot without trees. This probablymeant that a significant portion of the TCE was bound in the soils (Newman etal.1997b). At the end of the second growing season, 65-70% of the added TCEremained in the water stream in the control cells (Workshop on Phytoremediationof Organic Contaminants), indicating that the loss to the soil decreasedsubstantially after the soil became saturated with TCE. Still, during thesecond year over 97% of the added TCE was removed from the water stream in thecells containing trees (Newman 1997c). The investigators also foundproducts of anaerobic microbial dehalogenation, such as three isomers ofdichloroethylene and small amounts of vinyl chloride, in the water streams (LeeNewman, personal communication).
For those unfamiliar with groundwater treatment options,Table 11 lists many of the other technologiesthat have been used to treat plumes of chlorinated solvents. Table 11 includesthe cost of cleaning a plume of chlorinated solvents using these technologiesunder an idealized set of conditions. Unfortunately, phytoremediation was notincluded in this economic analysis. However, phytoremediation would be somewhatmore expensive than natural attenuation because it involves tree planting andmaintenance as well as monitoring, and it would be significantly less expensivethan pump and treat due to decreased energy needs. Therefore, the costs of apoplar phytoremediation system would likely fall somewhere in the middle ofTable 11, and probably near the less expensive end of the spectrum.
Since poplar phytoremediation systems are primarily used ashydraulic barriers and solar powered pumps, the most closely relatedengineering technology is a pump and treat system. The major advantage ofpoplars over pumps is that poplars provide their own energy, where pumps oftenconsume large amounts of electricity. This could save tremendous amounts ofmoney over the course of a long remediation project. On the other hand, a majordisadvantage of poplars is the fact that their pumping rates vary over thecourse of a year. In addition, poplar systems will only work at sites where thegroundwater contamination is within reach of their roots.
Some analytical work has been done on the tree tissues at the site,but this type of information is still in the early stages of collection. Datafrom November of 1996 indicated a TCE signature in the whips that were plantedover an area where the groundwater was the shallowest. This indicates that theyoung trees were capable of evapotranspirating TCE after just one growingseason. Now that the trees have been on site for more than an entire growingseason, site managers at Carswell plan to increase monitoring at the site toinclude a whole suite of water, soil, air, and tree tissue sample analysis.Some of the more unique data they plan to collect (in relation to the othercase study sites) are analyses of microbial populations and assays of TCEdegrading enzymes in the trees. 041b061a72