Phytoremediation of Volatile Organic Compounds in ... - CLU-IN

PERFORMANCE TO DATE
Phytoremediation of Volatile Organic Compounds in Groundwater
Groundwater flows radially from underneath the former toxic burning pits (where military
weapons testing and disposal practices led to concentrated chemical release) to discharge in the
freshwater marsh. Water table elevation is highest in winter, and fluctuates diurnally in the
summer due to evapotranspiration. The summer growing season also yields an upward hydraulic
gradient (visible up to 25 feet below the ground surface), as the tree roots increase their
groundwater intake. The increased transpiration rates of the poplar trees and adjacent native
woodlands cause a cone of depression to form under the poplars and in the center of the
plantation. As Figure 5 demonstrates, this depression causes a reversal of groundwater flow into
the plantation in the summertime, rather than outwards to the marsh.
Figure 5. September 2001 Water Table Map (Schneider, 2002)
Transpiration rates for trees have been estimated using sap flow analysis and local weather
(temperature, humidity, wind speed, and solar radiation) data. Results indicate that it will take 10
to15 years to achieve maximum transpiration rates of 2000 gallons per day. The leaf area index
method was also used as a means of predicting the time of canopy closure, which coincides with
the peak transpiration rate. This method approximated three to six more years to attain almost
complete canopy closure. Further volatilization research revealed that leaves do not transpire the
bulk of the contaminants. Finally, the degree of volatilization of harmful gases from the trees was
found to be minimal.
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