Name: Neil O.
I see from an article on the Argonne home page
today (2001 Feb 26) that the lab has received a
Facility Progress Award from USEPA for its
innovative work in removing contamination from
soil and groundwater. The article mentions the
technique of "phytoremediation -- using plants to
break down and remove contaminants" and shows a
photograph of some of the 800 poplar and willow
trees that were planted to implement this
However, another Argonne web article states:
Similar plantings of hybrid poplars in Washington
state indicate that of the organic contaminants
taken up by the trees, about 70 percent are
converted to non-volatile compounds and held in
the plant. The other 30 percent is vaporized from
the leaves along with the transpired water.
I'd like to know more about the consequences of
using phytoremediation. In particular, why is it
better to transport contaminants from a relatively
controlled, low-mobility environment (e.g., soil,
groundwater) into a relatively uncontrolled,
high-mobility environment (e.g., atmosphere,
surface water)? Do the plants involved in
phytoremediation become contaminated and hazardous
as they absorb contaminants from the soil? What
happens to wildlife that may feed on the plants?
What environmental risks arise when the plant
sheds leaves, blossoms, seeds, twigs, or other
material that may be carried by wind or surface
water far from the original contaminated location?
Similarly, what environmental risks arise when the
plant transpires to the atmosphere 30 percent of
the absorbed contaminants in unconverted form?
Anthony R. Brach
I agree that the sun looks like a disk when viewed from earth, but
I would think that it could be treated as a point source in terms of light
reaching the earth's surface. By "brightness on the disk for a given
wavelength", I assume you mean intensity as a function of wavelength. My
understanding is that the distribution of energy as a function of
wavelength (or frequency) is that of a black-body radiator with a
temperature of 6000 degrees Kelvin. Some further discussion on this can be
found in a book called "The Refrigerator and the Universe: Understanding
the Laws of Energy" by Martin Goldstein.
The use of phytoremediation is a classic case of being stuck between a rock
and a hard place. The better option is not to have put the contaminants
there in the first place, but it looks like we are only slowly learning the
lesson that blithely assuming most new compounds we introduce are harmless
only means that later we spend a lot of time and money cleaning up the mess.
Groundwater is actually not that controlled or contained an environment,
some of it often ends up seeping into lakes and rivers, but of course the
primary concern in keeping groundwater clean is human health- we are
perhaps the only animal species where a significant portion of our water
supply comes from ground rather than surface water.
The plants absorbing the contaminants often do in fact become hazardous,
with concentrations reaching levels requiring disposal in hazardous waste
facilities in some cases. However, in cases where the contaminant is
widely dispersed, particularly at lower concentrations, it is somewhat
preferable and less labor intensive than other cleanup methods (e.g. pump
out and treat all the water in the aquifer, removal of millions of cubic
yards of soil). That the vegetative material from the phytoremediative
plant becomes the primary transport mechanism of these contaminants into
the biosphere is true, supplanting the somewhat slower dispersal through
groundwater into surface waters, or the less well-characterized
"phytoremediation" that would occur anyway as the result of native
vegetation taking up some of the groundwater. It's a matter of picking
your poison. The primary advantage that I see in purposeful
phytoremediation is that there is some anticipation and monitoring of where
the contaminants go, and thus the more hazardous uses of contaminated plant
material (e.g. as food material, whether for humans or wildlife) are
generally better avoided.
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Update: June 2012