Environmental Engineering Reference
In-Depth Information
at military sites where TNT has been manufactured, stored,
used, and disposed of, and these sites can be characterized
with TNT-contaminated soils, sediments, and aquifers. In
addition, the manufacture of TNT does not produce a 100%
theoretical yield, and often TNT-contaminated sites contain
the TNT impurities 2,4-dinitrotoluene (2,4-DNT) and
2,6-dinitrotoluene (2,6-DNT). All these compounds are pri-
ority pollutants as defined by the USEPA. Other explosives
compounds that are based on nitroaromatics include
hexahydro-1,3,5-trinitro-1,3,5-triazine (Royal Demolition
Explosive, or RDX), and octahydro-1,3,5,7-tetranitro-
1,3,5,7-tetrazocine (high-melting-point explosive, or HMX).
The USEPA maximum contaminant
level
for RDX is
1.05 mg/L.
Nitroaromatic compounds can undergo a wide variety of
biologically mediated transformation reactions in groundwa-
ter. Microbes in a TNT-contaminated aquifer near Weldon
Spring, Missouri, have been shown to transform TNT, 2,4-
DNT, and 2,6-DNT to amino-nitro intermediate compounds,
as well as be completely mineralized to CO
2
(Bradley et al.
1994). That site is typical of many TNT-contaminated sites,
where TNT is present in surface soils up to percent-level
concentrations, as well as dissolved TNT concentrations
in groundwater that receives recharge through these
contaminated surface soils. 2,4-DNT and 2,6-DNT also are
present in groundwater at such sites. For example, at Weldon
Spring these concentrations were as high as 44 and 61.4
Fig. 13.19
Plant and groundwater interactions at a site characterized
by TCE-contaminated groundwater. TCE(g) is the volatile phase.
m
g/
L, respectively (Bradley et al. 1997).
The linkage between such microbial degradation of
TNT with similar contaminant degrading capability of the
microbes present in the rhizosphere around trees at
phytoremediation sites was shown by Schnoor et al.
(1995). They showed that the TNT-degrading nitro-
reductases originally believed to be related to soil bacteria
were shown to be, in fact, derived from plant roots. TNT can
induce plant toxicity, however, and was observed as a
decrease in the transpiration of poplar trees exposed to
TNT (Thompson et al. 1998b).
Another nitrogenous contaminant compound that has
been detected in groundwater systems is N-nitrosodi-
methylamine (NDMA). NDMA is a probable human carcin-
ogen. NDMA has both manmade and natural sources, being
a byproduct of rocket-fuel production but also can be formed
during the chlorination of water, respectively.
Perchlorate, as ammonium perchlorate, or NH
4
ClO
4
, also
is associated with rocket fuel and also can be found in arid
climates where it is formed naturally in evaporate deposits.
If consumed, perchlorate inhibits the uptake of iodine by the
thyroid; perchlorate is a probable human carcinogen. Per-
chlorate, however, has no federal drinking-water standard,
but health advisories as low as 24.5
could be achieved using transgenic poplar trees that contain
mammalian genes that code for cytochrome P-450, as has
been shown by researchers at the University of Washington.
A summary of the fate of a representative chlorinated
solvent TCE in plants is shown in Fig.
13.19
.
13.6
Plant Interactions with Nitroaromatics,
NDMA, Dioxane, Perchlorate,
and Tritium
Whereas the aromatic ring structure of nitroaromatics
is essentially the same reduced carbon-ring of aromatic
hydrocarbons, the nitrogenous functional groups are par-
tially oxidized. Therefore, these nitrogenous groups can be
reduced, whereas the aromatic core can be oxidized. In some
ways, these compounds reflect both major classes of degra-
dation pathways, oxidation and reduction, acting both as a
source of electrons and a sink for electrons.
Perhaps the most commonly known nitroaromatic to be
released to groundwater is trinitrotoluene, or 2,4,6-trinitro-
toluene, also known as TNT—TNT also is the most toxic of
the nitroaromatics. TNT was first synthesized in 1863, for
use as a red-colored dye. It is often found in surface soils at
munitions manufacturing plants. This is particularly the case
g/L have been man-
dated. Perchlorate is soluble in water and has been detected
in surface- and groundwater.
m
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