Published online October 27, 2006

Dissolution and Transport of TNT, RDX, and Composition B in Saturated Soil Columns
Katerina M. Dontsova,* Sally L. Yost, Jiri Šimunek, Judith C. Pennington, and Clint W. Williford
2001b; Brannon et al., 2002; Brannon et al., 2005).
However, explosives are typically formulated in combinations of two or more compounds with waxes,
stabilizers, and binders. Binders and waxes have beenshown to decrease dissolution rates of individual explosive compounds from formulations (Lynch et al., 2002b;
Phelan et al., 2002). The effects of formulation on otherfate and transport processes are unknown.
Solid particles ranging in size from small (,2mm) tolarge (up to the diameter of the projectile) may be deposited on the soil surface when projectiles fail todetonate completely (a low-order detonation) (Pennington et al., 2005; Jenkins et al., 2006). Similar residues canbe deposited when duds or unexploded ordnance thatare disposed of by detonation in place fail to detonate
completely. These failures result in locally scatteredchunks of Comp B on the soil surface (Jenkins et al.,2006). An understanding of transport behavior of thecomponents of this residue is necessary because of detections of one component of Comp B, RDX, in groundwater of military training ranges (USEPA, 1997).
This study focused on Comp B, one of the primary
explosive formulations currently used in artillery projectiles and many other munitions. To accurately represent field conditions, residues from deliberate low-orderdetonations of Comp B-filled artillery shells were used.
The objective was to determine transport parameters for
TNT and RDX from solid phase Comp B under saturated conditions. The parameters for solid Comp B werecompared with those for pure RDX and TNT; RDX andTNT in dissolved Comp B; and with adsorption coefficients determined in batch tests.

Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.

ABSTRACT
Low-order detonations and blow-in-place procedures on militarytraining ranges can result in residual solid explosive formulations toserve as distributed point sources for ground water contamination. This
study was conducted to determine if distribution coefficients from batchstudies and transport parameters of pure compounds in solution
adequately describe explosive transport where compounds are present
as solid particles in formulations. Saturated column transport experiments were conducted with 2,4,6-trinitrotoluene (TNT), hexahydro1,3,5-trinitro-1,3,5-triazine (RDX), and the explosive formulation,
Composition B (Comp B) (59.5 6 2.0% RDX, 39.5 6 2.3% TNT, and
1% wax) in solid and dissolved forms. The two soils used were Plymouth loamy sand (mesic, coated Typic Quartzipsamments) from Camp
Edwards, MA and Adler silt loam (coarse-silty, mixed, superactive,
thermic Fluvaquentic Eutrudepts) from Vicksburg, MS. Interrupted
flow experiments were used to determine if explosives were at equilibrium distribution between soil and solution phases. The HYDRUS-1D
code was used to determine fate and transport parameters. Results
indicated that sorption of high explosives was rate limited. The behavior
of dissolved Comp B was similar to the behavior of pure TNT and RDX.
Behavior of solid Comp B was controlled by dissolution that dependedon physical properties of the Comp B sample. Adsorption coefficientsdetermined by HYDRUS-1D were different from those determinedin batch tests for the same soils. Use of parameters specific to formulations will improve fate and transport predictions.

L

OW-ORDER(INCOMPLETE) DETONATIONS and blow-inplace procedures, unlike high-order (complete) detonations, can leave considerable explosives residue on
military training ranges (Pennington et al., 2002). Such
residues are potentially available to impact environmental and human receptors. Accurate transport parameters, such as soil-water partition coefficients (kds) anddissolution and degradation rates are essential for predicting transport to groundwater.
Batch-determined fate and transport parameters for
explosives commonly used by the military were reviewed by Brannon and Pennington (2002). Existingmodels for predicting environmental fate of explosives
rely on information obtained in batch tests with pure
explosive compounds (Pennington and Patrick, 1990;
Comfort et al., 1995; Xue et al., 1995; Price et al., 1997;
Brannon et al., 1998; Price et al., 2001a; Price et al.,

MATERIALS AND METHODS
Soils
Plymouth loamy sand (mesic, coated Typic Quartzipsamments) and Adler silt loam (coarse-silty, mixed, superactive,
thermic Fluvaquentic Eutrudepts) were the soils used forthis experiment. Adler silt loam samples were collected inVicksburg, MS, whereas Plymouth loamy sand samples werecollected at Camp Edwards, MA (Massachusetts Military
Reservation [MMR]). Soil samples were collected to a depthof 20 cm. Soils were air-dried, ground, and passed through a
2-mm sieve, and analyzed for cation exchange capacity (CEC)
using the NaAc method (USEPA, 1986) and organic carbon(OC) by combustion after acid pretreatment. Analyses were

K.M. Dontsova and C. Williford, Dep. of Chemical Engineering, Univ.
of Mississippi, Oxford, MS. K.M. Dontsova, duty station: U.S. ArmyEngineer Research and Development Center, 3909 Halls Ferry Rd.,
Vicksburg, MS 39180. S. Yost, Computer Sciences Corporation, Vicksburg, MS. J. Šimunek, Univ. of California, Riverside. J. Pennington,
Environmental Processes Branch, Environmental Laboratory, U.S.
Army Engineer Research and Development Center, Vicksburg, MS.
Received 5 Jan. 2006. *Corresponding author (Kateryna.Dontsova@
gmail.com).

Abbreviations: ADNTs, amino-dinitrotoluenes (2ADNT, 2-amino-4,6dinitrotoluene and 4ADNT, 4-amino-2,6-dinitrotoluene); ARAMS,
Army Risk Assessment Modeling System; CEC, cation exchange capacity; Comp B, Composition B; DNX, hexahydro-1,3-dinitroso-5-nitro1,3,5-triazine; HMX, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine;
HPLC, high performance liquid chromatography; kd, adsorption coefficient; MMR, Massachusetts Military Reservation; MNX, hexahydro1-nitroso-3,5-dinitro-1,3,5-triazine; OC, organic carbon; OM, organic
matter; RDX, hexahydro-1,3,5-trinitro-1,3,5-triazine; TNT, 2,4,6-trinitrotoluene; TNX, hexahydro-1,3,5-trinitroso-1,3,5-triazine.

Published in J. Environ. Qual. 35:2043–2054 (2006).
Technical Reports: Ground Water Quality
doi:10.2134/jeq2006.0007ª ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA

2043

Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.

2044

J. ENVIRON. QUAL., VOL. 35, NOVEMBER–DECEMBER 2006

performed by the Environmental Chemistry Branch of ERDC
in Omaha, NE. Particle size was determined by the hydrometer method (Gee and Or, 2002). The Mississippi State University Extension Service Soil Testing Laboratory analyzed
soil pH in 1:1 soil/water slurry, exchangeable cations, and basesaturation. Results of characterizations are listed in Table 1.
Soils had similar CEC despite differences in texture, possibly
due to the greater OC content of Plymouth soil. Both soils had
relatively low OC and clay content. High pH and Ca (notshown) indicated the presence of carbonates in Adler soil.

Column Design
Six flux-controlled flow-through columns were used in thisstudy. They consisted of 10.16-cm internal diameter by
17.00-cm height stainless steel cylinders fitted with Media
Grade 100 porous 316 stainless steel plates (Mott Corporation,
Farmington, CT) and caps on the top and the bottom. Aqueous
tracer or explosives solution was input via a valve in the top.
Flow from top to bottom was selected so that solid test material(Comp B) could be easily introduced. The top was also equippedwith a bleeder valve. A valve in the bottom cap allowed forfree drainage of solution. A Fisherbrand Ultralow-Flow Peristaltic Pump (Fisher Scientific Houston, TX) was attached tothe top of the column to supply solution at target solution flux.
Outflow samples were collected continuously into 20-mL vials
using an automatic fraction collector with 50-vial capacity.
To ensure even distribution of the soil and to minimize
preferential flow, soils were poured into columns in small
amounts at a time and manually pressed. Average packed bulk
density (BD) was 1.27 g cm23 for Adler soil, and 1.60 g cm23
for Plymouth soil. Bulk density was determined from the dry
mass of soil used to pack a column of known volume.

Saturated Flow Experiments
Columns were saturated from the bottom with 0.005 M
CaBr2 solution for 4 h. Pore volume was determined during
saturation as the volume of solution necessary to fill the packed
column. Columns were then connected to the pump on the top
and flow was started. Outflow was monitored to determine
volumetric flow rate. Once a constant flow rate was established(target flow rate was 1.18 mL min21, or solution flux of 0.87 cm
h21; measured average flow rate was 1.09 mL min21, or solution
flux of 0.8 cm h21), one pore volume of solution was pumped
through the column to assure uniform flow (Gaber et al., 1995).
The selected water flux was lower than the saturated hydraulicconductivities for both soils and about 20% of the average 1-yr
rainfall intensity (4 cm h21) for the eastern United States. Flowthen was switched to solutions of RDX, TNT, or Comp B; solidComp B was placed directly on the soil surface. Target concentrations of military grade RDX and TNT were 10 mg L21;
measured concentrations were 10.03 mg L21 and 9.4 mg L21,
respectively. Composition B solution had an RDX concentration of 6.87 mg L21, TNT of 3.28 mg L21, and octahydro-1,3,
Table 1. Selected properties of the studied soils.
Soil

BD†
23

Adler silt
loam
Plymouth
loamy
sand

CEC

OC
21

Sand

Silt

Clay

pH

BS
%
100

21

g cm
1.18

cmolc kg
7.83

2.0

g kg
114 841

45

8.2

1.43

7.05

7.8

875

50

5.1

75

3.74

† BD 5 field bulk density; CEC 5 cation exchange capacity by sodium
acetate method; OC 5 organic carbon by combustion after acid treatment; BS 5 base saturation.

5,7-tetranitro-1,3,5,7-tetrazocine (HMX) of 0.58 mg L21. Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine is an impurity inComp B typically at 1 to 10% (3.5% in our Comp B).
Radiolabeled 14C-RDX and 14C-TNT were added as tracers
to the bulk solutions (specific activity of RDX was 7.68 mCi
mmol21 and TNT 4.08 mCi mmol21) at 0.145% for RDX in
solution and 0.278% for TNT. The radiotracer allowed easy
monitoring of explosives concentrations in outflow as theexperiment progressed.
For the solid Comp B treatment, steady-state flow was briefly
stopped so that 10.0 g of low-order detonation residues containing solid Comp B (65% Comp B and 35% nonexplosive
debris, primarily soil particles) between 2 layers of glass wool
could be placed on the surface of the soil. Solid Comp B was acomposite of the 0.25- to 2-mm size fractions of residue from
low-order detonation experiments conducted for anotherproject (Pennington et al., 2005). In that project, detonations
were performed on a large tarp so that residue could be recovered by sweeping. Therefore, residues were relatively clean
except for uncontaminated soil introduced via occasional
shrapnel tears in the tarp. The explosives component included
57.6% RDX, 38.9% TNT, and 3.5% HMX. Flow was resumed
at the same rate as soon as solid Comp B residues were in place.
After four to six pore volumes of explosive solutions, flow
was switched back to the background solution (or solid CompB was removed), which was applied for another four to six
pore volumes. For every soil and explosive treatment two
experiments were conducted. In one, flow was uninterrupted,
whereas in the other, flow was interrupted for 12 to 24 h toallow explosives to equilibrate with the soil. During flowinterruption both inflow and outflow from the column were
stopped and the column remained saturated. Flow interruption techniques are often used to elucidate rate-limited
sorption processes (Murali and Aylmore, 1980; Fortin et al.,
1997; Šimunek et al., 2002).
Outflow samples were analyzed using high performanceliquid chromatography (HPLC) (Waters HPLC, GenTech Scientific, Arcade, NY) and liquid scintillation (Tri-Carb 2500TX
Liquid Scintillation Analyzer, PerkinElmer Life and Analytical
Sciences, Boston, MA). Every fifth sample (approximately
every 0.1 L) was analyzed by scintillation counting and every
20th (approximately every 0.4 L) by HPLC. Standard USEPA
Method 8330 (HPLC) (USEPA, 1994) was used with the addition of HMX and the following degradation products: monoand diamino-nitrotoluenes and azoxy dimers of TNT, andmono-, di-, and trinitroso derivatives of RDX. Detection limits
were 20 mg L21 for TNT, HMX, 2-amino-4,6-dinitrotoluene(2ADNT), and 4-amino-2,6,-dinitrotoluene (4ADNT); 200 mg
L21 for RDX and 50 mg L21 for RDX degradation products.
Concentrations of explosives in solid Comp B residue were
determined by shaking 5 g of residue with 150 mL acetonitrilein 3 sequential extractions followed by filtering (45-mm GF
filter) and HPLC analysis.
To determine the longitudinal dispersivity (l) for each soil
and monitor for signs of preferential flow, all solutions wereprepared with 3H2O tracer (Selim et al., 1995). Specificactivity of 3H2O was 2.18 mCi mmol21 with 1.239 3 1026% of
3
H2O in solution. Liquid scintillation was used to measuretritium activity.

Numerical Analysis
Experiments were analyzed using the HYDRUS-1D codefor simulating the one-dimensional movement of water, heat,and multiple solutes in variably saturated porous media(Šimunek et al., 2005). Minimization of the objective functionthat includes the sum of squared deviations between measured

DONTSOVA ET AL.: TNT, RDX, AND COMPOSITION B IN SATURATED SOIL COLUMNS

Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.

and simulated concentrations was performed by the LevenbergMarquardt nonlinear minimization algorithm (Marquardt,
1963). The following models were used in analysis: convectiondispersion equation for 3H2O tracer and two-site sorption
model (with decay) (chemical nonequilibrium model) for explosives and their degradation products.
Transport of Nonreactive Solute
Transport of the 3H2O tracer can be described using theconvection-dispersion equation for constant water content,
flux density, and dispersion coefficient:

u

]c
]2 c
]c
5 uD 2 2 q
]t
]z
]z

[1]

where c is the solute concentration (mg cm23), q is the convective flux (cm h21), u is the water content (cm3 cm23) equal
to porosity for saturated experiments, z is the spatial coordinate (cm), t is time (h), and D is the dispersion coefficient(cm2 h21) assumed to be the product of the longitudinal dispersivity, l (cm), and q divided by u.
Two-Site Sorption Model (with Decay)
The concept of two-site sorption (Selim et al., 1976; van
Genuchten and Wagenet, 1989) that permits consideration of
nonequilibrium adsorption–desorption reactions was used todescribe the transport of explosives. The two-site sorption concept assumes that the sorption can be divided into two parts:s 5 se 1 sk

[2]

21

Sorption, se (mg g ), on one fraction of the sites (the Type-1
sites) is assumed to be instantaneous, whereas sorption, sk(mg g21), on the remaining (Type-2) sites is considered to betime-dependent.
Sorption on the Type-2 nonequilibrium sites is assumed to bea first-order kinetic rate process. The mass-balance equation
for the Type-2 sites in the presence of degradation is given by:]sk
5 v [(1 2 f )kd c 2 sk ] 2 ms sk]t[3]

where f is the fraction of exchange sites assumed to be in
equilibrium with the solution phase (2), kd is the linear distribution coefficient (cm3 g21), ms is first-order rate coefficient in
the solid phase (h21), and v is a first-order kinetic rate constantfor sorption to kinetic sites (h21). A linear distribution coefficient was used because the majority of explosive compounds
exhibit linear sorption by soils (Pennington et al., 1999) andour preliminary batch studies showed that 3 out of 4 studied
isotherms were linear. The governing transport equation isthen as follows for constant water content, flux density, dispersion coefficient, bulk density, and distribution coefficient:

u

]c
]c
]sk
]2 c
]c
5 uD 2 2 q
1 qfkd1q
2 mw uc
]t
]t
]t
]z
]z
2 ms rfkd c 2 ms rsk 1 gu
23

[4]

where r is bulk density (g cm ), mw is first-order rate coefficient in the liquid phase (h21), and g is the zero-order rate
coefficient that we used to account for dissolution of solidComp B (mg cm23 h21). In Eq. [4] the first-order rate coefficients ms and mw lump all removal mechanisms of the parental
compound, such as irreversible sorption and decay. Complicated attenuation processes of TNT that include covalentbonding of decomposition products to soil organic matterfunctional groups make separation of irreversible sorption anddecay difficult.

2045

Coefficients mw and ms were set to be equal for this experiment because adsorbed explosives can decay (Achtnich andLenke, 2001; Balakrishnan et al., 2004) and setting decay coefficients equal decreased the number of fitted parameters.
Only mw is reported further in the paper. Dissolution was modeled as a zero-order production process of limited duration.
It was assumed that during the time when Comp B was present
at the top of the profile, there was a zero-order production
process in a small layer close to the top boundary.
Parameter Estimation
The numerical analysis of experimental data was performed
as follows. First, the conservative tracer breakthrough curves
were used to estimate the longitudinal dispersivity, l (cm). The
chemical nonequilibrium model was then used to analyze
explosives breakthrough curves. Longitudinal dispersivity was
fixed at a value determined for tracer. The following parameters
were estimated for explosives: fraction of sites with instantaneous adsorption, f, adsorption coefficient, kd (g21cm3), firstorder rate coefficient for dissolved phase (degradation rate),mw (h21), and first-order rate coefficient for two-site nonequilibrium adsorption, v (h21). Radiotracer results gave an
estimate of the movement of the total mixture of explosives
without considering individual parent and/or daughter products,
whereas HPLC measurements gave an estimate of the movement of TNT, RDX, and their transformation products. In
14
C radiotracer experiments the rate coefficients (mw and ms)
corresponded to irreversible attenuation, whereas in HPLC experiments they corresponded to the sum of attenuation anddegradation. In this paper, reversible adsorption was defined
operationally as the process that allowed explosives to be transported further as a result of the desorption, whereas irreversible
attenuation was defined as the process that made explosives
unavailable for further transport. The production coefficient,g, was fitted in HYDRUS-1D simulations for experiments withsolid Comp B.
Mass-balance calculations were performed for the 14C
radiotracer and HPLC results (Table 2) by integration ofeach breakthrough curve. The accuracy of mass-balance estimates was evaluated using recovery of tritiated water as aconservative tracer.
The R2 values and confidence intervals for fitted parameters
were obtained by analyzing the correspondence between measured and fitted breakthrough concentrations and the behavior of the objective function around its minimum, respectively(Šimunek and Hopmans, 2002). Parameter estimates wereconsidered significant if they were different from zero(confidence intervals did not intersect with zero). If parameter
estimates were consistently nonsignificant, parameters were
removed from the model. To improve the confidence in adsorption coefficients and dissolution rates, kinetic parameters( f and v) were removed from the model for solid Comp B
experiments. However, if only a few parameter estimates werenonsignificant, these parameters were kept in the model, but
not used to draw conclusions.
Interrupted and uninterrupted flow experiments weretreated as replicates for the purposes of statistical analysis oftransport parameters. For comparison between treatments,
differences were considered significant when greater than
the sum of standard errors of the means multiplied by 1.96
(for 95% probability).

Batch Adsorption Studies
Linear distribution coefficients determined from HYDRUS1D simulations were compared to kd s determined by five-point

2046

J. ENVIRON. QUAL., VOL. 35, NOVEMBER–DECEMBER 2006

Table 2. Fraction of solutes recovered in outflow in the column transport experiments involving 3H2O, 14C-TNT, 14C-RDX, and dissolved
Composition B (Comp B) (57.6% RDX, 38.9% TNT, and 3.5% HMX) in Adler and Plymouth soils.
Soil

Treatment

3

H2O

14

C-RDX

RDX†

RDX 1 products

Adler

RDX
RDX IF‡RDX
RDX IFTNTTNT IFTNTTNT IF
Comp B
Comp B IF
Comp B
Comp B IF

0.99
1.05
0.94
1.00
0.99
1.04
0.97
0.94
0.97
1.05
0.98
1.11

0.98
1.05
0.88 (0.91)§
0.81
NA#
NA
NA
NA
1.10
1.03
1.05
1.05

NA
NA
NA
NA
NA
NA
NA
NA
0.86
0.81
0.89
0.90

NA
NA
NA
NA
NA
NA
NA
NA
0.88
0.85
0.89
0.91

Adler
Plymouth
Adler
Plymouth

C-TNT

TNT

TNT 1 products

HMX

NA
NA
NA
NA
0.73
0.75
0.36 (0.42)§
0.41
NA
NA
NA
NA

NA
NA
NA
NA
0.07
0.15
0.01
0.03
0.06
0.03
0.20
0.14

NA
NA
NA
NA
0.29
0.35
0.04
0.05
0.22
0.07
0.25
0.20

NA
NA
NA
NA
NA
NA
NA
NA
1.02
0.96
1.01
1.03

† Mass balance was based on weight concentrations because of the similarity of molecular weights of TNT and some of its measured products. In addition,using molar concentrations for dimers, like azoxy compounds, would underestimate amount of TNT products.
‡ IF 5 interrupted flow.
§ Breakthrough was not complete, extrapolated value in parentheses was determined assuming the same slope for adsorption and desorption legs of the
breakthrough curve.
# Not applicable.

batch studies using RDX and TNT with 14C radiolabel. Adsorption partitioning was conducted at a 4:1 solution/soil mass
ratio in 25-mL glass centrifuge tubes. Triplicate samples and ablank were spiked at concentrations of 0.001, 0.0025, 0.005,
0.0075, and 0.01 mCi mL21 (37, 92.5, 185, 277.5, and 370 Bq mL21).
Total explosive concentrations were 1 to 10 mg L21 RDX and
1 to 10 mg L21 TNT. The samples were shaken for 24 h on areciprocating shaker at 180 excursions per minute, centrifugedat 8288 relative centrifugal force (RCF) for 60 min, and theaqueous phase removed and analyzed by liquid scintillation
counting. Linear adsorption isotherms were determined fromthe solution concentrations and calculated sediment concentrations. Equilibration time was determined from kinetic adsorption curves established by shaking 10 mg L21 radiolabeled
RDX and TNT solutions with soils at 4:1 solution/soil mass
ratio for 0, 0.5, 1, 2, 6, 12, and 24 h, and 5 d.

g21) (Brannon et al., 1992; Ainsworth et al., 1993; Xueet al., 1995; Myers et al., 1998; Pennington et al., 1999;
Price et al., 2000).
The TNT sorption isotherm was linear for Plymouthsoil and nonlinear for Adler (Freundlich kf 5 3.9, n 5
1.4, R2 5 0.99). However, to make comparisons between
the soils and explosives, linear regression was fitted
through the data. The TNT kds (2.4 and 1.6 cm3 g21 for
Adler and Plymouth, respectively) agree with values
reported in the literature (Brannon and Pennington,2002) for the same equilibration time and for soils withsimilar properties (average 1.8 cm3 g21, median 1.1 cm3g21, range 0.27 to 4.5 cm3 g21) (Brannon et al., 1992;
Ainsworth et al., 1993; Xue et al., 1995; Myers et al.,
1998; Pennington et al., 1999; Price et al., 2000).

RESULTS AND DISCUSSION
Batch Adsorption Studies

Conservative Tracer

The kinetic tests showed that RDX reached sorption
equilibrium between soil and water within less than 2 h.
Fast sorption of RDX by soils was also reported previously (Xue et al., 1995; Singh et al., 1998). Therefore,
24-h equilibration was used in subsequent sorption
studies for convenience. The RDX blank recovery was
101.1 6 1.0%, indicating no sorption by the glassware orloss through other pathways. The TNT blank recovery
was also complete (99.3 6 1.3%). However, TNT distribution between soil and water did not stabilize over time,
but continued to decline slightly even at 120 h. Processes
affecting TNT fate in soils may include TNT decomposition to products that tend to covalently bind to the soilorganic matter resulting in their removal from solution(Thorn et al., 2002). Partition coefficients were determined at 24 h for comparisons with RDX coefficients.
The RDX sorption isotherms were linear (Fig. 1) inagreement with published reports (Xue et al., 1995; Singhet al., 1998; Tucker et al., 2002; Yamamoto et al., 2004).
The kd values determined from the isotherms (0.48 cm3 g21
for Adler soil and 0.65 cm3 g21 for Plymouth soil) weresimilar to values reported from shake tests with soilsof comparable texture, OM content, and CEC (0.16 to
2.2 cm3 g21, average of 0.93 cm3 g21, median 0.92 cm3Hollow triangles in Fig. 2 and 3 represent measured
values for outflow concentrations of the conservative
tracer, 3H2O, labeled on figures as “measured water.”18

y = 2.4x
R2 = 0.90

RDX Adler
16

RDX Plymouth
TNT Adler

14

TNT Plymouthy = 1.6x
R2 = 0.98

12

S, mg kg -1

Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.

Plymouth14108y = 0.65x
R2 = 1.00

64

y = 0.48x
R2 = 0.96

2
0
0

2

4

6

8

10

C, mg L-1

Fig. 1. Adsorption isotherms for TNT and RDX with Adler andPlymouth soils. C, measured solution concentration; S, calculated
soil concentration. Error bars equal standard error of the mean.

2047

DONTSOVA ET AL.: TNT, RDX, AND COMPOSITION B IN SATURATED SOIL COLUMNS

1.2

1.2

Adler RDX

1

Measured water

0.8

C / C0C / C0

0.8
0.6
0.4

kd

0.2

= 0.16 cm3 g-1

Fitted water

0.6

Measured RDX

0.4

Fitted RDX

0.2

kd = 0.21 cm3 g-1

0

0
0

20

40

60

80

100

120

0

20

40

Time, h

60

80

100

120

Time, h1.2

1.2

Adler RDX IF

1

Plymouth RDX IF

1

0.8

0.8

C / C0C / C0

Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.

Plymouth RDX

1

0.6
0.4

0.6
0.4

0.2

kd

= 0.18 cm3 g-1

kd = 0.17 cm3 g-1

0.2

0

0
0

20

40

60

80

100

0

120

20

Time, h

40

60

80

100

120

Time, hFig. 2. Breakthrough curves for 14C-RDX and 3H2O for Adler (left) and Plymouth (right) soils for continuous (top) and interrupted (bottom) flow
experiments. IF, interrupted flow.

Dispersivity was larger for the coarser Plymouth soil
(0.62 to 2.29 cm) than for the finer Adler soil (0.07 to
0.17 cm) (Table 3), but generally small as expected forshort repacked columns.
Breakthrough of the conservative tracer occurred at
one pore volume (711 mL for Adler or 11 h and 519 mL
for Plymouth or 8 h), indicating the absence of preferential pathways for flow. No air entrainment within
the column was observed. Breakthrough curves wereplotted on a time basis to accurately present interrupted
flow experiments. Outflow concentrations of 3H2O resumed at similar values after the flow interruption, indicating that no or only limited physical nonequilibrium
occurred in the soil column. Mass-balance calculations(Table 2) indicated that all tritiated water was recovered(100 6 5%).

RDX
Solid diamonds represent measured values for the
C-RDX radiotracer concentration for the two soils
with and without interrupted flow (Fig. 2). For 14C-RDX,
breakthrough was observed later than for the conservative tracer indicating adsorption to the solid phase. The
average kd value determined from the curves was slightly
larger for Plymouth than Adler soil, but the difference
was not significant.
The 14C-RDX radiotracer exhibited little retardation,
as is generally reported in the literature (Selim et al., 1995;
Brannon and Pennington, 2002; Tucker et al., 2002). The
kd values for 14C-RDX determined from HYDRUS-1D(Table 3) were smaller than values determined in thebatch experiments (Fig. 1). Batch adsorption coefficientvalues for Plymouth soil were similar to values reportedby Speitel et al. (2002) for this soil. High mobility of
14

14

C-RDX was supported by the mass balance (Table 2).
For Adler soil, sorption was reversible; recovery of
14
C-RDX varied between 98 and 105%. Furthermore,
irreversible attenuation was not statistically significant
when modeled by HYDRUS-1D. However, for Plymouthsoil, some irreversible attenuation was observed, as
indicated by lower 14C-RDX recovery in outflow (81 to91%) and significant numbers for irreversible attenuation
rate coefficient (0.012 6 0.002 h21). Breakthrough curves
also indicated that the outflow concentration was less thaninflow for Plymouth soil, but similar to inflow concentration for Adler soil. Incomplete recovery of 14C-RDX
in Plymouth soil is consistent with 8% unextractable
14
C-RDX after 168 d observed by Singh et al. (1998) andirreversible attenuation of RDX in Plymouth (7.4%)
reported by Speitel et al. (2002).
The asymmetric shape of the breakthrough curves,particularly for Plymouth soil, and results of HYDRUS1D simulations suggest rate-controlled adsorption anddesorption. An estimated 30% of adsorption sites inAdler soil and 33% in Plymouth soil exhibited kinetic
adsorption, with the rate of exchange in 0.12 to 0.16 h21
range (Table 3). The decrease in concentration following
flow interruption also indicated chemical nonequilibrium.
Outflow was not analyzed for RDX by HPLC because
it was assumed that 14C-RDX accurately represents
RDX in soil as there is little RDX degradation underaerobic conditions (Pennington and Brannon, 2002;
Speitel et al., 2002).

TNT
14

The C-TNT radiotracer exhibited greater attenuation than 14C-RDX as indicated by a delay in the breakthrough curve and smaller outflow concentrations (Fig. 3).

2048

J. ENVIRON. QUAL., VOL. 35, NOVEMBER–DECEMBER 2006

1.2

1.2

Adler TNT

1

C / C00.4

kd = 0.50 cm3 g-1

0.2

Measured water

0.8

Fitted water

0.6

Measured TNT
Fitted TNT

0.4

kd = 0.71 cm3 g-1

0.2
0

0
0

20

40

60

80

100

120

0

20

40

Time, h

60

80

100

120

Time, h1.2

1.2

Adler TNT IF

0.8
0.6

kd = 0.55 cm3 g-1

0.4

Plymouth TNT IF

1

C / C01

C / C0

Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.

C / C00.8
0.6

Plymouth TNT

1

0.8
0.6

kd = 0.56 cm3 g-1

0.4
0.2

0.2

0

0
0

20

40

60

80

100

120

Time, h

0

20

40

60

80

100

120

Time, hFig. 3. Breakthrough curves for 14C-TNT and 3H2O for Adler (left) and Plymouth (right) soils for continuous (top) and interrupted (bottom) flow
experiments. IF, interrupted flow.

Part of the attenuation was attributed to reversible adsorption, and part to irreversible attenuation.
Average kds were 3.2 times greater for 14C-TNT thanfor 14C-RDX in both soils (Table 3) suggesting a greateraffinity of TNT for adsorption sites. This difference wasclose to the 2.4-fold difference in log Kow of the compounds (2.06 and 0.87 for TNT and RDX, respectively)
suggesting that partitioning of TNT to the OM may be asignificant mechanism for reversible adsorption.
For both soils, 14C-TNT kds were small compared tovalues reported in the literature (Brannon and Pennington, 2002) (0.53 and 0.63 cm3 g21 for Adler andPlymouth, respectively, difference not statistically significant). However, the irreversible attenuation contribution was considerable with the attenuation rate
significantly smaller in Adler (0.023 6 0.005 h21) thanin Plymouth (0.101 6 0.022 h21) soil. A 4.4-fold difference in rate between soils was consistent with differencein OC content (0.2 and 0.78% for Adler and Plymouthsoils, respectively), which is a matrix for irreversible
attenuation (Thorn et al., 2002). The mechanism forirreversible attenuation of TNT involves reduction toamino transformation products, followed by covalentbonding to the functional groups, e.g., carboxylic acid, ofOM (Thorn et al., 2002).
The 14C-TNT kds determined from batch shake tests(2.4 and 1.6 cm3 g21 for Adler and Plymouth, respectively) and column experiments (0.53 and 0.63 cm3 g21
for Adler and Plymouth, respectively) differed. Batch
kds were probably larger because they included irreversible attenuation and TNT is known to have a contribution of covalent bonding to total retention by soils(Thorn et al., 2002). Selim et al. (1995) found that adsorption coefficients for TNT obtained from batch ex-periments were larger than those obtained from column
experiments and explained it by more complete mixing
in batch experiments.
Asymmetric breakthrough curve, as well as the decrease in concentration when flow was interrupted, indicated rate-controlled sorption of 14C-TNT in agreementwith Selim et al. (1995). Similar to 14C-RDX, 14C-TNT
adsorption was instantaneous to a greater fraction of sites
in Adler (0.63) than in Plymouth (0.38) soils. Both kinetic
sorption and retarded nonlinear intra-particle diffusion(Fesch et al., 1998) may be causing nonequilibrium effects
in TNT transport in soils. Rate of sorption (v) accounts
for both forms of the nonequilibrium.
Outflow concentrations of HPLC-measured TNT in
Plymouth soil were small and inconsistent and, therefore, were not used for the simulation. However, for
Adler soil TNT kds were similar to the values for the
14
C-TNT radiotracer, whereas degradation was considerably greater, because HPLC-measured TNT included
both irreversible attenuation and degradation (Table 3).
Estimates for kinetic parameters ( f and v) in HPLCmeasured TNT breakthroughs were nonsignificant. The
limited number of samples analyzed by HPLC combinedwith the selection of samples with non-zero TNT
concentrations (and error) resulted in larger confidence
intervals and lower R2.
Mass-balance calculations for 14C-TNT were consistent between similar treatments (Table 2). Seventy-three
to seventy-five percent of 14C-TNT was recovered in
Adler and 41 to 42% in Plymouth soil. Speitel et al.
(2002) reported that 74 to 85% of 14C-TNT radiotracer
was recovered for the subsoil coming from MMR, whichis the same soil as Plymouth. For other soils, 20 to 50%
of added TNT was reported to be unextractable (Comfort

2049

DONTSOVA ET AL.: TNT, RDX, AND COMPOSITION B IN SATURATED SOIL COLUMNS

Table 3. Solute transport parameters obtained by HYDRUS-1D for column saturated flow experiments involving 3H2O, 14C-TNT, 14CRDX, and dissolved and solid Composition B (Comp B) (57.6% RDX, 38.9% TNT, and 3.5% HMX) in Adler and Plymouth soils(u was determined from the weight of water needed to saturate the column, l was estimated from 3H2O, whereas f, kd, mw, v, andg from explosives).
Parameter

u†

l

2

R

f

kd
3

Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.

cm
RDX solution phase
14C-RDX
Adler
Plymouth
TNT solution phase
14C-TNT
Adler
Plymouth
TNT
AdlerComp B solution phase
14C-RDX
Adler
Plymouth
HMX
Adler
PlymouthRDX
Adler
Plymouth
TNT
Adler
PlymouthComp B solid
HMX, noAdler
degradation
Plymouth
RDX, fitted
Adlerdegradation#
Plymouth
TNT, fitted
Adlerdegradation#
Plymouth
TNT, fixedAdlerdegradation†† Plymouth

cm g

mw
21

v
h

21

R

g
mg cm

23

h

2

21

0.53 6 0.008‡
0.37 6 0.013

0.17 6 0.004
0.73 6 0.065

0.99
0.99

0.70 6 0.01
0.67 6 0.10

0.17 6 0.01
0.19 6 0.02

0.000 6 0.000
0.012 6 0.002

0.12 6 0.04
0.16 6 0.07

NA§
NA

0.99
0.99

0.51 6 0.015
0.38 6 0.015
0.51 6 0.015

0.082 6 0.018
0.62 6 0.22
0.082 6 0.018

0.99
0.99
0.99

0.63 6 0.00
0.38 6 0.11
0.46 6 0.02

0.53 6 0.02
0.63 6 0.08
0.47 6 0.07

0.023 6 0.005
0.101 6 0.022
0.194 6 0.057

0.09 6 0.00
0.41 6 0.22
0.82 6 0.51

NA
NA
NA

0.98
0.97
0.78

0.51 6 0.000
0.38 6 0.002
0.51 6 0.000
0.38 6 0.002
0.51 6 0.000
0.38 6 0.002
0.51 6 0.000
0.38 6 0.002

0.17 6 0.08
0.86 6 0.22
0.17 6 0.08
0.86 6 0.22
0.17 6 0.08
0.86 6 0.22
0.17 6 0.08
0.86 6 0.22

0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99

0.92 6 0.04
0.54 6 0.05
0.87 6 0.00
0.51 6 0.12
0.80 6 0.03
0.50 6 0.04
0.38
0.42 6 0.13

0.17 6 0.00
0.22 6 0.02
0.48 6 0.01
0.43 6 0.03
0.19 6 0.01
0.21 6 0.02
0.60
0.98 6 0.15

0.000 6 0.000
0.000 6 0.000
0.001 6 0.001
0.000 6 0.000
0.012 6 0.002
0.013 6 0.005
0.268
0.210 6 0.015

0.04 6 0.02
0.16 6 0.11
0.06 6 0.01
0.13 6 0.09
0.19 6 0.12
0.12 6 0.06
0.45
0.10 6 0.04

NA
NA
NA
NA
NA
NA
NA
NA

0.99
0.99
0.99
0.98
0.995
0.99
0.84
0.98

0.51 6 0.003
0.37 6 0.013
0.51 6 0.003
0.37 6 0.013
0.51 6 0.003
0.37 6 0.013
0.51 6 0.003
0.37 6 0.013

0.068 6 0.006
2.29
0.068 6 0.006
2.29
0.068 6 0.006
2.29
0.068 6 0.006
2.29

0.99
0.96
0.99
0.96
0.99
0.96
0.99
0.96

NA¶
NA
NA
NA
NA
NA
NA
NA

0.35 6 0.11
0.20 6 0.03
0.18 6 0.04
0.05 6 0.00
0.35 6 0.11
0.29 6 0.00
0.26 6 0.02
0.50 6 0.01

NANA
0.003 6 0.002
0.002 6 0.000
0.020 6 0.001
0.008 6 0.004
NA
NA

NA
NA
NA
NA
NA
NA
NA
NA

2.8 6 1.8
6.4 6 3.7
29 6 2276 6 47
39 6 1665 6 41
56 6 25634 6 445

0.64
0.84
0.71
0.95
0.75
0.85
0.59
0.83

† u 5 water content; l 5 longitudinal dispersivity; f 5 fraction of sites with instantaneous adsorption; kd 5 adsorption coefficient; mw 5 first-order
rate coefficient for dissolved phase, degradation rate (for 14C radiotracer experiments indicates irreversible attenuation); v 5 first-order rate coefficientfor two-site nonequilibrium adsorption; g 5 zero-order rate coefficient for dissolution of solid Comp B.
‡ (means 6 standard error; n 5 2).
§ Not applicable.
¶ Because of increased uncertainty in Solid Comp B experiments, values for kinetic sorption parameters (f and v) were largely nonsignificant and therefore
they were removed from the model.
# Degradation rate was determined from product concentrations.
†† Degradation rate was determined as a sum of rates determined from product concentrations (“fitted degradation”) and the irreversible attenuation
21
rate for pure TNT (0.042 and 0.109 h for TNT for Adler and Plymouth soils, respectively).

et al., 1995; Kaplan and Kaplan, 1982b; Selim et al.,1995). Since degradation cannot be estimated fromradiotracer data, unrecovered 14C-TNT is assumed to becovalently bound to the soil OM by reactions reportedby Thorn et al. (2002). Also, TNT may form coplanarcomplexes with K-exchanged phyllosilicate clays in soils(Weissmahr et al., 1999); however, in studied soils only
0.5 to 1.2% of exchange sites were occupied by K. Selim
et al. (1995) reported that irreversible attenuation was adominant retention mechanism of TNT.
In Adler soil, the amount of TNT recovered by HPLC
was much lower than 14C-TNT recovered since 14C-TNT
elutes as both TNT and its decomposition products.
HPLC showed recovery of 7 to 15% as TNT and 29 to
35% as a sum of TNT and its measured products. In
Plymouth, HPLC TNT recovery was even lower than in
Adler soil with less than 5% of inflow. Measured products included ADNTs and azoxy compounds in agreement with Comfort et al. (1995) who observed thata significant fraction of TNT was recovered as amino
degradates of TNT.
Forty to forty four percent (14C-TNT recovery minus
HPLC TNT and its measured products) in Adler soil and
36 to 39% in Plymouth soil were unaccounted for, likely
as undefined or unmeasured degradation products. TNT
mineralization to CO2 is unlikely (Kaplan and Kaplan,
1982a; Speitel et al., 2002).

Solution Phase Composition B
TNT and RDX from dissolved Comp B behaved likepure TNT and RDX (Fig. 4) with kds, irreversible attenuation rate, degradation rate, fraction of sites withinstantaneous adsorption, and rate of exchange forkinetic sites for 14C-RDX radiotracer, RDX, and TNT in
Comp B not significantly different from the ones obtained for pure explosives (Table 3).
The only difference was in the rate of irreversible
attenuation and mass balance of 14C-RDX radiotracer
in Plymouth soil. Unlike pure 14C-RDX in Plymouththat exhibited a statistically significant rate of irreversible
attenuation (0.012 6 0.002 h21), 14C-RDX in Comp B had
no significant irreversible attenuation (irreversible attenuation rate, mw, was not significantly different from zero).
In addition, all of 14C-RDX was recovered in Comp B(105 6 0%), whereas in pure 14C-RDX some was retained (8 to 18%). Some competition for irreversible attenuation sites between TNT and RDX is possible, since
products of TNT transformation actively react with OM.
The HMX, which was not studied separately, wasstable with no significant degradation or irreversible
attenuation and 100% recovery (96 to 103%). The
HMX kds (0.43 to 0.48 cm3 g21) were in general agreement with batch values summarized by Brannon andPennington (2002). Close agreement between reported
batch kds for HMX (0.086 to 5.02 cm3 g21, median2050

J. ENVIRON. QUAL., VOL. 35, NOVEMBER–DECEMBER 2006

Concentration, mg L-1

Concentration, mg L-1

7Adler Comp B

6

RDX
TNT
HMX
4A-DNT
2A-DNT
2,2'-Azoxy
4,4'-Azoxy
TNX
MNX

5
4
3
2
1
0

Plymouth Comp B

6

RDX

5

TNT

4

HMX

3

2A-DNT

4A-DNT
MNX

2
1
0

0

20

40

60
Time, h

80

100

120

0

7
RDX
TNT

5

HMX

4

4A-DNT
2A-DNT

3

MNX
DNX

2

20

40

60
Time, h

80

100

120

7

Adler Comp B IF

6

TNX

1

Concentration, mg L-1

Concentration, mg L-1

Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.

7

Plymouth Comp B IF

6

RDX

5

TNT
HMX

4

4A-DNT

3

2A-DNT

2

MNX

24-DNT

1
0

0
0

20

40

60
Time, h

80

100

120

0

20

40

60
Time, h

80

100

120

Fig. 4. Breakthrough curves for liquid phase Composition B (Comp B) for Adler and Plymouth soils. IF, interrupted flow; RDX, hexahydro1,3,5-trinitro-1,3,5-triazine; MNX, hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine; DNX, hexahydro-1,3-dinitroso-5-nitro-1,3,5-triazine; TNX,
hexahydro-1,3,5-trinitroso-1,3,5-triazine; HMX, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine; TNT, trinitrotoluene; 2-ADNT-2-amino-4,
6,-dinitrotoluene; 4-ADNT, 4-amino-2,6,-dinitrotoluene; 2,29-Azoxy, 4,49,6,69-tetranitro-2,29-azoxytoluene; 4,49-Azoxy, 2,29,6,69-tetranitro-4,
49-azoxytoluene.

0.77 cm3 g21, average 1.62 cm3 g21) (Myers et al., 1998;
Brannon et al., 1999; Pennington et al., 1999) and columndetermined values can be explained by the absence ofirreversible attenuation.
Degradation products of TNT accounted for 65% ofoutflow in Adler soil and 25% in Plymouth soil. They
included ADNTs, which predominated, in agreementwith Selim et al. (1995), and azoxy compounds. Therewas more 4ADNT than 2ADNT in agreement with
Comfort et al. (1995). Furthermore, 4ADNT exhibitedan earlier breakthrough. This can be explained by higher
partition coefficients for 2ADNT (Pennington and
Patrick, 1990), and greater production of 4ADNT because biotic reduction of nitro group in para position ismore prevalent. Kaplan and Kaplan (1982a) reported
that 4ADNT accounted for the majority of the twoamines in compost spiked with TNT.
Degradation products of RDX were only a small partof the outflow (Table 2): 0.4 to 4% of RDX input (0.4 to
5% of the RDX outflow). The fraction of products
doubled in interrupted flow experiments, as additionaltime allowed degradation to proceed further. Hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine (MNX) was the
only detected degradation product in Plymouth soil. In
Adler soil, DNX (hexahydro-1,3-dinitroso-5-nitro-1,3,5triazine) and TNX (hexahydro-1,3,5-trinitroso-1,3,5triazine) were detected. Concentrations of all degradation
products increased in the outflow after flow interruption. Degradation pathways of RDX and TNTwere mod-eled, but are not reported, since parameter estimates
were nonsignificant.

Solid Composition B
Breakthrough curves from solid Comp B indicatedthat despite the small range of particle sizes, variability
in dissolution rate of Comp B among the experiments
was significant (Fig. 5). For two experiments outflow
concentrations decreased in time, possibly due to a decrease in dissolution rate as smaller particles weredissolved. Smaller particles with a larger surface area
dissolve faster due to greater contact with solution(Lynch et al., 2002a). The sharp initial peak observed forthe interrupted flow experiment in Adler soil can be
explained by the presence of microscopic Comp B particles on the surface of the principle particles as observedby Phelan et al. (2003). As dissolution is strongly affected by particle size, microscopic particles will dissolve
fastest and result in a sharp increase in solution concentration. Another prominent feature of the breakthroughcurves was similarity in outflow concentrations of TNT
and RDX, contrary to what was observed for dissolvedComp B (Fig. 4) and to observations of groundwater
concentrations in the field (Clausen et al., 2004). Variance from the field results was explained by the small
length of the column and limited residence time of solution. Longer residence would cause a greater change
in solution concentration of TNT than RDX due to2051

Concentration, mg L-1

Concentration, mg L-1

40
35
30
25
20
15
10
5
0

Adler solid Comp B
RDX
TNT
HMX
4A-DNT
2A-DNT
24-DNT4,4'-Azoxy
2,2'-Azoxy
MNX

0

20

40

60
Time, h

80

100

RDX
TNT
HMX
4A-DNT
2A-DNT
24-DNT4,4'-Azoxy
2,2'-Azoxy
MNX

30
25
20
15
10
5
0

Plymouth solid Comp B
RDX
TNT
HMX
4A-DNT
2A-DNT
24-DNT4,4'-Azoxy
2,2'-Azoxy
MNX
DNX
TNX

0

20

40

60
Time, h

80

100

120

40

Concentration, mg L-1

Adler solid Comp B IF

35

40
35
30
25
20
15
10
5
0

120

40

Concentration, mg L-1

Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.

DONTSOVA ET AL.: TNT, RDX, AND COMPOSITION B IN SATURATED SOIL COLUMNS

Plymouth solid Comp B IF

35

RDX

30

TNT
HMX

25

4A-DNT

20

2A-DNT

15

24-DNT4,4'-Azoxy

10

2,2'-Azoxy

5

MNX

0
0

20

40

60

80

100

120

Time, h

0

20

40

60
Time, h

80

100

120

Fig. 5. Breakthrough curves for solid phase Composition B (Comp B) for Adler and Plymouth soils. IF, interrupted flow; RDX, hexahydro1,3,5-trinitro-1,3,5-triazine; MNX, hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine; DNX, hexahydro-1,3-dinitroso-5-nitro-1,3,5-triazine; TNX,
hexahydro-1,3,5-trinitroso-1,3,5-triazine; HMX, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine; TNT, trinitrotoluene; 2-ADNT, 2-amino-4,6,dinitrotoluene; 4-ADNT, 4-amino-2,6,-dinitrotoluene; 2,29-Azoxy, 4,49,6,69-tetranitro-2,29-azoxytoluene; 4,49-Azoxy, 2,29,6,69-tetranitro-4,49azoxytoluene.

differences in degradation rates. Longer soil profiles
enhance the contribution of reversible sorption and irreversible attenuation. Difference between solid anddissolved Comp B experiments indicated that TNT has agreater dissolution rate than RDX.
As an excess of Comp B was used, no reliable measure of a source term was available to model transport
using HYDRUS-1D. Therefore, degradation had to be
determined to estimate dissolution of Comp B. Outflow
concentrations of HMX in dissolved Comp B experiments were similar to inflow concentrations. Based on
this and the absence of HMX degradation products indissolved and solid Comp B tests, degradation of HMX
from solid Comp B was assumed to be negligible andwas, therefore, set at zero (“no degradation” in Table 3).
For RDX, which had some decomposition, but no irreversible attenuation in dissolved Comp B experiments,
degradation was determined from the concentration of
degradation products assuming that the products did notfurther decompose (“fitted degradation” in Table 3). For
TNT, degradation rate was also determined from product concentrations. However, unlike RDX, TNT exhibited irreversible attenuation. Therefore, the irreversible
attenuation rate observed for pure TNT was added tothe degradation rate determined from product concentrations to give a total degradation rate (0.0424 and0.1091 h21 for TNT for Adler and Plymouth soils, respectively) (“fixed degradation” in Table 3). Irreversible
attenuation was assumed to be the same for TNT insolid Comp B as for pure TNT because total degradation rates in the pure system and in the dissolved CompB system were similar. Degradation rates of TNT and
RDX determined from product outflow concentration
were lower than determined for dissolved Comp B andpure systems (Table 3), but this was supported by thefact that concentrations of products relative to parent
compounds were smaller in the solid Comp B experiments. Greater concentrations of TNT and RDX in thesolid Comp B experiment may have caused a reduction
in microbial populations (Comfort et al., 1995) and suppression of biotic reduction that exceeds abiotic reduction of TNT (Pennington and Patrick, 1990).
When the above approach was applied to the data,
simulated dissolution rates (Table 3) varied between theexperiments, reflecting variability in outflow. However,a highly significant correlation (P . F , 0.01) existed
between HYDRUS-1D-estimated dissolution rates of
HMX and RDX and a significant correlation (P . F ,0.05) between RDX and TNT. This supported theaccuracy of HYDRUS estimates of dissolution ratesof Comp B and indicated that dissolution rates of explosives in a mix are linked. Dissolution rate was thehighest for TNT and lowest for HMX. According to
Brannon and Pennington (2002) when tested alone, TNT
has the highest dissolution rate (4164 mg cm22 h21),
followed by HMX (702 mg cm22 h21) and RDX (361 mg
cm22 h21). Several studies indicated that the dissolution
rate of individual explosives is decreased by being pres-Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.

2052

J. ENVIRON. QUAL., VOL. 35, NOVEMBER–DECEMBER 2006

ent in formulations, whereas solubility is not affected(Lynch et al., 2002b; Phelan et al., 2002; Taylor et al.,2004). In this study, the link between dissolution rates ofdifferent components of Comp B was analyzed by comparing slopes of regressions between dissolution rates of
RDX, TNT, and HMX (Table 4) with other relevant
properties of studied explosives normalized to RDX:
individual dissolution rate, measured content in CompB, and solubility. None of the listed properties gave agood explanation for the dissolution rates; however, aproduct of individual dissolution rate and explosives
content in Comp B was in very good agreement with
HYDRUS-1D estimates of dissolution rates in the formulation. This can be explained by dissolution being
related to the surface area, that for each individualcompound decreased proportionally to its content in
the formulation.
Adsorption coefficients for HMX, RDX, and TNT
determined from solid Comp B were generally smaller
than in dissolved Comp B. Differences were significant
for Plymouth soil and for TNT in Adler soil. Two processes can explain this difference, saturation of sorption
sites at high concentrations (inflow in dissolved Comp B
was below 10 mg L21 whereas in solid Comp B outflow
was as high as 40 mg L21), and competition between
RDX and TNT at high concentrations for nonspecificsorption sites. Both possibilities are supported by previous reports. Comfort et al. (1995) observed nonlinear
sorption of TNT and higher sorption coefficients at low
concentrations, whereas Haderlein et al. (1996) reported
competitive adsorption of nitroaromatic compounds.

CONCLUSIONS
A saturated column transport study was conducted toevaluate transport parameters for RDX, TNT, and solution and solid Comp B for two soils. Sorption coefficients were estimated and contribution of transformationand irreversible sorption was established. RDX was moremobile than TNT because RDX had smaller reversible
sorption, irreversible attenuation, and degradation.
Combining column flow-through studies with numerical analysis of outflow concentrations of explosives andan inert tracer allowed separation of the contributionsof physical and chemical nonequilibrium processes.
Whereas for studied soils with uniform particle-size
physical nonequilibrium was negligible, chemical nonTable 4. Estimated high explosives dissolution rate from solidComp B and other relevant properties of studied explosives
normalized to RDX.
Explosives properties

RDX

HMX

TNT

SourceEstimated dissolution
rate from solid Comp B
Individual dissolution rate

1.00

0.08

7.68

current study

1.00

1.94

11.53

Content in Comp B
Solubility at 20°C

1.00
1.00

0.06
0.07

0.68
2.30

(Brannon andPennington, 2002)
current study(Brannon andPennington, 2002)

Individual dissolution
rate 3 explosive
content in Comp B

1.00

0.12

7.79

equilibrium (kinetic sorption) contributed to transportand distribution of both RDX and TNT. The contribution of chemical nonequilibrium was confirmed by interrupted flow experiments.
The behavior of dissolved Comp B was similar to thatof pure RDX and TNT. Estimated adsorption and degradation rates for RDX, TNT, and HMX were generally
smaller for solid Comp B than for dissolved explosives
in column and batch studies. Therefore, using distribution coefficients from batch studies and transport parameters determined for pure explosives in solution canunderestimate transport of formulations. However,great variability in outflow concentrations in experiments with solid phase Comp B indicated that dissolution was controlling transport processes. The dissolution
rate of Comp B was affected by the dissolution rate ofpure compounds and by their fraction in Comp B.
ACKNOWLEDGMENTS
This study was funded by the U.S. Army Corps of Engineers, Environmental Quality Technology Program. M. John
Cullinane, Program Manager. Work of J. Šimunek was supported by the Terrestrial Sciences Program of the Army Research Office (Terrestrial Processes and Landscape Dynamics
and Terrestrial System Modeling and Model Integration).
Authors are grateful to T. Myers of ERDC, Vicksburg, MS for
his review of the manuscript. Disclaimer: Permission has been
granted by the Chief of Engineers to publish this report.

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