United States
Environmental Protection
Agency
National Exposure
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-96/141
December 1996
SEPA Project Summary
Investigations of Coupled
Geochemical and Transport
Modeling for the Hazardous
Waste Identification Rule
Louise J. Criscenti
Research to define both allowable
metal concentrations in waste streams
for landfill disposal and allowable land-
fill leachate compositions is being con-
ducted by the U.S. Environmental Pro-
tection Agency. Leachate interaction
with the subsurface may reduce or in-
crease the impact of disposed metals
on water quality. For regulatory pur-
poses, it is important to develop mod-
els that encompass the full spectrum
of landfill leachate migration scenarios
possible in the United States. The cal-
culations presented here were per-
formed to establish reasonable high-
end exposure scenarios.
The Chemical Transport Model was
used to perform calculations to illus-
trate the interaction between landfill
leachate and the subsurface. The ma-
jor inorganic constituents of both the
ground water and the landfill leachate
are included in aqueous speciation/
solubility calculations. Transport is de-
scribed by a 1-D advection/dispersion
equation. Cadmium was selected as an
example metal for the calculations, and
its adsorption onto ferric hydroxide Is
described by a surface complexation
model.
Preliminary results showed that the
continual influx of an acidic (pH = 5.9)
landfill leachate into the subsurface
may deplete the acid buffering capac-
ity of alkaline soil and ground water
(pH = 8.0) within 20 to 30 years.
Changes in solution pH led to changes
in aqueous speciation and metal ad-
sorption. In the simulations performed,
cadmium desorbed as pH decreased,
leading to the migration of a Cd front
through the subsurface with higher Cd
concentrations than those in the Initial
landfill leachate. Additional calculations
will be required to determine whether
changes in landfill leachate composi-
tion over the lifetime of the landfill or
changes in the subsurface redox envi-
ronment due to leachate migration will
significantly affect the model results.
This Project Summary was developed
by the National Exposure Research
Laboratory's Ecosystems Research Di-
vision, Athens GA, to announce key
findings of the research project that Is
fully documented in a separate report
(see Project Report ordering Informa-
tion at back).
Synopsis of Report
The objectives of the Hazardous Waste
Identification Rule (HWIR) are to define
allowable constituent concentrations in
waste streams for landfill disposal and to
define allowable landfill leachate concen-
trations. A methodology for organic con-
stituents has been developed in which a
ground water chemical transport code is
used to determine the dilution attenuation
factor (DAF), or the ratio of the leachate
concentration at a landfill to the concen-
tration at a receptor well, for each con-
stituent. Using the maximum allowable
concentration level (MCL) at the receptor
well, one can back-calculate the allowable
concentration in the leachate. A distribu-
tion of DAFs representing the range of
possible outcomes at landfills nationwide
can be calculated by applying the trans-
port code in a Monte Carlo mode.
Generating appropriate DAFs for metal
constituents in waste streams is more dif-
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ficult than generating DAFs for organics
due to variations in metal behavior with
changes in solution and soil chemistry. In
the HWIR methodology, the chemistry and
transport of metals from the landfill through
the vadose and saturated zones to a po-
tential receptor well is simulated with the
use of a loosely coupled, deterministic
transport and chemistry model. This re-
port reviews the assumptions made in the
HWIR methodology for metals, summa-
rizes the compositional data available for
landfill leachates, and explores (through
the use of an aqueous speciation/solubil-
ity code and a one-dimensional coupled
reaction/transport code) the "conservative
nature" of the HWIR assumptions (i.e., do
the HWIR assumptions provide conserva-
tive estimates of potential contamination
at receptor wells).
In the HWIR methodology, geochemical
reaction is coupled with transport through
the use of sorption isotherms for several
metal contaminants (beryllium, cadmium,
chromium, mercury, nickel, and lead). Four
master variables—ground water pH, or-
ganic matter from the landfill leachate,
iron oxide adsorbent concentration, and
natural organic matter—are considered to
play significant roles in the behavior of
metals in the subsurface environment.
Each of these four master variables is
assigned three values (high, medium, and
low) to generate a grid of geochemical
environments that spans the conditions
that might be observed in landfill sce-
narios. This grid is accessed by the trans-
port code whenever geochemical param-
eters are required.
This study examines whether the four
master variables selected for the HWIR
methodology characterize the landfill
leachate and the soil matrix/ground water
system sufficiently to predict metal con-
taminant transport in the subsurface. The
HWIR assumptions regarding landfill
leachate compositions, soil buffering ca-
pacity, redox reactions, competition be-
tween metals for adsorption sites, and
sorption by clay minerals are reviewed. A
major concern with the HWIR methodol-
ogy is that the development of the
geochemical model separately from the
transport model in the "loosely coupled"
code may not sufficiently characterize the
interactions that arise as leachate is gen-
erated and introduced into the subsur-
face.
Coupled reaction/transport simulations
were performed using the Chemical Trans-
port Model (CTM) in order to demonstrate
the importance of the leachate and soil
compositions on the transport and trans-
formation of metals. Five simple scenarios
illustrate the effects of variable amorphous
hydrous feme oxide (HFO) concentrations,
the presence of calcite, and competitive
adsorption between a metal and other sol-
utes.
CTM requires a path line length, aver-
age pore water velocity, and the
dispersivity of a conservative tracer along
the path line. These input parameters were
calculated using the values representative
of the 85 percentile reported in prior HWIR
studies, so that the results are represen-
tative of 15% of the HWIR scenarios. A
representative landfill leachate composi-
tion and an HWIR ground water composi-
tion at pH 8.0 are used. Cadmium is used
as a representative HWIR metal.
Cadmium may be retarded by both ad-
sorption and solubility reactions in the sub-
surface environment. The adsorption
model in CTM is a surface complexation
model developed to calculate the adsorp-
tion behavior of selected aqueous con-
stituents on amorphous hydrous ferric ox-
ide. A thermodynamic database was
adopted and acetate complexes from the
MINTEQA2 database and phosphate and
ammonium complexes were added. Cad-
mium adsorption to organic matter was
not considered.
The simulations show that both HFO
and calcite contribute to the acid-buffering
capacity of soil. Competitive adsorption
between Cd2* and the major cations in the
leachate and ground water decreases the
concentration of sorbed Cd2*, thereby in-
creasing the concentration of Cd2* that
migrates downstream.
The simulations also illustrate that one
reasonable high-end scenario arises when
the continual influx of an acidic (pH = 5.9)
landfill leachate into the subsurface de-
pletes the acid buffering capacity of the
soil and ground water (pH = 8.0). Changes
in solution pH led to changes in aqueous
speciation and metal adsorption. In the
simulations performed, Cd desorbed as
the pH of the subsurface decreased due
to mixing with the infiltrating leachate. This
led to the migration of a Cd front through
the subsurface with higher Cd concentra-
tions than those in the initial landfill
leachate.
Additional calculations will be required
to determine whether changes in landfill
leachate composition over the lifetime of
the landfill or changes in the subsurface
redox environment due to leachate migra-
tion will significantly affect the model re-
sults.
*U.S GOVERNMENT PRINTING OFFICE. 1997 .
54MOI/60IM
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