United States National Exposure

Environmental Protection Research Laboratory

Agency Research Triangle Park, NC 27711

Project Summary

Research and Development EPA/600/SR-96/141 December 1996

Research to define both allowable

metal concentrations in waste streams

for landfill disposal and allowable landfill leachate compositions is being conducted by the U.S. Environmental Protection Agency. Leachate interaction

with the subsurface may reduce or increase the impact of disposed metals

on water quality. For regulatory purposes, it is important to develop models that encompass the full spectrum

of landfill leachate migration scenarios

possible in the United States. The calculations presented here were performed to establish reasonable highend exposure scenarios.

The Chemical Transport Model was

used to perform calculations to illustrate the interaction between landfill

leachate and the subsurface. The major inorganic constituents of both the

ground water and the landfill leachate

are included in aqueous speciation/

solubility calculations. Transport is described 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 capacity 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 adsorption. In the simulations performed,

cadmium desorbed as pH decreased,

leading to the migration of a Cd front

Investigations of Coupled

Geochemical and Transport

Modeling for the Hazardous

Waste Identification Rule

Louise J. Criscenti

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 composition over the lifetime of the landfill or

changes in the subsurface redox environment due to leachate migration will

significantly affect the model results.

This Project Summary was developed

by the National Exposure Research

Laboratory’s Ecosystems Research Division, Athens GA, to announce key

findings of the research project that is

fully documented in a separate report

(see Project Report ordering information 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 concentrations. A methodology for organic constituents 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 concentration at a receptor well, for each constituent. Using the maximum allowable

concentration level (MCL) at the receptor

well, one can back-calculate the allowable

concentration in the leachate. A distribution of DAFs representing the range of

possible outcomes at landfills nationwide

can be calculated by applying the transport code in a Monte Carlo mode.

Generating appropriate DAFs for metal

constituents in waste streams is more dif-
2

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 potential receptor well is simulated with the

use of a loosely coupled, deterministic

transport and chemistry model. This report reviews the assumptions made in the

HWIR methodology for metals, summarizes the compositional data available for

landfill leachates, and explores (through

the use of an aqueous speciation/solubility code and a one-dimensional coupled

reaction/transport code) the “conservative

nature” of the HWIR assumptions (i.e., do

the HWIR assumptions provide conservative 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, organic 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 scenarios. This grid is accessed by the transport code whenever geochemical parameters 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 contaminant transport in the subsurface. The

HWIR assumptions regarding landfill

leachate compositions, soil buffering capacity, redox reactions, competition between metals for adsorption sites, and

sorption by clay minerals are reviewed. A

major concern with the HWIR methodology 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 generated and introduced into the subsurface.

Coupled reaction/transport simulations

were performed using the Chemical Transport Model (CTM) in order to demonstrate

the importance of the leachate and soil

compositions on the transport and transformation of metals. Five simple scenarios

illustrate the effects of variable amorphous

hydrous ferric oxide (HFO) concentrations,

the presence of calcite, and competitive

adsorption between a metal and other solutes.

CTM requires a path line length, average 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 representative of 15% of the HWIR scenarios. A

representative landfill leachate composition and an HWIR ground water composition at pH 8.0 are used. Cadmium is used

as a representative HWIR metal.

Cadmium may be retarded by both adsorption and solubility reactions in the subsurface environment. The adsorption

model in CTM is a surface complexation

model developed to calculate the adsorption behavior of selected aqueous constituents on amorphous hydrous ferric oxide. A thermodynamic database was

adopted and acetate complexes from the

MINTEQA2 database and phosphate and

ammonium complexes were added. Cadmium 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 increasing 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 depletes 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 concentrations 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 migration will significantly affect the model results.
3

Louise J. Criscenti is with Pacific Northwest Laboratory, Richland, WA 99352.

Gerard F. Laniak is the EPA Project Officer (see below).

The complete report, entitled “Investigations of Coupled Geochemical and Transport Modeling for the Hazardous Waste Identification Rule,” (Order No. PB97-

121164; Cost: $21.50, subject to change) will be available only from:

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

Telephone: 703-487-4650

The EPA Project Officer can be contacted at:

Ecosystems Research Division

National Exposure Research Laboratory

U.S. Environmental Protection Agency

Athens, GA 30605-2700

• •

United States

Environmental Protection Agency

Center for Environmental Research Information (G-72)

Cincinnati, OH 45268

EPA/600/SR-96/141

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