Updated: April 9, 2001. Copyright © 2001 by Walt W McNab, Concord, CA, U.S.A.. All Rights Reserved.
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MULTISPECIES REACTIVE TRANSPORT IN GROUNDWATE |
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Dr Walt McNabLawrence Livermore National LaboratoryLivermore, California, U.S.A. |
TOPIC C: INTRODUCTION TO REACTIVE TRANSPORT MODELING WITH PHREEQC: EXAMPLE APPLICATIONS
The
phenomenon of acid mine drainage is a familiar environmental problem in the
western United States (Rahmatian, 1990). The process involves the oxidation of
sulfide-rich surface mine tailings by atmospheric oxygen, yielding sulfuric acid
that may, for example, facilitate the leaching of metals into solution as a
result of mineral dissolution reactions or desorption. Tailings water that
migrates offsite then becomes a source of surface water and groundwater
contamination (Narasimhan, White, and Tokunaga, 1986, Kimball and others, 1994,
Gray, 1996).
The primary reaction of concern in acid mine drainage is the oxidation of sulfide by oxygen. Pyrite, FeS2, the most common of the sulfide minerals, is oxidized to ferric ion and sulfuric acid by the following reaction:
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(Eq.-1)
Once the products of this reaction ñ Fe3+, SO42-, and H+ ñ enter the aquifer, a host of other reactions may take place. These may include:
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An
example
Narasimhan, White, and Tokunaga (1986) made one of the first attempts to use a reactive transport modeling approach to simulate some of the processes associated with acid mine drainage. With PHREEQC, it is relatively straightforward to assemble such a model. Consider, for example, the following problem:
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Figure 1. Idealized one-dimensional model for simulation of simple acid mine drainage phenomena.
We describe this model to PHREEQC using the following input file:
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TITLE Infiltration of rainwater in equilibrium with the atmosphere through mine tailings and underlying aquifer
SOLUTION 0
temp 25
pH 7
pe 4
units mol/kgw
redox pe
density 1
-water 1 # kg
EQUILIBRIUM_PHASES 0
CO2(g) -3 10
O2(g) -0.7 10
Pyrite 0 0.01
SAVE Solution 0
END
SOLUTION 1-2
temp 25
pH 7
pe -3
units mmol/kgw
redox pe
density 1
-water 1
EQUILIBRIUM_PHASES 1-2
Pyrite 0 1
SOLUTION 3-10
temp 25
pH 7
pe -3
units mmol/kgw
redox pe
density 1
-water 1
EQUILIBRIUM_PHASES 3-10
Kaolinite 0 1
Calcite 0 1
Gypsum 0 0
Goethite 0 0
TRANSPORT
-cells 10
-shifts 20
-time_step 1 #dt = 10m / 10m/yr --> 1 shift per year
-flow_direction forward
-boundary_conditions flux flux
-lengths 10*10
-dispersivities 10*1.0
-diffusion_coefficient 0.0
-print_frequency 1
-punch_frequency 1
SELECTED_OUTPUT
-totals Fe S
-molalities SO4-2 Ca+2
-equilibrium_phases Pyrite Calcite Gypsum Goethite
END
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.gif)
Figure 2. Aqueous chemistry along the column after two pore volumes..
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Figure 3. Iron-bearing minerals present along the column after two pore volumes.
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Figure 4. Calcium-bearing minerals present along the column after two pore volumes.
The simulation results are shown on Figures 2 through 4. Briefly, after two pore volumes have passed through the model domain (after 20 years elapsed time), the geochemical system has evolved as expected. pH is substantially lowered, while dissolved iron and sulfate concentrations are elevated, within the tailings material, whereas buffering of the water chemistry through equilibration with calcite and goethite precipitation mitigates these effects in the aquifer material downgradient of the tailings.
Your
assignment
The simple model we have set up in PHREEQC to mimic acid mine drainage phenomena illustrates only a portion of the problem. Acid from the acidic pH, acid mine drainage is also responsible for the mobilization and transport of metal species of environmental concern such as Cd, Cu, Pb, Mn, U, and Zn. These metals can exist initially in tailings piles as sulfides (e.g., covellite, or CuS, and sphalerite, ZnS) and thus undergo oxidation in much the same way as pyrite. They may also exist as carbonates (e.g., cerrusite, PbCO3) or sulfates (e.g., anglesite, PbSO4), or even oxides or oxyhydroxides. These minerals may also dissolve (or, in some cases, precipitate) in conjunction with acid mine drainage. Moreover, desorption reactions involving the exchange of H+ ions and metal ions on mineral surfaces can also play a major role in influencing the mobility of these metals in such environments.
You assignment is to expand upon the simple example problem set up in this lecture to include reactions involving some of these metals. The MINTEQ thermodynamic database, an alternative to the PHREEQC database that is also supplied with the program, contains data on a large number of mineral phases for these metals, as well as some data for use in calculating surface reactions. Try adding some of these complexities to the model presented here. In each case, what are the major controls on solubility? Which metals seem to be more of a threat (in terms of continued downgradient migration) that others?
Gray, N.F., 1996, Field assessment of acid mine drainage contamination in surface and ground water, Environmental Geology, 27(4), 358-361.
Kimball, B.A., R.E. Broshears, K.E. Bencala, D.M. McKnight, 1994, Coupling of hydrologic transport and chemical reactions in a stream affected by acid mine drainage, Environmental Science and Technology, 28(12), 2065-2073.
Narasimhan, T.N., A.F. White, and T. Tokunaga, 1986, Groundwater contamination from an inactive uranium tailings pile, Water Resources Research, 22(13), 1820-1834.
Rahmatian, M., 1990, The incidence of environmental degradation - A case study of acid mine drainage, Journal of Environmental Management, 30(2), 145-155.
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You are now ready to continue to
TOPIC C: INTRODUCTION TO REACTIVE TRANSPORT MODELING WITH PHREEQC: EXAMPLE APPLICATIONS.
LECTURE
2: Weatherng of primary Ore Bodies and Secondary Enrichment: Supergene Copper
Ore Deposit Formation.
You may e-mail me questions and comments.
Walt W. McNab
E-mail address: mcnab1@llnl.gov.