United States
                     Environmental Protection
                     Agency
                                                             Environmental Research
                                                             Laboratory
                                                             Athens GA 30613
                     Research and Development
                                                             EPA/600/S3-86/030  Nov. 1986
 SEPA         Project  Summary
RECEIVED
MOV 2 5 1986
                     Development of Land Disposal
                     Decisions for Metals  Using
                     MINTEQ  Sensitivity  Analyses

                     David S. Brown, Roger E. Carlton, and Lee A. Mulkey
                      A metal speciation modeling ap-
                     proach was developed for evaluating
                     potential mobilities of arsenic, barium,
                     cadmium, chromium, lead, mercury,
                     nickel, selenium, silver and thallium in
                     ground waters under conditions reflect-
                     ing leachate contamination from a
                     failed land disposal facility. A modified
                     version of metal  speciation model
                     MINTEQ was used in combination with
                     a set of generic ground-water specifi-
                     cations and an En/pH uncertainty win-
                     dow to determine metal solubility limi-
                     tations. Metal speciation results were
                     interpreted in combination  with an
                     infinite-source, steady-state advective
                     dispersion model used to estimate dilu-
                     tion during transport to a down-
                     gradient exposure point.
                      The ten metals  were divided into
                     "mobile" and "relatively  immobile"
                     groups. The mobile group included ar-
                     senic, barium, cadmium, lead, nickel
                     and thallium. At least one Eh/pH combi-
                     nation within the uncertainty window
                     was found for which each of mobile
                     metals was dissolved up to concentra-
                     tions sufficient to exceed health-based
                     thresholds at the hypothetical down-
                     gradient exposure point. The relatively
                     immobile group included  chromium,
                     mercury, selenium and silver.  These
                     four metals had  limited  solubilities at
                     all  Eh/pH combinations within the un-
                     certainty window. After allowing for
                     dispersion, predicted concentrations at
                     the down-gradient exposure point
                     were below the drinking water stand-
                     ards. Chromium, mercury and selenium
                     were, however, more soluble under
                     conditions more oxic than those re-
                     flected by the uncertainty window. Ad-
                                                             ditional sensitivity tests were per-
                                                             formed to establish the Eh range above
                                                             which the respective solubilities were
                                                             predicted to increase significantly. The
                                                             order of dissolution with increasing Eh
                                                             was selenium > mercury > > chromium.
                                                               This Project Summary was devel-
                                                             oped by EPA's Environmental Research
                                                             Laboratory, Athens, GA, to announce
                                                             key findings of the research project that
                                                             is fully documented in a separate report
                                                             of the same title (see Project Report
                                                             ordering information at back).

                                                             Background
                                                               The U.S. Environmental  Protection
                                                             Agency often has the responsibility for
                                                             dealing with pollutants in situations
                                                             where the specific  environment avail-
                                                             able for interaction with the pollutant at
                                                             each individual site is unknown. In these
                                                             cases, the decision process must in-
                                                             volve a generic component. The full re-
                                                             port outlines a generic approach for
                                                             evaluating the speciation and transport
                                                             of metals in ground-water environ-
                                                             ments. The primary emphasis  was on
                                                             the simultaneous solution of metal spe-
                                                             ciation equations that determine dis-
                                                             solved phase metals concentrations.
                                                               Speciation and transport of  arsenic,
                                                             barium, cadmium, chromium, lead,
                                                             mercury, nickel, selenium, silver, and
                                                             thallium in ground  waters are consid-
                                                             ered in an  example setting designed to
                                                             mimic the behavior of the leachate from
                                                             a failed land disposal site. The proce-
                                                             dures used are designed to be compat-
                                                             ible with recently proposed methods for
                                                             treating organic leachates. Expected
                                                             variations in site-specific chemical back-
                                                             grounds relative  to the presumed

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generic ground-water  chemical envi-
ronment were accommodated using an
Eh/pH uncertainty window approach.
The uncertainty window was developed
from statistical data  on existing wells
around the country.
  Potential transport of equilibrium-
dissolved-phase metal species resulting
from solution of the speciation equi-
libria are  estimated  using a back-
calculation screening procedure. Sim-
ply stated, the goal of the  screening
procedure was to establish health-
based concentration thresholds for dis-
solved metal  species that would be ap-
plicable to extracts of  raw waste and
that were assured of being protective of
human health and the environment at a
down-gradient exposure point (possible
drinking water source). Health-based
concentration thresholds (CAoi) applica-
ble to the point of exposure served  as
the starting point. These  were appor-
tioned  such  that exposure to other
sources (surface water, air) were taken
into account.  Elaboration of the detailed
conceptual approach used in establish-
ing the CAD|'s was outlined in the Fed-
eral Register, 40 CFR, Part 260, January
14, 1986. The CADI'S were used  in con-
cert with  a  steady-state advective-
dispersion model and the metal specia-
tion  model (MINTEQ) results  to
back-calculation  allowable concentra-
tions in leachates from  the landfill.
Leachate concentrations were assumed
to be mimicked by pollutant concentra-
tions observed in extracts of the raw
waste material.
  The primary emphasis  of this work
was on evaluating the  impact of metal
speciation equilibria on  transport of
metals in a generically defined ground-
water environment.

Methods
  A schematic  representation of the
plume from a failed land disposal unit is
shown  in  Figure 1.  Metals  in the  dis-
posal unit leachates are  presumed to
equilibrate geochemically with the satu-
rated zone solution  instantaneously at
the point of  contact. Dissolved metals
concentrations in the resulting  mixture
would be subject to reduction by precip-
itation of solid phases as well as dilution
and dispersion.  Dilution due to disper-
sion  was  estimated using the model
 represented  in Equation 1 in conjunc-
tion with a Gaussian  boundary condi-
tion. This model also was used to esti-
 mate transport  of organic leachates,
and it  was  designed to account  for
                    Land Disposal
                        Unit
                                                      Down Gradient
                                                           Well
             Saturated
               Zone
                                    o» • • • Ground Water
                                                                A  I
                              Cross Section
                                   Plume Boundary
                              Plan View


Figure 1.    Schematic representation of plume from a failed waste disposal unit.
chemical-specific decay rates and veloc-
ity retardation due to sorption consis-
tent with several assumptions regard-
ing chemical behavior of the wastes and
characteristics of the transport medium.


Dxx^ + DVV 3y^+ Dzz a? ~*~~ =
                                       where:
                                               x,y,z = spatial coordinates in
                                                      the longitudinal,  lat-
                                                      eral, and vertical direc-
                                                      tions, respectively (m)

                                                  c = dissolved concentra-
                                                      tion of chemical (mg/l)
                                           x, Dyy, DH = retarded dispersion co-
                                                      efficients  in the x, y,

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               and z direction, respec-
               tively (m2/yr)
           V = groundwater seepage
               velocity assumed to be
               in the x direction  (m/
               YD
           Rf = retardation factor  (di-
               mensionless)
            t = elapsed time (yr)
           X = effective first order de-
               cay constant (yr~1)
            I = dilution rate due to net
               recharge (yr"1)
  The retardation factor, Rf, and the ef-
fective decay constant, X, are defined as
follows:
R, =
                                 (2)
and,
          x = -
                   X2pbKd
                  !-PbKd
where:

  Pb=bulk density  of the porous
      medium (g/cm3)
  Kd = distribution coefficient (cm3/g)
   9 = volumetric water content (cm3/
      cm3)
  XT = decay constant  for dissolved
      phase (yr1)
  X2 = decay constant for sorbed phase
      (yr1)
  Because the behaviors of individual
waste constituents are highly depend-
ent on the chemical properties of the
compounds as well as on the input vari-
ables  relating to  the transport environ-
ment, selection  of "reasonable worst
case" values would  involve consider-
able uncertainty. To avoid this ambiguity,
a Fortran computer code, EPASMOD-P,
was developed for the dispersion model
that allowed monte carlo simulation of
the input parameter distributions. Back-
calculated  concentration values were
determined for a large number  of ran-
domly selected combinations of the input
variables (typically 5000). Results could
then  be  expressed as distributions of
back-calculated dispersion factors keyed
to relative  probabilities  of occurrence.
These dilution factors combined with
results of the metal speciation modeling
procedures described herein formed
the basis for back-calculating allowable
leachate concentrations. The dispersion
model is described in more detail in the
Federal Register, CFR 40, Part 260, Jan-
uary 14, 1986.
  Two additional constraints on the dis-
persion model  were employed in apply-
ing it to  the transport of metals. The
degradation parameter was set to zero
(X = 0) in accordance with the assump-
tion that metals are conservative. This
also negated the consideration of sorp-
tion for metals because the retardation
effect would not change the concentra-
tion of a conservative pollutant ulti-
mately  arriving at the down-gradient
exposure point.
  The geochemical model MINTEQ is a
third generation equilibrium model that
uses the equilibrium constant approach
to solving the chemical equilibrium
problem. Its use in this work required
several modifications and additions.
Thermodynamic data for chromium,
mercury, thallium, and selenium were
not present in the original data base and
had to be added. Also, because the orig-
inal model would not handle more than
20  input components, the code was
modified for matrix expansion up to 50
components.  Numeric underflows,
which caused the program to abort oc-
casionally when individual component
concentrations  became very low
(=10~30 mol/1), were eliminated  by
modifying the  program to set the con-
centration to  a lower limit of 10 30
whenever this level was reached.  This
allowed the executions to continue.
  Because chemical properties of
ground waters underlying existing and
future waste disposal sites were  not
known, generic specifications were sub-
stituted. These were derived from large
volumes of well data extracted from the
EPA STORET data retrieval system. Be-
cause of the undue influence of extreme
values on the  means, the medians for
each variable were selected for  use.
These are tabulated in Table 1. All speci-
ation model runs were performed in the
chemical environment indicated,  with
the exception of the Eh and pH.
  Clearly, a single set of generic chemi-
cal  specifications could not be  derived
that would accurately reflect conditions
existing at all waste sites. To accommo-
date uncertainties associated with the
generic  specifications, an uncertainty
window encompassing several combi-
nations  of Eh  and pH values was de-
fined. The window spanned the range of
Eh and pH variations one standard devi-
ation (a) either side of the median
                                       Table 1.    Median Ground- Water Chemical Specifications
                                                                             Analytical Concentrations (mg/l)
Component
Arsenic
Barium
Cadmium
Chromium
Lead
Mercury
Selenium
Silver
Thallium
Mean
.021
.235
.030
.044
.035
.0006
.007
.019
.068
Stand. Dev.
.174
.251
.207
.121
.078
.0008
.008
.030
.037
Median
.010
.200
.005
.200
.010
.0005
.005
.010
.010
                                       Aluminum
                                       Calcium
                                       Iron
                                       Magnesium
                                       Manganese
                                       Potassium
                                       Sodium

                                       Bicarbonate
                                       Bromide
                                       Carbonate
                                       Chloride
                                       Nitrate
                                       Phosphate
                                       Sulfate
                                       Sulfide

                                       Organic carbon
                                                          1.134
                                                         79.000
                                                          2.780
                                                         51.240
                                                           .242
                                                          5.670
                                                        182.610

                                                        189.730
                                                          3.400
                                                         22.300
                                                        161.470
                                                          2.660
                                                          1.450
                                                        153.810
                                                           .471

                                                         12.762
                                                2.620
                                              118.478
                                                7.365
                                              134.840
                                                 .615
                                               11.390
                                              642.780

                                              111.000
                                               12.800
                                               30.037
                                              778.530
                                                5.100
                                                5.210
                                              347.350
                                                1.334

                                               15.603
                               .200
                             48.000
                               .200
                             14.000
                               .040
                              2.900
                             22.000

                            190.000
                               .300
                               0
                             15.000
                              1.000
                               .090
                             25.000
                               .200

                              7.2
                                       Temperature (°C)
                                       pH
                                       Eh (mv)
                                                         14.403
                                                          6.621
                                                        -10.535
                                                5.287
                                                1.285
                                              155.786
                             14
                              6.8
                            -50

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values obtained from the STORET data.
The resulting window is illustrated by
the small rectangle in Figure 2. Shifts in
metal speciation equilibria possible
within the window were mapped by ini-
tiating MINTEQ  model calculations
using nine different Eh/pH combinations
selected from within the window. The
nine combinations were formed by
combining each of three Eh values with
each of three pH values. The three val-
ues of each variable consisted  of the
median value, the median value plus 
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 for chromium were at low pH (5.5) and
 high Eh (106 mv) and the solubility was
 limited (0.08 mg/l) by formation of the
 Cr2O3 solid. Additional tests at Eh levels
 above the Eh window (Figure  4) indi-
 cated  dissolved chromium concentra-
 tions increased substantially above 700
 mv. Values this high are  however, ap-

                    5.5
parently, unlikely to be based upon the
STORE! data base.
  Predicted dissolved mercury levels
were limited to 0.024 mg/l by the forma-
tion of Hg°(l) at the highest Eh available
from the uncertainty window. The tran-
sition to Hg(ll)  ion  began to occur at
about  300  mv  (Figure 5). Above this
                                                                8.1
                                                                      106 mv
ffe







1
Eh= 106 mv
pH - 5.5
IV
Eh = -50 mv
pH = 5.5
VII
Eh = -206 mv
pH = 5.5

II
Eh= 106 mv
pH = 6.8
V
Eh = -50 mv
pH = 6.8
1
1
VIII
Eh - -206 mv
pH = 6.8
1
1
III
Eh= 106 mv
PH = a.i
VI
Eh = -50 mv
pH = 8.1
IX
Eh = -206 mv
pH = 8.1

                                                                     -206 mv
             where:  £nm = median £h = -50 mv
                   pHm = median pH- 6.8
                     0E = standard deviation of Eh= 156 mv
                     
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Table 3. Worst Case MINTEQ Output Concentrations Compared to Total Metal Input tainty window. Considering the wide
Concentrations (mg/l)
pH,Eh
mv

Arsenic 8.1, 106


Barium 6.8, -206

Cadmium 6.8, 706

Chromium 5.5, 106

Lead 5.5, 706

Mercury 8.1, 106

Nickel 5.5, 106

Selenium 8.1, 106

Silver 6.8, -206
(Undeter-
minant)
Thallium 6.8, -026

DWS + back
Input/
/Output
0.060
3.58 x /O'7

7.70
7.70
0.070
0.070
0.140
0.00513
0.030
1.25 x 70~6
0.0035
0.0078
0.760
0.760
0.050
0.046
0.700
~9x 70~7

0.059
0.059

25 DWS
Input/
/Output
1.25
1.18

37.5
37.5
0.125
0.120
3.00
0.079
0.500
0.398
0.0750
0.0243
3.750
3.748
1.13
0.0808
2.25
=9x70~7

0.480
0.480
variability of reported groundwater Et,
50 DWS 700 DWS 10,000 DWS values, the ambiguity of interpreting E*
Input/ Input/ Input' data and the strong evidence for lack of
/Output /Output /Output redox equilibrium in many ground-
250 5.00 500 water systems, the expected mobilities
2.43 4.93 $00 °f these three metals remain very un-
certain. Better understanding of
75.0 750 75,000 ground-water redox processes is a criti-
75.0 750 13,967 cal need for future progress in predict-
ing subsurface geochemical phenom-
0.250 0.500 50.0 ena
0.250 0.500 50.0
6.00 72.00 7,200.0
0.079 0.079 0.079
7.00 2.00 200
0.887 7.67 799.7
0.7500 0.300 30.0
0.0243 0.0243 0.0243
7.500 15.00 1,500
7.497 14.97 1,500
2.25 4.50 450
0.0808 0.0808 0.0805
4.50 9.00 900
=9x70~7 =9x70~7 =9x70~7

0.960 7.92 792
0.950 7.29 81.3
Potential Chromium Mobility

3-
E 2.5-
.3
0
h*
•c 2 -
1
"5
8 1.5-
<3
-••.
1 ;-
0.5-
l











I 	 B 	
o 4 	 r —
-200
pH = 5.5, 25 x DWS. Median Background











— B 	 B —
I 1
0











	 B-
200
p










	 B — B— B— B-BD
1











400 600 500
                                   Eh, Measured in mv
                                D     In (mg/l)

Figure 4.     Sensitivity of dissolved chromium concentration to Eh at pH = 5.5.

                                      6

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    0.08
    0.07-
 I
 ta  0.06-
 I
 "5
 .8
 2  0.05
 o
    0.04-
    0.03-
                                   Potential Mercury Mobility
                             pH = 8.1, 25 x DWS. Median Background
                           \        I        i
                          200               400

                                  Eh, Measured in mv
                               D   In (mg/U
                                  600
                                                   800
figure 5.    Sensitivity of dissolved mercury concentration to E h atpH=8.1.
     1.00
     0.00-
.1  -1.00-
c
1
    -2.00-
    -3.00-
 
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     The EPA authors D. S. Brown (also the EPA Project Officer, see below), R. E.
       Carlton, and L. A. Mulkey are with the Environmental Research Laboratory,
       Athens, GA 30613.
     The complete report, entitled "Development of Land Disposal Decisions for Metals
       Using MINTEQ Sensitivity Analyses," (Order No. PB 86-233 186/AS; Cost:
       $11.95, 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:
            Environmental Research Laboratory
            U.S. Environmental Protection Agency
            Athens, GA 30613
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S3-86/030
                                                                                        '"- ?3^i:: n  fi •> -
                                                                                            " - *.* ,-•> f.  -
          0000329   PS
          U S  ENVIR  PROTECTION  AGENCY
          REGION 5 LIBRARY
          230  S  DEARBORN  STREET
          CHICAGO               IL   60604

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