Seepage/Backfill Interactions Rev 00, ICN 00

ANL-EBS-MD-000039

April 2000

1. PURPOSE 
As directed by written development plan (CRWMS M&O 1999a), a sub-model of 
seepage/backfill interactions is developed and presented in this document to support the 
Engineered Barrier System (EBS) Physical and Chemical Environment Model. The purpose of 
this analysis is to assist Performance Assessment Operations (PAO) and the Engineered Barrier 
Performance Department in modeling the geochemical environment within a repository drift. In 
this analysis, a conceptual model is developed to provide PAO a more detailed and complete indrift 
geochemical model abstraction and to answer the key technical issues (KTI) raised in the 
NRC Issue Resolution Status Report (IRSR) for the Evolution of the Near Field Environment 
(NFE) Revision 2 (NRC 1999). 
The development plan calls for a sub-model that evaluates the effect on water chemistry of 
chemical reactions between water that enters the drift and backfill materials in the drift. The 
development plan specifically requests an evaluation of the following important chemical 
reaction processes: dissolution-precipitation, aqueous complexation, and oxidation-reduction. 
The development plan also requests the evaluation of the effects of varying seepage and drainage 
fluxes, varying temperature, and varying evaporation and condensation fluxes. Many of these 
effects are evaluated in a separate Analysis/Model Report (AMR), Precipitates Salts Analysis 
AMR (CRWMS M&O 2000), so the results of that AMR are referenced throughout this AMR. 
2. QUALITY ASSURANCE 
The Quality Assurance (QA) program applies to the development of this analysis. The 
Performance Assessment Operations responsible manager has evaluated the technical document 
development activity in accordance with QAP-2-0, Conduct of Activities. The QAP-2-0 activity 
evaluation, Conduct of Performance Assessment (CRWMS M&O 1999b), has determined that 
the preparation and review of this technical document is subject to Quality Assurance 
Requirements and Description (QARD) DOE/RW-0333P (DOE 2000) requirements. 
Preparation of this analysis did not require the classification of items in accordance with QAP-2- 
3, Classification of Permanent Items. This activity is not a field activity. Therefore, an 
evaluation in accordance with NLP-2-0, Determination of Importance Evaluations was not 
required. 
3. COMPUTER SOFTWARE AND MODEL USAGE 
3.1 COMPUTER SOFTWARE 
The software used in this analysis include: 
 Microsoft Excel 97, a commercially available spreadsheet software package. This 
software was used to tabulate and chart results. No calculations or data manipulations 
were performed in the spreadsheets. Validation of the Excel spreadsheet application was 
done by comparing the input data (Table 3 in Section 6.4) to the output (Figure 1 and 
Figure 2, which are graphic representations of the input data). Visual inspection 
confirms that the spreadsheet application provided correct results.

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The EQ3/6 code is discussed in Section 4.1.2 but not used in this AMR. Only EQ3/6 results 
from the Precipitates/Salts AMR (CRWMS M&O 2000) are used. 
3.2 MODELS 
The conceptual model used for this analysis and its validation is documented in Precipitates 
Salts Analysis AMR (CRWMS M&O 2000). The conceptual model documented in Precipitates 
Salts Analysis AMR (CRWMS M&O 2000) is considered appropriate and valid for this analysis 
because the model is applied to a specific case of the possible dissolution of quartz backfill in 
that system with the understanding that quartz stability is already quantified in the model 
calculations. 
4. INPUTS 
4.1 DATA AND PARAMETERS 
The Seepage/Backfill Interactions analysis uses the following input: 1) the composition of 
backfill and 2) a set of data developed in Precipitates Salts Analysis AMR (CRWMS M&O 
2000). These are described below. 
4.1.1 Backfill 
The EBS design backfill is composed of more than 99 percent quartz sand. The non-quartz 
fraction has predominantly feldspar mineralogy, and the dry backfill material contains less than 
0.1 percent combined halides and less than 0.1 percent organic material (CRWMS M&O 1999c). 
Current specifications are displayed in Table 1. These input data remain to be verified (TBV). 
Table 1. Backfill Composition 
Property Value 
Quartz (SiO2) content (by 
mass) 
> 99 percent 
Combined total content of 
Al2O3, Na2O, K2O, and CaO 
(by mass) 
< 1 percent 
(CRWMS M&O 1999c) 
4.1.2 Quartz Saturation Indices for Various Backfill Environment Conditions 
The quartz saturation index data used in the current AMR are obtained from a set of EQ3/6 
input/output files (DTN: MO0003MWDTAB45.013) that was developed in Precipitates Salts 
Analysis AMR (CRWMS M&O 2000). These data remain to be verified (TBV). From these files, 
the saturation indices (defined in Section 6.3) for quartz were obtained from the steady state 
results of every combination of temperature (T), fugacity of carbon dioxide (fCO2), and relative 
evaporation rate (Res, defined below) simulated. These cases included the following parameter 
values: T varied between four values (95C, 75C, 45C, and 25C), fCO2 varied between 10-1, 
10-3, and 10-6, and Res varied between 0, 0.1, 0.5, 0.9, 0.99, and 0.999.

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Only quartz saturation indices were retrieved because the backfill is greater than 99 percent 
quartz. The saturation indices of other silica minerals that might precipitate were not obtained 
because precipitation is not a seepage/backfill interaction. Precipitation is addressed in the 
Precipitates/Salts AMR (CRWMS M&O 2000). According to the development plan, the scope 
of the Seepage/Backfill Interactions AMR is limited to effects on water chemistry caused by 
seepage/backfill interactions. 
The relative evaporation rate (or flux) (Res) refers to the steady state evaporation flux (Qe) 
divided by (i.e., relative to) the incoming seepage rate (or flux) (Qs). Res, which may apply to a 
specific location and time, may be expressed as: 
s
e 
es 
Q
Q 
R = (Eq. 1) 
Representative water from well J-13 was used as the composition of the incoming seepage in the 
data obtained from CRWMS M&O (2000). 
The software used to generate the results referenced in this AMR was EQ3/6 v7.2b [CSCI: 
URCL-MA-110662 V7.2b, Wolery 1992a and 1992b, Wolery and Daveler 1992] with the solidcentered 
flow-through addendum [CSCI: URCL-MA-110662 V7.2b, MI: 30084-M04-001 
(Addendum Only) (CRWMS M&O 1998)]. This software was obtained from Configuration 
Management for Precipitates Salts Analysis AMR (CRWMS M&O 2000) and installed on an 
IBM-compatible computer. It was appropriate for the application in that AMR and was used only 
within the range of validation in accordance with AP-SI.1Q, Software Management. 
The thermodynamic database used to generate the referenced results is the PT4 database 
developed in the In-Drift Precipitate/Salts AMR (CRWMS M&O 2000). This is a Pitzer 
database that allows calculation of equilibrium chemistry at high ionic strength, but it is not 
direct input for the current AMR. It is cited here for information purposes only. For more 
information on its development and use, refer to CRWMS M&O (2000). 
4.2 CRITERIA 
Section 4.2.1 provides a summary of the applicable NRC review and acceptance criteria outlined 
in the Issue Resolution Status Report (IRSR) that apply to model development for the following 
NFE KTI sub-issue effects: (a) coupled thermal-hydrologic-chemical processes on the waste 
package chemical environment, (b) coupled thermal-hydrologic-chemical processes on the 
chemical environment for radionuclide release, and (c) coupled thermal-hydrologic-chemical 
processes on radionuclide transport through engineered and natural barriers (NRC 1999). A 
listing of the project features, events, and processes (FEPs) that potentially apply to this report is 
provided in Section 4.2.2. 
4.2.1 NRC IRSR Criteria 
The wording for the IRSR criteria listed below is generally taken from Section 4.3.1 of NRC 
(1999).

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4.2.1.1 Data and Model Justification Acceptance Criteria 
1. Consider both temporal and spatial variations in conditions affecting coupled 
thermohydrological-chemical (THC) effects on the chemical environment for 
radionuclide release. [NRC (1999), Sections 4.1.1, 4.2.1, 4.3.1, and 4.4.1] 
2. Consider site characteristics in establishing initial and boundary conditions for 
conceptual models and simulations of coupled processes that may affect the chemical 
environment for radionuclide release. [NRC (1999), Sections 4.1.1, 4.2.1, 4.3.1, and 
4.4.1] 
3. Collect sufficient data on the characteristics of the natural system and engineered 
materials, such as the type, quantity, and reactivity of material, in establishing initial 
and boundary conditions for conceptual models and simulations of THC coupled 
processes that affect the chemical environment for radionuclide release. [NRC (1999), 
Sections 4.1.1, 4.2.1, 4.3.1, and 4.4.1] 
4. Use sensitivity and uncertainty analyses (including consideration of alternative 
conceptual models) to determine whether additional new data are needed to better 
define ranges of input parameters. [NRC (1999), Sections 4.1.1, 4.2.1, 4.3.1, and 4.4.1] 
5. If the testing program for coupled THC processes on the chemical environment for 
radionuclide release from the engineered barrier system is not complete at the time of 
license application, or if sensitivity and uncertainty analysis indicate the additional data 
are needed, identify specific plans to acquire the necessary information as part of the 
performance confirmation program. [NRC (1999), Sections 4.1.1, 4.2.1, 4.3.1, and 
4.4.1] 
4.2.1.2 Data Uncertainty and Verification Acceptance Criteria 
1. Use reasonable or conservative ranges of parameters or functional relations to 
determine effects of coupled THC processes on the chemical environment for 
radionuclide release. Parameter values, assumed ranges, probability distributions, and 
bounding assumptions must be technically defensible and reasonably account for 
uncertainties. [NRC (1999), Sections 4.1.1, 4.2.1, 4.3.1, and 4.4.1] 
2. Consider uncertainty in data due to both temporal and spatial variations in conditions 
affecting coupled THC effects on the chemical environment for radionuclide release. 
[NRC (1999), Sections 4.1.1, 4.2.1, 4.3.1, and 4.4.1] 
3. Properly consider in the evaluation of coupled THC processes the uncertainties in the 
characteristics of the natural system and engineered materials, such as the type, 
quantity, and reactivity of material, in establishing initial and boundary conditions for 
conceptual models and simulations of THC coupled processes that affect the chemical 
environment for radionuclide release. [NRC (1999), Sections 4.1.1, 4.2.1, 4.3.1, and 
4.4.1]

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4. Use available data to ensure consistency in the initial conditions, boundary conditions, 
and computational domain used in sensitivity analysis involving coupled THC effects 
on the chemical environment for radionuclide release. [NRC (1999), Sections 4.1.1, 
4.2.1, 4.3.1, and 4.4.1] 
5. Assess in the performance confirmation program whether the natural system and 
engineered materials are functioning as intended and anticipated with regard to coupled 
THC effects on the chemical environment for radionuclide release from the EBS. [NRC 
(1999), Sections 4.1.1, 4.2.1, 4.3.1, and 4.4.1] 
4.2.1.3 Model Uncertainty Acceptance Criteria 
1. Use appropriate models, tests, and analyses that are sensitive to the THC couplings 
under consideration for both natural and engineering systems as described in the 
following examples. The effects of THC coupled processes that may occur in the 
natural setting or due to interactions with engineered materials or their alteration 
products include (i) thermohydrologic (TH) effects on gas and water chemistry; (ii) 
hydrothermally driven geochemical reactions, such as zeolitization of volcanic glass; 
(iii) dehydration of hydrous phases liberating moisture; (iv) effects of microbial 
processes; and (v) changes in water chemistry that may result from interactions 
between cementitious, or WP, materials and groundwater, which, in turn, may affect the 
chemical environment for radionuclide release. [NRC (1999), Sections 4.1.1, 4.2.1, 
4.3.1, and 4.4.1] 
2. Investigate alternative modeling approaches consistent with available data and current 
scientific understanding, and appropriately consider their results and limitations. [NRC 
(1999), Sections 4.1.1, 4.2.1, 4.3.1, and 4.4.1] 
3. Provide a reasonable description of the mathematical models included in its analyses of 
coupled THC effects on the chemical environment for radionuclide release. The 
description should include a discussion of alternative modeling approaches not 
considered in its final analysis and the limitations and uncertainties of the chosen 
model. [NRC (1999), Sections 4.1.1, 4.2.1, 4.3.1, and 4.4.1] 
4.2.1.4 Model Verification Acceptance Criteria 
1. The mathematical models for coupled THC effects on the chemical environment for 
radionuclide release must be consistent with conceptual models based on inferences 
about the near-field environment, field data and natural alteration observed at the site, 
and expected engineered materials. [NRC (1999), Sections 4.1.1, 4.2.1, 4.3.1, and 4.4.1] 
2. Appropriately adopt accepted and well-documented procedures to construct and test the 
numerical models used to simulate coupled THC effects on the chemical environment 
for radionuclide release. [NRC (1999), Sections 4.1.1, 4.2.1, 4.3.1, and 4.4.1] 
3. Abstracted models for coupled THC effects on the chemical environment for 
radionuclide release must be based on the same assumptions and approximations shown 
to be appropriate for closely analogous natural or experimental systems. Abstracted

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model results are verified through comparison to outputs of detailed process models and 
empirical observations. Abstracted model results are compared with different 
mathematical models to judge robustness of results. [NRC (1999), Sections 4.1.1, 4.2.1, 
4.3.1, and 4.4.1] 
4.2.2 FEPs 
Table 2 lists the Yucca Mountain Project FEPs (CRWMS M&O 1999d) that are addressed in this 
analysis. Any resolution of these FEPs is discussed in Section 7.4. 
4.3 CODES AND STANDARDS 
4.3.1 Codes 
This AMR was prepared to comply with the DOE interim guidance (Dyer 1999) which directs 
the use of specified Subparts/Sections of the proposed NRC high-level waste rule, 10 CFR Part 
63. Relevant requirements for performance assessment from Section 114 of that document are: 
"Any performance assessment used to demonstrate compliance with Sec. 113(b) shall: (a) 
Include data related to the geology, hydrology, and geochemistry ... used to define parameters 
and conceptual models used in the assessment. (b) Account for uncertainties and variabilities in 
parameter values and provide the technical basis for parameter ranges, probability distributions, 
or bounding values used in the performance assessment. ... (g) Provide the technical basis for 
models used in the performance assessment such as comparisons made with outputs of detailed 
process-level models ... ." 
4.3.2 Standards 
ASTM C 1174-97, Standard Practice for Prediction of the Long-Term Behavior of Materials, 
Including Waste Forms, Used in Engineered Barrier Systems (EBS) for Geological Disposal of 
High-Level Radioactive Waste, was used as guidance in the preparation of this model.

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Table 2. FEPs Potentially Applicable to the Seepage/Backfill Interactions Analysis 
YMP FEP 
Number 
NEA 
Category FEP Name 
2.1.04.01.00 2.1.04u Preferential pathways in the backfill 
2.1.04.02.00 2.1.04au Physical and chemical properties of backfill 
2.1.04.02.09 2.1.09ay Water chemistry, tunnel backfill 
2.1.04.03.00 2.1.04r Erosion or dissolution of backfill 
2.1.04.04.00 2.1.04az Mechanical effects of backfill 
2.1.04.04.00 2.1.07w Mechanical impact/failure, buffer/backfill 
2.1.04.05.00 2.1.04b Backfill evolution 
2.1.04.06.00 2.1.11g Small pieces of backfill undergo phase 
changes when heated and welded together 
2.1.04.08.00 2.1.04t Diffusion in backfill 
2.1.04.09.00 3.2.07r Radionuclide transport through backfill 
2.1.09.05.04 3.2.03s Sorption on filling material 
2.2.10.06.07 2.1.09j Precipitates from dissolved constituents of tuff 
and repository materials form by evaporation 
during thermal period 
ACC: MOL.19991214.0518; MOL.19991214.0519 
5. ASSUMPTIONS 
5.1 PROCESSES MODELED 
As explained in the introduction to Section 6, the only seepage/backfill interaction evaluated in 
this AMR is the potential dissolution of quartz backfill. 
5.2 GENERAL ASSUMPTIONS 
5.2.1 Standard State of Water or Brine 
Water in this analysis is assumed to be standard state. That is, the properties of the water are 
assumed to be unaffected by interface curvature or other effects resulting from contact with solid 
surfaces. For a more detailed explanation, refer to Precipitates Salts Analysis AMR (CRWMS 
M&O 2000, Section 5.2.1). This assumption, used throughout, does not affect the uncertainty in 
the model and is therefore not designated TBV. 
5.2.2 Redox Conditions 
The redox conditions within the drift are expected to be oxidizing at all times except possibly for 
the first several hundred years after the waste packages would be covered by the backfill. 
During these early times, it is possible that reducing conditions could occur. However, reducing 
conditions actually reduce corrosion rates for the drip shield and waste package container. In 
addition, they do not affect interactions with quartz backfill because silica is not sensitive to 
redox conditions. For these reasons, redox conditions are assumed to be oxidizing at all times.

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This assumption, used throughout, is reasonable and conservative for the scope of this AMR and 
is therefore not TBV. 
5.2.3 Equilibrium Conditions 
The model assumes equilibrium chemical conditions except for a few minerals that are not 
allowed to precipitate. Chemical reactions are assumed to occur rapidly compared to anticipated 
seepage and evaporation rates. The assumption of equilibrium conditions will not affect the 
uncertainty in the model and is therefore not designated TBV. This assumption is used 
throughout. 
Several slow-forming minerals in the database are not allowed to precipitate. The suppressed 
minerals include albite high, albite low, albite, corundum, cristobalite(alpha), cristobalite(beta), 
quartz, tridymite, diaspore, dolomite, hercynite, ice, illite, k-feldspar, maximum microcline, and 
spinel. For an explanation, see Section 5.4 of Precipitates Salts Analysis AMR (CRWMS M&O 
2000). These mineral suppressions are TBV in the referenced AMR. The only suppressed 
minerals that became supersaturated in the calculations were quartz, maximum microcline, Kfeldspar, 
albite, albite low, tridymite, diaspore, and dolomite. In a separate set of EQ3/6 
calculations documented in CRWMS M&O (2000), albite low was allowed to precipitate. 
However, this change affected the predicted concentrations by less than one percent for any of 
the aqueous components. 
6. MODEL 
As stated in Section 1, the development plan calls for an evaluation of the effects of 
seepage/backfill interactions on water chemistry resulting from the following important 
processes and parameters: 
 Dissolution-precipitation, 
 Aqueous complexation, 
 Oxidation-reduction, 
 Seepage and drainage fluxes, 
 Temperature variation, and 
 Evaporation and condensation fluxes. 
Most of these processes and parameters are incorporated in the development of the 
Precipitates/Salts Model documented in Precipitates Salts Analysis AMR (CRWMS M&O 2000). 
The model is used in that AMR to assess the effects of these processes and parameters (except 
for oxidation-reduction and reaction with backfill) on water chemistry within the drift. Thus, 
output from that AMR is used to assess the additional potential interaction with the backfill. In 
this way, dissolution, precipitation, aqueous complexation, and varying temperatures, seepage 
fluxes, and evaporation fluxes are evaluated in a coupled manner. Oxidation-reduction is 
addressed separately in Section 5.2.2. 
Seepage/backfill interaction under varying conditions is investigated by examining the quartz 
saturation indices (defined in Section 6.3) predicted by the EQ3/6 Pitzer model developed in

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Precipitates Salts Analysis AMR (CRWMS M&O 2000). The backfill is composed of more than 
99 percent quartz sand (Table 1). A potentially important seepage/backfill interaction that might 
significantly affect water chemistry in the backfill is the dissolution of this quartz. Quartz 
saturation indices (defined in Section 6.3) from the referenced data set indicate whether quartz 
dissolution is thermodynamically favored over a full range of possible temperature, seepage, and 
evaporation conditions and possible carbon dioxide fugacities within the backfill. 
Seepage interaction with the non-quartz fraction of the backfill is not evaluated in this AMR 
because of insufficient information on the quantity and mineralogy of this fraction. The nonquartz 
fraction is less than one percent of the backfill mineralogy (Table 1) and thus may not be 
abundant enough to significantly affect the water chemistry in the backfill such that performance 
of the EBS is affected. Nevertheless, an analysis of the seepage interaction with this small 
fraction should be performed when more precise information on the composition and quantities 
of this fraction becomes available. 
According to the development plan, the scope of the Seepage/Backfill Interactions AMR is 
limited to effects on water chemistry caused by seepage/backfill interactions. Potential 
seepage/backfill interactions other than dissolution of quartz backfill are not evaluated. These 
interactions include adsorption, hydrolysis of minerals such as feldspar in the non-quartz fraction 
of the backfill, and others. Before these additional interactions can be appropriately evaluated, 
additional characterization of the properties and composition of the backfill is needed. Effects 
on backfill permeability are outside of the defined scope. 
6.1 GEOCHEMISTRY OF SILICA 
6.1.1 Solubility of Silica Species 
The solubility of quartz and other silica minerals is affected by pH, temperature, pressure, and 
ionic strength (Dove and Rimstidt 1994, pp. 262-265). At 25C, 1 bar atmospheric pressure, and 
a pH between 1 and 9, the dominant aqueous silica species is SiO2 (aq) (also written SiO2H2O or 
H4SiO4) (Dove and Rimstidt 1994, p. 264; Langmuir 1997, p. 246; Drever 1988, p. 100). 
Above pH 9, the solubility of silica increases by several orders of magnitude as the soluble 
species H3SiO4
- and H2SiO4
2- become increasingly stable. In the pH range of approximately 9.82 
to 13.1, H3SiO4
- is the dominate silica species, followed by H2SiO4
2- at pH values greater than 
13.1 (Langmuir 1997, p. 246). For comparison, another source indicates that the pH values of 
these transitions are closer to 9.9 and 11.7, respectively (Drever 1988, p. 100). The differences 
are due to differences in the adopted dissociation constants at 25C. 
In addition to the uncertainty of the values of the dissociation constants, these constants are 
affected by changes in temperature and pressure. As illustrated in Krauskopf and Bird (1995, p. 
88) at pH values less than 9, a temperature increase from 25C to 100C causes the solubility of 
SiO2 (aq) to increase from about 0.002 molal to about 0.006 molal for a solution equilibrated 
with amorphous silica. This figure also shows that for a solution equilibrated with quartz, the 
same temperature increase causes the solubility of SiO2 (aq) to increase from about 0.0001 molal 
to about 0.0008 molal. At temperatures over 200C, solutions are in equilibrium with quartz 
(Dove and Rimstidt 1994, p. 267). Below this temperature, factors such as flow rate and 
precipitation rate are important and may cause sustained supersaturation with respect to quartz

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due to the slow formation of quartz relative to other silica minerals (Dove and Rimstidt 1994, pp. 
267-268; Rimstidt 1997, p. 490-491). Silica solubility is increased by Na+ and/or Cl- (Dove and 
Rimstidt 1994, p. 265). The effects of temperature and solution composition on solubility are 
accounted for by equilibrium geochemical models such as EQ3/6. These effects factor into the 
calculated saturation indices for each mineral phase (see section 6.4). 
6.1.2 Dissolution and Precipitation Rates 
Quartz dissolution and precipitation rates are controlled by temperature, solution composition 
(catalysis by alkali cations, ionic strength, trivalent metals) and pH (Dove and Rimstidt 1994, pp. 
278-287). An increase in temperature causes an increase in the dissolution rate. For example, a 
rate increase of three orders of magnitude occurs as the temperature increases from 25C to 
100C. An increase in dissolved salts also increases the dissolution rate. For relatively dilute 
solutions, dissolution rates increase by a factor of five to eight with the addition of sodium and 
potassium chloride for neutral pH and temperatures below 70C. The changes in dissolution 
rates are cation-dependent. In 0.09 to 1.0 molal NaCl, silica maintains its dissolution rate for a 
pH of 5 to 8 over a temperature range of 60C to 120C. However, dissolution rates increase by 
an order of magnitude as the solution reaches 4 molal NaCl (Dove and Rimstidt 1994, pp. 281- 
282). 
The precipitation or dissolution of lower order silica oxides, such as chalcedony, is not greatly 
inhibited by kinetic factors over a temperature range of 25C to 100C; however, the dissolution 
of quartz is very slow (Klein and Hurlbut 1999, p.571). At pH 5 and 25C, the predicted mean 
lifetime of a 1-millimeter cube of quartz is 24 million years (Langmuir 1997, p. 233). This 
prediction is based upon a dissolution rate of 10-13.39 mol/m2/s (Langmuir 1997, p. 233). As pH 
increases to 12, the rate remains low but may rise to approximately 10-10.5 mol/m2/s (Langmuir 
1997, p. 78). Thus, the lifetime of the same cube of quartz would decrease to about 35,000 
years. 
Quartz is a relatively stable mineral at low temperatures and pressures (Klein and Hurlbut 1999, 
pp. 526-532). It melts at over 1728C but may undergo polymorphic transitions above 573C at 
atmospheric pressure. Based upon estimates of repository conditions, quartz will not undergo 
solid-solid transitions to its polymorphs. 
6.2 CONCEPTUAL MODEL 
The conceptual model for the potential dissolution of quartz is essentially identical to the EQ3/6 
Pitzer model developed in Precipitates Salts Analysis AMR (CRWMS M&O 2000) except that 
the stability of quartz sand is monitored. Seepage/backfill interactions cannot be adequately 
investigated without incorporating the effects of evaporation on the composition of the water in 
contact with the backfill. The only difference in the current application is that the potential 
dissolution of the quartz backfill is investigated. 
Dissolution of the quartz backfill is investigated by examining the quartz saturation indices 
(defined in Section 6.3) for each of the cases simulated by the EQ3/6 Pitzer model. These cases 
include the set of all simulations for conditions of 85 percent relative humidity and higher (DTN: 
MO0003MWDTAB45.013). Lower relative humidity cases were ignored because the increased

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evaporation at lower relative humidity favors precipitation of silica and continued saturation or 
supersaturation of quartz (CRWMS M&O 2000). Although silica salts are predicted to form at 
relative humidity values below 85 percent, precipitation of these salts is not considered a 
seepage/backfill interaction (see Section 4.1.2). 
6.3 MODEL DESIGN AND IMPLEMENTATION 
For the design, implementation, and evaluation of the EQ3/6 Pitzer model used to investigate the 
potential for backfill dissolution, the reader is referred to Precipitates Salts Analysis AMR 
(CRWMS M&O 2000). Saturation indices for quartz were tabulated and evaluated with respect 
to backfill dissolution. 
The saturation index (SI) is defined as the logarithm of the ratio of the reaction quotient (or ion 
activity product) to the equilibrium constant (Langmuir 1997, p. 8). A zero value for SI indicates 
equilibrium with respect to the mineral. Strictly interpreted, a positive value indicates the water 
is supersaturated with respect to the mineral and a negative value indicates undersaturation. 
Supersaturation favors precipitation of the mineral and undersaturation favors dissolution. 
However, because there is some uncertainty in the equilibrium constants, the EQ3/6 code 
interprets an SI value between -0.2 and +0.2 as an indication of approximate saturation with 
respect to the mineral. 
6.4 MODEL RESULTS 
Based upon results of the Precipitates/Salts AMR (CRWMS M&O 2000), over 97% of the 
predictions for various conditions with J-13 incoming water have a quartz saturation index 
greater than zero. Part of this result stems from suppression of quartz and tridymite, which 
allows chalcedony to be the precipitating phase that controls silica solubility. The predictions are 
shown in Table 3, Figure 1, and Figure 2. In 84 simulations of J-13 water, only two results 
predicted a quartz saturation index less than zero. In these cases the quartz saturation indices 
were -0.092 and -0.052. In each case, the temperature was 95C and the fugacity of carbon 
dioxide was 10-6 which are consistent with predictions of conditions within the first several 
hundred years. In these two cases, the seepage rate was very high relative to the evaporation rate 
(Res of 0 and 0.1, respectively). However, these low evaporation rates relative to seepage rates 
are not likely to occur at early times. The likelihood of such conditions is predicted in other 
AMRs. Regardless, these quartz saturation indices are within the range that EQ3/6 associates 
with saturated conditions (see Section 6.3). 
These results indicate that none of the likely combination of conditions in the backfill will create 
a considerable tendency for quartz to dissolve. This observation is consistent with the fact that 
evaporation tends to favor silica precipitation, not dissolution. For the unlikely case of zero 
evaporation, incoming seepage water is expected to already be approximately saturated with 
respect to silica (as is the case for average J-13 well water (Table 1)), which will also inhibit 
quartz dissolution.

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Table 3. Quartz Saturation Indices by Case. 
Temperature 
(C) Res 
Quartz Saturation 
Index (fCO2 = 10-6) 
Quartz Saturation 
Index (fCO2 = 10-3) 
Quartz Saturation 
Index (fCO2 = 10-1) 
25 0a 0.271 0.271 0.271 
25 0.1 0.271 0.271 0.271 
25 0.5 0.271 0.271 0.271 
25 0.9 0.271 0.271 0.271 
25 0.99 0.271 0.271 0.271 
25 0.999 0.271 0.271 0.271 
25 >=1b 0.271 0.271 0.271 
45 0 0.191 0.254 0.254 
45 0.1 0.243 0.254 0.254 
45 0.5 0.254 0.254 0.254 
45 0.9 0.254 0.254 0.254 
45 0.99 0.254 0.254 0.254 
45 0.999 0.254 0.254 0.254 
45 >=1 0.254 0.254 0.254 
75 0 0.018 0.232 0.232 
75 0.1 0.055 0.232 0.232 
75 0.5 0.232 0.232 0.232 
75 0.9 0.232 0.232 0.232 
75 0.99 0.232 0.232 0.232 
75 0.999 0.232 0.232 0.232 
75 >=1 0.232 0.232 0.232 
95 0 -0.092 0.066 0.136 
95 0.1 -0.052 0.110 0.220 
95 0.5 0.160 0.220 0.220 
95 0.9 0.220 0.220 0.220 
95 0.99 0.220 0.220 0.220 
95 0.999 0.220 0.220 0.220 
95 >=1 0.220 0.220 0.220 
DTN: MO0003MWDTAB45.013 
a no evaporation but seepage occurs 
b evaporated to an ionic strength of 10 molal

Seepage/Backfill Interactions 
ANL-EBS-MD-000039 REV 00 18 April 2000 
-0.2 0.0 0.2 0.4 
25 
25 
25 
45 
45 
75 
75 
95 
95 
95 
Temperature (Celsius) 
Quartz Saturation Index 
fco2=1E-1 
fco2=1E-3 
fco2=1E-6 
"Saturated with respect to mineral" "Supersaturated" 
DTN: MO0003MWDTAB45.013 
Figure 1. Plot of Quartz Saturation Indices for Each Case 
0 
12 
24 
36 
48 
60 
72 
84 
<-1.00 to -0.20 >-0.20 to 0.00 >0.00 to 0.20 >0.20 to 0.28 
Quartz Saturation Index 
Number of Cases 
0% 
10% 
20% 
30% 
40% 
50% 
60% 
70% 
80% 
90% 
100% 
Percentage of Cases 
Frequency 
Cumulative % 
. DTN: MO0003MWDTAB45.013 
Figure 2. Histogram of Quartz Saturation Indices.

Seepage/Backfill Interactions 
ANL-EBS-MD-000039 REV 00 19 April 2000 
7. CONCLUSIONS 
This document and its conclusions may be affected by technical product input information that 
requires confirmation. Any changes to the document or its conclusions that may occur as a result 
of completing the confirmation activities will be reflected in subsequent revisions. The status of 
the input information quality may be confirmed by review of the Document Input Reference 
System database. 
7.1 MODELS AND RESULTS 
The results indicate that dissolution of quartz backfill by seepage derived from evaporated J-13 
well water is not likely to occur, and that if any dissolution does occur, it will likely be 
negligible. Dissolution of quartz, which makes up more than 99 percent of the backfill, is 
generally not favored except only weakly in the most unlikely scenarios. 
Two of the 84 cases investigated are shown to have quartz saturation indices below zero (Table 
3, Figure 1). The values of these indices are -0.092 and -0.052, which the code EQ3/6 generally 
considers to be saturated conditions, given the uncertainty in thermodynamic data. Moreover, 
these two cases are for conditions of high temperature (95C) and high seepage rate relative to 
the evaporation rate (a factor of 10 or more higher). Such a combination of conditions is 
unlikely. For these reasons, dissolution of the quartz backfill is not likely and will not likely 
affect water chemistry within the backfill. These results and conclusions are consistent with the 
fact that evaporation tends to favor mineral precipitation, not dissolution. 
7.2 UNCERTAINTY AND LIMITATIONS 
For the analysis of quartz backfill dissolution, the greatest uncertainties are likely the predicted 
inputs (e.g., the seepage rate and seepage composition) that feed the Precipitates/Salts analysis 
that generated the input used in this analysis. The analysis was limited to a conceptual model in 
which J-13 well water was the seepage. However, the conclusion that quartz dissolution is not 
favored, except perhaps in the most unlikely scenarios, is reasonable because evaporating water 
tends to favor mineral precipitation, not dissolution. For the unlikely case of zero evaporation, 
incoming seepage water is expected to already be approximately saturated with respect to silica 
(as is the case for average J-13 well water), which will also inhibit quartz dissolution. 
Potential seepage/backfill interactions other than dissolution of quartz are not evaluated in this 
AMR. These interactions include adsorption, hydrolysis of minerals such as feldspar in the nonquartz 
fraction of the backfill, and others. Before these interactions can be appropriately 
evaluated, additional characterization of the properties and composition of the backfill is needed. 
7.3 TBV IMPACT 
The EQ3/6 data were acquired from the results of EQ3/6 Pitzer model developed in Precipitates 
Salts Analysis AMR (CRWMS M&O 2000). These acquired data remain to be verified (TBV) 
until that AMR becomes qualified. Additional information on the TBV status and implications 
can be found in Precipitates Salts Analysis AMR. 
The other TBV is the composition of the backfill.

Seepage/Backfill Interactions 
ANL-EBS-MD-000039 REV 00 20 April 2000 
7.4 FEPs 
Table 4 lists the FEPs in Table 2 and the conclusions based on the Seepage/Backfill Interactions 
analysis. 
Table 4. FEPs Potentially Applicable to the Seepage/Backfill Interactions Analysis 
YMP FEP 
Number 
NEA 
Category 
FEP Name AMR Conclusions 
2.1.04.01.00 2.1.04u Preferential pathways in the 
backfill 
This FEP not addressed in this AMR. 
2.1.04.02.00 2.1.04au Physical and chemical 
properties of backfill 
Based on the results of this AMR and the Precipitates/Salts 
AMR (CRWMS M&O 2000), the water chemistry within the 
backfill will likely be primarily controlled by evaporative 
processes and not by interactions with backfill. Affects of 
backfill on water chemistry are limited. This AMR 
considered the potential for dissolution of quartz backfill 
and the potential effects on water chemistry. 
2.1.04.02.09 2.1.09ay Water chemistry, tunnel 
backfill 
The seepage composition considered in this AMR and the 
Precipitates/Salts AMR (CRWMS M&O 2000) is J-13 water. 
Water chemistry indicates saturation with respect to silica. 
These AMRs predict that backfill dissolution will likely be 
small or negligible. 
2.1.04.03.00 2.1.04r Erosion or dissolution of 
backfill 
Dissolution of quartz backfill will likely be small or 
negligible. Erosion was not addressed by this AMR. 
2.1.04.04.00 2.1.04az Mechanical effects of 
backfill 
This FEP is not addressed by this AMR. 
2.1.04.04.00 2.1.07w Mechanical impact/failure, 
buffer/backfill 
This FEP is not addressed by this AMR. 
2.1.04.05.00 2.1.04b Backfill evolution This FEP is not addressed by this AMR. 
2.1.04.06.00 2.1.11g Small pieces of backfill 
undergo phase changes 
when heated and welded 
together 
This FEP is not addressed by this AMR. 
2.1.04.08.00 2.1.04t Diffusion in backfill This FEP is not addressed by this AMR 
2.1.04.09.00 3.2.07r Radionuclide transport 
through backfill 
This FEP is not addressed by this AMR. 
2.1.09.05.04 3.2.03s Sorption on filling material This FEP is not addressed by this AMR. 
2.2.10.06.07 2.1.09j Precipitates from dissolved 
constituents of tuff and 
repository materials form by 
evaporation during thermal 
period 
This FEP is not addressed by this AMR. 
ACC: MOL.19991214.0518; MOL.19991214.0519 
8. INPUTS AND REFERENCES 
8.1 DOCUMENTS CITED 
CRWMS M&O (Civilian Radioactive Waste Management Services Management and 
Operations) 1998. Software Qualification Report (SQR) Addendum to Existing LLNL Document 
UCRL-MA-110662 PT IV: Implementation of a Solid-Centered Flow-Through Mode for EQ6

Seepage/Backfill Interactions 
ANL-EBS-MD-000039 REV 00 21 April 2000 
Version 7.2B. CSCI: UCRL-MA-110662 V 7.2b. SCR: LSCR198. Las Vegas, Nevada: CRWMS 
M&O. ACC: MOL.19990920.0169. 
CRWMS M&O 1999a. Provide Sub-Models for the Physical and Chemical Environmental 
Abstraction Model for TSPA-LA. TDP-WIS-MD-000006 REV 00. Las Vegas, Nevada: CRWMS 
M&O. ACC: MOL.19990902.0450. 
CRWMS M&O 1999b. Conduct of Performance Assessment. Activity Evaluation, September 30, 
1999. Las Vegas, Nevada: CRWMS M&O. ACC: MOL.19991028.0092. 
CRWMS M&O 1999c. Request for Repository Subsurface Design Information to Support TSPASR. 
Input Transmittal PA-SSR-99218.Ta. Las Vegas, Nevada: CRWMS M&O. ACC: 
MOL.19990901.0312. 
CRWMS M&O 1999d. YMP FEP Database Rev. 00C. Las Vegas, Nevada: CRWMS M&O. 
ACC: MOL.19991214.0518; MOL.19991214.0519. 
CRWMS M&O 2000. Precipitates Salts Analysis AMR. Input Transmittal 00097.T. Las Vegas, 
Nevada: CRWMS M&O. ACC: MOL.20000309.0497. 
DOE (U.S. Department of Energy) 2000. Quality Assurance Requirements and Description for 
the Civilian Radioactive Waste Management Program. DOE/RW-0333P REV 9. Washington, 
D.C.: U.S. Department of Energy. ACC: MOL.19991028.0012. 
Dove, P.M. and Rimstidt, J.D. 1994. "Silica-Water Interactions." Reviews in Mineralogy, 29, 
262-287. Washington, D.C.: Mineralogical Society of America. TIC: 246644. 
Drever, J.I. 1988. The Geochemistry of Natural Waters. 2nd Edition. Englewood Cliffs, New 
Jersey: Prentice-Hall. TIC: 242836. 
Dyer, J.R. 1999. "Revised Interim Guidance Pending Issuance of New U.S. Nuclear Regulatory 
Commission (NRC) Regulations (Revision 01, July 22, 1999), for Yucca Mountain, Nevada." 
Letter from Dr. J.R. Dyer (DOE/YMSCO) to Dr. D.R. Wilkins (CRWMS M&O), September 3, 
1999, OL&RC:SB-1714, with enclosure, "Interim Guidance Pending Issuance of New NRC 
Regulations for Yucca Mountain (Revision 01)." ACC: MOL.19990910.0079. 
Klein, C. and Hurlbut, C.S. 1999. Manual of Mineralogy. 21st Edition. New York, New York: 
John Wiley & Sons. TIC: 246258. 
Krauskopf, K.B. and Bird, D.K. 1995. Introduction to Geochemistry. 3rd Edition. New York, 
New York: McGraw-Hill. TIC: 239316. 
Langmuir, D. 1997. Aqueous Environmental Geochemistry. Upper Saddle River, New Jersey: 
Prentice Hall. TIC: 237107. 
NRC (U.S. Nuclear Regulatory Commission) 1999. Issue Resolution Status Report Key 
Technical Issue: Evolution of the Near-Field Environment. Rev. 2. Washington, D.C.: U.S. 
Nuclear Regulatory Commission. ACC: MOL.19990810.0640.

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ANL-EBS-MD-000039 REV 00 22 April 2000 
Rimstidt, J.D. 1997. "Gangue Mineral Transport and Deposition." Chapter 10 of Geochemistry of 
Hydrothermal Ore Deposits. 3rd Edition. Barnes, L.H., ed. Pages 487, 490-491. New York, New 
York: John Wiley & Sons. TIC: 246637. 
Wolery, T.J. 1992a. EQ3/6, A Software Package for Geochemical Modeling of Aqueous Systems. 
Package Overview and Installation Guide (Version 7.0). UCRL-MA-110662 PT I. Livermore, 
California: Lawrence Livermore National Laboratory. TIC: 205087. 
Wolery, T.J. 1992b. EQ3NR, A Computer Program for Geochemical Aqueous Speciation- 
Solubility Calculations. Theoretical Manual, Users Guide, and Related Documentation (Version 
7.0). UCRL-MA-110662 PT III. Livermore, California: Lawrence Livermore National 
Laboratory. TIC: 205154. 
Wolery, T.J. and Daveler, S.A. 1992. Theoretical Manual, User's Guide, and Related 
Documentation, Version 7.0. Volume IV of EQ6, A Computer Program for Reaction Path 
Modeling of Aqueous Geochemical Systems. UCRL-MA-110662. Draft 1.1. Livermore, 
California: Lawrence Livermore National Laboratory. TIC: 238011. 
8.2 DATA LISTED BY DATA TRACKING NUMBER 
MO0003MWDTAB45.013. EQ3/6 Input/Output Files for In-Drift Precipitates/Salts Lookup 
Tables. Submittal date: 03/06/2000. 
8.3 CODES, STANDARDS, REGULATIONS, AND PROCEDURES 
AP-SI.1Q, Rev. 2, ICN 4. Software Management. Washington, D.C.: U.S. Department of 
Energy, Office of Civilian Radioactive Waste Management. ACC: MOL.20000223.0508. 
ASTM C 1174-97. 1997. Standard Practice for Prediction of the Long-Term Behavior of 
Materials, Including Waste Forms, Used in Engineered Barrier Systems (EBS) for Geological 
Disposal of High-Level Radioactive Waste. West Conshohocken, Pennsylvania: American 
Society for Testing and Materials. TIC: 246015. 
NLP-2-0, Rev. 5. Determination of Importance Evaluations. Las Vegas, Nevada: CRWMS 
M&O. ACC: MOL.19981116.0120. 
QAP-2-0, Rev. 5. Conduct of Activities. Las Vegas, Nevada: CRWMS M&O. ACC: 
MOL.19980826.0209. 
QAP-2-3, Rev. 10. Classification of Permanent Items. Las Vegas, Nevada: CRWMS M&O. 
ACC: MOL.19990316.0006.