3 rd Response Submittal to the EPA Enclosure 1

1 EPA Comment 1-23-3 Region ROMPCS

The Agency agreed to the adopted par ameter values used of the panel cl osure change request to isolate the effects and facilitate a comparison of the propos ed panel closure design on the baseline PA and, at the time of the planned changed request, there was uncertai nty in the emplacement technique to be used. The Agency would like DOE to address the following comments on several parameters related to panel closures.

1. Identify and technically justify that ranges of porosity and permeability for the ROM salt PCS during the time period 0 to 100 years (material PCS_T1) are consistent with initial emplacement of the ROM salt without wetting or compaction. 2. Provide technical justification for applying a capillary pressure model that assumes zero threshold pressure to region ROMPCS during tim e periods 100 to 10,000 years (T2 and T3). 3. Provide a technical justification for selecti ng the ranges for the residual brine and gas saturations (SAT_RBRN and SAT_RGAS) during tim e periods T2 and T3; the justification should include adopting a zero value as the low end. 4. Provide technical justification for using the same value for the bulk compressibility of ROM salt during the T1, T2, and T3 time periods (from 0 to 100, 100 to 200, and 200 to 10,000 years).

DOE Response

The responses to EPA’s four comments are as follows:

1. The ranges of porosity and permeability assumed fo r the run-of-mine (ROM) salt panel closure system (ROMPCS) for use in the PCS-2012 Pe rformance Assessment (PA) were discussed in considerable detail in Camphouse et al. (2012a), as well as the exhaustive discussion of porositypermeability correlations in the U.S. Department of Energy’s (DOE’s) response to an EPA comment dated June 7, 2012, regarding the ROMPCS Planned Change Request (DOE 2012). These parameters were agreed to by EPA in an email from Tom Peake to Russ Patterson dated June 19, 2012. As discussed below, these va lues are consistent with the planned initial emplacement of the ROM salt, and DOE is not aw are of any additional information which would give reason to modify them.

The main component of the ROMPCS at the Waste Isolation Pilot Plant (WIPP) is 100 feet of ROM salt, which will be produced by ongoing mi ning operations, stored at convenient locations in the mine, and then installed in multiple lifts. Underground testing has demonstrated that frontend loaders and other existing equipment can push the ROM salt against the ribs and back of the panel access drifts, thereby filling the drift (Zimmerly et al. 2012).

DOE does not intend to establish criteria for the initia l condition of the salt pile or to monitor the as-emplaced conditions in the ROM salt. The grain size distribution of the ROM salt, its moisture content, and the degree of initial compaction will not be measured or controlled during installation. It is noted that the ROM salt will contain some moisture from inclusions in the grains and absorbed from humidity in the ventila tion air, which may vary depending on the time of year it is emplaced. There are no plans to compact the salt in the pile, although there is expected to be some compaction of lower lifts from the multiple passes of emplacement equipment, such as front-end loaders. In this s ituation, the initial conditions of the ROM salt are uncertain.3 rd Response Submittal to the EPA Enclosure 1

2 After installation, the ROM salt pile will consolid ate due to closure of the panel access drifts. The rate of this consolidation, a nd hence of the change in porosity and permeability over time, will vary depending on a number of factors. Moisture content may vary depending on the time of year and the varying humidity of the ventilation air. Closure rates of the panel access drifts will vary between panels depending on the local loading conditi ons and the time that the panel entries have been open before emplacing the salt. These factors add additional uncertainty to the timing of the reconsolidation process for ROM salt in the ROMPCS.

Given these uncertainties, DOE chose an appropria te range of initial porosities for the ROM salt for the PCS-2012 PA. As described in Camphouse et al. (2012a), the range of initial porosity chosen had a minimum of 20% and a maximum of 35%. The maximum initial porosity of 35% is assumed from typical data for unconsolidated granular materials such as sand or gravel (e.g., Bear 1972, Table 2.5.1). The minimum initial porosity of 20% is intended to represent ROM salt with a higher level of moisture content and with some compaction during the emplacement process, and represents a lower bound establishe d using engineering judgment.

As discussed in Camphouse et al. (2012a), the porosity of the ROM salt during the first time period of 0 to 100 years will decrease rapidly due to compaction driven by closure of the entries. A series of JAS3D calculations has been run to estimate the porosity as a function of time for different initial values (Herrick 2012). Given the rapid change in porosity, it was considered reasonable to define the range of porosity for th is time period based upon the calculated values at the mid-point, or 50 years. For an assumed initia l porosity range of 20% to 35%, the 50-year values calculated by JAS3D are 6.6% to 18.7%. No new data or analyses are available, so there are no reasons to modify this range of values at this time.

The permeability of the ROM salt during the T1 period, from 0 to 100 years, is based on laboratory data for crushed salt cores from WIPP and from the Asse mine in Germany (Camphouse et al. 2012a, Section 5.1.2 and Ta bles 3 and 4; DOE 2012). The range of permeability during the T1 period is based on the por osity range at the midpoint of the T1 period (50 years) described above of 6.6% to 18.7%. The corresponding range of permeability based on the laboratory data is 1 × 10 -12 m 2 to 2 × 10 -19 m 2 (Camphouse et al. 2012a, Table 5 for PCS_T1). This range of permeability is consistent with th e assumed initial porosity range of the ROM salt, captures the uncertainties in the initial state and reconsolidation process, and is appropriate for performance assessment. No new data are availabl e, so again there is no reason to modify these values at this time.

2. The capillary threshold pressure model used for the PCS-2012 PA is discussed by Camphouse in his June 7 memorandum (2012b). As he explains, values for threshold pressure can be estimated using a best-fit power curve to capillary threshold pr essure data for several types of consolidated rock (Vaughn 1996):

P T = (5.6×10 -7 [MPa]) k -0.346 , (1)

where P T is the capillary threshold pressure [MPa] and kis the intrinsic permeability in the xdirection [m 2 ] (see also Hurtado 1997, Section 3.2.1.1) . Equation (1) has been included in the BRAGFLO code since the Compliance Certification Application (CCA) (Vaughn 1996) and provides a dynamic dependence of capillary th reshold pressure on permeability. Using this relationship, separate values for the threshold pressu re can be set for times T1, T2 and T3 based on their respective sampled values for permeability.3 rd Response Submittal to the EPA Enclosure 1

3 However, as explained in Camphouse (2012b), in BRAGFLO:

“Instantaneous changes in permeability, due to material changes in a grid region, cause difficulty in numerically satisfying the c onvergence criterion for the capillary pressure constraint equation. When the convergence criterion cannot be satisfied, the capillary pressure model is disabled for the responsible materials so that a convergent solution can be obtained from BRAGFLO. The flag for this is to set PCT_A = 0, which results in the capillary pressure component to be disabled.”

He goes on to note that:

“BRAGFLO grid regions used to represent the ROM salt panel closures in the PCS-2012 will undergo instantaneous changes in perm eability. Specifically, the permeability prescribed to these BRAGFLO grid regions w ill change instantaneously at 100 and 200 years as closure materials change from PCS_T1 to PCS_T2 and from PCS_T2 to PCS_T3 (Camphouse et al. 2012).” 1

Because of these numerical issues, the capilla ry pressure model was disabled in the PCS-2012 PA.

It should be noted that the capillary threshold pressu re is not a sensitive parameter for the flow of gases through the ROMPCS for the following reasons:

 In the higher permeability range, from 10 -16 m 2 to 10 -18 m 2 , capillary threshold pressure calculated from Equation (1) varies between 0.19 MPa (28 psi) and 0.95 MPa (140 psi). These low values of threshold pressure will ha ve minimal impact on hydrologic response because typical repository gas pressures are al most always greater than 0.95 MPa (140 psi). For example, waste panel pressures in the S1, S2, and S4 scenarios are almost always greater than 1 MPa in the perfo rmance assessment for CRA-2014 (DOE 2014, Appendix PA-2014, Figures PA-35, PA-50, and PA-56). In this case, setting P T to zero has no impact on hydrologic response because g as pressure almost always exceeds the threshold pressure and gas can therefor e flow into and through the ROM salt.

 In the lower permeability range, from 10 -19 m 2 to 10 -22 m 2 , the capillary threshold pressure calculated from Equation (1) varies between 2.1 and 23 MPa, and could provide a significant impediment to gas penetrating th e ROMPCS. However, hydrologic flow rates for brine or gas through any material with intrinsic permeability less than 10 -19 m 2 are sufficiently low that the material is essentia lly impermeable. In this situation, the gas flow through the ROMPCS is insignificant for any value of P T , and setting P T to zero has no impact on the hydrologic response.

1 The quoted reference corresponds to Camphouse et al. (2012a) in this response.3 rd Response Submittal to the EPA Enclosure 1

4 3. The residual saturations for brine and gas, which have been used since th e CCA when they were established for use in the shaft seals, are based on extensive data from a literature search for geologic materials. The literature search and r esulting parameter values are described by Hurtado et al. (1997, Section 3.2.2) and the relevant informat ion is repeated here. It should be noted that calculated repository performance is not sens itive to these parameters, as discussed below.

Residual liquid saturations for geologic materi als were found in four references: Brooks and Corey 1964; Lappala et al. 1987; Parker et al . 1987, and Rawls et al. 1982. Brooks and Corey (1964) determined residual saturations for five unconsolidated samples based on measured values of liquid saturation as a function of capillary pressu re. Lappala et al. (1987) determined residual moisture content for 11 soils by obtaining best fits to measured moisture content versus pressure head data using three models. The residual mois ture contents from the three models were then averaged and divided by the reported porosity to determine residual liquid saturation for each soil. Parker et al. (1987) fit their saturation- pressure relationship to observed data to obtain residual saturations for a sandy and clayey porous medium. Residual water contents reported by Rawls et al. (1982) for 11 soil texture classes were divided by the reported porosity to obtain residual saturation.

The values of the residual saturation from the litera ture search are summarized in graphical form in Figure 1 (from Hurtado et al. 1997, Figure 3-2). All values in Figure 1 fall within the range of 0.0 to 0.6 and all but two values fall within the range of 0.0 to 0.4. Hurtado et al. (1997, Section 3.2.2.2) recommended an expected value of 0.2 and a range of 0.0 to 0.6 for the residual liquid saturation for all shaft seal materials, including crushed salt. For compliance calculations, this has been represented as a cumulative distribution with a minimum value of 0.0, a maximum value of 0.6, and a mode of 0.2 at the 50 th percentile (DOE 2004, Appendix PA, Attachment PAR, Parameter 62). This is the recommended distribu tion for the ROM salt in the ROMPCS and has been used in the PCS-2012 PA and for the CRA-2014 PA. The lower value for this distribution of 0 comes from a “Sandy Porous Media” reported as 0.000 by Parker et al. (1987). The upper bound of 0.6 comes from two samples and could be interpreted as too high since most data fall below 0.4. However, given the natural uncertain ties and variation in the residual brine saturation and the fact that PA is not sensitive to it (as discussed next), the value of 0.6 is retained.

Repository performance is not sensitive to the residual brine saturation or the residual gas saturation of the ROMPCS, as confirmed by the sensitivity study for the CRA-2014 PA (Kirchner 2013). More specifically, PCS_T i :SAT_RBRN and PCS_T i :SAT_RGAS ( i = 1, 2, or 3) are not identified as correlated with mean normalized rele ase for Replicates 1, 2, or 3 of the CRA-2014 PA (Kirchner 2013, Tables 6 through 13), with a single exception. The exception is for PCS_T1:SAT_RBRN, which explains 2% of the va riability for Culebra releases in Replicate 3 (only) (Kirchner 2013, Table 13). This correlati on is not important because it only enters the stepwise ranked regression analysis for one of the three replicates, and because Culebra releases are not significant to overall performance of the repository.3 rd Response Submittal to the EPA Enclosure 1

5 Source: (Hurtado et al. 1997, Figure 3-2) 

### Figure 1. Distribution of residual liquid saturation values by geologic material

Literature values for residual gas saturation of ge ologic materials were not found by Hurtado et al. (1997). A single value of 0.18 was found for nor mal concrete. Given the lack of data, the expected value was set at 0.2 and the reco mmended range was 0.0 to 0.4 with a uniform distribution. This is the recommended distribu tion for the ROM salt in the ROMPCS and has been used in the PCS-2012 PA and for the CRA-2014 PA.

4. The recommended value for the bulk compressibility of the ROM salt is 8×10 -11 Pa -1 during the T1 (0-100 years), T2 (100-200 years), and T3 (200-10,000 years) time periods (Camphouse et al. 2012a, Section 5.1.5 and Table 8). It is appropriate to use this value for all time periods because BRAGFLO only uses bulk compressibility to comput e changes in storativit y due to changes in pore pressure, and those changes are not signifi cant during the T1, T2, or T3 time periods. Changes in porosity due to ex ternal loading are calculated sep arately using fully mechanistic modeling with JAS3D (Herrick 2012). Given this use of the bulk compressibility, the use of a constant value of 8×10 -11 Pa -1 is appropriate because it is repr esentative in all time periods. A more detailed explanation follows.

The recommended value is based on a nonlinear model relating bulk modulus to fractional density (Hurtado et al. 1997, pages 2-20 and 2-21 ) that was originally developed by Sjaardema and Krieg (1987):3 rd Response Submittal to the EPA Enclosure 1

6 

### , e ) MPa

. ( , Ae K

)

. ( B





10 14 

### 0176 0   (2)

where K is the bulk modulus, 

###  is the fractional density, and A and B are fitting parameters. The
value of 8x10 -11 Pa -1 is appropriate for a porosity of 0.05, or fractional density as defined by Hurtado et al. (1997) (equation 2-10) of 0.956. The porosity of 0.05 corresponds to the mean porosity for ROM salt during the T2 time period and is slightly less than the lower end of the porosity distribution during the T1 time period, and slightly less than the upper end of the porosity distribution for ROM salt during the T3 time period (Camphouse et al. 2012a, Table 2). The resulting value for bulk compressibility of ROM salt is close to the median value for the bulk (rock) compressibility of intact halite, 9.75×10 -11 Pa -1 (DOE 2004, Chapter 6, Table 6-16). A value of 8×10 -11 Pa -1 for the bulk compressibility of ROM salt is therefore appropriate for the T1, T2, and T3 time periods.

Throughout the regulatory period of 10,000 years, the ROM salt porosity used by BRAGFLO in the three time periods is obtained by samp ling a porosity range developed from JAS3D mechanical calculations, which are used to pr edict the time-dependent compaction of the ROM salt as a function of initial porosity. This process is discussed in Camphouse (2012a) and is noted in the answer to the first question. As note d in Camphouse (2012a), the porosity ranges sampled will vary by time period. During T1 the range is 6.6% to 18.7%, during T2 2.5% to 7.5%, and during T3 0.1% to 5.19%.

Bulk compressibility in BRAGFLO, and its related value of pore volume compressibility, is used to account for porosity changes due to pore pressu re changes in the halite surrounding the waste rooms, and thus changes in storativity. The bul k compressibility is defined by Freeze and Cherry (1979, page 54):



### , d dV
V c

e T

T 

###  1

  (3)

where c is the compressibility, V T is the total volume of the medium, and  e is the effective stress.

The BRAGFLO code uses a modified form of Equa tion (3) to define porosity changes in a porous medium:



##  

. p p c exp r      

  0 0 0    (4)



### where  is porosity [unitless], p is the brine pressure in the pores [Pa], c r is the bulk compressibility [Pa -1 ], and the subscript 0 denotes initial conditions (DOE 2014, Appendix PA2014, Equations (PA.30) and (PA.37)). This form ulation is appropriate for the halite around the waste panels, where changes in the external confining stress will be minimal, so the only significant change in effective stress comes from changes in pore pressures.

For the ROM salt in the panel closure both the external loading and the pore pressures will change as a function of time, so Equation (4) will have a time-varying effect on the porosity. However, it can be shown that this effect is extremely small. During the T1 time period, the numerical values for the parameters in Equation (4) are as follows:3 rd Response Submittal to the EPA Enclosure 1

7  The pore pressure, p , is set to its maximum value, 7×10 5 Pa, during the T1 time period.

 The initial pore pressure, p 0 , is set to atmospheric pressure, 1.01×10 5 Pa.

 The rock or bulk compressibility, c r , is equal to 8×10 -11 Pa -1 , as discussed above.

Table 1 presents the calculated changes in porosity where th e initial porosity,  0 , is 0.20, 0.25, 0.30, or 0.35 (Camphouse et al. 2012a, Table 1).



### Table 1. Porosity changes predicted by Equation (3) during the T1 time period



0 0.20 0.25 0.30 0.35

0 

###   1.00024 1.00019 1.00016 1.00014

% change 0.024% 0.019% 0.016% 0.014%

The results in Table 1 demonstrate that the porosity changes in time T1 induced by pore compressibility are less than 0.03% of the in itial porosity for all initial conditions. This conclusion is also valid during the T2 and T3 time periods. For example, using the bulk compressibility of 8×10 -11 Pa -1 , the mean porosity for time T2 of 0.05, and a maximum pore pressure of 14 MPa, the porosity change induced by pore compressibility is around 1.8% of the initial porosity.

These calculated porosity changes are minimal co mpared to the chang es built into the assumed porosity ranges from the JAS3D calculations, so it follows that the porosity changes computed from bulk compressibility are not significant during the T1, T2, or T3 time periods, and the use of a constant value for rock compressibility of 8×10 -11 Pa -1 is appropriate for compliance calculations.

References:

Brooks, R.H., and A.T. Corey. 1964. Hydraulic Properties of Porous Media. Hydrology Paper No. 3. ERMS 241117. Colorado State University, Fort Collins, CO. 2

Bear, J. 1972. Dynamics of Fluid Flow in Porous Media. Elsevier, NY, 1972. *

Camphouse, R.C., M. Gross, C.G. Herrick, D.C. Kicker, and B. Thompson. 2012a. Recommendations and Justifications of Paramete r Values for the Run-of-Mine Salt Panel Closure System Design Modeled in the PCS-2012 PA. Memorandum to WIPP Records dated May 3, 2012. ERMS 557396. Sandia National Laboratories, Carlsbad NM.

Camphouse, R.C. 2012b. An overview of the BRAGFLO two-phase flow parameters used to model the run-of-mine salt panel closures impl emented in the PCS-2012 PA. Memorandum to WIPP Records dated June 7, 2012. ERMS 557653. Sandia National Laboratories, Carlsbad, NM.

Freeze, R.A. and J.A. Cherry. 1979. Groundwater . Prentice Hall, Englewood Cliffs, NJ. *

2 Brooks and Corey (1964) was submitted to EPA as a reference for Appendix BRAGFLO of the CCA.

* Copyrighted reference not provided in Enclosure 2.3 rd Response Submittal to the EPA Enclosure 1

8 Herrick, C.G. 2012. JAS3D Calculations Performed in Support of the PCS-2012 PA Parameters Selections . April 30, 2012. ERMS 557354. Sandia National Laboratories, Carlsbad, NM.

Hurtado, L.D., M.K. Knowles, V.A. Kelly, T.L. Jones, J.B. Ogintz, and T.W. Pfeifle. 1997. WIPP Shaft Seal System Parameters Recomme nded to Support Compliance Calculations

. SAND97-1287. Sandia National Laboratories, Albuquerque, NM.

Kirchner, T. 2013. Sensitivity of the CRA-2014 Perfor mance Assessment Releases to Parameters . May 17, 2013. ERMS 560043. Sandia National Laboratories, Carlsbad, NM.

Lappala, E.G., R.W. Healy, and E.P. Weeks. 1987. Documentation of Computer Program VS2D to Solve the Equations of Fluid Flow in Variably Saturated Porous Media . Water-Resources Investigations Report 83-4099. U.S. Geological Survey, Denver, CO.

Parker, J. C., R.J, Lenhard, and T. Kuppusamy. 1987. A Parametric Model for Constitutive Properties Governing Multiphase Flow in Porous Media . Water Resources Research. Vol. 23, no. 4, 618-624. *3

Rawls, W.J., D.L. Brakensiek, and K.E. Saxton. 1982. Estimation of Soil Water Properties

. Transactions of the ASAE (American Society of Agricultural Engineers). Vol. 25, no. 5, 1316- 1320 and 1328. *

Sjaardema, G.D. and R.D. Krieg. 1987. A Constitutive Model for the Consolidation of WIPP Crushed Salt and Its Use in Analyses of Backfilled Shaft and Drift Configurations . SAND88- 1977. Sandia National Laboratories, Albuquerque, NM.

U.S. Department of Energy (DOE). 2004. Title 40 CFR Part 191 Compliance Recertification Application for the Waste Isolation Pilot Plant (March). 10 vols. DOE/WIPP 2004-3231. Carlsbad Field Office, Carlsbad, NM.

U.S. Department of Energy (DOE). 2012. Response to EPA Comment Dated June 7, 2012: Review of Alternate Data Sets for Defining Permeability of Crushed Salt. Transmitted via EMail from Russell Patterson, DOE, to Tom Peake, EPA, on June 15, 2012. Carlsbad Field Office, Carlsbad, NM.

U.S. Department of Energy (DOE). 2014. Title 40 CFR Part 191 Subparts B and C Compliance Recertification Application-2014 for the Waste Isolation Pilot Plant (March). DOE/WIPP-14- 3503. Carlsbad Field Office, Carlsbad, NM.

Vaughn, P. 1996. KPT Modeling Feature. Memo randum to Martin Tierney dated February 19, 1996. ERMS 235271. Sandia National Laboratories, Carlsbad, NM.

Zimmerly, B., T. Moffatt, and J. Zavicar IV. 2012. Construction Methods Assessment for Compacted Salt Panel Closures (December). Nuclear Waste Partnership LLC, Carlsbad, NM.

* Copyrighted reference not provided in Enclosure 2.
3 Parker et al. (1987) was submitted to EPA as a reference for Appendix C, Fluid Flow Analysis, which is part of Appendix SEAL of the CCA.3 rd Response Submittal to the EPA Enclosure 1

9 EPA Comment 1-23-4 Region DRZ_PCS

The Agency would like DOE to add ress the following comments relate d to the parameter values adopted for the DRZ_PCS:

1. Provide technical justification for assigning the same sampled value of porosity to the material in DRZ_PCS region for both early and late time periods (T2 and T3) when the DRZ is undergoing consolidation and healing. 2. Provide technical justification for the apparen t discrepancy created by independently sampling the permeability of material D RZ_PCS, representing a healed DRZ, from a distribution that can provide a sampled permeability as much as seven orders of magnitude higher than the permeability of intact halite. 3. Provide technical justifications for assign ing zero values to the residual brine and gas saturations (SAT_RBRN and SAT_RGAS) in CRA- 2014 for the region DRZ_PCS during the T3 time period (200 to 10,000 years). 4. Provide technical justification for applying a capillary pressure model that assumes zero threshold pressure in region DRZ_PCS during the T3 time period (200 to 10,000 years). 5. Provide technical justification for the value of the bulk compressibility of the DRZ_PCS region, and applying that same value during both early and late time periods (T2 and T3) when the material in that region is undergoing consolidation.

DOE Response

The various items in this comment refer to the re gion in the BRAGFLO grid identified as DRZ_PCS, primarily during the T2 and T3 time periods (100-200 years and >200 years). The DRZ_PCS corresponds to the areas above and below the pane l closures where the surrounding halite has been disturbed during mining, but where, as external stress is applied due to consolidation of the ROMPCS, the disturbance, and hence the porosity will be healed. St rictly speaking, the region identified as DRZ_PCS is only differentiated in the BRAGFLO grid for the T3 period; for the T1 and T2 periods the regions representing the DRZ_PCS have properties that are identical with the DRZ around the waste panels. Figures MASS-7, MASS-8, and MASS-9 (DOE 2014, Appendix MASS-2014), reproduced below, illustrate the evolution of the DRZ_PCS regions durin g the T1, T2, and T3 time periods, respectively. In the following response, the nomenclature “ DRZ_PCS ” will be used more loosely to refer to the DRZ above and below the panel closur es during any of the time periods.3rdResponse Submittal to the EPA Enclosure 110Figure MASS-7. CRA-2014 PA BRAGFLO grid and material map, years 0 to 1003rdResponse Submittal to the EPA Enclosure 111Figure MASS-8. CRA-2014 PA BRAGFLO gridand material map, years 100 to 2003rdResponse Submittal to the EPA Enclosure 112Figure MASS-9. CRA-2014 PA BRAGFLO grid and material map, years 200 to time of intrusion3 rd Response Submittal to the EPA Enclosure 1

13 The responses to EPA’s five comments are as follows:

1. The DRZ_PCS region covers the area above and below the panel closure where the surrounding halite has been disturbed during mining, but wh ere, as external stress is applied due to consolidation of the ROMPCS, the disturbance, and hence the porosity will be healed. The porosity of the DRZ around the waste emplacement area is calculated from the porosity of the intact halite, which is sampled from a range of 0.1% to 5.19% (Ismail 2007). The DRZ porosity is calculated by adding 0.0029 to the sampled valu e of porosity for intact halite (Vaughn 1996a). The value 0.0029 was determined prior to the Co mpliance Certification Application (CCA) as the difference between the median value of intact halite porosity and the median DRZ porosity value obtained from the Q-Room expe riments (Christian-Frear 1996).

During times T1 and T2, before the DRZ_PCS is fully loaded due to consolidation of the ROMPCS, the porosity of the DRZ_PCS regions are equal to the porosity of the DRZ surrounding the waste emplacement areas. During time T3, while it is recognized that the DRZ will have experienced some healing due to conso lidation of the ROMPCS, the porosity is kept unchanged for two primary reasons:

 There is considerable uncertainty in the poros ity values for intact halite and for the DRZ, which are available from laboratory and in situ testing. With an assumed range of halite porosity of 0.1% to 5.19%, the value of the DRZ porosity has a range of 0.39% to 5.48% (Ismail 2007). This range adequately covers the likely porosity both before and after full load is applied to the DRZ (i.e., for times T1, T2 and T3).

 The results of PA calculations are not sensitive to the porosities of the DRZ and DRZ_PCS within their given ranges. Both of these valu es are derived from the sampled value of intact halite porosity, as noted above, by adding a cons tant value of 0.0029. The sensitivity study for the CRA-2014 PA (Kirchner 2013) indicates that for total mean normalized release, the intact halite porosity contributes 1% of the va riability in Replicates 1 and 2 and 3% of the variability in Replicate 3 (Kirchner 2013, Ta bles 6 and 7). Since the DRZ porosity is calculated from this sampled value, an identi cal result also holds for the DRZ porosity. In addition, the porosity of the DRZ_PCS during the T1 , T2, or T3 time periods is not within the top 8 sensitive parameters for Replicates 1, 2, or 3 for mean total normalized release (Kirchner 2013, Tables 6 and 7). In this situa tion, creating and using a time-dependent model for porosity of the DRZ or the DRZ_PCS adds no value to the assessment of long-term repository performance.

2. Initially, it is assumed that the DRZ around the PCS will be no different from that around the disposal rooms since there will be only a small back stress from the consolidating ROM salt in the ROMPCS. Geomechanical calculations for clos ure of panel entries and consolidation of the ROM salt predict that the back stress from the ROM salt does not become appreciable until approximately 200 years after the ROM salt is em placed (Herrick 2012, Section 5.2). While the back stress is low, the state of the DRZ surroun ding the PCS will be similar to the state of the DRZ surrounding the disposal rooms, and the values of porosity and permeability of the DRZ_PCS during the T1 (0 to 100 years) and T2 (100 to 200 years) time periods have been set equal to those for the disposal room DRZ. The permeability of the DRZ_PCS therefore has a range of 10 -12.5 m 2 to 10 -19.4 m 2 for the first 200 years after repository closure. Further details are provided in Camphouse et al. (2012a, Section 5.2.1).

At 200 years (the start of the T3 time period), back stress builds up to values between 6 and 10 MPa, and fractures in the DRZ above the ROMPCS should begin to heal; by 500 years the back3 rd Response Submittal to the EPA Enclosure 1

14 stress reaches about 12 MPa (Camphouse et al . 2012a, Figures 9 and 10 and discussion on pages 21 and 22). The recommended range of permeability for the DRZ_PCS is 2×10 -21 m 2 to 1×10 -17

m 2 during the T3 time period. This recommendation is based on permeability measurements for cores of crushed salt and the range represents the uncertainty in the degree and timing of reconsolidation in the DRZ_PCS. This range is significantly greater than the permeability range for intact halite, which is 10 -24 m 2 to 10 -21 m 2 , for the following reasons:

 As noted above, the DRZ begins to heal wh en the back stress builds up to 6 to 10 MPa after 200 years. However, the timing for comp lete healing of the DRZ is uncertain. The JAS3D calculations predict that the back st ress approaches 12 MPa by 500 years (Herrick 2012, Figures 9 through 13) and may slow ly increase to full lithostatic stress, approximately 14 MPa, thereafter. Full hea ling of the DRZ may require thousands of years to approach the extremely low permeability of intact salt. There is no discrepancy if the DRZ, even after some healing, has a permeability somewhat higher than the very low values for intact halite strata that have been undisturbed for millions of years.

 Given the uncertainty in the timing of the h ealing process, PA represents a range of flow rates through the DRZ_PCS. At the high end of the permeability range, 1×10 -17 m 2 , the permeability of the DRZ_PCS is similar to that of concrete. At the low end of the permeability range, 2×10 -21 m 2 , the DRZ_PCS is essentially impermeable (i.e., no flow) and similar to the intact salt. This approa ch provides an appropriate range of hydrologic responses for the DRZ_PCS, thereby capturing the uncertainty in the timing of the healing process for performance assessment.

An alternative to this approach is to use the permeability range for intact halite for the DRZ during T3. This is not appropriate b ecause DOE does not have data or analyses to prove that the DRZ fully heals by 200 years.

 Independent analytic work by EPA has show n that mean total normalized releases are insensitive to the range of DRZ_PCS perm eability during the T3 time period. EPA performed an independent PA using a revi sed permeability range for the DRZ_PCS of 1×10 -22 m 2 to 1×10 -20 m 2 during the T3 time period (Economy 2013). The results of the EPA’s independent PA demonstrated that mean total normalized releases are essentially equal for the EPA’s or the DOE’s distribu tions for DRZ_PCS permeability in the PCS2012 PA (Economy 2014).

3. The residual saturations for brine and gas, which have been used since th e CCA when they were established for use in the shaft seals, are based on extensive data from a literature search for geologic materials. The literature search and r esulting parameter values are described by Hurtado et al. (1997, Section 3.2.2) and included in the answer to item #3 of our response to Comment 1-23-3, and are not repeated here.

All values from this literature search fall within the range of 0.0 to 0.6, and all but two values fall within the range of 0.0 to 0.4. For complia nce calculations, this has been represented as a cumulative distribution with a minimum value of 0.0, a maximum value of 0.6, and a mode of 0.2 at the 50 th percentile (DOE 2004, Appendix PA, Attach ment PAR, Parameter 62). This is the recommended distribution for the DRZ_PCS during T1, T2, and T3, and has been used in the PCS-2012 PA and for the CRA-2014 PA.

The lower value for this distribution of 0 comes from a “Sandy Porous Media” reported as 0.000 by Parker et al. (1987), and is the lower bound of the distribution, rather than a fixed value used3 rd Response Submittal to the EPA Enclosure 1

15 in the calculations. It should be noted that re pository performance is not sensitive to the residual brine saturation or the residual gas saturation of the DRZ_PCS during T3, as confirmed by an independent PA performed by the EPA. The EPA chose a residual brine saturation of 0.1 and a residual gas saturation of 0.05 for the DRZ_PCS during T3 (Economy 2014, Table 1) and compared the results to those from the PCS-2014 PA (Camphouse 2012a, Table 11). The results of the EPA’s independent PA demonstrated that mean total normalized releases are essentially equal for the EPA’s values or the DOE’s values of residual saturation in the DRZ_PCS (Economy 2014, Figure 4).

4. The capillary threshold pressure model used for the PCS-2012 PA is discussed by Camphouse in his June 7 memorandum (Camphouse 2012b). As he explains, values for threshold pressure can be estimated using a best-fit power curve to capillary threshold pressure data for several types of consolidated rock (Vaughn 1996b):

P T = (5.6×10 -7 [MPa]) k -0.346 , (1)

where P T is the capillary threshold pressure [MPa] and k is the intrinsic permeability in the x-direction [m 2 ] (see also Hurtado 1997, Section 3.2.1.1). Equation (1) has been included in the BRAGFLO code since the CCA (Vaughn 1996b) and provides a dynamic dependence of capillary threshold pressure on permeability. Us ing this relationship, separate values for the threshold pressure can be set for times T1, T2 and T3 based on their respective sampled values for permeability.

However, as explained in Camphouse (2012b), in BRAGFLO:

“Instantaneous changes in permeability, due to material changes in a grid region, cause difficulty in numerically satisfying the c onvergence criterion for the capillary pressure constraint equation. When the convergence criterion cannot be satisfied, the capillary pressure model is disabled for the responsible materials so that a convergent solution can be obtained from BRAGFLO. The flag for this is to set PCT_A = 0, which results in the capillary pressure component to be disabled.”

He goes on to note that:

“BRAGFLO grid regions used to represent the ROM salt panel closures in the PCS-2012 will undergo instantaneous changes in perm eability. Specifically, the permeability prescribed to these BRAGFLO grid regions w ill change instantaneously at 100 and 200 years as closure materials change from PCS_T1 to PCS_T2 and from PCS_T2 to PCS_T3 (Camphouse et al. 2012).” 1

Because of these numerical issues, the capillary pressure model was disabled in the PCS-2012 PA and in the CRA-2014 PA.

It should be noted that the capillary threshold pressu re is not a sensitive parameter for the flow of gases through the DRZ_PCS for the following reasons:

 In the higher DRZ_PCS permeability range, from 10 -18 m 2 to 10 -17 m 2 , capillary threshold pressure calculated from Equation (1) varies between 0.19 MPa (28 psi) and 0.95 MPa

1 The quoted reference corresponds to Camphouse et al. (2012a) in this response.3 rd Response Submittal to the EPA Enclosure 1

16 (140 psi). These low values of threshold pressure will have minimal impact on hydrologic response because typical repository gas pressur es are almost always greater than 0.95 MPa (140 psi). For example, waste panel pr essures in the S1, S2, and S4 scenarios are almost always greater than 1 MPa in the performance assessment for CRA-2014 (DOE 2014, Appendix PA-2014, Fi gures PA-35, PA-50, and PA -56). In this case, setting P T to zero has no significant impact on the hydrol ogic response because gas pressure almost always exceeds the threshold pressure and g as can therefore flow into and through the DRZ_PCS.

 In the lower permeability range, from 10 -21 m 2 to 10 -19 m 2 , the capillary threshold pressure calculated from Equation (1) varies between 2.1 and 23 MPa, and could provide a significant impediment to gas penetrating the DRZ_PCS. However, hydrologic flow rates for brine or gas through any material with intrinsic permeability less than 10 -19 m 2 are sufficiently low that the material is essentia lly impermeable. In this situation, the gas flow through the DRZ_PCS is insignificant for any value of P T , and setting P T to zero has no impact on the hydrologic response.

5. The recommended value for the bulk (or rock) compressibility of the DRZ_PCS is 7.41×10 -10 Pa -1 during the T1 (0-100 years), T2 (100-200 years), and T3 (200-10,000 years) time periods (DOE 2004, Appendix PA, Attachment PAR, Table PAR-33) . This value has been used since the CCA as noted by Vaughn (1996a), and this bulk compressi bility is only used to determine changes in storativity in BRAGFLO.

The permeability of the DRZ_PCS is the dominan t parameter in determining flow through the DRZ_PCS, not changes in storativity; therefore, since permeability is not affected by compressibility effects, the use of a constant va lue for compressibility is appropriate. Any minor changes in porosity from bulk compressibility w ill not have a significant impact on hydrological response of the DRZ_PCS. A more detailed explanation follows.

Bulk compressibility in BRAGFLO, and its related value of pore volume compressibility, is used to account for porosity changes due to pore pressu res changes in the halite surrounding the waste rooms, and thus changes in storativity. The bul k compressibility is defined by Freeze and Cherry (1979, page 54):



### , d dV
V c

e T

T 

###  1

  (2)

where c is the compressibility, V T is the total volume of the medium, and σ e is the effective stress.

The BRAGFLO code uses a modified form of Equa tion (2) to define porosity changes in a porous medium:



##  

. p p c exp r 

###      

  0 0 0 

###    

### (3)

where  is porosity [unitless], p is the brine pressure in the pores [Pa], c r is the bulk compressibility [Pa -1 ], and the subscript 0 denotes initial conditions (DOE 2009, Appendix PA-2009, Equations (PA.30) and (PA.37)). For the DRZ surrounding the panel closure both the external loading and the pore pressures will cha nge as a function of time, so Equation (3) will3 rd Response Submittal to the EPA Enclosure 1

17 have a time varying effect on the porosity. Howe ver, it can easily be shown that this effect is extremely small.

Table 1 presents the calculated changes in porosit y during the T2 and T3 time periods for the bulk compressibility of 7.41×10 -10 Pa -1 and a maximum pore pressure of 14 MPa, and starting porosities covering the range calculated from the intact halite range. The magnitude of the porosity changes for initial porosities equal to or gr eater than 0.01 is less than 0.02 and will not have a significant impact on the hydrologic response of the repository. This is clear by considering the volume of the DRZ_PCS above and below a panel closure in Figure MASS-9 (DOE 2014, Appendix MASS-2014; see the fourth page in this response). The DRZ_PCS region has an orange color in Figure MASS-9. The combined volume above and below the ROM PCS between the south rest-of-repository and the north rest-of-repository is:



### V DRZ_PCS 

### = (0.69+0.69+2.62+4.53+4.53)(15.48+15.48)(40+40) = 31,850 m 3 

### . (4)

The porosity change for  0 = 0.01 is 0.018 (see Table 1) and the corresponding increase in free volume is (31,850 m 3 )(0.018) = 570 m 3 . This increase is not significant for the overall volumes of brine and gas in the repository, and will not be important for repository performance. For example, the total excavated volume of the repository is 436,000 m 3 (SNL 1992, Section 3.1.1) and the approximate volume of a single panel is (0.1)(436,000 m 3 ) = 43,600 m 3 , which is much greater than the pore volume change in the DRZ_PCS.



### Table 1. Porosity changes predicted by Equation (3) during the T2 and T3 time periods



0 0.0548 0.03 0.01 0.0038

0 

###   1.207 1.410 2.801 15.05

0    

  0.011 0.012 0.018 0.053

For the minimum initial porosity of 0.0038, the maximum change in porosity is 0.053 and the corresponding volume increase is (31,850 m 3 )(0.053) = 1,700 m 3 . Again, this increase is not significant for the overall volumes of brine and gas in the repository, and will not be important for repository performance.

References:

Camphouse, R.C., M. Gross, C.G. Herrick, D.C. Ki cker, and B. Thompson. 2012a. Recommendations and Justifications of Parameter Values for the R un-of-Mine Salt Panel Closure System Design Modeled in the PCS-2012 PA. Memorandum to WIPP Record s dated May 3, 2012. ERMS 557396. Sandia National Laboratories, Carlsbad, NM.

Camphouse, R.C. 2012b. An overview of the BRAG FLO two-phase flow parameters used to model the run-of-mine salt panel closures implemented in th e PCS-2012 PA. Memorandum to WIPP Records dated June 7, 2012. ERMS 557653. Sandia National Laboratories, Carlsbad, NM.3 rd Response Submittal to the EPA Enclosure 1

18 Christian-Frear, T.L. 1996. Salado Parameter Principal Investigator Document Package for: DRZ Porosity from Room Q Analysis. Memorandum to SW CF-A Records Center Dated February 13, 1996. ERMS 419199. Sandia National Laboratories, Albuquerque, NM.

Economy, K. 2014. EPA’s Confirmatory PA Analysis of DOE’s Proposed Run-of-Mine Salt Panel Closure System for the WIPP . Paper number 14058. Waste Mana gement 2014 Conference, March 2-6, 2014, Phoenix, AZ. *

Freeze, R.A. and J.A. Cherry. 1979. Groundwater . Prentice Hall, Englewood Cliffs, NJ. *

Herrick, C.G. 2012. JAS3D Calculations Performed in Support of the PCS-2012 PA Parameters Selections . April 30, 2012. ERMS 557354. Sandia National Laboratories, Carlsbad, NM.

Hurtado, L.D., M.K. Knowles, V.A. Kelly, T.L. Jones, J.B. Ogintz, and T.W. Pfeifle. 1997. WIPP Shaft Seal System Parameters Recommende d to Support Compliance Calculations . SAND97-1287. Sandia National Laboratories, Albuquerque, NM.

Ismail, A.E. 2007. Revised porosity estimates for th e DRZ. Memorandum to File Dated April 10, 2007. ERMS 545755. Sandia National Laboratories, Carlsbad, NM.

Kirchner, T. 2013. Sensitivity of the CRA-2014 Performan ce Assessment Releases to Parameters

. ERMS 560043. Sandia National Laboratories Carlsbad, NM.

Parker, J.C., R.J, Lenhard, and T. Kuppusamy. 1987. A Parametric Model for Constitutive Properties Governing Multiphase Flow in Porous Media . Water Resources Research. Vol. 23, no. 4, 618-624. *2

Sandia National Laboratories (SNL). 1992. Preliminary Performance Assessm ent for the Waste Isolation Pilot Plant . SAND92-700/3. Vol. 3: Model Parameters. December 1992. Sandia National Laboratories, Albuquerque, NM.

U.S. Department of Energy (DOE). 2004. Title 40 CFR Part 191 Compliance Recertification Application for the Waste Isolation Pilot Plant (March ). DOE/WIPP 2004-3231, 10 vols. Carlsbad Field Office, Carlsbad, NM.

U.S. Department of Energy (DOE). 2009. Title 40 CFR Part 191 Compliance Recertification Application for the Waste Isolation Pilot Plant (Mar ch). DOE/WIPP-09-3424. Carlsbad Field Office, Carlsbad, NM.

U.S. Department of Energy (DOE). 2014. Title 40 CFR Part 191 Subparts B and C Compliance Recertification Application-2014 for the Waste Isolation Pilot Plant (March). DOE/WIPP-14-3503. Carlsbad Field Office, Carlsbad, NM.

Vaughn, P. 1996a. Revision to DRZ and TZ properti es to be used in the BRAGFLO CCA calculations. Memorandum to Margaret Chu Dated January 24, 19 96. WPO 32288. Sandia National Laboratories, Albuquerque, NM.

Vaughn, P. 1996b. KPT Modeling Feature. Memorandum to Martin Tierney dated February 19, 1996. ERMS 235271. Sandia National Laboratories, Carlsbad, NM.

* Copyrighted reference not provided in Enclosure 2.
2 Parker et al. (1987) was previously submitted to EPA as a reference for Appendix C, Fluid Flow Analysis, which is part of Appendix SEAL of the CCA.3 rd Response Submittal to the EPA Enclosure 1

19 EPA Comment 1-23-6 Probability of Encountering a Castile Brine Pocket. Please address the following comments:

1. TDEM results are site specific and indicate the presence of potentially large volumes of brine beneath some waste panels. Explain why TDEM data is not used in DOE’s proposed approach for estimating PBRINE. 2. DOE’s approach ignored the presence of high elect rical conductivity zones identified beneath four of the ten WIPP waste panels. Please explain how this omission affected the comparison of the TDEM approach with DOE’s newl y derived drilling data approach. 3. ERMS 558724 asserts that brine encounters of su fficient size to impact the repository would be noticed and logged by a driller. The appr oach does not acknowledge the potential of encountering a low yield and high volume brine po cket which would not be noticed by the driller in calculating PBRINE. Please address the basis fo r not considering the low yield, high volume brine pockets. 4. DOE reported the same count of 34 brine intrusi ons out of 678 Castile wells in 2008. It is unclear whether 2008 was the last time a brine in trusion was observed at the time of collecting data for the CRA-2014 or if 2008 was the cut-off date for recoding a brine intrusion. Please clarify. 5. The circular regions in Figure 5 of ERMS 558724 were selected to include a known brine pocket encounter. Please provide information as to wh ether this radius would bias the results and the sensitivity of the results to the radius size. 6. Provide information as to how the well depths, for each well that did and did not encounter a brine pocket, were incorporated into the drilling data analysis. 7. The ratios in Table 2 of ERMS 558724 include do uble-counting of many wells. Please provide information as to how this affected the results. 8. ERMS 558724 states that pressurized brine is associ ated with near-vertical fractures. However, WIPP-12 yielded large volumes of brine that is assumed to have been stored and transmitted through the extensive and primarily interconnected horizontal to sub-horizontal fractures. Please provide an explanation as to how the near-vert ical fracture model adopted by DOE’s modified PBRINE parameter is consistent with evidence t hat WIPP-12 releases are attributed to horizontal and sub-horizontal fractures.

DOE Response

1. The goal in estimating parameters for use in the WIPP PA is, to the extent feasible, to provide the best estimate of the parameter based on empirical evid ence and to represent the uncertainty on those estimates by assigning a distribution to the parameter. The form and magnitude of dispersion represented by the distribution ideally could be dete rmined from the empirical data, but in practice, additional sources of uncertainty must be considered as, for example, when the empirical data are few or not entirely representative of the site in questio n. In some cases uncertainties on parameters can be ignored, as when the parameter has little influence on th e result or when the uncertainty is negligible. In some cases parameters must be estimated by expert elicitation, drawing on the experience of knowledgeable experts to subjectively define a dist ribution for a parameter. Since the goal in risk assessment generally is to minimize the chance of a false-negative conclusion, parameters deemed low-influence or well-known can be assigned conservative (risk maximizing) values with no uncertainty. In a probabilistic risk assessment, like that employed in WIPP PA, the use of conservative assumption for parameters must be we ighed carefully since excessive conservatism will transform the results from the best estimates with uncertainties that capture the possible range of outcomes, to conservative estimates with uncertain ties that likely range into regions of highly improbable or impossible outcomes. In addition, implausible predictions from a probabilistic model can cast doubt on its overall credibility.3 rd Response Submittal to the EPA Enclosure 1

20 This update of the probability of encountering a pr essurized brine pocket (PBRINE) is consistent with the requirement in 40 CFR 194.15(a)(2), which stat es that recertification applications include “...all additional monitoring data, analys is, and results.” The monitoring data collected by the Delaware Basin Drilling Surveillance Program (DBDSP) show a dramatic increase in drilling activity over the last 10 years, with no additional occurrences of pr essurized brine within the monitoring area. The DOE believes that these new data should be recogni zed and incorporated as appropriate into the CRA-2014, and that the distribution derived from the se data is more credible than the distribution previously derived using time-domain electromagne tic (TDEM) data (Peake 1998). It will be shown below that:

 the distribution developed from th e time-domain electromagnetic data is based on questionable assumptions,

 the assignment of a uniform distribution to intr oduce conservatism is questionable on theoretical grounds,

 the uniform distribution is a poor representation of the results from a Monte Carlo-based analysis conducted by the EPA, and

 the subjectively assigned uniform distribution could be less conservative than a more realistic distribution based on statistical theory.

Thus the previous uniform distribution of PBRINE mandated by EPA both impacts the credibility of the results and impairs their interpretation with in the probabilistic framework of the compliance assessment. The distribution used in the CRA-2014 is based on a direct measurement of the frequency of brine encounters, is consistent with the pr obabilistic framework of compliance assessment, and does not require the use of a conceptual model havi ng many uncertainties to derive the probability of encountering brine from the measurements. Furtherm ore, a subset of the well data from the region was used to estimate PBRINE. These data came from a reference region near WIPP that was selected because it had a higher than average rate of brine encounters. Thus, the nominal estimate of PBRINE used in the CRA-2014 is at least twice as large as th e regional rate and more than three times the rate encountered in wells around the WIPP site. In add ition, the shape of the CRA-2014 distribution is derived from statistical theory and reduces the wei ght associated with low-probability events. Based on these features, the DOE considers the CRA-2014 distri bution to be the more defensible alternative. This conclusion is de rived from review of:

 The data on pressurized brine around the WIPP site

 The on-site drilling data

 The TDEM data and their interpretation

 The theoretical expectation for the form of the distribution

 Assumptions made in the stoc hastic simulation of depths

 Using drilling data to estimate PBRINE3 rd Response Submittal to the EPA Enclosure 1

21 Pressurized brine in the region of the WIPP

The occurrence of pressurized brine in the Castile formation was known at the time that the DOE was evaluating sites for the WIPP. The potential for encountering pressurized brine became an issue during the site suitability investig ation process when a brine release occurred from the WIPP-12 test well (Popielak et al. 1983). Of three well s drilled within the WIPP boundary, only WIPP-12 encountered pressurized brine. The parameter PBRI NE is potentially influential because it is the influx of brine into the repository that can cause gas pressures to rise and subsequently give rise to releases of contaminated brine to the surface. Thus, it is a parameter that one would want to estimate as accurately as possible with an appropriate level of uncertainty. However, the actual sensitivity of direct brine releases (DBR) to PBRINE is quite low (Fig. 1), as documented in Kirchner (2013). Here DBR was only weakly correlated with PBRINE (cont ributing about 2% to the variability) in one of three replicates. This is undoubtedly because it tak es only one drilling event where pressurized brine is encountered to flood the repository, so additiona l intrusions into pressurized brine have little or no impact. We observed an average of 7.44 drilling intr usions per future in the CRA-2014 simulations. This value is stored in an output file from the CCDFGF code, but for verification the expected value can be computed as

7.43 = 0.00673 intrusions-km –2 -yr –1 ൈ 0.1115 km 2 ൈ 9900 yr

where the area of 0.1115 km 2 is the footprint of the repository and 9900 is the number of years simulated after the period of institutional controls . Thus, low values of PBRINE can result in no brine pockets being penetrated in a future. For example, if PBRINE is 0.01, then the probability of seeing at least 1 brine encounter in 7 trials is only about 0.07. If PBRINE is 0.6, that probability is nearly 1 (0.998). This nonlinear response means that the impa ct of the change in the distribution cannot be judged merely by comparing the ranges of the old an d new distribution. It also means that the shape of the distribution, particularly the weight in the lower tail, is important.

Herein we explain why we are more confident in the distribution of PBRINE developed from the drilling data than we are in the distribution derive d from the TDEM data. Our lack of confidence in the TDEM-derived distribution arises from:

 The lack of any data that could be used to correlate TDEM data with the probability of encountering brine

 The assumption that the proportion of the area of the repository underlain by regions of high conductivity is a good estimate of the probability of encountering brine

 The assumption that the error bar on TDEM depth of ±75 m was one sigma

 The discounting of expert opinion that brine poc kets would be containe d within the anhydrite

 The use of a uniform distribution that ignores bot h the results obtained from stochastic simulation of uncertainties in the depth estimates and th e expected distribution for a Bernoulli process

Each of these observations is explained in the following text.3 rd Response Submittal to the EPA Enclosure 1

22 

### Figure 2. Scatter plot of rank direct brin e release vs. rank PBRINE for CRA2014-BL and CRA2014-TP

The on-site drilling data, TDEM data and flaws in their interpretation

Given the limitation that direct measurement of th e probability of encountering brine in a drilling event over the WIPP repository cannot be made, one might consider alternative methods for estimating this probability. An ideal experiment would be to find a non-intrusive measurement that could be used to predict the rate of pressurized brine encounters, make those measurements over the repository, then calibrate the non-intrusive meas urements to the probability of encountering pressurized brine on a similar off-site area where both the non-intrusive measurements and drilling experiments could be conducted. Ho wever, this was not an experime nt that was conducted. What is available are data from non-intrusive TDEM measure ments over the repository, data from three wells drilled on the WIPP site and well logs from comme rcial drilling operations in the area surrounding the WIPP site. The TDEM measurements can be used to provide a map of regions of high conductivity, which are likely associated with th e presence of water. The TDEM data can be interpreted to estimate the depth of the high-c onductivity regions. Thus, the TDEM data were valuable in alerting DOE to the possibility of th ere being pressurized brine pockets beneath the WIPP repository. The TDEM data are also valuable in th at they detected no areas of high conductivity at depths of less than 1000 m (The Earth Technolog y Corporation 1988). However, TDEM results provide a two-dimensional map of areas of high conductivity and cannot be used to estimate the volume of brine, particularly at the low levels of resolution that result from the requirement of obtaining measurements at the depth of the Castile. Thus, the portion of the question that asserts that “the TDEM results ... indicate the presence of poten tially large volumes of brine” is not accurate.

In order to make measurements to the depth of th e Castile, the TDEM loop had to be a square with the length of the side about equal to the depth of interest. The TDEM loops in this case were 500 m GLOBAL:PBRINE 0.0 0.2 0.4 0.6Direct Brine Release (EPA Units)

1e-4 1e-3 1e-2 1e-1 CRA2014-BL CRA2014-TP3 rd Response Submittal to the EPA Enclosure 1

23 by 500 m on a side (The Earth Technologies Corporation 1988). This large size also reduced noise in early time channels as compared to smaller sized loops (The Earth Technologies Corporation 1988). Thus, these are necessarily low-resolution data a nd they cannot provide a detailed map of brine pockets, which are expected to have cross-sectiona l areas considerably smaller than 500 m by 500 m, nor can TDEM data distinguish pressurized from non- pressurized fluids. Expert opinion is that water contained wholly within the anhydrite layers of the Castile formation is likely to be pressurized brine, whereas water within the Bell Canyon formation, which permits lateral flow, is unlikely to be pressurized (Peake 1998; Sandia 1992 page 5-4; Po wers et al. 1996; Powers et al. 1978). The conceptual model for the brine pockets is that they were formed within near-vertical cracks in the anhydrite of the Castile formati on during periods of deformation (P opielak et al. 1983). Borns (1983) confirmed the near-vertical orientation of brine- producing fractures through study of core from the WIPP-12 borehole. Popielak et al. (1983) found no evidence of hydraulic or chemical connection between reservoirs, but did conclude that low permeability microfractures around the large fractures contained the majority of the brine. Evidence for the lack of connectivity between reservoirs (Popielak et al. 1983) includes:

 Hydraulic heads that are substantially different between reservoirs

 The containment of gas by the reservoirs

 Different brine and gas chemistries among the reservoirs

 Geographic separation and heterogeneity in the spatial distribution of reservoirs

 Bulk chemical equilibria between brine, gas and reservoir rock

This conceptual model provides the foundation for making assumptions to translate the TDEMgenerated map of conductivity into the probability of encountering pressurized brine. In particular, it suggests that it is wrong to assume that any well dr illed in an area identified by TDEM as being an area of high conductivity will necessarily encounter pressu rized brine. However, it is probably fair to say that a well drilled in an area where TDEM showed no areas of high conductivity within the Castile is highly unlikely to encounter a brine reservoir.

There are two sets of site-specific data: TDEM data showing areas of high conductivity and data from three drilling events. The distribution of PBRINE used in the previous CRAs was derived from the TDEM data with no consideration of the rates of encounters of pressurized brine pockets observed either on the WIPP site or in the area surrounding WIPP. The translation from TDEM measurements to a probability of encountering brine relied on stoc hastic modelling of the uncertainties in the TDEM data and formation depths, and so me conservative assumptions about the interpretation of the TDEM data (Peake 1998). An important consideration in this stochastic modeli ng was the assumption about the location of pressurized brine in the Castile. Two possibilities were considered: that the pressurized brine was contained within the anhydrite, so the b ase of Anhydrite III was the lower bound of depth, or that the pressurized brine could extend to the base of the Castile. Both limits were used in the analysis and each produced a distribution. These dist ributions were widely separated from each other, having ranges of about 0.02 to 0.06 (base of Anh ydrite III) and 0.35-0.58 (base of Castile) based on Fig. 5-5 in Peake (1998). A bimodal composite distribution was suggested in Peake (1998) that combined the two distributions by assuming each wa s equally likely (Peake 1998, Fig. 5-5). The stochastic modeling also considered the limiti ng cases of 1) there being no spatial correlations between areas associated with high TDEM conductivi ty measurements and 2) there being very high correlation between such areas. These two cases were labeled the random model and the block model, where the block model derived its name from the spatial aggregation of areas surrounding each of 363 rd Response Submittal to the EPA Enclosure 1

24 measurement points with the aggregate being assigned the measurement value, thus yielding 36 blocks of elevations. The block model was consid ered to be more conservative than the random model. The block model was chosen to represent the data even though the TDEM measurements showed no indication of spatial correlation.

Peake (1998) notes that “... the TDEM results do not account for the possibility of obliquely oriented or oblong-shaped brine pockets...”. Thus, the areas mapped out using the TDEM data do not represent areas of high probability of encountering a pressurized brine pocket but rather areas of high conductivity in the Castile formation defined at a ve ry low level of spatial resolution. There are no data available to use to translate from drilling in an area of high conductivity to the probability of encountering pressurized brine, so this important co rrelation was treated as being perfect, i.e., having a coefficient of 1, in spite of the core data that sugg est brine resides in nearly vertical cracks. In 1997, the EPA mandated that the parameter PBRINE be assigned a uniform distribution over the range 0.01 to 0.60. The uniform distribution was selected unde r the assumption that there were no data on which to base an expected or modal value and the end points were set subjectivel y, with the upper end primarily based on an interpretation of the TDEM data. This uniform distribution was based on the stochastic modeling of the depth of the TDEM c onductive layers but ignored the suggestion that a bimodal distribution be used. As a result, the uniform distribution assigns non-zero probability to values of PBRINE between 0.08 and 0.35 in spite of the lack of any sampled data in this range from the stochastic analysis. Furthermore, the equal wei ghting given to each distribution does not appear to be justified by the opinions of the experts (Popielak et al. 1983) that pressurized brine would be found primarily, and probably exclusively, in the anhydrite layers.

The other site-specific data are the drilling da ta from three wells, one of which encountered pressurized brine. EPA has previously argued that the WIPP site’s geology with respect to the abundance of brine pockets could be different from th at of neighboring areas, based in part on this one encounter with pressurized brine. These data are too sparse to derive a correlation between TDEM results and the probability of encountering brine. However, encountering pressurized brine reservoirs can be assumed to be a Bernoulli process because for each drilling event the probability of encountering pressurized brine can be assumed to be independent of previous drilling events. A 90% confidence interval on the observed rate of encount ering pressurized brine on the WIPP site, 0.33, is approximately 0.02 to 0.86 (Hahn and Meeker 1991). This interval represents the range of PBRINE that, with 90% confidence, could give rise to exactly 1 out of 3 brine encounters. In Kirchner et al. (2012) we showed that the range of PBRINE that could give rise to at least one brine encounter in three trials to be 0.02 to 0.63, thus showing that the observed rate of encounters within the WIPP boundary was not inconsistent with the average ra te of encounters in the vicinity of WIPP.

Conservatism of the uniform distribution

The TDEM-based interval has a lower maximum th an the observation-based estimate based on the three on-site wells, but the uniform distribution has a higher frequency of the lowest values as compared to the distribution expected for a Be rnoulli process based on data from only three wells (Fig. 2). In other words, the distribution derive d from the TDEM data and a set of assumptions could well be less conservative than a distribution based on the site-specific drilling data in spite of the uniform distribution’s smaller range because of the greater influence of low values than high values. However, basing an estimate of PBRINE on data from only three wells obviously introduces both a coarseness and a high degree of uncertainty on the value; the only possible estimates of PBRINE that can be obtained from three trials are 0, 1/3, 2/3 and 1. Furthermore, a rate of 33% far exceeds the rate of encounters of pressurized brine in the area su rrounding WIPP. Thus, these data were also judged to be inadequate for providing a creditable estimate of PBRINE.3 rd Response Submittal to the EPA Enclosure 1

25 Flaws in the stochastic simulation of uncertainty in depth measurements

A study of the TDEM analysis report (Peake 1998) al so raised questions about the methodology used to map areas of high conductivity under the vari ous panels. The method employed a stochastic simulation model to try to incorporate the measureme nt uncertainty, ± 75 m, into the assessment. This uncertainty was sampled and applied to the nominal depth measurement values, and then the sampled depths were compared to the extent of the C astile to assess whether they could be included or excluded as indicators of potential Castile brine pocke ts. One major concern was the interpretation of the measurement uncertainty as representing one sigma. Normally, a measurement uncertainty expressed using ± is assumed to represent an absolu te limit or two standard deviations (sigma) unless specifically noted to be an estimate of dispersion with a specific “coverage factor” (the multiplier on sigma) (Taylor and Kuyatt 1994; ANSI 1997). The Earth Technology Corporation (1988) stated that “...the accuracy of determining depth to brine in th is survey was , ± 75 m,” thus lending credence to the interpretation that ± 75 m re presented an absolute interval a nd not a measure of variability or dispersion. Furthermore, -75 m and +75 m are e xplicitly labelled the “m inimum depth” and the “maximum depth” on the TDEM depth estimates in Borns (1996). If sigma was reduced, then the area of regions of high conductivity with in the Castile would probably be reduced as well. Presumably, the assignment of areas possibly containing brine pocke ts to panels where the nominal value did not indicate brine was related to this interpretation of uncertainty. However, there were insufficient details in the report to reproduce this analysis w ith the interpretation of the measurement error as being two-sigma.



### Figure 3. Previous uniform distribution based on a subjective analysis of the TDEM data and the distribution of uncertainty based on encountering pressurized brine once in three trials PBRINE 0.0 0.2 0.4 0.6 0.8 1.0 1.2Probability

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Observation-based distribution Subjectively assigned distribution3 rd Response Submittal to the EPA Enclosure 1

26 Estimating PBRINE based on the drilling data

Given the subsequent increase in the number of commercial wells in the area surrounding WIPP and based on the number of encounters of pressurized br ine recorded in the well logs for those wells, DOE’s position is that the uniform distribution assigned to PBRINE and used prior to the CRA-2014 fails to properly characterize the probability of enco untering brine. On one hand, a rate of 60% of wells encountering brine far exceeds the observed rate of brine encounters in the region, even within an area where the frequency of encountering brine is high compared to the average rate across the region surrounding WIPP (Kirchner et al. 2012). On the other hand, a nominal probability of 0.01 for encounters seems low given the drilling data now ava ilable. Furthermore, the drilling data provide information that can be used to define a “mos t likely” value to use as the mode of a unimodal distribution to replace the uniform distribution, hence reducing the weight in the tails of the distribution.

One advantage of using the drilling data over the TD EM data is that the drilling data do not require any consideration of the distribution and geometry of pressurized brine pockets. There are only two assumptions that need to be made about the da ta: 1) the drilling logs identify encounters of pressurized brine pockets having characteristics like those modeled in BRAGFLO, and 2) the area from which the data are tabulated has an equivalent or higher density of pressurized brine pockets as compared to the area under the repository. Having a higher density of brine pockets would lead to a conservative estimate of PBRINE. It is our opinio n that the commercial drilling data are consistent with both of these assumptions. Thus, the drilling data from commercial wells in the vicinity of WIPP were judged to give direct measurements for the probability of encountering brine, thus eliminating many of the uncertainties and unknowns associated with deriving PBRINE from the TDEM data. Arguments in favor of these assumptions were addressed in Kirchner et al. (2012), but will be reiterated and expanded upon herein.

There was an early concern as to whether the freque ncy of brine pocket intrusions in the region around the WIPP site was a suitable estimate for the WIPP site in particular. Powers et al. (1996) addressed that concern by doing an extensive geosta tistical analysis on the drilling data available at that time. They looked for spatial autocorrelations in the data and correlations with factors such as the thickness of the formations. Powers et al. (1996) concluded that the probability of encountering pressurized brine under the waste panels was 0.08, with no estimate of uncertainty on that value. In Kirchner et al. (2012) we selected an area that w ould maximize the observed rate of brine encounters to ameliorate concern about using non-site-specifi c data. Furthermore, we constructed a confidence interval for the estimate of the true probability based on the observed frequency drawing upon statistical theory for Bernoulli pro cesses. What we did not do explicitly in that report was to compare the rate from the reference area to the rate obser ved on and in a band around the WIPP site. That comparison follows.

The number of wells on which to base PBRINE can be expanded from the three on the site by using wells near the WIPP boundary. For example, usi ng the most recent data on wells within a square having borders about 2.5 miles from the WIPP boundar y extends the number of wells to 517 (Fig. 3). Of these, 18 encountered pressurized brine, giving a value of PBRINE of 0.035, or 3.5%. This is lower than the ratio for the region currently used, which is about 5%. However, restrictions on drilling imposed by the state to protect potash resources re sult in some areas near the WIPP showing few or no wells. For that reason we chose to reduce the ch ance of estimating a rate of brine encounters that is lower than that which might be encountered on the WIPP site by analyzing a sub-region to the northeast of the WIPP (Kirchner et al. 2012). This area showed a rate of brine encounters of 12.7% and the distribution employed in CRA-2014 was derive d using that rate and the theory of Bernoulli processes (Kirchner et al. 2012). Thus, the nominal estimate of PBRINE used in the CRA-2014 is at3 rd Response Submittal to the EPA Enclosure 1

27 least twice as large as the regional rate and more th an three times the rate encountered in wells around the WIPP site. We believe that this conservatism, in addition to the uncertain ty associated with that value, should encompass any tendency to underreport brine encounters.



### Figure 4. Wells in close vicinity to WIPP3 rd Response Submittal to the EPA Enclosure 1

28 To summarize:

 The TDEM data only provide a low-resolution, 2-di mensional map of the depth to regions of high conductivity

 The site-specific well data are too sparse to de velop a correlation between TDEM results and the probability of encountering brine

 There are no TDEM data in the areas surrounding WIPP to use to derive a correlation between encountering brine and being in a TDEM-identif ied zone of high conductivity in the Castile

 The assumption that the proportion of the area of the repository underlain by regions of high conductivity ignores the near vertical orientation of the brine reservoirs, and thus overestimates the correlation between encountering brine and being in a TDEM-identified zone of high conductivity in the Castile

 The stochastic analysis, which expanded the propor tion of the area of the repository underlain by regions of high conductivity, assumed that th e error bar on TDEM depth of ±75 m was one sigma, whereas it should have been assumed to be two sigma

 The composite bimodal distribution derived from the analysis assumed equal weighting for using the base of the anhydrite and the base of the Castile , in spite of the expert opinion that brine pockets would be contained within the anhydrite

 The uniform distribution derived from the analysis assigned probabilities to values of PBRINE not represented in the results of the stochastic sampling, which yielded a bimodal distribution

 The uniform distribution has more mass in the left tail than would a modal distribution, like a normal distribution, and hence is less con servative than such a distribution would be.

 The drilling data, in contrast to the TDEM data, provide a direct measurement of the rate of encountering brine that does not depend on a conceptual model for brine reservoirs

 The drilling data, although not site-specific, were selected from an area showing 2 to 3 times the rate of encounters of brine as compared to th e area immediately surrounding the WIPP site, thus providing conservatism

 The distribution of uncertainty on PBRINE estimated from the drilling data can be derived from the theory of Bernoulli processes, and thus is totally objective

EPA has previously argued that the WIPP site’s geology with respect to the abundance of brine pockets could be different from that of neighboring areas, based in part on having an encounter with pressurized brine in one of three wells drilled on the s ite. We have shown that this observed rate has an uncertainty, based on statistical theory, wh ich encompasses the much lower rate of brine encounters in the region. In the absence of evidence to the contrary, we have no reason to believe that the probability of encountering brine on the WIPP s ite would be greater than that observed in neighboring regions and expect it to be considerably lower than that observed in the reference area chosen for our derivation of PBRINE. Given this evaluation, we concluded that 1) the TDEM data method for estimating PBRINE was less credible th an the method involving use of the commercial drilling data, and 2) we could see no way that the TDEM data could contribute to improving the estimate of PBRINE based on the drilling data.3 rd Response Submittal to the EPA Enclosure 1

29 2. This comment appears to be about Kirchner et al. (2012), where we attempted to estimate a conditional probability of encountering brine given th at a well was in an area identified by TDEM as having a zone of high conductivity within the Cas tile. To be clear, that part of the analysis has nothing to do with our approach in deriving a ne w distribution for PBRINE. The reinterpretation of the TDEM data was done simply to make the distributions directly comparable, since in our opinion the calculation of the previous estimate of PBRINE failed to incorporate a reasonable assumption for the conditional probability, and hence was the proba bility of being in an area of high conductivity rather than the probability of encountering brine. Omitting four of the ten panels in our assessment was an error, based on a misinterpretation of th e text, and including them and their estimates of TDEM areas would expand the TDEM-derived distribu tion to the right and also affect the shape of the distribution. However, the Kirchner et al. (2012) analysis also ignored the potential error in interpreting measurement uncertainty (the interpre tation of ±75 m) since there was no way given the available information to reproduce the stochastic an alysis with which the panel-specific areas were derived. Because the position of the maximum value of the distribution is expected to have little influence on releases, as discussed under comment 1, including the area estimates for the four panels when properly deriving the TDEM-based distribution would have little impact of a practical nature and would not improve the credibility of the TDEM-derived distribution.

3. A low-yield, high-volume brine pocket originating from the Castile is not consistent with our understanding of the characteristics of this formation. Ductile rock units behave plastically and exert pressures upon any resident liquids at or approachi ng lithostatic for that unit (Beauheim and Roberts 2002). Therefore low-yield (read: low-pressure) br ine flows from this unit are not observed or reported from drillers in the area. That is, in th e rare instances when brine is encountered from the Castile, it is always found to be at high pressure. In our opinion the conservatisms in our mean rate (a factor of 2-3 larger than the observed rate around the WIPP site) and the uncertainty on that rate should encompass any likely truncation of the detection rate of brine pockets.

4. Although there was a significant increase in the nu mber of wells drilled in the region since 2008, none of the new wells encountered brine. The last brine occurrence recorded was in 2004; therefore, the count remains at 34.

5. To be clear, the neighborhood analysis of drilling events with and without brine encounters was not used in the development of the distribution of PBRINE in the CRA-2014. The purpose of that analysis was to try to derive a conditional probab ility for being in an area having brine pockets and actually encountering brine. The radius of 500 m was shown to maximize the average ratio between wells with brine encounters to wells without enco unters. It was stated in Kirchner et al. (2012) “Although the 1000 m diameter of the circles is somewhat arbitrary, examination of Fig. 5 shows that increasing the diameter will most likely add more wells without brine intrusions than wells with intrusions, and thus drive the average ratio lower. Circles of a smaller diameter would result in many more cases of there being only a single well contained and thus be inadequate for estimating the ratio. Finally, smaller circles would not reflect minimum well spacing requirements for drilling in the Delaware Basin, and would exclude effectively all neighboring wells.” To specifically answer this question, the selection of the radius would have a profound impact on the analysis of the conditional probability for the TDEM data, but we are not advoca ting using the neighborhood analysis to rectify problems with the TDEM data. In our opinion combin ing the TDEM data with this crude estimate of the conditional probability linking TDEM areas to the probability of encountering brine would not improve the credibility of the resulting distributi on given some of the other issues discussed in comment 1.

6. The DBDSP researches the well files of new wells dr illed in the New Mexico portion of the Delaware Basin each year looking for instances of encounter s with pressurized brine. The program also sends3 rd Response Submittal to the EPA Enclosure 1

30 out an annual survey to operators of new wells asking if they encountered pressurized brine during the drilling process. Because stratigraphy changes over the survey area, the depth of brine intercept is not as important as the formation in which brine is encountered. Therefore, the specific depths of brine intercepts are not incorporated into the anal ysis. It is also recognized by industry experience that pressurized brine flows rarely originate from units deeper than the Castile in the WIPP vicinity. Consistent with this, all brine flows reported by area drillers originate from the Castile Formation. Therefore, only Castile brine occurrences are counted by the DBDSP, and only these were used in the data analysis. The wells that did not encounter pressurized brine were all deep boreholes that transcend the Castile to the deeper oil and gas producing zones.

7. To be clear, the neighborhood analysis of drilling events with and without brine encounters was not used in the development of the distribution of PBRINE in the CRA-2014. Table 2 of Kirchner et al. (2012) provides data on the frequency of encountering brine in another intrusion within 500 m of a known brine intrusion. The assumption is that the dr illing/encounter events are independent. The fact that a well having a brine intrusion is near two or more wells also having brine intrusions, hence being counted twice or more, has no impact on the es timated frequencies. Indeed, if there were strong spatial correlations in the locations of wells enc ountering brine, the existence of multiple neighbors having brine would be expected. The motive behind th is neighborhood analysis was to try to convert the previous TDEM distribution into one that w as a better estimate of PBRINE because it included a more realistic value for the correlation between PBRINE and TDEM-identified areas of high conductivity. However, given the flaws in the TD EM analysis addressed in comment 1 and the large uncertainty associated with the estimate of correlati on, we do not see this approach as a viable way to improve the credibility of the estimate of PBRINE based on the TDEM data.

8. With regard to the EPA’s assertion that WIPP-12 brine is assumed to have been transmitted via horizontal to sub-horizontal fractures, we respectfully request the source of this information, as it directly conflicts with information and analyses pr esented in Popeilak et al. (1983) and Borns (1983), which are the primary sources of Castile brine characterization. We are not aware of any characterization data that conflict or refute the curre nt understanding that brine in the Castile is within near-vertical, high-angle fractures.

However, the modified distribution for PBRINE is based on a direct observation of the rate of brine encounters and does not depend on a conceptual mode l of the orientation of brine pockets nor the presence of horizontal or sub-horizontal fractures. PBRINE is not related to the volume of brine encountered, it is just the probability of encount ering brine by wells drilled through the Castile. Indeed, one advantage of using the drilling data is that these data are used to measure PBRINE based on whatever the system of fractures is, rather than a model of what those fractures are likely to be.

We have not attempted to alter the conceptual model for pressurized brine within performance assessment, although many of the assumptions used in the pressurized brine conceptual model are quite conservative, and could benefit from more realism. However, changes of this nature (conceptual model changes) go beyond what we understand to be allowable through the recertification process. Therefore, we have attempted to stay within the content constraints for recertification applications, and have limited changes to an update of the parameter PBRINE. This parameter update is based on our obligation to comp ly with 40 CFR 194.15(a)(2), where it states that recertification applications include “...all additiona l monitoring data, analysis, and results.” The monitoring data collected by the DBDSP show a dramatic increase in drilling activity, but no additional occurrences of pressurized brine within the monitoring area over the last 10 years. The DOE believes that these new data should be recogni zed and incorporated as appropriate into the CRA-2014.3 rd Response Submittal to the EPA Enclosure 1

31 References:

American National Standards Institute (ANSI). 1997. American National Standard for Expressing Uncertainty – U.S. Guide to the Exp ression of Uncertainty in Measurement . ANSI/NCSL Z540-2-1997. American National Standards Institute, NY. *

Beauheim, R.L. and R.M. Roberts. (2002). Hydrol ogy and hydraulic properties of a bedded evaporite formation. Journal of Hydrology 259 (2002): 66-88. *

Borns, D.J. 1983. Petrographic Study of Evaporite Deformation Near the Waste Isolation Pilot Plant (WIPP) . SAND83-0166. June 1983. Sandia Na tional Laboratories, Albuquerque, NM.

Borns, D.J. 1996. Proportion of the waste panel area, WIPP site, t hat is underlain by brine reservoirs in the Castile Formation as Inferred from Transie nt Electromagnetic Method (TEM or TDEM) Surveys

. WPO # 39121. Sandia National Laboratories, Albuquerque, NM.

Hahn, G.J. and W.Q. Meeker. 1991. Statistical Intervals: A Guide for Practitioners . 392 pp. Wiley and Sons, NY. *

Kirchner, T.B., T. Zeitler and R. Kirkes. 2012. Evaluating the data in ord er to derive a value for GLOBAL:PBRINE . ERMS 558724. Sandia National Laboratories, Carlsbad, NM.

Kirchner, T.B. 2013. Sensitivity of the CRA-2014 performan ce assessment releases to parameters

. ERMS 560043. Sandia National Laboratories, Carlsbad, NM.

Peake, T. 1998. Technical Report Review of TDEM Analysis of WIPP Brine Pockets . Prepared for U.S. Environmental Protection Agency, Office of Radiation and Indoor Air, 401 M. Street, S. W., Washington, DC.

Popielak, R.S., Beauheim, R.L., Black, S.B., Coons , W.E., Ellingson, C.T., and R.L. Olsen. 1983. Brine Reservoirs in the Castile Formati on, Waste Isolation Pilot Plant (WIPP), Project Southeastern New Mexico. TME-3153. Vols. 1 and 2. Westinghouse Electric Corporation, Carlsbad, NM.

Powers, D.W., Lambert, S.J., Shaffer, S-E., Hill, L.R., Weart, W.D. (eds.). 1978. Geological Characterization Report Waste Isolation Pilot Plant (WIPP) Site. Southeastern New Mexico . SAND78- 1596, Volume II. Sandia National Laboratories, Albuquerque, NM.

Powers, D., J.M. Sigda and R.M. Holt. 1996. Probability of intercepting a pressurized brine reservoir under the WIPP . ERMS 523414. Sandia National Laboratories Albuquerque, NM.

Sandia. 1992. Preliminary performance assessment for the Waste Isolation Pilot Plant, December 1992. Volume 3: Model Parameters . SAND92-0700/3. Sandia National Laboratories, Albuquerque, NM.

Taylor, B.N. and C.E. Kuyatt. 1994. Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results, NIST Technical Note 1297 . U.S. Department of Commerce, National Institutes of Standards and Technology, Gaithersburg, MD. *

The Earth Technologies Corporation. 1988. Final Report for Time Domain Electromagnetic (TDEM) Surveys of the WIPP Site . SAND87-7144. Sandia National Laboratories, Albuquerque, NM.

* Copyrighted reference not provided in Enclosure 2.3 rd Response Submittal to the EPA Enclosure 1

32 EPA Comment 1-23-8 Fluid Flow in Repository Operations and Experimental Areas. There have been numerous refinements of conceptual and numerical models of repository fluid flow since the 1994-95 time frame as well as changes to the panel closure system that may al so change repository fluid flow. Please provide updated technical justifications for the following par ameter values adopted for the experimental and operations areas:

1. Setting the permeability of the operations and experi mental rooms to a constant value of value 10 -11 m 2 throughout the modeled period. 2. Setting the porosity of the operations and experime ntal rooms to a constant 18% throughout the modeled period. 3. Setting the porosity and permeability of the D RZ adjacent to the operations and experimental room to the same sampled value as the DRZ surrounding a waste panel throughout the modeled period.

DOE Response

1. To remain consistent with previous, peer-review ed conceptual models for the repository and for the Salado, performance assessment assigns cons tant porosity and permeability to the operations and experimental areas. Constant values for por osity and permeability in the experimental and operations areas are based on the concept that it is conservative in terms of releases if these areas have high permeability and low por osity. High permeability ensu res that these regions do not impede brine and gas flow that might contribute to a borehole release and low porosity minimizes the storage capacity for gas in these regions, thus resulting in higher pressures in the waste-filled regions of the repository. The operational and experimental regions in the CRA-2014 PA are assigned a permeability of 10 -11 m 2 and a porosity of 0.18 for the duration of the 10,000-year regulatory period, the same values established a nd used in the CCA, as well as the CRA-2004 and CRA-2009.

Repository performance has been repeatedly shown to be insensitive to dynamic closure in the experimental and operations areas:

1. During the development of the CCA, the DOE investigated whether it was important to include a dynamic model of creep closure in the experimental and operations areas. Calculations demonstrated that performan ce was quite insensitive to whether dynamic closure in these areas was included or not (Vaughn 1996).

2. In response to a similar comment from the EPA (C-23-4), supplemental calculations to the CRA-2004 submittal demonstrated that the assignment of constant porosity and permeability in the experimental and ope rations areas remained a conservative assumption (Detwiler 2004).

3. Following the CRA-2009 PABC, an impact assess ment was undertaken to investigate the impact of additional mined volume in th e WIPP experimental region on repository performance. An additional 60,335 m 3 were added to the CRA-2009 PABC BRAGFLO representation of the WIPP experimental region. It was found that increased volume in the experimental region had no impact on regulatory compliance curves obtained in the CRA-2009 PABC (Camphouse et al. 2011).

4. The Salado flow analysis performed for the CRA-2014 PA compared results from the CRA-2009 PABC, the PCS-2012 PA, and Case CRA14-BL. In other words, it compared3 rd Response Submittal to the EPA Enclosure 1

33 Salado flow baseline results where the Option D PCS was implemented, results where the Option D PCS was replaced by the ROMPCS, and results where the ROMPCS was implemented with 60,335 m 3 of additional excavated volume added to the experimental region in BRAGFLO. Waste region pressure a nd brine saturation results for intrusion scenarios were found to be similar in all three cases (Camphouse 2013).

There have been no new conditions or changes th at render the current representation inadequate or inconsistent with our current understanding of reposito ry behavior. There is no reason at this time to modify the constant porosity and permeability values assi gned to the operations and experimental areas.

2. The response to item 2 is contained in the response to item 1.

3. Parameter values for the DRZ adjacent to waste panels, and the operati ons and experimental regions, are defined for two time periods. The fi rst time period corresponds to the time associated with waste emplacement in a fresh panel, and endu res for 5 years prior to repository closing (-5 to 0 years). The second time period corresponds to the sealed repository time period from 0 to 10,000 years. At the start of the first time peri od (the instant the excavation is made), the DRZ is assumed to be fully brine-saturated and in hydros tatic equilibrium with the rest of the Salado Formation. For the second time period, the DR Z is assumed to be a well-developed region characterized by vertical fractures. Because of gas generation and the consequently increasing pressure in waste panels, the DRZ above and be low repository waste regions is assumed not to heal. The effect of gas pressure is to reduce back stress applied by the waste. Moreover, pressure is gradually transmitted from the repository w aste panels to the northern operations and experimental regions. As a result, the DRZ a bove and below the operations and experimental regions is also assumed not to heal. As incr easing pressure is the driving mechanism inhibiting healing of the DRZ, and pressure increase is seen in repository waste regions as well as the northern operations and experimental areas, the por osity and permeability of the DRZ adjacent to the operations and experimental regions are set to the same sampled values as the DRZ surrounding waste panels. This implementation rema ins consistent with previous, peer-reviewed conceptual models for the reposito ry and for the DRZ. There is no reason to modify these values at this time.

References:

Camphouse, R.C. 2013. Analysis Package for Salado Flow Modeling Done in the 2014 Compliance Recertification Application Performance Assessment (CRA-2014 PA) . ERMS 559980. Sandia National Laboratories, Carlsbad, NM.

Camphouse, R.C., D.C. Kicker, T.B. Kirchner, J.J. Long, and J.J. Pasch. 2011. Impact Assessment of SDI Excavation on Long-Term WIPP Performance . ERMS 555824. Sandia National Laboratories, Carlsbad, NM.

Camphouse, R.C., D.C. Kicker, T.B. Kirchner, J.J. Long, B. Malama, and T.R. Zeitler. 2012. Summary Report and Run Control for the 2012 WIPP Pane l Closure System Performance Assessment . ERMS 558365. Sandia National Laboratories, Carlsbad, NM.

Camphouse, R.C., D.C. Kicker, S. Kim, T.B. Kirchner, J.J. Long, B. Malama, and T.R. Zeitler. 2013. Summary Report for the 2014 WIPP Compliance Rec ertification Application Performance Assessment

. ERMS 560252. Sandia National Laboratories, Carlsbad, NM.3 rd Response Submittal to the EPA Enclosure 1

34 Detwiler, R.P. 2004. Partial Response to Envir onmental Protection Agency (EPA) May 20, 2004 Letter on CRA. ERMS 537372. Sandia National Laboratories, Carlsbad, NM.

Vaughn, P. 1996. Phase I FEPS Composite FEP Records Screening Analysis . ERMS 418211. Sandia National Laboratories, Carlsbad, NM.3 rd Response Submittal to the EPA Enclosure 1

35 EPA Comment 2-43-1 Changes in Passive Institutional Controls (PICs). Recent Nuclear Energy Agency and International Atomic Energy Agency reports descri be changes and developments in international approaches to PICs. These are referenced in INIS-US-13-WM-13145 which states “The DOE/CBFO WIPP PIC’s program in place today meets the re gulatory criteria, but complete feasibility of implementation is questionable, and may not be in conformance with the international guidance being developed.” Please explain this feasibility concern . Please also provide the complete INIS-US-WM13145 report (the Web link only provides an Abstract) and any other recent studies or reports that may impact PICs planning in the future.

Reference:

INIS-US-13-WM-13145, “The Revised WIPP Passive Int ernational Controls Program – A Conceptual Plan – 13145, Dated 2013-07-01, Web link: http://www.osti.gov/scitech/biblio/22225507

DOE Response

The reference INIS-US-13-WM-13145 is a Waste Mana gement Symposia 2013 paper that outlines some preliminary thoughts on developing a revised WIPP PI Cs program. The paper is provided with this response and discusses the feasibility concerns in EPA comment 2-43-1.

DOE will discuss with EPA the current status of th e development of international guidelines for PICs, which is outside of the context of the CRA-2014 completeness review.3 rd Response Submittal to the EPA Enclosure 1

36 EPA Comment 2-55-1 Incorrect Reference. IGP-2014, Section IGP-2.1 makes reference to 194.55(b)(1), should this be 194.54(b)(1) “Existing boreholes...”?

DOE Response

This is a typographical error in Appendix IGP-2014. The text will be corrected to read “Existing boreholes, as required by 40 CFR § 194.54(b)(1)...”.

This typographical error has been added to Enclosure 4, CRA-2014 Errata Tracking.3 rd Response Submittal to the EPA Enclosure 1

37 EPA Comment 2-C-7 MgO Hydration Rate Data File. Please provide a copy of the Microsoft Excel file “hydration kinetics Q & HY2 & HH djc 5-1-07.xls” used by Nowak and Clayton (2007) to calculate the MgO hydration rates.

References:

Nowak, E.J. and D. Clayton. 2007. Analysis of MgO Hydration Laboratory Results and Calculation of Extent of Hydration and Resulting Water Uptake versus Time under Postulated WIPP Conditions. Sandia National Laboratories ERMS 546769.

DOE Response

The requested documentation is included in Enclosure 2.3 rd Response Submittal to the EPA Enclosure 1

38 EPA Comment 2-C-8 Iron and Lead Corrosion Rate Data. Please provide spreadsheets containing the iron and lead corrosion data listed in Appendix A, Tables A-1 and A-2 from Roselle (2013).

References:

Roselle, G.T. 2013. Determination of Corrosion Rates from Iron/Lead Corrosion Experiments to be Used for Gas Generation Calculations. Sandia National Laboratories ERMS 559077.

DOE Response

The requested documentation is included in Enclosure 2.