NAUGHTON POWER PLANT NORTH ASH POND INITIAL CLOSURE PLAN

NAUGHTON POWER PLANT NORTH ASH POND INITIAL CLOSURE PLAN Prepared for: PacifiCorp Energy Project Manager: Chad Tomlinson Date: 01 September 2016 Document No.: NP-TR-022 Quality Assurance Statement Office Address Prepared by Reviewed by Approved for Issue by Revision Schedule MWH 2890 Cottonwood Parkway, Suite 300 Salt Lake City, UT 84121 Luis Rodriguez Mike Witler Chad Tomlinson Rev No. Date Description Prepared By Reviewed By Approved By 0 09/01/16 Issued For Use L. Rodriguez M. Witler C. Tomlinson Comments: This document has been prepared for the benefit of PacifiCorp in accordance with the generally accepted practices and standards in use at the time it was prepared. No liability is accepted by this company or any employee or sub-consultant of this company with respect to its use by any other person. This disclaimer shall apply notwithstanding that this document may be made available to other persons for an application for permission or approval to fulfill a legal requirement.

CERTIFICATION I hereby certify that this Initial Written Closure Plan for North Ash Pond located at PacifiCorp s Naughton Power Plant has been prepared in accordance with 257.102 Criteria for Conducting the Closure or Retrofit of CCR Units of the Coal Combustion Residuals (CCR) Rule - 40 CFR Parts 257 and 261, Hazardous and Solid Waste Management System; Disposal of Coal Combustion Residuals From Electric Utilities; Final Rule published in the Code of Federal Regulations (CFR) on April 17, 2015 with an effective of October 19, 2015. By: Date: 09/01/2016 Chad Tomlinson, P.E. Wyoming P.E.# 15375

TABLE OF CONTENTS 1.0 INTRODUCTION... 1 1.1 PURPOSE AND SCOPE... 1 1.2 REPORT ORGANIZATION... 1 2.0 PROJECT BACKGROUND... 2 2.1 NAP CLOSURE DESIGN DETAILS... 2 2.2 CLOSURE REQUIREMENTS FOR ACTIVE SURFACE IMPOUNDMENTS... 2 3.0 PRELIMINARY CLOSURE DESIGN... 4 3.1 DEWATERING AND EXCAVATION OF ASH WASTE MATERIALS... 4 3.2 COVER SYSTEM... 4 3.2.1 Cover Geometry... 4 3.2.2 Erosion Protection Layer... 5 3.2.3 Stormwater Controls... 5 3.3 COVER CONSTRUCTION... 5 3.3.1 Cover Subgrade Installation... 5 3.3.2 Cover Soil Placement... 5 3.3.3 Seeding... 5 4.0 PRELIMINARY CLOSURE SCHEDULE... 7 5.0 REFERENCES... 8 APPENDICES Appendix A Preliminary Closure Schedule LIST OF TABLES Table 2.1 Applicable CCR Criteria for Active Surface Impoundments (257.100(b))... 2 Table 2.2 Key Operating Parameters for the NAP... 2 LIST OF FIGURES Figure 2-1 Site Vicinity Map Figure 2-2 North Ash Pond Plan View Map Figure 3-1 North Ash Pond Closure Schematic Status: Rev. 0 i 01 September 2016 Document No: NP-TR-022

1.0 INTRODUCTION This Initial Written Closure Plan (Closure Plan) has been prepared to describe the activities to be performed to obtain final closure of PacifiCorp s Naughton Power Plant (the Plant) North Ash Pond (NAP). The site is located approximately six miles southwest of Kemmerer, Wyoming. The Pond has been used to store bottom ash and fly ash material, which is sluiced with process water from the Unit 3 power generation facility through high density polyethylene (HDPE) pipes and open concrete channels. This Closure Plan has been prepared for PacifiCorp by MWH Americas, Inc. to meet the requirements of 257.102(b)(2) of the Hazardous and Solid Waste Management System; Disposal of Coal Combustion Residuals From Electrical Utilities; Final Rule (CCR Rule; EPA, 2015). The closure of the NAP is proposed to consist of the following tasks: Cessation of flows to the Pond following shutdown of the plant; Dewatering of ash in the Pond via evaporation and mechanical dewatering; Construction of a soil cover over the NAP; Construction of stormwater conveyance systems; and Post-closure maintenance and monitoring. 1.1 PURPOSE AND SCOPE The purpose of this document is to present the closure plan for the NAP in conformance with applicable regulatory requirements. This document provides a detailed description of the activities to be performed as part of the proposed closure plan, to close and cover the NAP with the ash in place. These activities include decanting of water in the NAP, dewatering and potential consolidation of the fly ash, construction of a soil cover and associated stormwater controls, and revegetation of cover surface. The cover system associated with the closure of the NAP will be designed to meet the EPA regulations for Disposal of Coal Combustion Residuals from Electric Utilities (EPA, 2015) as discussed in Section 2.2. 1.2 REPORT ORGANIZATION Section 2.0 Project Background Section 3.0 Preliminary Closure Design Section 4.0 Preliminary Closure Schedule Section 5.0 References Status: Rev. 0 1 01 September 2016 Document No: NP-TR-022

2.0 PROJECT BACKGROUND The Naughton Plant is located approximately six miles southwest from the town of Kemmerer, Wyoming as shown in Figure 2-1. The Plant consists of three coal-fueled units (Units 1, 2, and 3) rated with a net dependable capacity of 156 megawatt (MW), 201 MW, and 280 MW, respectively. Coal combustion by-products at the Plant include several types of materials: bottom ash, fly ash, and flue gas desulfurization materials. 2.1 NAP CLOSURE DESIGN DETAILS The closure of the NAP will include consolidating/dewatering the ash and constructing a soil cover in accordance with 257.102(d) as shown in Figure 2-2. Therefore, the closure parameters presented in Table 2.1. Design High Water Level Elevation (feet above mean sea level) Table 2.1 - Approximate Closure Parameters, NAP Assumed Closure Parameters Maximum Operating Capacity (acreft) at High Water Level Maximum Volume of CCR Stored (cubic yards) Maximum Area of CCR to be Capped (acres) 6,900.50 2,235 2,600,000 ~45 2.2 CLOSURE REQUIREMENTS FOR ACTIVE SURFACE IMPOUNDMENTS On April 16, 2015, the EPA established nationally applicable minimum criteria for the safe disposal of coal combustion residuals (CCR) in landfills and surface impoundments. The NAP is considered an active surface impoundment since it will be receiving CCR after the effective date. This closure plan will be released in accordance to the EPA rule regarding the completion of a closure plan for an operational CCR storage facility by October 17, 2016. Specifically, the new EPA rule includes the following requirements, presented in Table 2.2, for the closure of an active CCR surface impoundment: Table 2.2 - Applicable CCR Criteria for Active Surface Impoundments Section 257.102(d) (1)(i) (1)(ii) Description of Requirement Control, minimize, or eliminate, to the maximum extent feasible, post-closure infiltration of liquids into the waste and releases of CCR, leachate, or contaminated run-off to the ground or surface waters or to the atmosphere Preclude the probability of future impoundment of water, sediment, or slurry Naughton NAP Closure Design The cover will act to prevent infiltration of liquids into the lined pond and prevent runoff from contacting the CCR. The cover and surrounding area will be graded to shed stormwater away from the cover. Diversion channels will be maintained upstream of the pond to prevent run-on from precipitation. Status: Rev. 0 2 01 September 2016 Document No: NP-TR-022

Section 257.102(d) (1)(iii) (1)(iv) (2)(i) (2)(ii) (3)(i)(A) (3)(i)(B) (3)(i)(C) Description of Requirement Include measures to provide for major slope stability to prevent the sloughing or movement of the final cover system Minimize the need for further maintenance of the CCR unit Eliminate free liquids by removal or solidifying remaining wastes and waste residues Stabilize remaining wastes to sufficiently support final cover system Design and construct a final cover system with a permeability less than or equal to the permeability of any bottom liner system the NAP is unlined), or natural subsoils present, or a permeability no greater than 1x10-5 cm/sec, whichever is less Minimize infiltration of liquids through the closed CCR unit by the use of an infiltration layer that contains a minimum of 18 inches of earthen material Minimize erosion of the final cover system by the use of an erosion layer that contains a minimum of six inches of earthen material capable of sustaining native plant growth Naughton NAP Closure Design A slope stability assessment will need to be completed as part of the closure plan and will identify any changes that need to be made to the design in order to meet this requirements. The cover will be vegetated with a native seed mix. Once established, the cover will require little or no longterm maintenance. The closure of the NAP will include the decanting of the pond to remove standing water followed by dewatering/stabilization and consolidation of the ash prior to constructing the final cover. Given that the NAP is unlined, the final cover will be designed to achieve a permeability less than the compacted subgrade or no greater than 1x10-5 cm/sec, whichever is less The design of the final cover system will consist of at least 18-inches of earthen material. The design of the soil cover includes a 6-inch thick erosion protection layer. The erosion protection layer will be fertilized and seeded with a native seed mix to establish vegetation. (3)(i)(D) Minimize disruption of the integrity of the final cover system through a design that accommodates settling and subsidence Source: US EPA CCR Regulations (EPA, 2015) The soil cover will be designed with a sufficient slope to account for settling and subsidence. Status: Rev. 0 3 01 September 2016 Document No: NP-TR-022

3.0 PRELIMINARY CLOSURE DESIGN The following sections contain an overview of the anticipated closure activities for the Pond. The closure approach for the NAP has been developed in accordance with 257.102(d) Closure performance standards when leaving CCR in place to achieve the following performance objectives: Eliminate fugitive dust from the dried fly ash and bottom ash; Reduce infiltration of precipitation into the ash material; Minimize disruption of the integrity of the final cover system by accommodating expected settling and subsidence; Provide protection from wind and water erosion, including run-on and run-off; Eliminate potential long-term impact on groundwater; Minimize long-term operation and maintenance. Closure Plan activities to achieve the performance objectives include dewatering of the waste material, construction of a soil cover over the Pond, establishing vegetation on the soil cover, and construction of stormwater control features. In addition, site monitoring will be continued to track the performance of the implemented closure. The proposed closure approach is illustrated in Figure 3-1, and the major Closure Plan activities are outlined in the following subsections. 3.1 DEWATERING AND EXCAVATION OF ASH WASTE MATERIALS Dewatering of ash will first be accomplished by decanting the existing sluiced water over the ash. The decanted water will be evaporated via natural evaporation and if necessary through mechanical evaporation. Following removal of standing water, the ash will be dewatered prior to construction of the final cover. The exact method of dewatering will be developed as part of the detailed closure design for the NAP. 3.2 COVER SYSTEM The cover system will be designed to minimize infiltration of precipitation into the ash, control runoff, sustain native vegetation, minimize erosion, and require minimal maintenance in accordance with 257.102(d)(3). The cover system surface grading and vegetation requirements will be designed to provide protection against wind and surface water erosion by incorporating a minimum of 6-inch thick protective soil layer. 3.2.1 Cover Geometry A minimum 18-inch thick cover system is proposed for the NAP. The cover will be constructed with a minimum slope of 1.0% to reduce the potential for long-term erosion. The cover will be vegetated with native plant species with sufficient rooting depths and overlapping growth seasons to maximize Status: Rev. 0 4 01 September 2016 Document No: NP-TR-022

evapotranspiration. Design drawings for the final cover will be developed as part of the detailed design. 3.2.2 Erosion Protection Layer At a minimum, the cover will include a 6-inch erosion protection layer consistent with 257.102(3)(c) to provide a total cover thickness of 24-inches. 3.2.3 Stormwater Controls The sloped cover area and the surrounding bases of the slope will need to be constructed with stormwater control structures in order to divert water away from the graded cover to prevent pooling and infiltration. This collection system will be designed to safely convey the runoff from a 100-year 24-hour storm event. 3.3 COVER CONSTRUCTION The activities required to construct the soil cover are described in the following subsections. The activities are described in the order that they will be performed. Design drawings and construction specifications for the soil cover and associated stormwater controls will be as part of the detail design. 3.3.1 Cover Subgrade Installation Soil from the removed embankments of the NAP will be used to construct the subgrade of the soil cover and if necessary will be augmented by a borrow source. Although constructed of the similar material as the cover soil layer, the soil placed as part of the cover subgrade (general fill) will be compacted at a higher density to reduce the potential of settlement within the soil cover itself. The soil cover subgrade will be constructed to the design grades and placement specifications that will be developed as part of the detailed design. 3.3.2 Cover Soil Placement The cover soil will be generated from a borrow area in as close a location as practical. Given that the main objective of the cover soil layer is to provide a suitable bedding surface for vegetation, the cover soil layer will be compacted to a lower density than the cover subgrade. To achieve a uniform slope throughout the soil cover, and limit compaction, the cover soil layer will be placed in lifts using a lowpressure tracked dozer. An additional six inches of soil will be placed over the 18-inch cover soil to provide growth media for the vegetative cover seed and erosion protection. 3.3.3 Seeding Following tilling of the final cover, seed will be drilled evenly over the entire area. Seeding will be drilled in late Fall (mid-october or later) before the soil freezes or is covered with snow or in early Status: Rev. 0 5 01 September 2016 Document No: NP-TR-022

Spring (before the first of May) when the ground thaws. Reclamation seed mixtures and application rates will be developed as part of the detailed cover design to return the area to a pre-disturbance condition. Status: Rev. 0 6 01 September 2016 Document No: NP-TR-022

4.0 PRELIMINARY CLOSURE SCHEDULE A schedule for completing all activities necessary to complete the closure of the NAP in accordance with 257.102(c) is provided in Appendix A. The schedule is based on the Naughton Plant being shutdown in December 31, 2029, and has been developed to show the sequential order of closure activities as well as major milestones. Status: Rev. 0 7 01 September 2016 Document No: NP-TR-022

5.0 REFERENCES EPA, 2015. Hazardous and Solid Waste Management System; Disposal of Coal Combustion Residuals From Electric Utilities; Final Rule. Federal Register Vol. 80, No 74, Part II 40 CFR Parts 257 and 261, April17, 2015 Status: Rev. 0 8 01 September 2016 Document No: NP-TR-022

Figures

310 14 120 IDAHO 89 14 Basin 14 90 Worland Gillette Newcastle 25 Jackson Thermopolis 20 26 189 287 220 Wheatland 191 G 0 5000 Feet Lusk 87 Douglas 26 80 Evanston Casper Green River E 20 Lander Kemmerer Sundance Buffalo 16 Pinedale 27 Jun 2016 Sheridan SOUTH DAKOTA Cody 191 UTAH 30 Rawlins 80 Laramie Torrington 25 85 Cheyenne 189 NEBRASKA DRAWN BY D. Severson MONTANA COLORADO PROJECT LOCATION 30 C:\Data\MWH\NAUGHTON POWER PLANT\FIGURES_14Jun2016\FIGURES\Fig 1-1_Site Vicinity Map_27Jun2016.mxd Kemmerer North Ash Pond Naughton Power Plant 30 189 PACIFICORP NAUGHTON POWER PLANT SITE VICINITY MAP Figure 2-1

DRAWN BY D. Severson E 0 800 Feet FGD Pond 1 FGD Pond 2 8 Jul 2016 Approximate area of NAP to be closed Approximate Location of NAP/NCWP Discharge Structure C:\Data\MWH\HOLDEN\HOLDEN_BarrierWall_Jun2016\FIGURES\Fig 2-2_North Ash Pond Layout_8Jul2016.mxd North Ash Pond Area to be used for ash consolidation during retrofit of NAP North Clear Water Pond NAP Intermediate Dike NCWP Discharge Structure South Clear Water Pond East Saddle Dike NAP/NCWP Main Dike PACIFICORP NAUGHTON POWER PLANT NORTH ASH POND LAYOUT Figure 2-2

C:\Data\MWH\NAUGHTON POWER PLANT\FIGURES_Oct2015\FIGURES\Fig 3-1_North Ash Pond_Initial Written Closure_8Jul2016 8 Jul 2016 DRAWN BY D. Severson STEP 1 Cease ash flows to NAPL following shutdown of Units 1 and 2 STEP 2 Evaporate and/or decant standing water STEP 3 AND 4 Dewater and consolidate ash NORTH ASH POND Embankment to be used as general fill and augmented by onsite borrow source COVER SUBGRADE STEP 5 Utilize soil from embankment to develop slope for cover over ash FINAL COVER COVER SUBGRADE STEP 6 Construct final cover and stormwater control PACIFICORP NAUGHTON POWER PLANT PRELIMINARY CLOSURE SCHEMATIC Figure 3-1

Appendices

Appendix A Preliminary Closure Schedule

ID Task Mode Task Name Duration Start Finish 1 Final Receipt of CCR into NAP 0 days Mon 31/12/29 Mon 31/12/29 Preliminary Closure Schedule North Ash Pond Naughton Power Plant 2030 2031 2032 2033 2034 Qtr 4 Qtr 1 Qtr 2 Qtr 3 Qtr 4 Qtr 1 Qtr 2 Qtr 3 Qtr 4 Qtr 1 Qtr 2 Qtr 3 Qtr 4 Qtr 1 Qtr 2 Qtr 3 Qtr 4 Qtr 1 Qtr 2 Qtr 3 Qtr 4 31/12 2 NAP Closure Design and Permitting 60 days Mon 9/02/32 Mon 3/05/32 3 Develop Detailed Closure Plan 20 days Mon 9/02/32 Mon 8/03/32 4 Submit Detailed Closure Plan to Wyoming Department of Environmental Quality 0 days Mon 8/03/32 Mon 8/03/32 8/03 5 WDEQ Review of Detailed Closure Plan 6 Amend Detailed Design Plan per WDEQ Comments 20 days Mon 8/03/32 Mon 5/04/32 10 days Mon 5/04/32 Mon 19/04/32 7 Submit Revised Detailed Design Plan to WDEQ 8 WDEQ Approval of Detailed Design Plan 0 days Mon 19/04/32 Mon 19/04/32 10 days Mon 19/04/32 Mon 3/05/32 19/04 3/05 Task Project Summary Inactive Milestone Manual Summary Rollup Deadline Project: Preliminary Closure Sched Date: Tue 23/08/16 Split Milestone External Tasks External Milestone Inactive Summary Manual Task Manual Summary Start only Progress Manual Progress Summary Inactive Task Duration only Finish only Page 1

ID Task Mode Task Name Duration Start Finish 9 NAP Closure Activities 1285 days Mon 31/12/29Fri 1/12/34 Preliminary Closure Schedule North Ash Pond Naughton Power Plant 2030 2031 2032 2033 2034 Qtr 4 Qtr 1 Qtr 2 Qtr 3 Qtr 4 Qtr 1 Qtr 2 Qtr 3 Qtr 4 Qtr 1 Qtr 2 Qtr 3 Qtr 4 Qtr 1 Qtr 2 Qtr 3 Qtr 4 Qtr 1 Qtr 2 Qtr 3 Qtr 4 10 Decant and Evaporate Surface of NAP 11 Closure Contractor Mobilization 600 days Mon 31/12/29 Fri 16/04/32 5 days Mon 3/05/32 Fri 7/05/32 7/05 12 Dewater Ash 137 days Mon 10/05/32 Tue 16/11/32 13 Construct Cap Subgr131 days Wed 18/05/33 Wed 16/11/33 14 Construct Final Cove131 days Thu 18/05/34 Thu 16/11/34 15 Seed Final Cover 10 days Fri 17/11/34 Thu 30/11/34 16 Final Inspection 1 day Fri 1/12/34 Fri 1/12/34 1/ 17 Closure Certification 10 days Mon 4/12/34 Fri 15/12/34 Task Project Summary Inactive Milestone Manual Summary Rollup Deadline Project: Preliminary Closure Sched Date: Tue 23/08/16 Split Milestone External Tasks External Milestone Inactive Summary Manual Task Manual Summary Start only Progress Manual Progress Summary Inactive Task Duration only Finish only Page 2