B08_-Section_J_-_Attachment_10-Geotechnical_Data_Report_Part1.pdf

SUBMITTED TO:

Jacobs Government Services Company 9191 Jamaica Street Englewood, CO 80112

BY:

Shannon & Wilson 5900 W. 38th Avenue Wheat Ridge, CO 80212

(303) 825-3800 www.shannonwilson.com

GEOTECHNICAL DATA REPORT

Replace and Rehabilitate WWTPs at Yellowstone National Park

WYOMING

July 23, 2024

Shannon & Wilson No: 107560-005

Replace and Rehabilitate WWTPs at Yellowstone National Park Geotechnical Data Report

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CONTENTS

1 Introduction

1.1 Scope of Services

1.2 Authorization

2 Description of Project Areas

2.1 Project Areas

2.2 Canyon Village Project Area

2.2.1 Government Lift Station and Collection System

2.2.2 Canyon Village WWTP

2.2.3 Brink of the Upper Falls Lift Station

2.3 Grant Village Project Area

2.3.1 Original WWTP Area

2.3.2 Grant Village WWTP

2.3.3 Campground Lift Station

2.4 Old Faithful Project Area

2.4.1 Old Faithful WTP

2.4.2 Finished Water Main

3 Existing Geotechnical Information

3.1 Canyon Village

3.1.1 Northern Testing Laboratories, Inc

3.1.2 Chen Northern, Inc

3.2 Grant Village

3.3 Old Faithful

3.3.1 WTP Site

3.3.2 West of Finished Water Main Site

4 Preliminary Design Geotechnical Survey

4.1 Field Reconnaissance and Test Pits

4.2 Geophysics

4.2.1 Canyon Village WWTP Geophysics

4.2.2 Canyon Village WWTP Government Lift Station Geophysics

4.2.3 Grant Village WWTP Geophysics

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4.2.4 Old Faithful WTP and Finished Water Main Geophysics

5 Field Exploration and Laboratory Testing

5.1 Subsurface Explorations

5.2 Slug Testing

5.3 Laboratory Testing

6 Geologic Setting

6.1 Canyon Village

6.1.1 Canyon Rhyolite Flow and Tuff of Sulfur Creek

6.1.2 Sediments of Lower Falls

6.1.3 Sediments of Inspiration Point

6.1.4 Pinedale Till

6.1.5 Pinedale Stream Deposits

6.2 Grant Village Geologic Conditions

6.2.1 Shoshone Lake Tuff Member

6.2.2 Aster Creek Rhyolite Flow

6.2.3 Pinedale Ice-Dammed Sediments

6.2.4 Open Lake Sediments

6.3 Old Faithful Geologic Conditions

6.3.1 Pleistocene - Scaup Lake Flow

6.3.2 Late Pleistocene - Pinedale Glaciation Deposits

6.3.3 Holocene - Siliceous Hot-Spring Deposits

7 Subsurface Conditions

7.1 Canyon Village Subsurface Conditions

7.1.1 Government Lift Station

7.1.2 WWTP Site

7.1.3 Brink of the Upper Falls Lift Station

7.2 Grant Village Subsurface Conditions

7.2.1 Original WWTP Site

7.2.2 WWTP Site

7.2.3 Campground Lift Station

7.3 Old Faithful Subsurface Conditions

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7.3.1 WTP Site

7.3.2 Finished Water Main

7.4 Subsurface Variation

8 Closure

9 References

Tables Table 1: Summary of Borings Table 2: Summary of Groundwater Levels

Figures Figure 1: Vicinity Map Figure 2: Canyon Village Site and Exploration Plan Figure 3: Grant Village Site and Exploration Plan Figure 4: Old Faithful Site and Exploration Plan Figure 5: Canyon Village Geologic Map Figure 6: Grant Village Geologic Map Figure 7: Old Faithful Geologic Map

Appendices Appendix A: Canyon Village Site Reconnaissance Appendix B: Grand Village Site Reconnaissance Appendix C: Old Faithful Site Reconnaissance Appendix D: Geophysical Results Appendix E: Subsurface Explorations Appendix F: Laboratory Test Results Important Information

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1 INTRODUCTION

Geotechnical explorations are required for the proposed rehabilitation/replacement of three water and wastewater treatment plants (WTPs/WWTPs) and associated collection systems at Yellowstone National Park (Park) in Wyoming. The Project includes the following aspects and project areas:

Canyon Village Wastewater Treatment Plant and Collection System (Canyon Village);

Grant Village Wastewater Treatment Plant and Collection System (Grant Village); and

Old Faithful Water Treatment Plant and Related Infrastructure (Old Faithful).

Jacobs Government Services Company (Jacobs) is leading the civil engineering aspects of this Project on behalf of the National Park Service (NPS). Boring locations were selected in conjunction with Jacobs and Park personnel, based on the schematic design (SD) level plans for each project area (Jacobs, 2022a-2022c).

1.1 Scope of Services

Shannon & Wilson’s scope of work for the Project included a geotechnical survey (site reconnaissance and geophysical survey) performed in 2021 during preliminary design (PD).

Upon completion of this task, an initial interpretation of anticipated geologic conditions was presented in a Report of Anticipated Geologic Conditions (Shannon & Wilson, 2022a). The geologic interpretation presented in that report are superseded by those included in this report.

Shannon & Wilson performed a geotechnical investigation (subsurface exploration program and laboratory testing program) in 2022 upon completion of SD. The subsurface investigation included drilling 34 borings and installing six groundwater monitoring wells.

Our initial scope of work included 38 borings, but four of them were unable to be performed due to access constraints (borings SW-C-02 and SW-C-03), utilities (boring SW-C-04), or required offset from the existing pipeline (SW-OF-11). Our subsurface investigation was developed to support development of a design-build request for proposal. At that time, design details including the type and location of required structures and associated structure loads were unknown. For this reason, the number and location of borings is limited. The project delivery method was later changed to design-bid-build.

We also performed laboratory tests on selected soil and rock samples collected from the borings. Shannon & Wilson updated the geotechnical information based upon the results of this subsurface exploration program (Shannon & Wilson, 2022b, 2023a, and 2023b). The

107560-005 July 23, 2024 geologic interpretation and information presented in those reports are superseded by those included in this report.

No engineering recommendations are provided in this report; a separate Geotechnical Report has been prepared for this purpose (Shannon & Wilson, 2024).

1.2 Authorization

Shannon & Wilson prepared this report and participated in this Project as a subconsultant to Jacobs. Our scope of services was specified in Professional Services Task Order Number 148027717 (dated October 18, 2021), Professional Services Task Order Number 148045034 (dated August 22, 2023), Professional Services Task Order Number 148047195 (dated November 29, 2023), Professional Services Task Order Number 148045034 Revision 01 (dated April 25, 2024), and Professional Services Task Order Number 148047195 Revision 02 (dated May 14, 2024).

2 DESCRIPTION OF PROJECT AREAS

2.1 Project Areas

The Project involves replacement and/or rehabilitation of wastewater treatment, water treatment, and collection system components in three project areas located throughout Yellowstone National Park (Figure 1). The project areas are identified as:

Canyon Village Wastewater Treatment Plant and Collection System (Canyon Village);

Grant Village Wastewater Treatment Plant and Collection System (Grant Village); and

Old Faithful Water Treatment Plant and Related Infrastructure (Old Faithful).

Jacobs provided Shannon & Wilson with overview maps, map books, and site plans of the various project areas (Jacobs, 2021a-2021j) during the PD phase in 2021, followed by SD plans in 2022 (Jacobs 2022a-2022c), Design Development (DD) plans in 2024 (Jacobs, 2024a- 2024d), and Construction Document (CD) plans in 2024 (Jacobs, 2024e and 2024f). The proposed improvements are summarized below and shown in plan in Figures 2, 3, and 4.

2.2 Canyon Village Project Area

The Canyon Village project area is located approximately 13 miles northwest of Yellowstone Lake. This geotechnical investigation included examination of three sites, as shown in Figure 2. The first site is the existing Government Lift Station and associated collection system, which is located north of Norris Canyon Road. The second site is the existing

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Canyon WWTP site north of North Rim Drive. The third and final site is the Brink of the Upper Falls Lift Station. For mapping purposes, an additional site was established along North Rim Drive to evaluate bedrock exposures along the roadway. Site observations and photographs are included in Appendix A.

Both the Government Lift Station, the Brink of the Upper Falls Lift Station, and the bedrock exposures along North Rim Drive are accessed using roads open to the public. Access to the existing Canyon WWTP is through a gate that is not generally open to the public.

The WWTP was constructed in the early 1990s. According to NPS, the system is showing continued signs of deterioration. The collection system experiences high levels of inflow and infiltration, which can overwhelm the WWTP during peak runoff.

2.2.1 Government Lift Station and Collection System

This site is located approximately 700 feet north of the Canyon Village Visitor Center, north of Norris Canyon Road. Access to this site is from Norris Canyon Road or Grand Loop Road, both of which are closed to regular wheeled traffic during the winter. The topography at this site is generally flat and heavily wooded.

The existing force main in this area is approximately 1,000 feet long. It extends from the Government Lift station near manhole CA2041 at its western end to manhole CA2307 at its eastern end. The eastern end of the force main is located adjacent to the parking lot of a gas station, which is approximately 300 feet north of the intersection.

Based on draft CD plans (Jacobs, 2024e), a new lift station will be constructed adjacent to the existing lift station along with a generator building; the existing lift station will be demolished. The new lift station is proposed to have portions of the foundation up to four feet below finished grade. Additionally, the existing force main will be replaced with approximately 1,500 feet of sanitary sewer south of Norris Canyon Road. The new section of sanitary sewer will include two trenchless road crossings under Norris Canyon Road and Grand Loop Road.

2.2.2 Canyon Village WWTP

The Canyon Village WWTP is located approximately 200 to 1,400 feet north of North Rim Drive. The site is approximately 400 feet south of the Canyon Village WTP; the water plant is not included in this scope of work. Access to the WWTP site is from the north via Norris Canyon Road and/or Grand Loop Road, both of which are closed to regular wheeled traffic during the winter. From Grand Loop Road, access to the WWTP is from service roads

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(including Fleabane Service Road) from the north down towards the headworks building.

Directly west of the headworks building is the location of the gate for access to the site.

A series of structures and treatment system components are located south and east of the Fleabane Service Road which extends from the north to the southwest, including headworks building and septage receiving, Lagoon 1, sludge holding pond, blower building, drying beds, Lagoon 2, AWT building (advanced water treatment), and Lagoon 3. The lining of Lagoon 1 and the sludge holding pond have reportedly experienced uplift due to high groundwater (Shannon & Wilson, 2021). In addition, the west side of Lagoon 1 experienced a slope failure during construction, when the approximately 5H:1V (horizontal to vertical) hillside was excavated to a steeper 3H:1V slope. The topography at the WWTP site is dominated by gentle rolling hills sloping to the south towards the Grand Canyon of The Yellowstone.

The north end of the site is at the highest elevation, approximately 7,868 feet based on draft CD plans (Jacobs, 2024e). Site topography slopes downward towards the south and east, with the southernmost Lagoon 3 located at an approximate elevation of 7,832 feet. The slopes above (west of) the WWTP site exhibit hummocky topography typically associated with slope movement, along with scattered large glacial erratics (cobbles and boulders) visible along the hillside. Several landslide scarps were observed where older and active landslides were present; the slope angle was as flat as 10H:1V. The relatively flat topography observed at the lagoons was formed by making cuts on the uphill side to the north and west and placing fills on the downhill side to the south and east. See Figure A-1- 24 through Figure A-1-29 in Appendix A for photos of the landslide features.

Based on draft CD plans (Jacobs, 2024e), improvements will include construction of a new headworks facility, bioreactor and digestor facility, process facility, dewatering facility, sludge drying beds, intraplant pump station, generator building, and operations and maintenance (O&M) storage building. These structures are proposed to be located approximately within existing facility footprints, with exception to the bioreactor and digester facility, process facility, dewatering facility, and sludge drying beds which will be located within the existing sludge holding pond and Lagoon 2 which will be demolished.

Foundations for these structures are proposed to range between approximately 3 feet and 19 feet below finished grade. In addition, yard piping is planned throughout the project area.

2.2.3 Brink of the Upper Falls Lift Station

The existing Brink of the Upper Falls lift station is located approximately 3/4 of a mile southwest of the Canyon Village WWTP. Based on draft CD plans (Jacobs, 2024e), the existing lift station will be demolished, and a new lift station and wet well will be

107560-005 July 23, 2024 constructed in its place. The foundation for the new structure is proposed to be approximately 18 feet below finished grade.

2.3 Grant Village Project Area

The Grant Village project area is comprised of four sites based on geographic location, as shown in Figure 3. Each site is located on a glaciolacustrine terrace (Richmond, 1973). Site observations and photographs are included in Appendix B.

The first site is the original WWTP, which is located along the south shoreline of the West Thumb area of Yellowstone Lake. The second and third sites are associated with the newer WWTP that is located approximately 1,000 feet southwest of the original site. The final site is the Campground Lift Station associated with the collection system. This site is located approximately 1.3 miles northwest of the active WWTP, along the A-Loop Road in a developed campground.

Access to the Grant Village sites is via Grand Loop Road and US 89 south to Grant Village Road. The majority of Grand Loop Road is closed to regular wheeled traffic during the winter. Access to the original and current WWTPs are via Delusional Service Road through a gate. Access to the Campground Lift Station is via Grant Campground Road and Old Marina Road and is open to the public.

The WWTP was rehabilitated in the early 1980s. According to Jacobs, the system is beyond its useful design life, showing continued signs of deterioration. The collection system experiences inflow and infiltration, especially during peak runoff.

2.3.1 Original WWTP Area

The original WWTP is located on the lowest glacial terrace in relation to the other sites, approximately 20 feet above the lake surface elevation and less than 100 feet south of the shoreline. The site encompasses approximately 1.2 acres.

Several structures and treatment system components are located at this site, including Lakeside Lift Station, wet well building, overflow storage tank, and a vault manhole. The remaining area is primarily open with some areas used as storage. The overall site is sloped downwards to the north (towards the lake), with an average slope of 10H:1V.

Based on draft CD plans (Jacobs, 2024f), all existing facilities will be removed and a new headworks facility and O&M storage building will be constructed. Foundation depths for the proposed buildings are expected to range from approximately 4 to 11 feet below finished grade.

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2.3.2 Grant Village WWTP

The WWTP is located on approximately 9 acres of land, approximately 80 feet above the lake surface elevation. The site consists of sludge drying beds, two equalization ponds, three percolation ponds, and a leach field.

The sludge drying beds are located on a middle terrace (in relation to the other sites), approximately 800 feet southwest of the original WWTP area. These facilities are approximately 10 to 15 feet lower in elevation than the rest of the WWTP site.

The percolation ponds in the WWTP site are also located on a middle terrace, although it is located at a higher elevation than the sludge drying beds. Our observations in this area indicate that the equalization ponds, percolation ponds, and leach field are in an area with generally flat terrain with berms separating the wastewater treatment features.

The site is surrounded by a perimeter fence. The area within the fence is relatively flat with raised embankments separating the percolation ponds and aeration ponds. Along the northern perimeter fence, the ground slopes relatively steeply (2H:1V) for approximately 40 horizontal feet before becoming a much gentler slope towards Yellowstone Lake.

Based on draft CD plans (Jacobs, 2024f), Grant Village WWTP improvements will include a new process facility, bioreactor/splitter, bioreactor and digestor facility, dewatering facility, generator building, treatment wetlands, and leach field. The existing equalization and percolation ponds will remain in place. The proposed process facility and generator building are proposed in an area with dense vegetation. Yard piping is planned to be constructed throughout the site.

Foundation depths for the proposed buildings are expected to range from approximately 4 to 17 feet below finished grade.

2.3.3 Campground Lift Station

This site is located on an upper terrace (in relation to the other project areas) in a commercial campground approximately 1.3 miles west of the WWTP. Manhole GV2354 is in this area, west of Old Marina Road near the north end of the Grant Campground A-Loop. Site topography is relatively flat, and the ground surface is a mixture of wooded land along with developed campground improvements. Other sections of the collection system were not evaluated.

Based on draft CD plans (Jacobs, 2024f), this lift station will undergo minor improvements such as a manhole replacement. Existing structures and facilities are planned to remain in place.

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2.4 Old Faithful Project Area

The Old Faithful project area is comprised of two sites based on geographic location, as shown in Figure 4. Site observations and photographs are included in Appendix C.

The first site is the existing WTP site, which is located approximately one mile southeast of the Old Faithful Lodge (lodge). Access to this site is from a service road off US-89/Grand Loop Road located approximately 1.8 miles east of the Old Faithful interchange leading into the visitor center.

The second site is the proposed water main that begins at the WTP and extends towards the northwest across the Firehole River to the lodge. The east end of the water main is accessed from the WTP site and generally follows the alignment of the existing pipeline. The west end of the proposed water main is accessed from the lodge parking lot and will have a new alignment generally following the existing road. The 0.8-mile-long pipeline generally follows the path of the Kepler Cascades trail and existing pipeline between the Old Faithful area and US-89.

The WTP was constructed around 1980. According to Jacobs, the level of arsenic in the raw water fluctuates seasonally and can reach levels that are difficult for the WTP to remove.

The current treatment system design does not allow for consistent treatment of arsenic.

2.4.1 Old Faithful WTP

The WTP facility is located on approximately 3 acres of land. Several structures and treatment system components are located in this area, including main process building, chemical feed building, underground concrete clearwell, presedimentation pond, backwash ponds, and a decant/solids pump station. The presedimentation and backwash ponds are fenced.

The overall slope of the site is down to the northwest. The top of the presedimentation pond, ground level of the main treatment building and the backwash ponds are located at different elevations. The native slopes are relatively shallow, and the constructed embankments have slopes on the order of 2H:1V.

SD plans (Jacobs, 2022c) call for most of the existing structures to be demolished. New structures will include a water treatment process building, a generator building, stormwater detention ponds, sand drying beds, reinforced concrete ramps for the existing backwash ponds, and various utilities. Minor grading is anticipated.

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The water treatment process building will be two stories in height. Some localized areas have pits that may extend down to approximately 20 feet below grade. The generator building will be one story in height and be founded near finished grade.

2.4.2 Finished Water Main

The existing water main alignment extends generally southwest from the WTP toward US- 89, then turns northwest and generally follows the Kepler Cascades Trail toward Old Faithful. The water main crosses the Firehole River at a point approximately 120 feet south of the trail crossing.

The ground slope along the water main is relatively flat from the WTP to the trail. At that point the ground slopes down moderately for approximately 900 feet before leveling off again near the Firehole River. On the west side of the river, the ground is relatively flat adjacent to the river, then it slopes up steeply towards the Old Faithful Lodge parking lot.

Based on SD plans (Jacobs, 2022c), the proposed water main alignment will generally follow the existing pipeline alignment. The existing 12" pipe will be removed in areas where proposed 14" diameter pipe will be constructed with approximately 7 feet of cover. The existing 12" pipe will be abandoned outside the vicinity of the new pipe. The western-most portion of the new pipeline will cross the Firehole River and have a different alignment than the existing pipeline. Borings are located along the proposed water main alignment.

3 EXISTING GEOTECHNICAL INFORMATION

NPS and Jacobs provided Shannon & Wilson with existing geotechnical reports from the various project areas for review. The following sections summarize the subsurface investigations performed and the geologic conditions encountered in each project area.

3.1 Canyon Village

3.1.1 Northern Testing Laboratories, Inc.

Northern Testing Laboratories, Inc. (1970, 1971) performed two geotechnical investigations for the WWTP site in June 1970 and September 1971 before the original WWTP was constructed. The purpose of their 1970 investigation was to provide geotechnical recommendations for design and construction of foundations at the site. Their 1971 investigation was to provide additional geotechnical recommendations for a proposed “contact stabilization tank and holding pond.” Their investigations included four borings in 1970 and six borings in 1971. At the time of this investigation, the ground surface in the vicinity of the proposed structures was relatively flat with only a few feet of slope

107560-005 July 23, 2024 downward from west to east. The borings were advanced in the vicinity of the current sludge holding pond and the proposed Intraplant Pump Station. Approximate boring locations are shown in Figure 2.

The 1970 borings primarily encountered soft to firm clay to depths ranging from 20 to 39 feet below grade with isolated areas of clayey gravel, up to 7 feet thick, and topsoil, up to 1½ feet thick. The clay soils were described as “weathered volcanic deposits.” The 1971 borings encountered similar material to depths ranging from 8 to 21 feet below grade, underline by Rhyolite bedrock to depths ranging from 10 to 21 feet.

3.1.2 Chen Northern, Inc.

Chen Northern, Inc. (1990) performed a geotechnical investigation for the WWTP site in October 1990. The purpose of their investigation was to provide geotechnical recommendations for design and construction of the various system components in this project area (i.e. the headworks building, three lagoons, etc.). Their investigation included 14 borings drilled to depths ranging from 15 to 35 feet below existing ground surface (bgs).

Approximate boring locations of the most pertinent borings are shown in Figure 2.

The borings encountered glacial lacustrine deposits, glacial till, and altered volcanics consisting of the following general geologic units: 5 to 10 feet of stiff, lean clay with varying amounts of sand; up to 10 feet of stiff to very hard, clay till that is hydrothermally altered;

and 25 feet of hydrothermally altered volcanics (very stiff to hard clay to medium dense to dense, gravelly sand). The altered volcanics were generally described as bedrock units (rhyolitic flow rocks, ash-fall tuffs, and tuffaceous sandstone and siltstone) that were characterized as soil due to the high degree of alteration. However, less altered zones encountered at depth were characterized as being more rock-like.

Site soils were characterized by low dry densities, high water contents, and high plasticity.

Note that the report incorrectly characterized the upper clay soils as lean clays, but they are high plasticity clay based on liquid limit. Groundwater levels were noted on three of the 14 boring logs, at depths ranging from about 10 to 20 feet bgs.

3.2 Grant Village

Ellis and others (1962) provided a hydrology report for the groundwater at Grant Village.

Their investigations included six test wells and 10 auger holes. The test wells ranged in depths from 100 to 200 feet bgs. The auger holes ranged in depths from 65 to 103 feet bgs.

The closest test well to the Grant Village WWTP is approximately 3,500 feet west of the existing lift structure.

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J.T. Banner & Associates, Inc. (1971) provided engineering analysis for multiple sewage treatment facilities located in Yellowstone National Park in October 1971. Their analysis for Grant Village included a lift station and percolation ponds. They drilled a total of 50 test holes, 25 percolation holes and 25 exploratory holes at the site. The exploratory borings extended to a depth of approximately 10 feet bgs. These borings encountered two soil layers. The upper layer varied from two to four feet thick and generally consisted of a tan sand with significant amount of silt. The lower layer consisted of dark brown sand with very little silt.

Edward R. Cox (1976) performed a wastewater effluent monitoring report that included the area around the Grant Village WWTP. A total of 14 test wells were augured at the site, and the soil deposits were described as “sand, silt, clay and gravel.”

Chen and Associates, Inc. (1979) performed a geotechnical investigation for the Grant Village Domestic Water System in October 1978 and August 1979. The purpose of their investigation was to provide geotechnical recommendations for design and construction of the various system components of the Grant Village WTP. The WTP is located approximately ½ mile west-southwest of the active WWTP site and approximately ¾ mile from the original WWTP site. Their investigation included six borings drilled to depths ranging from 20 to 40 feet bgs.

The borings encountered glacial lacustrine deposits and glacial till consisting of 20 to 40 feet of medium dense silty sand to sand with silt. No groundwater or bedrock was indicated on the boring logs. The soils were found to compress up to 2% when loaded in laboratory swell/collapse tests.

3.3 Old Faithful

3.3.1 WTP Site

NTL Engineering & Geoscience, Inc. (2001) of Great Falls, Montana performed a geotechnical investigation at the Old Faithful WTP in August 2000. The purpose of their investigation was to provide geotechnical recommendations for the design and construction of the pre-sedimentation basin. Their subsurface investigation included two borings drilled to depths of approximately 25 feet bgs.

The borings, drilled south and east of the main process building, encountered glacial till consisting of 5 to 10 feet of medium dense, silty sand underlain by very dense, silty sand with gravel. Groundwater was noted in these borings at depths of 5.5 and 6.5 feet bgs.

Bedrock was not encountered.

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Wright-McLaughlin Engineers (1978) of Denver, Colorado prepared an engineering report for the overall domestic water system at Old Faithful. As part of this effort, they performed a geotechnical investigation for the main process building and the backwash ponds in October 1977. Their subsurface investigation included seven borings drilled to depths of approximately 40 feet bgs. Seismic refraction was also performed.

The borings encountered glacial till and kame deposits overlying volcanic bedrock. General geologic conditions observed included 3 to 11 feet of medium dense, silty sand underlain by rhyolite bedrock. Groundwater was noted in multiple borings, ranging in depth from 8 to 23 feet bgs.

3.3.2 West of Finished Water Main Site

Nelson Engineering (2002) of Jackson, Wyoming performed a geotechnical investigation for the water main in the government area of at the Old Faithful site (approximately 1.5 miles west of this project) in November 2001. The purpose of their investigation was to provide geotechnical recommendations for the design and construction of the water main. Their subsurface investigation included two borings drilled to depths of approximately 10 to 15 feet bgs.

The borings encountered glacial till sand consisting of 8 to 9 feet of medium dense, poorly graded sand underlain by dense sand with gravel and clay, with occasional boulders.

Groundwater and bedrock were not encountered.

4 PRELIMINARY DESIGN GEOTECHNICAL SURVEY

4.1 Field Reconnaissance and Test Pits

During the PD phase, field reconnaissance of the four project areas was carried out between October 18, 2021, and October 21, 2021, by two Shannon & Wilson geotechnical engineers and an engineering geologist (Ms. Peggy Ganse, Mr. Collin Cummins, and Mr. Daniel Markowski). Prior to commencing field work, Shannon & Wilson staff prepared background documents, including topographic maps, geologic maps, and site overviews and map books provided by Jacobs (Jacobs, 2021a through 2021j).

Working out of a base location in West Yellowstone, the reconnaissance team spent four field days observing the three different project areas. Within each project area, we recorded notes describing geologic conditions and other pertinent site features and collected representative site photographs. A total of 45 mapping points were collected and surveyed using a handheld EOS Arrow 100 GPS unit. In addition, 27 shallow test pits were excavated

107560-005 July 23, 2024 using hand tools to observe subsurface conditions near the ground surface. While the material encountered in the tests pits was described as soil, it is possible they are fill. In some cases, the findings of our test pits did not correlate to the geologic map or the previous borings. Field reconnaissance notes and site photographs are presented in Appendices A through C.

4.2 Geophysics

Shannon & Wilson contracted Collier Geophysics (Collier) to perform a geophysical investigation at the three WTP and WWTP sites to augment the data obtained from the subsurface investigation. The surveys were completed using 2-D Seismic Refraction Tomography (SRT). The SRT method is used to measure the compressional-wave (P-wave) velocity distribution of the subsurface material. The testing results include a P-wave velocity profile that can be used to approximate soil and rock contacts along the sounding line. A total of 12 SRT lines were collected at the three sites in November 2021 (Collier, 2021). Collier performed five additional SRT lines in June 2022 (Collier, 2022) and three additional SRT lines in June 2024 (Collier, 2024). Collier's reports are included in Appendix D.

4.2.1 Canyon Village WWTP Geophysics

As described by Collier, the Canyon Village WWTP SRT lines from 2021 identified a 10- to 15-foot-thick zone of stiff to very dense soil or soft rock material, underlain by a 25- to 60-foot layer of soft rock, followed by a hard rock layer to the effective depth of the SRT survey line 12, approximately 160 feet bgs. SRT lines 10 and 11, with effective depths of 70 and 40 feet bgs, respectively, did not record a hard rock layer. SRT Line 1 from the 2022 exploration identified a 20-foot-thick layer of dense soil to soft rock underlain by rock to a depth of approximately 40 feet followed by hard rock to depth. These results are consistent with the existing geotechnical borehole information from the previous subsurface investigations as described in Section 3 as well as our geotechnical investigation, which encountered overburden soils to a depth of 15- to 25 feet.

4.2.2 Canyon Village WWTP Government Lift Station Geophysics

As described by Collier, the Canyon Village WWTP SRT lines from 2024 identified a 10- to 25-foot-thick zone of relatively low velocity material, interpreted to be alluvium or completely to highly weathered rhyolite. Underlying this layer is a transitional zone from approximately 25- to 50-feet bgs which represents a gradual decrease in the amount of rhyolite weathering with depth. Starting at a depth of 50 feet bgs, a high velocity layer interpreted to be slightly weathered to unweathered rhyolite was observed. These results are consistent with boring SW-C-14 which was drilled in this area and encountered

107560-005 July 23, 2024 overburden soils and highly weathered altered rhyolite to the bottom of the boring (30.3 feet bgs).

4.2.3 Grant Village WWTP Geophysics

As described by Collier, the Grant Village WWTP SRT lines from 2021 identified a 15- to 75-foot-thick zone of stiff to very dense soil or soft rock material, underlain by a soft rock layer from depths of 25 to 150 feet, followed by a hard rock layer to the effective depth of the SRT survey line which ranges from 50 to 200 feet bgs. SRT Lines 1 and 5 did not record a hard rock layer. SRT Line 2 from the 2022 exploration identified low-velocity material to depths ranging from 18 feet on the eastern portion of the line to more than 55 feet on the west portion of the line. This layer is underlain by a soft rock layer that is 25 feet thick on the eastern portion to more than 70 feet thick on the western portion of the SRT line. A hard rock layer is present along the eastern portion of the line from approximately 50 feet to at least 80 feet below the ground surface. These results are consistent with the existing geotechnical borehole information from the previous subsurface investigations described in Section 3, as well as our geotechnical investigation, which encountered overburden soils to a depth of approximately 40 feet.

4.2.4 Old Faithful WTP and Finished Water Main Geophysics

As described by Collier, the Old Faithful WTP SRT lines from 2021 identified a 2- to 12-foot-thick zone of stiff to very dense soil or soft rock material, underlain by a 10-to 15-foot foot thick, soft rock layer, followed by a hard rock layer to the effective depth of the SRT survey lines, ranging from 40 to 170 feet bgs. Based on the SRT data, the transition from surficial soil deposits to underlying bedrock ranges from 10- to 30 feet below grade.

In the 2022 exploration, Collier identified an approximate 12- to 20-foot-thick layer of low-velocity (dense soil to soft rock) material along Line 3, the western end of Line 4, and the Eastern end of Line 5. This layer is underlain by a transitional layer that is approximately 10- to 30-feet thick along most areas of Lines 3, 4, and 5. Each of the lines identified a high-velocity layer beneath the transitional material to depth. These depths are consistent with the existing borings as described in Section 3 and our geotechnical explorations.

A previous geophysical study (Wright-McLaughlin Engineers, 1978) was performed near the Firehole River water intake structure, which is approximately 1 mile south of the water treatment facility.

107560-005 July 23, 2024

5 FIELD EXPLORATION AND LABORATORY TESTING

5.1 Subsurface Explorations

Shannon & Wilson conducted a field exploration program between July 18 and August 1, 2022 to explore subsurface conditions in the three project areas. The subsurface exploration program consisted of drilling and sampling 34 geotechnical borings, at the locations shown in Figures 2 through 4. Each boring is designated with a letter associated with each project area; “C” for Canyon Village, “G” for Grant Village, and “OF” for Old Faithful. The six borings designated with “W” indicate that a groundwater monitoring well was constructed at that location (SW-C-06W, SW-C-09W, SW-G-02W, SW-G-10W, SW-OF-05W, and SW-OF- 10W).

Our initial scope of work included 38 borings, but four of them were unable to performed due to access constraints (borings SW-C-02 and SW-C-03), utilities (boring SW-C-04), or required offset from the existing pipeline (SW-OF-11) (see Table 1). To augment our borings, Shannon & Wilson also relied upon previous explorations as described in Section

3.1. Specifically, we utilized six borings from Northern Testing Laboratories, Inc. (borings D.H. No. 01-D.H. No. 06) near the Bioreactor and Process Buildings, one boring from Chen Northern (boring L-3) near the Dewatering Facility, and one Chen Northern boring (boring D.H. No. 3) near the O&M Building.

Table 1 provides a summary of the completed borings, and Table 2 summarizes groundwater levels encountered during drilling and measured in the monitoring wells in September 2022. Appendix E presents a discussion of the drilling and sampling procedures used to complete the borings, along with monitoring well completion details. Appendix E also presents the individual borings logs and an explanation of the symbols and terminology used.

5.2 Slug Testing

In June 2023, Jacobs performed slug tests in monitoring well SW-OF-10W that was installed by Shannon & Wilson at the Old Faithful WTP site. Slug testing was conducted to estimate the hydraulic conductivity (K) of the shallow aquifer beneath the proposed WTP building.

Two falling and rising head slug tests were conducted.

Slug tests were performed by lowering a cylindrical slug of known volume into the well, imparting a near-instantaneous rise in head (water level) within the well casing. An electronic pressure transducer was used to monitor the recovering water level in the well as it declined to equilibrium (falling head slug test). The slug was then pulled from the well, imparting a decrease in head within the well casing, and the water level in the well was

107560-005 July 23, 2024 monitored as it recovered to equilibrium (rising head slug test). The result of the slug testing was a K value of 0.05 feet/day (Jacobs, 2023) at this location.

5.3 Laboratory Testing

Geotechnical laboratory tests were completed on selected samples retrieved from the borings to estimate index and engineering properties. Index tests included moisture content, unit weight, grain size analysis, and Atterberg limits. Engineering properties tests included corrosivity and one-dimensional swell/collapse. Laboratory test results and a discussion of the testing procedures are included in Appendix F. The natural water content, fines content, and Atterberg Limits are also shown on the individual boring logs in Appendix E.

6 GEOLOGIC SETTING

The project areas are all located within the Yellowstone Plateau, an extensive volcanic plateau created from repeated cycles of massive, violent eruptions. The geology underlying the site includes volcanic bedrock, glacial soils, and post-glacial deposits.

6.1 Canyon Village

The 1:64,000 Geologic Map of the Canyon Village Quadrangle, Yellowstone National Park, Wyoming (Christiansen and Blank, 1975) and Surficial Geologic Map of The Canyon Village Quadrangle, Yellowstone National Park, Wyoming (Richmond, 1977) indicate that five geologic units are present in this project area, as listed below:

Pre-Sacagawea Glaciation

Canyon Rhyolite Flow and Tuff of Sulfur Creek

Sediments of Lower Falls

Sediments of Inspiration Point

Pinedale Glaciation

Till

Alluvium Stream Deposits

Figure 5 includes the various sites within this project area overlain with the geologic map units. Each of the anticipated geologic units is described below.

107560-005 July 23, 2024

6.1.1 Canyon Rhyolite Flow and Tuff of Sulfur Creek

The oldest deposits at this site are Canyon Rhyolite Flow and Tuff of Sulfur Creek. These deposits were observed in some of the exposures located along North Rim Drive during our reconnaissance. This rock is highly geothermally altered, making it difficult to distinguish between rhyolite, tuff, and sediments of the Lower Falls and of Inspiration Point (Christiansen and Blank, 1975). According to Richmond (1977), the thickness of the tuff at Lower Falls is about 150 feet and is disconformably overlain by the Canyon Rhyolite flow.

This is the likely the bedrock unit encountered in the borings drilled during the previous subsurface investigations at the WWTP. This unit was also encountered during our subsurface explorations.

6.1.2 Sediments of Lower Falls

These sediments are described as cemented diamicton containing abundant large angular fragments of fine-grained, gray biotite diorite; sparse cobbles of white quartzite; and sparse cobbles of andesitic flow rock (Christiansen and Blank, 1975). These sediments were not observed at any of the field reconnaissance points or encountered while drilling.

6.1.3 Sediments of Inspiration Point

These sediments are described as intensively altered and silicified white, buff and pink fine to coarse, poorly to well-sorted alluvial rhyolitic sand, pebbly sand and silt (Christiansen and Blank, 1975). These sediments were not observed at any of the reconnaissance points or encountered while drilling.

6.1.4 Pinedale Till

The Pinedale till is the soil unit visible at the ground surface at the WWTP site. It is described as grayish brown to gray, silty, and unsorted loose to compact sand (Richmond, 1977). Richmond describes erratics of Precambrian granite, gneiss, and quartzite as being present in till in the northwestern part of the quadrangle. However, these descriptions are inconsistent with our shallow test pit explorations, surficial observations, existing geotechnical explorations and our current subsurface investigation which encountered a till diamicton with a high percentage of high plasticity clay fines and scattered erratics.

6.1.5 Pinedale Stream Deposits

These deposits were observed in the vicinity of the Government Lift Station and Collection System site. They are described as gray to grayish brown, sandy gravel, gravelly sand, and sand deposited by streams related to ice but mostly by drainage unrelated to ice in middle and late Pinedale time (Richmond, 1977). These descriptions are consistent with the shallow

107560-005 July 23, 2024 test pit explorations made at the time of our reconnaissance and with our subsurface investigation.

6.2 Grant Village Geologic Conditions

The 1:64,000 Geologic Map of the West Thumb Quadrangle, Yellowstone National Park, Wyoming (Christiansen and Blank, 1974b) and Surficial Geologic Map of the West Thumb Quadrangle, Yellowstone National Park, Wyoming (Richmond, 1973) indicate that two geologic units are present in the project area and two bedrock units are exposed in the vicinity of the project area, as listed below:

Pre-Sacagawea Ridge-Bull Lake Glaciation

Shoshone Lake Tuff Member

Aster Creek Rhyolite Flow

Pinedale Glaciation

Open Lake Sediments

Ice-Dammed Lake Sediments

Several lake terrace levels are mapped in the vicinity of the sites. The 110-foot terrace appears to be the most continuous and is coincident with the existing WWTP and sludge drying beds. The 40-foot terrace is coincident with the original WWTP site and the Campground Lift Station. Figure 6 includes the various sites within this project area overlain with the geologic map units. Each of the anticipated geologic units is described below.

6.2.1 Shoshone Lake Tuff Member

The majority of this project area is underlain by Quaternary rhyolite flows and tuff (Richmond, 1973). The tuff and rhyolite flows described in the west thumb area are believed to have developed after the 600,000-year-old caldera. Specifically, the Shoshone Lake Tuff is related to the 155,000-year-old eruption which formed the boundary of the West Thumb of Yellowstone Lake (The U.S. Geological Survey, 1972). Based on this map, the geologic contact is near the Campground Lift Station, suggesting it may be present at that site. This unit was not encountered during our subsurface investigation.

6.2.2 Aster Creek Rhyolite Flow

The Aster Creek Rhyolite Flow was not observed in the project areas during our field reconnaissance or during our subsurface explorations. However, exposures were noted west and southwest of West Thumb on the geologic maps. According to Richmond (1977), 107560-005 July 23, 2024 all the rhyolite flows in the area have erupted in the past 160,000 years, with the Aster Creek dated around 150,000 years old. It is thought that the Aster Creek Rhyolite is slightly younger than the Shoshone Lake Tuff based on K-Ar (Potassium-Argon dating) dates by J.D Obradovich. It is likely that these rhyolite flows underlie the younger Pinedale glaciation sediments; however, the depth at which these flows may be expected to be encountered below the ground surface is not known at this time.

6.2.3 Pinedale Ice-Dammed Sediments

Ice-dammed deposits are mapped in the location of the existing WWTP. Richmond (1977) describes these deposits as blue-gray, buff-weathering, massive to laminated, locally varved silt and describes other similar ice-dammed deposits as massive to bedded, medium to fine sand and beach gravel. These sediments were observed in our borings.

6.2.4 Open Lake Sediments

Theses deposits are mapped in the location of the original WWTP and at the Campground Lift Station. Richmond (1977) describes these deposits as gray to blue gray, varved, laminated or massive silt. These sediments were observed in our borings.

6.3 Old Faithful Geologic Conditions

The 1:64,000 Geologic Map of the Old Faithful Quadrangle, Yellowstone National Park, Wyoming (Christiansen and Blank, 1974a) and Surficial Geologic Map of the Old Faithful Quadrangle, Yellowstone National Park, Wyoming (Waldrop, 1975) indicate that three surficial geologic units and one bedrock unit are exposed in the vicinity of the project area, as listed below:

Pleistocene - Scaup Lake Flow

Late Pleistocene - Pinedale Glaciation Deposits

Till

Kame Deposits

Holocene - Siliceous Hot-Spring Deposits

Figure 7 includes the various sites within this project area overlain with the geologic map units. Each of the anticipated geologic units is described below.

6.3.1 Pleistocene - Scaup Lake Flow

The project area is characterized by Late Pleistocene Pinedale Glaciation deposits and more recent siliceous hot spring deposits near the Old Faithful Geyser. According to Christian

107560-005 July 23, 2024 and Blank (1974a), the Upper Member of the Scaup Lake Flow underlies the glacial deposits and hydrothermal deposits is this area. This bedrock unit is described as like the rhyolites of the Central Plateau Member, but also contain moderately abundant phenocrysts of sodic plagioclase, and clinopyroxene. This unit was encountered during our subsurface explorations.

6.3.2 Late Pleistocene - Pinedale Glaciation Deposits

The WTP site is characterized by Pinedale Till Deposits which were encountered at this site during drilling. This unit is described as subangular to subrounded boulders and cobbles in a sandy to silty matrix which is unsorted and nonstratified, with local erratics of basalt from the Absaroka Range (Waldrop, 1975).

The kame deposits mapped in the vicinity of the project area are exclusively located near the Old Faithful visitor center. They extend as far east as the Firehole River. These kame deposits are described as gray to brownish gray, poorly sorted to well-sorted, gravelly sand (Waldrop, 1975). Located at this geologic boundary are also several mapped hot springs and sinter-depositing hot springs described below.

6.3.3 Holocene - Siliceous Hot-Spring Deposits

Located at the Old Faithful Geyser and a few locations west, north, and northwest of the Old Faithful Geyser are Siliceous Hot Spring Deposits. They are described as white to light gray deposits of amorphous silica that form rims and cones of hot springs and geysers (Waldrop, 1975). These deposits were not encountered during our subsurface investigation.

7 SUBSURFACE CONDITIONS

The anticipated geologic and subsurface conditions are described below for each of the three project areas. For each project area, our reconnaissance observations and shallow test pit data were synthesized with existing geologic maps, geotechnical boring and lab data from prior studies completed by others, and our geotechnical exploration and laboratory testing program. Geophysical explorations were also completed by Collier at the WTP and WWTP sites to further characterize subsurface materials. Existing geotechnical data was used to calibrate the geophysical subsurface model for determination of different surface layers or transitional boundaries.

7.1 Canyon Village Subsurface Conditions

In this project area, we encountered granular glacial deposits (till and kame) along with altered volcanic-deposited bedrock consisting of rhyolite and tuff. A summary of specific

107560-005 July 23, 2024 conditions at each site is presented below. The WWTP site was the only project area where evidence of slope instability was observed. In addition, the lining of Lagoon 1 and the sludge holding ponds have reportedly experienced uplift due to high groundwater (Shannon & Wilson, 2021).

7.1.1 Government Lift Station

Two borings were drilled in the Government Lift Station area and for the force main (SW-C- 14 and SW-C-15). Boring SW-15 encountered glacial till at the ground surface to a depth of approximately 20 feet bgs. As encountered in the boring, glacial till consisted of very dense sand with varying amounts of silt and gravel (SP-SM and SM).