Appendix_A.pdf

Appendix A

Attachment A – Statement of work for field investigations on the Horseshoe and Carefree faults. Seismic hazard study for Horse Mesa, Mormon Flat and Bartlett dams.

Colin Chupik 12/5/2023

Introduction This document outlines sites that the Technical Services Center of the Bureau of Reclamation would like to permit for subsurface geologic investigations along the Horseshoe and Carefree faults in central Arizona (Figure 1). The purpose of this document is to provide information for compliance under the National Environmental Protection Act, the National Historical Preservation Act, and Endangered Species Act. This document will also provide details of the study for permitting documents for the Tonto National Forest, and the Scottsdale McDowell Sonoran Preserve, which is managed by the City of Scottsdale.

The Geomorphology and Seismology group at the Bureau of Reclamation is investigating active and potentially active faults in central Arizona for dams along the Salt and Verde rivers including Horse Mesa, Mormon Flat and Bartlett dams (Figure 1). There are three known active faults in the region that may pose a hazard to these structures (Figure 1). The goal of the seismic hazard study is to use evidence preserved in the recent (Quaternary) geologic record to refine the frequency, size, and location of large earthquakes on these faults. We have located sites on the Horseshoe and Carefree faults where ground disturbances are required for our work. The investigation involves digging soil pits by hand at 20 locations within the Tonto National Forest (Figure 4 & Figure 5). Additionally, there are 3 sites we would like to permit for trenching within the Tonto National Forest and Scottsdale McDowell Sonoran Preserve (Table 1; Figure 6 & Figure 7). All three trench sites will be dug with a backhoe. The goal of this document is to provide background on each location of interest to inform all stakeholders and landowners involved in the environmental compliance and permitting process.

Methodology Active faults are identified where there is evidence of faulting across a Quaternary (a time period within the last 2.58 million years) landform or deposit. One of the most diagnostic features of an active fault preserved in the landscape is a fault scarp (Caskey, 1995) (Figure 2). A fault scarp forms usually in large earthquakes when a landform is displaced and one side of the fault moves up in relationship to the other, forming a topographic escarpment (Figure 2). We plan to constrain the frequency, and size of large earthquakes in Arizona by digging trenches across these scarps and determining the age of the landforms that are displaced by digging soil pits. Our study plan for each fault is described in detail below:

For the Carefree fault, we plan to dig 3 trenches across scarps within the Tonto National Forest and Scottsdale McDowell Sonoran Preserve. The purpose of digging trenches is to determine the timing of the last earthquake and to establish how fast the fault is moving (Figure 2). The method is referred to as paleoseismic trenching (McCalpin and Nelson, 1996) which requires analyzing and dating deposits exposed in the trench that have been displaced or disturbed by past earthquakes. In each trench, samples are collected for numerical age dating techniques (e.g., optically stimulated luminescence dating, radiocarbon dating, and cosmogenic radionuclide dating) to establish an earthquake history for each fault.

For the Horseshoe fault, we have identified 20 sites where we want to conduct subsurface field investigations on the west side of Horseshoe Reservoir within the Tonto National Forest (Figure 4 & Figure 5). The goal of the study is to date surfaces that are displaced by the fault to estimate a slip rate for the fault. To date the landforms, we need to hand-excavate a small one- to two-meter-deep pit by hand and collect samples for radiometric dating. An example of a soil pit is shown in Figure 3. We will collect multiple samples to estimate the age of landforms in the study area and may use multiple dating methods such as optically stimulated luminescence dating, radiocarbon dating, and/or cosmogenic radionuclide dating to constrain the age of the surface that is faulted. If the deposits are not conducive for dating using these methods, we will collect cosmogenic exposure samples from boulders within each site area. This method requires chiseling off 3-4 cm of an exposed boulder and measuring cosmogenic radionuclides accumulated on the rock surface to establish an age of emplacement of that boulder. In addition to collecting samples for dating, we plan to describe the development of the soil in profile which is another indication of relative age for the surface that is faulted (Birkeland and others, 1991). By digging soil pits on either side of the fault and dating exposure ages of boulders, we can confirm that a surface or landform is the same on either side of the fault and determine how much the surface has been displaced over time.

This methodology will help determine if these faults are active and help determine a slip rate for these faults that can be used in a probabilistic seismic hazard assessment.

Objective and scope There are 20 soil pit sites and 3 trench location that we would like permitted for this investigation. The sites are summarized in Table 1 and the following five figures (Figure 4, Figure 5, Figure 6, and Figure 7). A shapefile and KML of the areas will be provided electronically for the locations of these sites. The polygons for trenching represent the areas where cultural and biological surveys are needed. For the soil pit investigation, our pits will be within a 25 m radius of the points highlighted in Figure 4 and 5. In general, the areas provided in this document are significantly larger than the area of ground disturbance to allow for flexibility in the selection of a site within the bounds of the polygons. There are also many back-up sites for the soil pit study to allow for maximum flexibility if cultural and environmental resources are encountered. The total ground disturbance will be much less than the total areas presented in this document.

There are 20 sites we would like to permit along the Horseshoe fault for our soil pit and bouldering sampling study. The locations of the sites are shown in Figure 4 & Figure 5. The sites are grouped into 8 general locations for groupings of sites that are generally on either side of the fault (Figure 4 and 5). All the sites are within the Tonto National Forest, but within lands withdrawn for Reclamation’s purpose. An example of a hand-dug soil pit is shown in Figure 3. The soil pits will be dug by hand with shovels, picks, and other hand-tools. Electric power tools such as a hammer drill may be used to assist with the digging.

The soil pit may be dug within a 25 m radius surrounding each point in Figure 4 and 5. The area of disturbance for each soil pit location may encompass a 4 m radius around each pit and the depth of the soil pits may reach greater than 2.5 m but will not exceed 3m. The soil pits will be benched on one side at about 1 m to comply with job hazard regulations. The soil pit may take multiple weeks to dig and may be left open for over two weeks to allow enough time for experts to visit the site. We will use orange fencing to enclose the site when unattended. ‘No trespassing’ signage will be posted on the fence surrounding the pits. Additional fencing may be required to deter animals at each site depending on landowner management needs. Fencing may include the installation of silt fences, and grass wattles, barbed wire, and/or smooth support wire. Fencing needs will be determined at a meeting with the landowner before the study commences. Cosmogenic exposure samples of large boulders may be collected within the 25 m radius of each soil pit to supplement the dating techniques in the pits. The sampling of the boulders requires chipping off a 3-4 cm rind from the top of the boulder. The sampling will be collected using a chisel or a hand-power tool. Each site will require multiple one-gallon bags of material for various dating techniques. Access to these sites will be by boat or by foot depending on the timing of the study and the water-level of the reservoir. Our group may use the USFS Ocotillo Boat Launch Facilities to launch a boat to access the site and/or as a staging area to park vehicles. The study may need to moor the boat on the western shore of the Horseshoe reservoir. The group may camp during the soil pit study on the western shore of the reservoir on the beach or within one of the 25 m radius polygons outlined in this document.

There are 3 sites we would like to permit for paleoseismic trenching on the Carefree fault. The locations of the sites are shown in Figure 6 & Figure 7. All the sites will be dug with a backhoe excavator. The two northern-most sites are located on the Tonto National Forest and the furthest south site is located on Sonoran McDowell Sonoran Preserve land. Road access to sites C1 and C2 (on Tonto National Forest) is along dirt roads shown in green and yellow on Figure 6. The green leading from Stagecoach Pass Road could be used for access to C1 and C2; the route would use an existing trail that is wide enough to accommodate a backhoe and would not need improvements; however, two gate would need to be accessed (one at the Stagecoach Pass Road to access McDowell Sonoran Preserve lands and the other at the boundary line with the Tonto National Forest). Once on Tonto National Forest lands, the access road would follow Forest Road 513 to C1 and C2. The access road for C3 (in the Sonoran McDowell Sonoran Preserve) would utilize Brown Ranch Road and the Corral Trail; foot traffic may also use the Maverick Trail. The backhoe route for C3 would use existing trails that are wide enough to accommodate a backhoe and would not need improvements. A backhoe with rubber tires or tracks will be used to reduce impacts along all access routes. The staging areas for unloading the backhoes is shown in the figures 6 and 7 which correspond to parking areas surrounding the site including the Brown Ranch Road trailhead.

Each staging areas encompasses an area less than a quarter of an acre.

A backhoe “off-shoot” access route will be needed for the backhoe to travel from the dirt road to the trenching polygon. These “off-shoot” routes are shown in Figure 6 and Figure 7 as bright green lines. A buffer of 30m on either side of these lines may be needed to allow for maximum flexibility to avoid heavily vegetated regions. Vegetation trimming may be needed to allow for the backhoe to navigate around trees and shrubs without removing them. Some slight ground disturbance may occur, although, the backhoe will be equipped with rubber traction to mitigate disturbance to the ground. The “off-shoot” route is 140 m for site C1, 150m for site C2, and 50 m for site C3 (Figure 6 and 7).

For the trench sites, a small all-terrain vehicle may be used to access the sites each day and to transport equipment to and from the trench sites. Other vehicle access may be allowed depending on the sensitivity of the site and permission from the landowners. Examples of paleoseismic trenches are shown in Figure 2.

Trenching excavations may exceed 4 m in depth but will not exceed 80 m in length. The walls will be benched at a depth of 1.5 m to comply with job hazard regulations specified by the Occupational Safety and Health Administration (OSHA). The width of the area of disturbance may be up to 30 m to accommodate benches and spoil piles. The trenches will be marked with high-visibility construction tape and fenced using orange construction fencing. ‘No trespassing’ signage will be posted on the fence surrounding the trench. Like the soil pit sites, additional fencing may be required to deter animals at each site depending on landowner management needs. Fencing may include the installation of silt fences, and grass wattles, barbed wire, and/or smooth support wire. The wattles and silt fences may be able to deter wild tortoise from entering the trench sites. Fencing needs will be determined at a meeting with the landowner before the study commences. The trenches may be opened for 1-2 months to allow time for experts to visit the site. The trenches will be backfilled using the same excavator and the site will be restored to its natural state. Wildfire and dust mitigation protocols required by the landowners will be followed upon opening of the trenches and for transportation of the backhoe. Additional mitigation steps may be requested by the landowner . A meeting with the National Forest and the Scottsdale McDowell Sonoran Preserve will take place at least a month before the proposed excavations to solidify needs for each site.

Table 1.—Site numbers for soil pits and trenches for the Horseshoe and Carefree faults.

Generalized locations for each site are provided. Refer to shapefiles for exact locations.

Fault Site number Coordinates Excavation Landownership

Horseshoe

H1 a/b 111.74577°W, 34.01819°N

Hand dug

Tonto National Forest

H2 a/b 111.74678°W, 34.01588°N

H3 a/b/c/d/e/f

111.74777°W, 34.01545°N

H4 a/b 111.74417°W, 34.00656°N

H5 a/b 111.7432713°W, 34.0036250°N

H6 a/b 111.7485893°W, 34.0160998°N

H7 a/b 111.7422227°W, 34.0156718°N

H8 a/b 111.7426391°W, 34.0002465°N

Carefree

C1 111/8289857°W, 33.8219041°N

Backhoe

C2 111.8292651°W, 33.8180100°N

C3 111.8307730°W, 33.7819940°N

Scottsdale McDowell Sonoran Preserve

Safety and health To ensure the safety of the investigators, personal protective equipment will be required on site near the hand-dug and excavator-dug trenches. Investigators will wear protective eyewear, face masks, steel-toed boots, and hard hats when digging and while in the trenches. Fencing and signage will be used to mark off the area of investigation. Subsurface investigations will follow OSHA requirements and regulations. Fire extinguishers and/or backpack pump will be available during backhoe excavation.

Environmental and Cultural Considerations:

All the soil pit sites will be hand-dug to minimize ground disturbance. An archeologist monitor during the excavations may be required upon digging of the soil pits due to the sensitivity of cultural sites surrounding the Horseshoe fault. A backhoe will be used to dig three trench sites which are located within the Scottsdale McDowell Sonoran Preserve and the Tonto National Forest. Pre-existing roads and trails will be used to minimize impacts when transporting the backhoe to the trench sites. Following the investigation, the pits and trenches will be filled in and restored to their natural state (Figure 3). If required, the area of disturbance can be re-seeded with native plants. Alternate locations to the ones presented in this statement of work are available if the proposed sites will impact sensitive species and their habitat or if the sites are near cultural sites. Large trees and bushes along with cacti, such as saguaros and cholla, will be avoided for this investigation.

Schedule If there are no delays in the permitting of this work, the exposures and trenches will be dug between the Fall of 2024 and the Winter/Spring of 2025. To account for unexpected delays, we would like the permit to be open through 12/31/2025. If there are delays to the project that extend beyond December 31, 2025, the Tonto National Forest and the Scottsdale McDowell Sonoran Preserve will be notified. Trenches may be open for multiple weeks. Tonto National Forest and the Scottsdale McDowell Sonoran Preserve will be notified when the trenches will be open and backfilled.

Figure 1.—Overview map showing potentially active faults (dashed red lines) and known active faults (solid red lines). The permits will be needed for sites on the Carefree, and Horseshoe faults.

Lidar for Tonto Basin is shown in green. Elevation datasets from the Arizona Geological Survey is shown in blue and red. Digital aerial photogrammetry (DAP) datasets are provided at 1m resolution for the entire state of Arizona (Carini, 2017). The locations of dams owned by Reclamation are shown by yellow squares.

Figure 2.—A) Lidar hillshade showing an example of fault scarps cutting across alluvial surfaces along the Sawatch fault zone in the Arkansas River Valley near Buena Vista, Colorado. B) Example of a benched paleoseismic trench across a fault scarp in Wyoming. C) Example of a trench excavation using a track-mounted excavator. The size of the excavator will depend on the sensitivity of the site, the expected sizes of the deposits, and the size of the trench required at the site.

Figure 3.— Example of a soil pit presented in a categorical exclusion document for a paleoflood study for Vallecito Dam (From Appendix B: authored by Giorgia Fulcheri). The bottom right image shows the soil pit covered after excavation.

Figure 4.—Map showing the location of soil pit sites H1, H2, H3, H6, and H7 in relationship to mapped scarps and other fault related features along the Horseshoe fault. Overview shows the location of Horseshoe Dam in relationship to the known and potentially active faults mapped in the region. Overview map shows the map extent in white.

Figure 5.— Map showing the location of soil pit sites H4, H5 and H8 in relationship to mapped scarps and other fault related features along the Horseshoe fault. Overview shows the location of study area in white and in relationship to Horseshoe Dam.

Figure 6.— Map showing the locations of the two trench locations that will be dug with a backhoe on the Tonto National Forest land. Our preferred access route is from E Stage Coach Pass Rd along the green line and the eastern yellow line. Overview shows the location of the sites and the access route. The mapped trace of the fault is from the Quaternary fault and fold database from the USGS (https://www.usgs.gov/programs/earthquake-hazards/faults).

Olsker, Nichole M Let's remove the orange line from the map

Figure 7.— Map showing the locations of one trench that will be dug with a backhoe on land owned by the Scottsdale Mcdowell Sonoran Preserve. Access to the site is shown by the yellow line which aligns with prominent trails that are greater than 6 ft wide. Overview shows the location of these trenches in relationship to other sites along the Carefree fault and a mapped trace of the fault from the Quaternary fault and fold database from the USGS (https://www.usgs.gov/programs/earthquake-hazards/faults).

References

Birkeland, P.W., Machette, M.N., and Haller, K.M., 1991, Soils as a tool for applied Quaternary geology, Utah Geological and Mineral Survey Salt Lake City, UT, USA.

Carini, K., 2017, NAIP Point Cloud (2017): University of Arizona,, University Libraries- Geospatial.

Caskey, S., 1995, Geometric relations of dip slip to a faulted ground surface: new nomograms for estimating components of fault displacement: Journal of Structural Geology, v. 17, no. 8, p. 1197- 1202.

McCalpin, J.P., and Nelson, A.R., 1996, Introduction to paleoseismology, Elsevier, v. 62, p. 1-32.

Appendix A