B08_22R0022_At8_Geophyiscal_Eval_Addendum.pdf

Terracon Consultants, Inc. 4685 South Ash Avenue, Suite H-4, Tempe, Ar izona 85282 P [480] 897-8200 F [480]-897-1133 terracon.com

August 25, 2020

SEH, Inc.

503 North Main Street, Suite 225 Pueblo, Colorado 81003

Attn: Mr. Jimmie Hayson Phone: (719) 423-7403 Email: jhayson@sehinc.com

Re: Geotechnical Engineering Services Report Addendum No. 1 Geophysical Evaluation Proposed BLM La Posa WW Lagoons La Posa South LTVA Quartzsite, Arizona Project No. 65195142

Dear Mr. Hayson:

At your request, Terracon Consultants, Inc. (Terracon) has prepared this addendum to the geotechnical engineering report titled Geotechnical Engineering Report, Proposed BLM La Posa WW Lagoons, La Posa South LTVA, Quartzsite, Arizona dated October 30, 2019. (Project No.

65195142) for the proposed project. We understand a new well is planned in order to provide potable water to the planned development and a geophysical survey has been requested by SEH to be performed to characterize the subsurface geologic conditions and identify any potential water bearing units at the referenced project. The purpose of this addendum is to provide the report of the geophysical evaluation performed by Atlas (formally Southwest Geophysics) that provides the results of the geophysical survey and recommendations of one or more potential well sites since the recently drilled well produces an inadequate water supply for the planned development.

Project Information/Description

We understand the proposed project will consist of new waste water treatment system, waste water lagoons, water well and well/pump house, water well, 15,000-gallon water storage tank and associated distribution lines, RV dump and fill stations, and electrical distribution. We understand one well has been drilled for the project and the well is not producing sufficient water needed for the planned development. Therefore, a new well will be needed for the project.

Addendum No. 1 to the Geotechnical Engineering Report Proposed BLM La Posa WW Lagoons ■ Quartzsite, Arizona August 25, 2020 ■ Terracon Project No. 65195142

Resourceful ■ Responsive ■ Reliable 2

Results of the Geophysical Evaluation

Terracon subcontracted Atlas (formally Southwest Geophysics) to evaluate the presence of prospective areas for groundwater resources and recommend one or more potential well sites, since the recently drilled well produces an inadequate water supply. As directed by the BLM and SEH, the geophysical survey was focused within the specified project area just east of Tyson Wash via collection of Hybrid Source Audis Magnetotellurics (HSAMT) data. The collected HSAMT data represents in-situ electrical resistivity measurements that were utilized to detect possible groundwater resources. The Atlas field services were performed from June 16 through June 25, 2020. The Atlas report titled Geophysical Evaluation, BLM La Posa Tyson Wash LTVA, HSMAT Groundwater Exploration, Quartzsite, Arizona dated August 25, 2020 describes the equipment used during their study, the methodology of how HSAMT is used to help locate groundwater, results of the analyses, and recommendations. The Atlas report is attached to this addendum. The Atlas report also provides recommendations for additional geophysical evaluations to detect more favorable groundwater resources at the project site.

Closure

All other recommendations presented in our geotechnical engineering report (Terracon Project No. 65195142, dated October 30, 2019) remain applicable for the design and construction of the planned project. This supplemental letter should be made part of the attached original geotechnical engineering report.

We appreciate the opportunity to be of service to you on this project. If you have any questions concerning this report, or if we may be of further service, please contact us.

Sincerely, Terracon Consultants, Inc.

Eddy F. Ramirez, P.E. Donald R. Clark, P.E.

Geotechnical Project Manager Sr. Consultant/Sr. Principal

Attachments: Atlas (formally Southwest Geophysics) report dated August 25, 2020.

Copies to: Addressee (1 via email)

8/25/2020

GEOPHYSICAL EVALUATION

BLM LA POSA TYSON WASH LTVA

HSAMT GROUNDWATER EXPLORATION

Quartzsite, Arizona

PREPARED FOR:

Mr. Eddy F. Ramirez, P.E.

Terracon 4685 South Ash Avenue, Suite H4 Tempe, Arizona 85282

PREPARED BY:

Atlas Technical Consultants, LLC 9185 South Farmer Avenue, Suite 111 Tempe, Arizona 85284 August 25, 2020

9185 South Farmer Avenue, Suite 111 Tempe, Arizona 85284

(480) 855-3201 | oneatlas.com

August 25, 2020 Atlas No. 220023SWG

Report No. 1 Rev. 2

MR. EDDY F. RAMIREZ, P.E.

TERRACON

4685 SOUTH ASH AVENUE, SUITE H4

TEMPE, AZ 85282

Subject: Geophysical Evaluation BLM La Posa Tyson Wash LTVA HSAMT Groundwater Exploration Quartzsite, Arizona

Dear Mr. Ramirez:

In accordance with your authorization, Atlas (formerly Southwest Geophysics) has performed geophysical services pertaining to the United States Bureau of Land Management's (BLM) La Posa Tyson Wash Long Term Visitor Area (LTVA) project located approximately 2 miles southwest of Quartzsite, Arizona. The purpose of our services was to evaluate the presence of prospective areas for groundwater resources and to recommend one or more potential well sites, because a recently drilled well produces an inadequate water supply. As per your and Short Elliott Hendrickson Inc. (SEH) specifications, our work was focused within the specified project area via collection of Hybrid Source Audio Magnetotellurics (HSAMT) data. Our field services were performed June 16 through June 25, 2020. This revised report presents our evaluation methodology, equipment used, analysis, and results. Revisions include a brief discussion of previous downhole geophysical logging performed by others and the reported water producing intervals within the recently completed water well located near Tyson Wash and within the current water production well located in the La Posa South LTVA, southeast of US95. Also discussed in this revision is the question of data resolution in resolving depth to bedrock.

We appreciate the opportunity to be of service on this project. Should you have any questions related to this report, please contact the undersigned at your convenience.

Sincerely, Mark D. Edwards, R.G. Paul Neuberger Principal Geologist/Geophysicist Staff Geophysicist

MDE/PN/pfl/mde/ds

Dstribution: Mr. Eddy Ramirez (eddy.ramirez@terracon.com)

Atlas No. 220023SWG Report No. 1 Rev. 2

Page | i

CONTENTS

1. INTRODUCTION

2. SCOPE OF SERVICES

3. SITE AND PROJECT DESCRIPTION

4. EVALUATION METHODOLOGY AND ANALYSIS

5. RESULTS

6. CONCLUSIONS

7. RECOMMENDATIONS

8. LIMITATIONS

9. SELECTED REFERENCES

FIGURES

Figure 1 Site Location Map Figure 2a Line Location Map (West of US95) Figure 2b Line Location Map (East of US95) Figure 3a Site Photographs Figure 3b Site Photographs Figure 4a HSAMT StrataGem Setup Figure 4b HSAMT Geode EM3D Setup Figure 4c HSAMT StrataGem Data Example Figure 4d HSAMT Geode EM3D Data Examples Figure 4e Line and Well Location Map Figure 5a "East Well" Depth Section (Well No. 566319), 4 to 30 ohm-meters Figure 5b "Bedrock" Depth Section, 4 to 400 ohm-meters Figure 5c Line 1 Depth Section, 4 to 30 ohm-meters Figure 5d Line 1 Depth Section, 4 to 400 ohm-meters Figure 5e Line 2 Depth Section, 4 to 30 ohm-meters Figure 5f Line 2 Depth Section, 4 to 400 ohm-meters Figure 5g Lines 1 and 2 Depth Sections, 4 to 30 ohm-meters, View to NW Figure 5h Lines 1 and 2 Depth Sections, 4 to 30 ohm-meters, View to SE Figure 5i Lines 1 and 2 Depth Sections, 4 to 400 ohm-meters, View to NW Figure 5j Lines 1 and 2 Depth Sections, 4 to 400 ohm-meters, View to SE Figure 5k Line 3 Depth Section, 4 to 30 ohm-meters Figure 5l Line 3 Depth Section, 4 to 400 ohm-meters Figure 5m Lines 1 and 2, "Bedrock", and "East Well" Depth Sections, 4 to 30 ohm-meters Figure 5n Lines 1 and 2, "Bedrock", and "East Well" Depth Sections, 4 to 400 ohm-meters

APPENDIX

HSAMT Evaluation Methodology

Page | 1

1. INTRODUCTION

In accordance with your authorization, Atlas (formerly Southwest Geophysics) has performed geophysical services pertaining to the United States Bureau of Land Management's (BLM) La Posa Tyson Wash Long Term Visitor Area (LTVA) Project located approximately 2 miles southwest of Quartzsite, Arizona (Figure 1). The purpose of our services was to evaluate the presence of prospective areas for groundwater resources and to recommend one or more potential well sites based on our geophysical results. We understand a recently drilled well just east of Tyson Wash produces an inadequate water supply. Our work was focused within your and Short Elliott Hendrickson Inc. (SEH) specified project area just east of Tyson Wash via collection of HSAMT data. Our collected HSAMT data represent in-situ electrical resistivity measurements that can be used to detect possible groundwater resources. Our field services were performed from June 16 through June 25, 2020. This revised report describes the equipment used during this study, the methodology of how HSAMT is used to help locate groundwater, and an analysis of the results. Revisions in this report include a brief discussion of previous downhole geophysical logging performed by others in the "New Well" completed this year just east of Tyson Wash, and a short discussion of the reported water producing intervals within the "New Well" and within BLM's separate water production well located in the La Posa South LTVA, southeast of US95.

Also discussed in this revision is the question of data resolution in resolving depth to bedrock using HSAMT methods.

2. SCOPE OF SERVICES

Our scope of services included:

• Performance of project stakeholder's requested three HSAMT evaluation lines (Lines 1 through 3) within your specified project footprint totaling approximately 16,000 feet.

HSAMT data were acquired along the three lines, two of which are approximately about 5,250 feet each and one line of about 5,500 feet using a station spacing of 250 feet on each line. This included 66 HSAMT stations.

• Collection of HSAMT data at two reference HSAMT stations along the "East Well" Line located near the existing production well (Well No. 566319) located southeast of US95 within the La Posa South LTVA for data modeling reference purposes. Since the well site is a producing well, we elected to collect data where geologic conditions are known to be conducive for groundwater production as a way of obtaining comparative data to the other project areas.

• Recording two reference HSAMT stations at an exposed bedrock area along the "Bedrock" Line located southwest of HSAMT Line 1 and your specified project footprint for data modeling reference purposes. Since the bedrock line is located at known surface exposures of bedrock, we elected to collect data at this location so that a measure of

Page | 2 bedrock resistivity values could be recorded to compare to other project areas and allow for a projection of interpreted bedrock depth to the east into, and slightly beyond, your specified project footprint.

• Data from a total of 70 HSAMT evaluation station locations were recorded.

• Compilation and analysis of the data collected.

• Preparation of this evaluation report presenting our data collection methods, results, conclusions and recommendations.

3. SITE AND PROJECT DESCRIPTION

The project site is located approximately 2 miles southwest of Quartzsite, Arizona (Figure 1). The study area consists of relatively undeveloped native desert crossed by Tyson Wash and dirt roads. Vegetation includes small bushes, creosote, cacti, and dense mesquite, palo verde and ironwood along Tyson Wash.

Based on the information that SEH provided us in meeting minutes for the July 15, 2020 teleconference project status meeting, and according to Gary Jaggers in those meeting minutes the recently drilled groundwater well on the edge of Tyson Wash (ADWR Well No. 55-232032), or "New Test Well" has a capacity of ±25 gallons per minute.

We received a copy of the interpretation email from SEH on August 4, 2020. The consultant's interpretation shows an interpreted "bedrock" surface at a depth of approximately 565 feet below ground surface (bgs) in the New Test Well. According to the e-log testing consultant's interpretation, the geophysical borehole log for the New Test Well indicates a "best producible zone in the well" between 400 and 520 feet bgs. SEH reports depths between 400 and 520 feet bgs correlate to observed drill cutting material consisting of fine sand.

In SEH's communications with us they interpret the fine sand interval from 400 to 520 feet bgs as "a potential water bearing material". Based on information provided to us by SEH on August 4, 2020 in the Well Driller Report and Well Log record filed with ADWR, the designed production interval in the New Test Well is screened casing from 480 to 520 feet bgs and perforated casing from 400 to 480 feet bgs.

The BLM has an existing producing groundwater well located east-southeast of US95 within the La Posa South LTVA (Well No. 566319). According to "after-the-fact" records filed with the Arizona Department of Water Resources (ADWR) in 1997 for this well, the water level in this well was at 535 feet bgs in 1989 at the time of well completion. The current water level in this well is unknown to us, but from the ADWR records the well reportedly has a screened interval 40 feet long starting at about 771 feet bgs.

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The New Test Well (Well No. 55-232032) location near Tyson Wash was selected without the use of surface based geophysical data. For current project purposes, you and SEH decided that prior to considering a siting of an alternate groundwater well location a surface based geophysical evaluation should be conducted. The purpose of the current study was to use HSAMT data to help determine if there was one or more additional potential well sites. Specifically, our study was designed to evaluate the potential for groundwater in the alluvial deposits and within the relatively shallow underlying bedrock. Our three HSAMT lines conducted near and within the specified project footprint were located as per SEH's verbal specifications provided to us via phone call at the time of our proposal (around April 5, 2020), which included lines passing near the "New Well", and also through a location of a test well previously drilled in 2002 (Well No. 595956). Our understanding is that the "2002" test well site was not a developed operating well at the time of our evaluation.

The current study included three HSAMT lines (Lines 1 through 3) totaling approximately 16,000 linear feet (Figure 2a). HSAMT data were acquired along the three lines. Two of the lines are approximately 5,250 feet long each (Lines 1 and 3), and the third is approximately 5,500 feet long (Line 2). Data was recorded at stations spaced 250 feet apart on each line. The current study was able to evaluate the subsurface resistivity down to a maximum depth of approximately 500 meters (approximately 1,600 feet), and likely recorded bedrock resistivity values below the alluvial deposits along Lines 1 through 3. To help calibrate and compare the data at these depths with real bedrock, two additional stations were added at an exposed bedrock area southwest of Line 1 (the "Bedrock" Line and B1 and B2 stations shown on Figure 2a). For a similar purpose, namely, to record resistivity values near the known "good" production well, two HSAMT stations were added near the existing production well (Well No. 566319) located east-southeast of US95 within the La Posa South LTVA as shown in Figure 2b. Figures 3a and 3b depict the general site conditions of the study area.

4. EVALUATION METHODOLOGY AND ANALYSIS

As previously indicated, the primary purpose of our services was to characterize the subsurface conditions relative to groundwater resource potential at locations specified by you and SEH through the collection of HSAMT data. Due to SEH's schedule concerns regarding field start date, data were acquired using an available StrataGem EH4 HSAMT system starting June 16 and continuing through June 18. Data acquired during the remaining seven field days were acquired using a Geometrics Geode EM3D HSAMT system which became available on June 19th. Both systems are manufactured by Geometrics, Inc. of San Jose, California. The StrataGem EH4 system was originally designed in 1995 and has proved to be a very useful instrument for groundwater and mineral exploration (the StrataGem unit used in this survey was built in 2003).

However, the StrataGem equipment had not been used in a long period of time and recalibrating it after so long in idle condition involved significant challenges. The Geode EM3D system design is less than three years old and is considered a significant update to the older outdated StrataGem

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EH4. The Geode EM3D system has improved data quality and better filtering of "noise" resulting in a much-improved signal-to-noise ratio relative to the older unit. Figures 4a and 4b show illustrations of the field setup and individual components of the StrataGem and Geode EM3D HSAMT instrumentation used in this project, respectively. Note that the transmitting assembly ("transmitter loop") employed was the same for each system.

The following is a brief description of the HSAMT method.

HSAMT is a hybrid electromagnetic (EM) geophysical method, commonly used in groundwater exploration and fault location studies. It evaluates the earth’s subsurface electrical resistivity distribution by measuring time dependent variations of the earth’s natural electric (E) and magnetic (H) fields (MT method), as well as the electric and magnetic fields resulting from high frequency induced waves sourced from a man-made transmitter. The HSAMT method is typically used to evaluate depths of approximately 50 to 2,500 feet (approximately 15 to 760 meters) bgs or greater.

Data were acquired in “high” frequency mode. Electric dipoles (Ex and Ey) and magnetometers (Hx and Hy) were laid out in perpendicular directions and both natural and transmitted frequencies were recorded from distant and non-polarized sources. With the StrataGem EH-4, time-series of the four components of data were recorded in three overlapping frequency bands: 10 to 1,000 Hz, 500 to 3,000 Hz, and 750 to 92,000 Hz. The Geode EM3D can record the four components of time-series data at three sample rates: 48 kHz, 3 kHz and 93.75 Hz resulting in a recorded range of 0.1 Hz to 20 kHz.

An initial test using all three sample rates was made, but it was determined that only the two higher sampling rates (resulting in a recorded range from about 2 Hz to 20 kHz) were necessary to reach the desired investigation depth of 500 meters, which is beyond the anticipated drilling depth of approximately 300 meters or less. For each StrataGem EH4 station, or sounding, the magnetic sensors and 50 meters long electric dipoles were oriented with a Brunton compass so that all components in each direction were parallel (i.e., Ex and Hx were parallel to each other as were Ey and Hy). Geode EM3D data acquisition was similar other than 76 meters electric dipoles were utilized.

Based on published geologic maps, we assumed the regional geologic strike (the average orientation of the major geologic structures such as faults) within the survey area was approximately zero degrees (i.e., north). Estimating a geoelectric strike direction is also a common procedure in HSAMT data analysis. Often the phase tensor is widely used, usually assuming deposits or geology having approximately non-varying electrical properties over distances shorter than the wavelength of the signals being measured. The phase tensor in recorded data tends to detect electrical responses which approximate the geologic structural strike or "geologic fabric" of the evaluation area. In other words, the phase tensor typically points the structural strike direction.

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This is because the phase tensor is insensitive to anisotropy itself; it mostly detects the spatial variation of resistivity.

If geoelectric strike (which may or may not be the same as geologic strike) is known then measured resistivities with the E field oriented parallel to strike are referred to as transverse electric (TE) mode measurements, while resistivities with the E field oriented perpendicular to strike are referred to as transverse magnetic (TM) mode measurements. Because of anticipated noise interference from the north-south-oriented power lines along both sides of US95, it was necessary to acquire the StrataGem EH4 data with the components oriented at 45° to minimize the interference. Thus, the Ey and Hy components were oriented at N45°E while the Ex and Hx components were oriented at N45ºW. Possibly due to its improved signal-to-noise ratio, and "noise" filtering, no power line interference was apparent within the Geode EM3D data from information available during data acquisition (i.e., an AC noise monitor is available to the Geode EM3D operator) and the data acquisition arrays were oriented with Ex and Hx parallel to the Line 1 and Line 2 orientations respectively. During processing the data from each system were rotated to zero degrees so that the data could be processed in the both the TE and TM modes.

Over a two-dimensional earth, the TE and TM modes give different apparent resistivity values and are sensitive to different aspects of the subsurface structure. The TE mode is most sensitive to conductors, whereas the TM mode is most sensitive to resistors and shallow structure. The TE mode is purely inductive, while the TM mode additionally has a galvanic component inherent in its response. This makes the TM mode higher resolution with respect to defining lateral contacts. When searching for vertical conductors the TM mode is only weakly excited, while the TE mode can show a very strong response with large spatial extent. Therefore, the interpretation weight of each mode depends on the target orientation (vertical, horizontal), the nature of the target (resistive, conductive) and the quality of the data. Because conductive targets are the primary concern for this evaluation, the data have been processed and interpreted in the TE mode.

During data acquisition, prior to any data being recorded, a weak mixture of table salt (NaCl) and potable water was poured around the stainless-steel electrodes at each end of a dipole and at the grounding electrodes at the StrataGem EHF AFE box, transmitter loop, and at the Geode EM3D EH6 data collection box. It is not possible to determine the contact resistances at the electrodes at the end of the electric dipoles with the StrataGem EH4 system but contact resistance values are available for the Geode EM3D system and were generally recorded as less than 500 ohms. Lower contact resistances enable higher quality data to be acquired and with the StrataGem EH4 system best field practice was employed to try to minimize the assumed values.

The ground surface consisted of dry and at times sandy soils that possessed very large contact resistances. The magnetometer sensors were buried in small trenches that were dug to eliminate potential signal noise from wind sourced micro-pulsations, or "rocking" of the sensors had they been otherwise left exposed to the wind (Figures 3a and 3b).

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Regarding the StrataGem EH4 unit, the instrument gains were independently set before collecting data at each station with identification of possible interference and other quality control procedures assessed before recording the data. After adjusting the gains, fourteen runs of time series data were separately obtained in all three frequency bands. The lowest frequency band recorded only natural signals, the middle band encompassed both natural signals and about five separate transmitted frequencies, and the highest band recorded signals from fourteen separate transmitted frequencies in addition to any natural signals present.

After initial field testing, the length of time for acquisition of sampling rates for the Geode EM3D unit were selected as 10 minutes for 48 kHz and 15 minutes for 3 kHz. Gains for both sampling rates were set at 24 decibels (dB). The Geode EM3D transmitter, operating in 10 steps from

1.2 kHz to 16 kHz, was recorded for the 48 kHz sampling rate. Both the StrataGem EH-4 and Geode EM3D instruments are broadband instruments and as such will record signals of any frequencies within their operating range. The transmitter is used to augment the typically lower amplitude signals from approximately 1.2 kHz to 16 kHz. The data from each StrataGem EH4 sounding were stored on the instrument’s hard drive and downloaded to a laptop PC several days after completion of the field data acquisition. This was because of small network card interface problem in the StrataGem instrument that was resolved successfully via a parts repair effected at the office. The Geode EM3D field data were acquired with a field laptop PC and stored within it and downloaded nightly to a separate analysis computer.

An example of raw data acquired with the StrataGem EH-4 is shown in Figure 4c, although not from this project area. The lower right portion of the figure presents a one-dimensional model, which is labeled as “true” resistivity. This model is generally only useful for simple, layered geologic environments and is only marginally applicable to this project area, but is useful in the field as a quick check on general data quality. The X-axis for each of the three plots within the upper right portion of the figure is logarithmic frequency and the top plot is scalar resistivity ranging logarithmically from 1 to 500 ohm-meters, the middle plot is scalar impedance phase from 0º to 90º, and the bottom plot is scalar impedance coherency from 0 to 1.0. On each plot, data from the X direction are shown as diamonds while the Y direction data are squares.

An example of the raw data acquired from this project with the Geode EM3D unit are shown in Figure 4d with logarithmic tensor resistivities from 0.1 to 1k ohm-meters, tensor impedance phase from -90° to 180°, and tensor impedance phase from 0 to 1.0. Under ideal circumstances and in areas with no appreciable geologic structures, the X and Y direction data should plot nearly on top of each other in the resistivity graph, have a phase close to 45º, and a coherency around 1.0.

A total of 70 separate HSAMT soundings were acquired along five lines as shown in Figures 2a, 2b, and 4e. Stations along our evaluation lines were located nominally 76 meters (about 250 feet) apart. Initial modeling of the Geode EM3D data was done daily to confirm the quality of the data;

however, the initial modeling was not the rigorous type done later and thus was only used as a guide. Because of downloading issues in the field with the StrataGem EH4 those data were not

Page | 7 initially modeled, but the resistivity curves, phase, and coherency were viewed in the field as a check on relative data quality at each station. Overall, the StrataGem EH4 data are considered relatively lower quality compare to the Geode EM3D data (which affects evaluation Line 3 only), while the Geode EM3D data are generally excellent quality. Low natural signal amplitudes adversely affected some of the low and middle frequency band data. Because of the relatively lower quality of the StrataGem EH4 data they were processed in the scalar mode; the excellent quality of the Geode EM3D enabled processing in the preferred tensor mode.

HSAMT measurements are adversely influenced by the presence of EM noise caused by overhead or underground power lines, grounded metal fences, metallic pipelines, other underground utilities, structures that contain metal (such as reinforced concrete) and other metallic objects. Therefore, a lateral distance of approximately 300 feet or more was maintained between suspected potential interferences such as the known power lines near US95, and powerlines and other infrastructure located near the "East Well" Line near Well No. 566319, and our HSAMT measurement stations.

HSAMT data from each station were reviewed and edited when appropriate. Editing of the data is somewhat subjective but is based upon experience, juxtaposition of X and Y direction resistivity data, phase differences from the optimum value of 45º, and coherency. Because of the relatively lower quality of the StrataGem EH4 data it was necessary to make more edits with those data, while only limited, if any, editing was necessary for the Geode EM3D data. Two-dimensional depth sections were then modeled along profiles using Schlumberger’s WinGLink software that calculates a two-dimensional smooth inversion using finite difference code. Each depth section consists of logarithmic resistivity versus depth along relatively straight lines. Subsequently, values for each sounding are converted into a format compatible with the Tecplot Focus 2019 R1 computer program (version 2019.1.0.99403, 64-bit) and presented as two- and three-dimensional cross-sections in depth format.

Please refer to Appendix I for a more detailed description of the HSAMT method.

5. RESULTS

Figures 2a and 2b show the approximate locations of the HSAMT lines on a Google Earth Pro aerial image. Figure 4e shows the locations of the lines and the approximate locations of groundwater wells near the lines. The approximate locations of the groundwater wells were provided to us by project stakeholders. Evaluation results discussed below are presented in cross-section figures of contoured resistivity values vs. depth bgs. For each line location, Line 1 through Line 3, there are two cross section figures presented. The first figure for each line uses a resistivity range of 4 to 30 ohm-meters to highlight the contrasts in resistivity between layers within the alluvial deposits above bedrock, and a second figure for each line uses a range of 4 to 400 ohm-meters to highlight the contrast in resistivity values between the alluvial deposits and the

Page | 8 underlying interpreted bedrock, some figures also display our interpreted approximate depth to bedrock bgs as a dashed line.

We begin our evaluation results description below with the "East Well" Line located east of US95 in the La Posa South LTVA.

Two HSAMT stations were located along the "East Well" Line near the existing production Well No. 55-566319. This well is located east of US95 within the La Posa South LTVA. Although outside the original line layout plan, results from this line allows us to compare subsurface resistivity conditions near the operating production well with the subsurface resistivity conditions within the La Posa Tyson Wash LTVA project area located west of US95.

Well No. 566319, located approximately 100 meters southwest of our "East Well" evaluation line, was drilled in 1989. According to the driller’s log, clay and gravel (described as ¼ to ½ inch or

0.635 to 1.27 centimeters) are present from the surface to 370 feet (~113 meters) with some cobbles and boulders from 20 to 90 feet (~6 to 27 meters). From 370 to 400 feet (~113 to 122 meters) clay was reported with a noted possible water zone, while from 400 to 525 feet (~122 to 160 meters) dry clay or mudstone was logged. From 525 to 600 feet (~160 to 183 meters) clay with fine gravel was reported, while from 600 to 690 feet (~183 to 210 meters) "clay and shale" was described. A noted possible water zone was reported from 690 to 730 feet (~210 to 222 meters) and is described as "shale, sand and some rock." Hard, rough drilling was reportedly encountered from 730 to 770 feet (~222 to 235 meters) with boulders or hardpan and some clay noted on the log. From 770 to 815 feet (~235 to 248 meters) shale and gravel was logged, while from 815 feet (~248 meters) to the total depth of 840 feet (~256 meters) the log describes blue clay or hard shale. The static water level when the well was drilled in 1989 was 535 feet (163 meters) bgs, a 40 feet screened interval extends from about 711 feet bgs, and this data was reported to ADWR "after-the-fact" in 1997. The current water level in Well No. 566319 is unknown to us.

Our results from the "East Well" HSAMT survey line conducted near Well No. 566319 are shown in Figure 5a. Resistivity values from approximately 30 to 14 ohm-meters are present from the surface to about 113 meters bgs. These values possibly correlate to “clay and gravel” in the driller’s log with the higher resistivity values possibly containing slightly more gravel. Cobbles and boulders noted in the lithologic log from about 6 to 27 meters correlate to approximately 30 to 24 ohm-meters with the higher values to the southeast. The possible water zone within clay noted in the well log from approximately 113 to 122 meters depth is seen as resistivities around approximately 14 ohm-meters. From 122 to 210 meters the resistivities vary from approximately 14 to 8 ohm-meters with the geology logged essentially as "clay" or "shale". A possible water zone from about 210 to 222 meters depth is described as "shale", sand and some rock and has resistivities of approximately 8 ohm-meters, which is quite low. The boulders or hardpan encountered in the drilling from about 222 to 235 meters depth should have slightly higher resistivities but are not visible within the resistivity data because that zone may be too thin to be

Page | 9 resolved with an electromagnetic method and TE mode which is more influenced by conductive versus resistive material. From approximately 235 meters to total depth of 256 meters the resistivities are around 7 ohm-meters which is interpreted as primarily clay with rare, if any, gravel.

Using the "East Well" Line results in Figure 5a near well No. 566319 and the accompanying lithologic log, the resistivity values from about 14 to 30 ohm-meters are interpreted to be clay and gravel, with the higher resistivity values in that range containing slightly more gravel. Zones with more gravel are considered to be better for groundwater production, but only if the pore spaces between the grains are not completely filled with clay. The possible water zone noted in the lithologic log corresponds to a resistivity value of about 14 ohm-meters and is considered to have low groundwater potential. The HSAMT data suggest that the groundwater here contains greater concentrations of total dissolved solids (TDS) (greater TDS concentrations tend to lower the resistivity of water; for example, saltwater conducts electricity much better than fresh water).

Resistivity values below about 14 ohm-meters are interpreted as clay or "shale" (to use the driller's term) and are not favorable for groundwater potential. Although no information is available regarding the current groundwater depth bgs or the water quality in well No. 566319, resistivity values above 14 ohm-meters are better groundwater targets. Zones exhibiting higher values, above 25 ohm-meters, are considered even better targets if these areas are below the water table. Near-surface zones with resistivities above 25 ohm-meters may look favorable at first, but they are probably above the water table and, thus, are probably dry.

Two additional HSAMT stations were added at an exposed bedrock area west of Tyson Wash.

Results for this "Bedrock" Line collected across a weathered bedrock area are shown in Figure 5b. As expected, the resistivity values here are much greater than those along the "East Well" Line near well No. 566319, located approximately 4.5 kilometers to the southeast. Exposed bedrock at the "Bedrock" Line in published maps of the area is mapped as mostly weakly metamorphosed Jurassic sediments (sandstone/quartzite with minor shale) and some granite units farther west. Resistivity values less than about 100 ohm-meters from the land surface to a depth of approximately 100 meters are interpreted to be highly fractured and/or weathered bedrock. Based on the much greater resistivity values in our results at depth, the bedrock may become more competent and less weathered with depth. Because we now understand what the resistivity of the known bedrock area is, we can more confidently interpret the depth to bedrock beneath the alluvial deposits eastward out into the valley along Lines 1 and 2.

Figure 5c shows the HSAMT cross-section for Line 1. It reveals a zone of low resistivities containing values less than about 10 ohm-meters extending across the line from a depth of about 130 meters near station 105 to at least 300 meters depth at the northeast end of the line. As discussed above, this zone of lower resistivities is not considered a target for groundwater production. The “new well” adjacent to Tyson Wash drilled earlier this year went to a depth of about 205 meters bgs and reaches a depth where the modeled maximum resistivity value is about 13 ohm-meters at the well's total depth (T.D.). Based on the well's less than satisfactory air-lift

Page | 10 production rate that was reported to us by SEH, and based on the results seen in Figure 5a for the "East Well" Line, we interpret the region of Line 1 above bedrock to be mostly clay or contain high TDS groundwater and, hence, is not a good target for groundwater production.

Figure 5d shows a cross-section for HSAMT Line 1 with logarithmic values from 4 to 400 ohm-meters to better highlight the depth to interpreted bedrock. Well No. 595956 was reportedly drilled in 2002 and is located along or near Line 1. Based on logs, this well is reported to have encountered gravel from about 45 to 90 meters, which correlates to resistivity values less than about 8 ohm-meters which, from the discussion above, is not considered a good groundwater production target. Farther to the northeast of well No. 595956 the resistivity values are much lower (generally less than approximately 5 ohm-meters) to depths of at least 200 meters bgs. Similarly, the material at these depths is likely also composed primarily of clay or might contain high TDS groundwater. The upper surface of the bedrock along Line 1 is interpreted to be fractured and/or weathered at a depth of approximately 250 meters bgs near the southwestern end of Line 1. This surface appears to dip to the east, dropping to deeper than 500 meters bgs. Results from Line 1 do not indicate any good groundwater targets. Resistivity contrasts indicative of bedrock are not present in our Line 1 HSAMT results at the depth interpreted by others from e-logs at the New Test Well location. Our results show a resistivity contrast interpreted to be consistent with results from our "Bedrock Line". Namely, at our "Bedrock Line" where known bedrock is exposed at ground surface, we interpret bedrock at the surface in the range of about 80 to 100 ohm-m which increases in value beyond 400 ohm-m at depths greater than 400 m. Accordingly, our interpreted bedrock depth at the New Test Well location is approximately 350 to 380 m (about 1,150 to 1,250 ft) bgs. However, it should be noted that because our data was analyzed in TE mode, which is more influenced by conductive versus resistive material (because the current study target is presence of groundwater and not the precise depth to resistive bedrock), the resolution of depth to bedrock bgs can have inherent error in HSAMT data, and will not be as precise as data recorded by downhole e-logs. However, based both on our prior experience with HSAMT methods and on published geologic data for this basin, the overall trend of deepening bedrock to the east as depicted in our results, in Figures 5c through 5f, is anticipated to be generally accurate, and our assessment of the non-presence of good groundwater targets in the study area is not affected by the depth to bedrock resolution issue.

Figure 5e shows the cross-section for HSAMT Line 2 displaying logarithmic resistivity values from 4 to 30 ohm-meters. Results along Line 2 are very similar to those along Line 1 (Figure 5c), showing a zone of low resistivities across most of the line that is not considered a good groundwater target for the reasons stated above. The depth to bedrock and the east-dipping bedrock surface along Line 2, shown in Figure 5f, is also similar to that seen along Line 1 (Figure 5d) although slightly deeper and with marginally lower resistivities than in Line 1. Results from Line 2 do not indicate any good groundwater targets. Like Line 1, resistivity contrasts consistent with our interpretation of bedrock are not present in our Line 2 HSAMT results at the depth interpreted by others from e-logs at the New Test Well location. Our results show a

Page | 11 resistivity contrast interpreted to be consistent with results from our "Bedrock Line" as described above similar to our Line 1 results interpretation. But like Line 1, the resolution of depth to bedrock bgs can have inherent error in HSAMT data and will not be as precise as data recorded by downhole e-logs. Also similar to Line 1, at Line 2 our assessment of the non-presence of good groundwater targets in the study area is not affected by the depth to bedrock resolution issue.

The relationships between the Line 1 and 2 cross-sections are best seen within Figures 5g through 5j that present both lines together in two views and resistivity contour intervals. A possible geologic explanation for the presence of the ubiquitous low resistivity zone along Lines 1 and 2 could be the presence of an ancient depression such as a lake or playa that filled with mostly clay deposits. The limited data along the "East Well" Line near well No. 566319 east of US95 (Figure 5a) has slightly higher resistivities, indicating that the low resistivity zone seen in Lines 1 and 2 does not extend as far east as the "East Well" Line, east of US95. However, with the current data set it is not possible to determine where the low resistivity zone ends or if it truncates or "pinches out" due to lateral changes in the grain size or geologic structure. Another pertinent question is whether the low resistivity zone observed at Lines 1 and 2 may act as a detriment to groundwater recharge. This is because the interpreted presence of a significant thickness of clay deposits would presumably correspond to low transmissivity and permeability for recharged groundwater.

Figures 5k and 5l present results from HASMT Line 3. As discussed earlier in this report, the data along Line 3 were acquired with the older StrataGem EH4 equipment, instead of the newer Geode EM3D unit. Because of the relatively lower quality of the data recorded by the older StrataGem EH4 unit, we processed the StrataGem data in scalar mode versus the preferred tensor mode.

The tensor mode was used for processing the Geode EM3D data. The different processing modes and the significant amount of editing of the "noisy" StrataGem data resulted in slight differences between the cross-sections of Lines 1 and 2. Because of the differences, the Line 3 cross-sections for both contour intervals (Figures 5k and 5l) are not included in the three-dimensional projection plots shown in Figures 5g through 5j. Caution should be used when interpreting the results in the Line 3 sections below about 200 meters bgs which exhibit higher resistivity values than are seen along Lines 1 and 2. The lower-frequency data along Line 3 were greatly impacted by the power lines along US95, even though they were at least 450 meters away. Note that Lines 1 and 2 (acquired with the newer Geode EM3D unit) are closer to the power lines and yet were not adversely impacted by them.

In our experience, acceptable data with the StrataGem can be acquired when the equipment is within 100 to maybe even 200 meters of large power lines. However, the adverse impact on the StrataGem EH4 data along Line 3 resulted in phase values decreasing to approximately zero degrees at frequencies less than approximately 1 kHz, which consequently manifests as anomalously higher resistivities in the Line 3 results when compared to results from Lines 1 and 2 at similar depths. The Geode EM3D data indicated phases approaching approximately 25° along

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Lines 1 and 2 but only for frequencies less than about 10 Hz (a harmonic of 60 Hz power line frequencies). Also, in our experience, phase can be depressed for frequencies of approximately less than 100 Hz in remote areas with lower signal levels. The current project area can be considered relatively remote and is at approximately 33.6° North latitude which has inherently lower signal levels than found closer to the Equator. However, even though lower signal levels are present in areas removed from the Equator acceptable data are often acquired. In addition, the Geode EM3D has a 24-bit analog to digital converter versus 18-bits for the StrataGem EH4, this represents a potential factor of 106 difference in recorded signal to noise ratio, where the Geode EM3D unit is very much better than the StrataGem. Better signal to noise ratio implies a much higher confidence level in the Geode EM3D results on Lines 1 and 2 compared to the StrataGem results obtained at Line 3. A relatively much lower signal to noise ratio for the StrataGem unit means the results from Line 3 are "fuzzier" in comparison to Lines 1 and 2, in other words results at Line 3 have less resolved resolution in layer thicknesses, depths bgs, and resistivity values.

6. CONCLUSIONS

Figures 5m and 5n show cross-sections along four lines (excluding Line 3 as discussed above) at two different contour intervals. The contour interval 4 to 30 ohm-meters is meant to emphasize differences in resistivity within the deposits shallower than bedrock. Figures using contour interval 4 to 400 ohm-meters are designed to emphasize differences in resistivity between the shallower alluvial deposits and the deeper bedrock. The resistivity results shown in the "East Well" cross-section (Figure 5a) near production well No. 566319 east of US95 is much different than the cross-sections along Lines 1 and 2 (Figures 5c through 5j). Very shallow intervals with resistivities greater than approximately 25 ohm-meters are probably above the water table and are thus assumed to be dry.

The ubiquitous low resistivity zone along Lines 1 through 3 probably reflects the presence of abundant clays, likely deposited in a dry lake, or playa, that occupied this valley before it became externally drained by Tyson Wash. The limited data from the "East Well" Line near well No. 566319 east of US95 (Figure 5a) has slightly higher resistivity values, indicating that the low-resistivity zone in Lines 1 through 3 does not extend as far east as the "East Well" Line and may pinch out in that direction. However, with the current data set it is not possible to determine where the low resistivity zone ends and if its eastern extent is truncated by changes in sediment type or geologic structure. In any case, the presence of abundant clays in the subsurface in the results from this current study does not provide any obvious good target for groundwater production or well siting. In addition, this clay probably acts as an impediment to groundwater recharge, because of its low transmissivity and low permeability.

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7. RECOMMENDATIONS

As per your and SEH's requests, we herein provide our additional recommendations for your consideration.

Based on the depth to bedrock map by Richards and others (2007) the depth-to-bedrock contours are spaced close together on the west side of the study basin and are spaced further apart on the east side of the basin. This implies a possible "shelf" of bedrock at relatively shallower depths is present near the west edge of the La Posa Basin (near Tyson Wash) with a much steeper slope to the bedrock on the western side of the basin, which suggests the presence of a north-south trending basin-bounding fault in the subsurface, probably at some currently unknown distance east of US95. The east-dipping bedrock surface identified in the HSAMT cross-sections might suddenly terminate at this implied fault. The sediments east of the fault, on the footwall or "down-dropped" side of the fault, might be much thicker than those on the west side of the fault. This is important because there is potentially a thicker "pile" of coarser grained sediments east of the fault which could potentially host more abundant sand and gravel deposits shed from the fault scarp. If so, then the area east of this surmised fault might provide a better target for groundwater exploration. These thicker and coarser sediments, if they exist, should be detectable using additional HSAMT data collections in this area. In the absence of such a fault, it is evident that the basin's surface stream channels drain from the east side of the basin across the present valley floor to Tyson Wash near the west edge of the basin. If this configuration has been long-lived, then the western side of the basin was probably the lowest lying part of the basin for a long time, as supported by the presence of the thick clay deposits. Therefore, moving farther eastward has the potential to encounter more frequent and possibly thicker, coarser sediments deposited further upstream from the low areas at the western edge of the basin.

To evaluate this possibility, we recommend two additional geophysical evaluations be conducted.

• First, a gravity evaluation should be conducted to detect the dip and depth of bedrock, the purpose of the evaluation is to more clearly delineate the depth to bedrock, the geometry of the buried bedrock surface, the presence or absence of one or more suspected buried faults, and their possible influence on groundwater conditions. This gravity evaluation will be conducted along a profile generally coincident with HSAMT Line 2 but extending farther to the east-southeast, beginning from bedrock areas west of Tyson Wash and slightly west of the current project footprint to a point a few hundred meters past the current production Well No. 566319 in the La Posa South LTVA area to the east. Once initiated in the field, field data acquisition for a gravity study may take no more time than one field-week, perhaps less.