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F Paleontological Resources Study F-1 Technical Report Paleontological Resource Assessment Hudson Ranch Power II Geothermal Project

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Page 1: F Paleontological Resources Study

F Paleontological Resources Study

F-1 Technical Report Paleontological Resource Assessment Hudson Ranch Power II Geothermal Project

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Page 3: F Paleontological Resources Study

F-1 Technical Report Paleontological Resource Assessment Hudson Ranch Power II Geothermal Project

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TECHNICAL REPORT PALEONTOLOGICAL RESOURCE ASSESSMENT HUDSON RANCH II GEOTHERMAL PROJECT

IMPERIAL COUNTY, CALIFORNIA

Prepared for:

ECOLOGY AND ENVIRONMENT, INC. 401 WEST A STREET, SUITE 775 SAN DIEGO, CALIFORNIA 92101

Prepared by:

DEPARTMENT OF PALEOSERVICES SAN DIEGO NATURAL HISTORY MUSEUM

P.O. BOX 121390 SAN DIEGO, CALIFORNIA 92112

THOMAS A. DEMÉRÉ, PH.D., DIRECTOR ERIC G. EKDALE, PH.D., STAFF PALEONTOLOGIST

14 JULY 2011

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TECHNICAL REPORT PALEONTOLOGICAL RESOURCE ASSESSMENT HUDSON RANCH II GEOTHERMAL PROJECT

IMPERIAL COUNTY, CALIFORNIA

INTRODUCTION

Hudson Ranch Power II LLC (Hudson Ranch) is proposing to construct and operate the Hudson

Ranch II Geothermal Development Project (Project), a 49.9 MW geothermal power plant and

wellfield project within the Salton Sea Known Geothermal Resource Area in Imperial County,

California. The Project area occupies the north half of Section 19 of Township 11 South, Range

14 East, Niland, CA Quadrangle. The private parcels comprising the Project area are located

about 3.5 miles west-southwest of the community of Niland, CA (Figure 1).

The Hudson Ranch II Geothermal Project will be operated by an affiliate of Hudson Ranch

Power II LLC’s parent company, EnergySource LLC (EnergySource). The Project area consists

of approximately 326 acres of private land either owned by EnergySource LLC or leased by the

geothermal mineral rights holders to EnergySource.

This technical report provides an assessment of issues related to paleontological resources that

occur within the project area. The purpose of this report is to assist Hudson Ranch staff in

planning and design efforts for the proposed project as it relates to paleontological resource

issues. Specifically, this report is intended to summarize existing paleontological resource data

in the vicinity of the project area, assess potential impacts to paleontological resources from

implementation of the project, and identify mitigation measures to avoid or reduce project-

related impacts to a level below significance. Additional discussion of the report methodology is

provided below. This report was prepared by Thomas A. Deméré, Rodney M. Hubscher, and

Eric G. Ekdale of the Department of PaleoServices, San Diego Natural History Museum

(SDNHM), San Diego, California under contract to Ecology & Environment, Inc.

DEFINITION AND SIGNIFICANCE OF PALEONTOLOGICAL RESOURCES

Paleontology is a multidisciplinary science that combines elements of geology, biology,

chemistry and physics in an effort to understand the history of life on Earth. Paleontological

resources, or fossils, are the remains, imprints or traces of once-living organisms preserved in

sedimentary rocks. Fossils include mineralized, partially mineralized, or unmineralized bones

and teeth, soft tissues, shells, wood, leaf impressions, footprints, burrows, and microscopic

remains. The fossil record is the only direct evidence that life on Earth has existed for more than

3.6 billion years. Fossils are considered non-renewable resources because the organisms they

represent no longer exist. Thus, once destroyed, a fossil can never be replaced. Fossils are

important scientific and educational resources because they are utilized to:

• Study the evolutionary relationships between extinct organisms, as well as their relationships

to modern groups.

• Elucidate the taphonomic, behavioral, temporal and diagenetic pathways responsible for

fossil preservation, including the biases inherent in the fossil record.

• Reconstruct ancient environments, climate change, and paleoecological relationships.

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• Provide a measure of relative geologic dating which forms the basis for biochronology and

biostratigraphy, and which is an independent and corroborating line of evidence for

radiometric dating.

• Study the geographic distribution of organisms and tectonic movements of land masses and

ocean basins through time.

• Study patterns and processes of evolution, extinction and speciation.

• Identify past and potential future human-caused effects to global environments and climates.

PROJECT DESCRIPTION

The proposed Project consists of the following main components:

• Geothermal well drilling and development of a geothermal wellfield;

• Construction and operation of a brine production facility (BPF);

• Construction and operation of a turbine generator facility (TGF); and

• Electrical connection to the Imperial Irrigation District’s (IID’s) planned interconnection

transmission line to the Hudson Ranch I Geothermal Project, which will transport power

generated from the power plant to the existing IID electrical transmission grid system.

Hudson Ranch proposes to drill and test up to eight geothermal wells. Up to four of the proposed

wells will be drilled as geothermal production wells from two production well sites located in the

west-central portion of the Project area. The purpose of the proposed development well program

is to locate, sample, drill, complete, test and monitor potential geothermal resource development

target zones on these geothermal leases. Up to three geothermal injection wells will be drilled

from two injection well sites located on the eastern edge of the Project area. A fourth injection

well (plant well), for the injection of geothermal steam condensate, cooling tower blowdown,

and aerated geothermal brines will be drilled on the power plant site.

Each of the well sites will include a clay- or plastic-lined containment basin for the storage of

waste drilling mud. Project wellfield activities will also include the improvement or construction

of required access roads. The proposed wells will be directionally drilled from the respective

well sites to explore specific geophysical or geologic targets, each to a total depth of

approximately 9,000 feet (into the geothermal zone) from one of the constructed well drilling

pads. After drilling, the wells will be flow-tested into portable storage tanks. Thereafter, the

wells will continue to be monitored for well pressure and other data until placed into commercial

service.

The production and injection well sites will each be constructed to be up to 500 feet by 400 feet

in size (about 4.6 acres each). Pad preparation activities include clearing, earthwork, drainage

and other improvements necessary for efficient and safe operation. Each site is designed to create

a level pad for the drill rig and a graded surface for the support equipment. Runoff from

undisturbed areas around the constructed sites will be directed into ditches and energy dissipaters

(if needed) around the site, consistent with Imperial County, IID, and California Regional Water

Quality Control Board, Colorado River Basin Region (CRWQCB) best management practices

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Figure 1. Index map showing general location of the Hudson Ranch II Geothermal Project Site. Base

map: Salton Sea, CA 30’ x 60’ quadrangle.

Project Site

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Figure 2. Location map showing Hudson ranch I (green box), Hudson Ranch II (pink box), “N” Drain

(doted blue line), and existing transmission line (doted orange line). Red circle represents fossil

observation discovered during Hudson Ranch II survey. Base map: Niland, CA USGS 7.5’

quadrangle.

for storm water. All machinery, drilling platforms, and oil and fuel storage will be in areas

tributary to the cellar(s) in order to prevent the movement of storm water from these areas off of

the constructed site. The well sites will be surrounded by a berm and graded to direct runoff from

the respective pad into the cellar which will be pumped as necessary.

An approximately 80-foot by 400-foot by 7-foot deep containment basin will be constructed on

each of the well sites for the containment and temporary storage of waste drilling mud, drill

cuttings, and storm water runoff from the constructed site. The containment basins are designed

to be lined with either a compacted clay liner or a 40-mil plastic liner with a 2-foot freeboard, in

accordance with requirements of the CRWQCB.

REGULATORY SETTING

This section summarizes federal and state laws and regulations that are related to paleontological

resources.

Federal Requirements

The Federal Highway Act of 1935 (20 United State Code [USC] 78) addresses paleontological

resources. Section 305 of the Act (20 USC 78, 78a) gives authority to use federal funds to

salvage archaeological and paleontological sites that are impacted by highway projects.

Although there are several other laws and regulations that address paleontological resources

either directly or indirectly, such as the Antiquities Act of 1906 (16 USC 431-433),

Archeological and Paleontological Salvage (23 USC 305), the National Environmental Policy

Act of 1969 (42 USC 138; 49 USC 1653), and the Paleontological Resources Preservation Act of

2009, those regulations apply to fossil resources on federally owned or controlled land, and are

not applicable to the Hudson Ranch II Geothermal Project.

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State Requirements

California state laws and regulations under the California Environmental Quality Act (CEQA)

and Public Resources Code (PRC) Section 5097.5 apply to paleontological resources and the

Hudson Ranch II Geothermal Project.

California Environmental Quality Act The CEC environmental review process required under the Warren-Alquist Act is considered

functionally equivalent to that of CEQA (Public Resources Code [PRC] 15064.5 (a) (2)) with

respect to paleontological resources. Guidelines for the Implementation of CEQA (Title 14,

Chapter 3, California Code of Regulations: 15000 et seq..) define procedures, types of activities,

persons, and public agencies required to comply with CEQA, and include as one of the questions

to be answered in the Environmental Checklist (Section 15023, Appendix G, Section XIV, Part

a) the following: “Would the project directly or indirectly destroy a unique paleontological

resource or site or unique geologic feature?”

Public Resources Code Section 5097.5 Section 5097.5 of the California Public Code Section protects historic or prehistoric ruins, burial

grounds, archaeological or vertebrate paleontological sites, or any other archaeological,

paleontological, or historical feature that is situated on land owned by, or in the jurisdiction of,

the State of California, or any city, county, district, authority, or public corporation, or any

agency thereof.

Local Requirements

The General Plan for the County of Imperial does not specify any requirements for

paleontological resources. Paleontological resources, however, are often considered a sub-

category of cultural resources. The Conservation and Open Space Element of the General Plan

contains requirements for cultural resources that involve the identification and documentation of

significant historic and prehistoric resources and the preservation of representative and worthy

examples. The Conservation and Open Space Element also recognizes the value of historic and

prehistoric resources and the need to assess current and proposed land uses for impacts upon

these resources.

METHODOLOGY

A review was conducted of relevant published and unpublished geologic reports (e.g., Morton,

1977; Dorsey, 2006; Landmark Consultants, Inc., 2011) and published and unpublished

paleontological reports (e.g., Ingram, 1947; Whistler et al., 1995). In addition, record searches

were conducted through the Colorado Desert District Stout Research Center at Anza-Borrego

Desert State Park and the Department of Paleontology at SDNHM. This approach was followed

in recognition of the direct relationship between paleontological resources and the geologic

formations within which they are found. Knowing the geology of a particular area and the fossil

yield potential of formations that occur in that area, it is possible to predict where fossils will, or

will not, be encountered.

A pedestrian field survey of the project was conducted on June 22, 2011 by SDNHM personnel

to field check the results of the literature and record reviews and to directly evaluate the

paleontological resource sensitivity of the geologic units that will be affected by the proposed

project. This work primarily focused on the northern and southern portions of the project site

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and involved inspection for bedrock outcrops, geologic contacts, and the presence of

paleontological resources (i.e., fossils).

EXISTING CONDITIONS

PHYSICAL GEOLOGICAL SETTING

The Hudson Ranch II Geothermal Project Site is located in the north central portion of Imperial

County, California, approximately 3.5 miles southwest of the community of Niland, California

and 1.1 miles east of the existing Hudson Ranch I Geothermal Power Plant. The Hudson Ranch

II Geothermal Project is located on ~326 acres of fallow agricultural lands approximately two

miles east of the southeast shoreline of Salton Sea. The project site varies in elevation between

205 and 215 feet below sea level.

Geologically, the project area lies within the southern portion of the Salton Trough, a

northwesterly-trending tectonic basin located between the Peninsular Ranges on the west and the

Chocolate Mountains on the east (Dorsey, 2006). The area is characterized by numerous

northwest-trending strike-slip faults, including from east to west, the San Andreas, San Jacinto,

and Elsinore faults. Roughly 2,000 square miles of the Salton Trough lie below sea level, and in

many respects, the area can be considered a landward extension of the Gulf of California. In

fact, if it were not for the tremendous volumes of sediment transported by the modern Colorado

River and its Pliocene and Pleistocene counterparts, the Gulf of California would still extend

northward as far as Riverside County. However, during the past five million years as the

ancestral and modern day Colorado River have cut down through the Colorado Plateau carving

out the Grand Canyon and carrying the eroded sediment load southward, the river has built a

sediment dam—the Colorado River delta—across the Salton Trough from east to west. At

various times during the history of the prograding Colorado River delta, the full discharge of the

river flowed north, forming a large, inland freshwater lake (actually a succession of ephemeral

lakes, see discussion below). Periodic changes in the river’s course would divert the flow to the

south and into the Gulf of California. Cut off from its freshwater supply, the prehistoric lake

would eventually dry up due to evaporation.

In point of fact, there has not been one, but a succession of ephemeral lakes in the area spanning

a period of almost three million years (Kirby et al., 2007). The oldest ephemeral lakes from

approximately 2.5 to 1.1 million years ago accumulated extensive deposits of claystone,

mudstone, and siltstone that are collectively referred to by geologists as the Borrego Formation

(Lutz et al., 2006). A younger succession of ephemeral, freshwater lakes that formed from

approximately 1.1 to 0.5 million years ago accumulated thick deposits of fine-grained sediments

referred to by geologists as the Brawley Formation (Steely et al., 2009). More recently,

including up to late prehistoric times (~450 years ago), a series of ephemeral freshwater lakes

accumulated sediments that today are exposed extensively across the central portion of the

Salton Trough and are referred to by geologists as Lake Cahuilla sediments (see discussion

below).

The geologically oldest rocks in the vicinity of the Hudson Ranch II Geothermal Project Area

consist of rhyolitic volcanic deposits collectively known as the Salton Buttes Lava Domes.

These rather unique geologic features consist of an aligned series of four small volcanoes that

include, from southwest to northeast, Obsidian Butte, Rock Hill, Red Island, and Mullet Island

(Robinson et al. (1976). These volcanoes, which last erupted approximately 16,000 years ago,

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are flanked by younger sedimentary deposits consisting of poorly consolidated gravels, sands,

and silts. Erosional features (e.g., sea cliffs and shorelines) around the margins of several of the

domes indicate former periods of inundation associated with prehistoric Lake Cahuilla (see

below). Sedimentary deposits of Lake Cahuilla and partially coeval modern alluvium underlie

major portions of the central Salton Trough, including the Hudson Ranch II Geothermal Project

Area.

Subsurface soils encountered during the geotechnical investigation by Landmark Consultants,

Inc. were reported to consist of the following sediment/rock types: 0-17 feet, fat clays; 17-20

feet, medium dense silty sands and silts; 24-98 feet, stiff clays; 97-99 feet, medium dense silty

sand; 99-100 feet, stiff to very stiff clay (Landmark Consultants, Inc., 2011).

Prehistoric Lake Cahuilla

The history of Lake Cahuilla involves a succession of prehistoric freshwater lakes that

periodically occupied a major portion of the Salton Trough during the latest Pleistocene and

Holocene between at least 12,000 and 450 years ago. There is debate over the exact timing and

duration of inundation. For example, Waters (1983) proposed that the main period of lake

formation was fairly recent with four intervals of inundation and desiccation occurring between

~2,000 and 450 years ago. In contrast, Whistler et al. (1995) reported a radiocarbon date of

6,000 years ago from Lake Cahuilla sediments and suggested that earlier periods of inundation

may have existed in the Salton Trough. Hubbs and Miller (1948) and Stanley (1963) went so far

as to suggest that the main period of inundation was older, perhaps as long ago as 10,000 years.

These latter authors recognized more recent intervals of lake formation, but suggested that they

were of short duration with minimal impact to the valley floor (i.e., limited erosion and

strandline formation). Hubbs and Miller (1948) also hypothesized that higher precipitation at the

end of the Wisconsin glacial interval (or during a substage of the Wisconsin) may have

contributed to a prolonged duration of Lake Cahuilla in the latest Pleistocene or early Holocene

when local precipitation was high. During later Holocene time, the duration of inundation was

shortened as local rates of decreased precipitation and increased evaporation resulted in complete

desiccation in less than 60 years following maximum highstand (Wilke, 1978).

Regardless of the exact timing of inundation, the former shoreline marking the maximum

Holocene highstand for Lake Cahuilla is well-preserved around the margins of the Imperial

Valley at an elevation of approximately 40 to 48 feet above sea level (Blake, 1914; Stanley,

1963). At this maximum lake level, Lake Cahuilla would have been over 300 feet deep, 105

miles long, and about 35 miles across at its widest point (Waters, 1983). As mentioned earlier,

the filling of Lake Cahuilla occurred several times during the Holocene (Waters, 1983). Each

time, the filling was the result of natural diversion of the Colorado River from its delta at the

head of the Gulf of California to the (below sea level) Salton Trough. Wilke (1978) estimated

that at present discharge levels, it would take 12 to 20 years for the full flow of the Colorado

River to fill the area of prehistoric Lake Cahuilla. Once filled, the lake would eventually

overflow its natural levee to the south, allowing the Colorado River to reestablish its flow into

the Gulf of California. Over time, the river would bypass the lake, which (now cutoff from

recharge) would gradually dry up. This cycle of flooding and desiccation is proposed to have

occurred several times in the past. An accidental breach in the man-made levee of the Colorado

River between 1905 and 1907 resulted in the formation of the present Salton Sea (Morton, 1977).

Based on the geologic mapping of Morton (1977), the entire project area is underlain by

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undifferentiated Quaternary alluvium and deposits of prehistoric Lake Cahuilla (Holocene and

Late Pleistocene age).

PALEONTOLOGICAL RESOURCE ASSESSMENT

The following section provides a general overview of the types of geologic deposits located

within the vicinity of the Hudson Ranch II Geothermal Project Area and their paleontological

resource potential.

Quaternary Alluvium

Much of the ground surface of the western portion of the Salton Trough in Imperial County is

covered by a thin veneer of recent sediments of variable thickness (0-20 feet), including aeolian

sand (in currently active sand dunes) and alluvial sand and gravel (in modern washes and alluvial

fans) (Morton, 1977). In general, these surficial deposits are undeformed by faulting and are

probably entirely Holocene in age. Quaternary alluvium typically is not considered to yield

significant fossils given the young age of the sediments.

Lake Cahuilla sediments

Sedimentary rocks mapped as Lake Cahuilla sediments underlie the entire Hudson Ranch II

Geothermal Project Area. Although the Hudson Ranch II Geothermal Project geotechnical

report (Landmark Consultants, Inc., 2011) does not specifically discuss Lake Cahuilla sediments,

it is likely that most of the deposits encountered in their exploratory boreholes actually represent

these deposits.

As described by Waters (1983) and Whistler et al. (1995) Lake Cahuilla sediments consist of a

thick, interbedded sequence of both freshwater lacustrine (lake) and fluvial (river/stream)

deposits. The lacustrine deposits typically consist of finely laminated claystones and mudstones,

while the fluvial deposits typically consist of laminated to cross-bedded, siltstones and fine-

grained sandstones.

Each time that Holocene Lake Cahuilla formed, it became home to a variety of freshwater

animals and plants. The remains of these organisms are preserved today over a large area of the

Imperial Valley. Fossil-bearing exposures may occur at the surface where local wind deflation

has been especially heavy. Temporary subsurface exposures resulting from utility line

excavations or geotechnical exploration trenching reveal interbedded lake and river sediments to

depths of at least 20 feet below the ground surface (Waters, 1983; Whistler et al., 1995).

The first mention of fossils in the lake and river deposits of prehistoric Lake Cahuilla was by

Blake (1854, 1857) who noted the widespread occurrence of shells of various kinds of freshwater

mollusks (clams and snails). Since then, numerous writers have discussed the occurrence of

these molluscan fossils (Orcutt, 1890; Stearns, 1901; Ingram, 1947; Hubbs and Miller, 1948;

Whistler et al., 1995; Bowersox, 2003). In addition, the occurrence of fossil fish remains in

these Holocene lake deposits has been reported by Hubbs and Miller (1948), Hubbs et al. (1960),

Myncklei (1979), and Whistler et al. (1995).

Whistler et al. (1995) documented freshwater molluscan assemblages from an interbedded

sequence of lacustrine (lake) and fluvial (river) sediments temporarily exposed in a series of

exploratory trenches near La Quinta in Riverside County, California. These sediments and their

contained fossil remains documented at least four cycles of Lake Cahuilla inundation and

desiccation. Molluscan diversity was high in both the lake and river sediments, suggesting

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sustained freshwater conditions. Diatoms recovered from the younger (~2,000 years old)

lacustrine sediments confirm the presence of a large persistent lake with low sedimentation rates.

Whistler et al. (1995) also reported on vertebrate fossils recovered from Lake Cahuilla

sediments, noting the occurrence of skeletal remains of both terrestrial reptiles (horned lizard,

spiny lizard, brush lizard, shovel-nosed snake, night snake, gopher snake, ground snake,

sidewinder, and rattlesnake) and mammals (e.g., cottontail rabbit, pocket mouse, kangaroo rat,

ground squirrel, and wood rat) in the fluvial strata. Freshwater fishes from these fluvial deposits

included desert pupfish, bonytail chub, and razorback sucker. These authors speculated that the

duration of lake highstands was too short to allow colonization of the freshwater lake intervals

by aquatic vertebrates (e.g., frogs, toads, turtles, and water snakes).

Bowersox (2003) examined shells of the freshwater mussel Anodonta californiensis from Lake

Cahuilla sediments and found that as the ancient lake desiccated, salinity levels slowly increased

from 0.7 ppt at highstand, to 6 ppt at -180 feet (180 feet below highstand), then rapidly increased

to >35 ppt at -280 feet (modern Salton Sea surface level). Because no in situ specimens of

Anodonta californiensis were found at localities that were -190 feet and lower, Bowersox (2003)

hypothesized that the salinity of Lake Cahuilla must have become lethal to this bivalve species at

a lake surface level above -185 feet if it lived in 7 feet of water.

In addition to the published accounts discussed above, recent paleontological mitigation work in

Imperial County has resulted in the discovery and recovery of diverse fossil assemblages from

temporary subsurface exposures of Lake Cahuilla lacustrine and fluvial sediments. During 2009,

trenching and slant drilling for the Southern California Gas Line 6914 Loop Imperial Valley

Project between Brawley and Calipatria exposed alternating layers of clayey siltstones and fine-

grained sandstones to a depth of 40 feet. Fossils recovered from these layers (some as shallow as

5 feet below ground surface) included well-preserved remains of freshwater mollusks (clams and

mussels), ostracods, and fish. Also during 2009, mass grading operations for the State Route

78/111 Brawley Bypass Project near Brawley exposed over 35 feet of alternating mudstone,

siltstone, and fine-grained sandstone of prehistoric Lake Cahuilla. Fossils recovered from these

layers (some as shallow as 3 feet below ground surface) included well-preserved remains of

freshwater algae, mollusks, ostracods, and fish.

Based on these previous discoveries and published reports, it is clear that Lake Cahuilla

sediments possess a high paleontological resource potential to contain fossil remains. In turn,

fossils preserved in the sedimentary deposits of prehistoric Lake Cahuilla are considered

significant and unique because of the paleoclimatic and paleoecological information they can

provide (Jefferson, 2006).

RESULTS OF FIELD SURVEY During the field survey, limited exposures of Lake Cahuilla sediments were observed along the

banks of the “N” Drain, northeast of the intersection of English Road and Schrimpf Road

(Figures 3 and 4). As summarized in Figure 3, the exposed stratigraphy consisted of 12 feet of

moderate brown, laminated mudstone interbedded with thin yellowish gray siltstone. The

siltstone intervals were spaced approximately 12 inches apart, were approximately ¼ inch thick,

and exhibited cross bedding. Small fossil shells of freshwater snails were observed in place

within mudstone strata at both 6 feet and 10 feet below grade level. The mudstone at 10 feet

below grade level exhibited extensive bioturbation. The top 18 inches of the stratigraphic section

consisted of sediments disturbed by previous agricultural tilling operations.

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Figure 3 Stratigraphic section of Lake Cahuilla sediments exposed in the “N” Drian on south side of

property. Note occurrences of fossil shells in mudstone strata at 6 and 10 feet below grade.

Figure 4 Lake Cahuilla sediments exposed along the banks of “N” Drain running parallel to Schrimpf

Road. A 12 foot-thick stratigraphic section was measured and described from this exposure.

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RESULTS OF INSTITUTIONAL RECORD SEARCHES

The paleontological archives of the Colorado Desert District Stout Research Center, Anza-

Borrego Desert State Park, the Department of Paleontology SDNHM do not document any

recorded fossil collecting localities within the Hudson Ranch II Geothermal Project Area.

However, the archives of SDNHM do document several recorded fossil collecting localities from

Lake Cahuilla sedimentary deposits discovered during paleontological mitigation monitoring of

construction activities for the Southern California Gas Line 6914 Loop Imperial Valley Project

between Brawley and Calipatria (see discussion above). These documented occurrences, as well

as those discovered during paleontological monitoring of the State Route 78/111 Brawley Bypass

Project, underscore the high paleontological resource potential of prehistoric Lake Cahuilla

sediments and illustrate the fact that project related excavation operations can impact buried

paleontological resources. Further, with implementation of a paleontological mitigation plan,

such impacts can be reduced to a level below significance.

IMPACT ANALYSIS

INTRODUCTION

Direct impacts to paleontological resources occur when earthwork activities, such as grading for

access road, well pads, containment basins, and surface drainage ditches, cut into the geologic

deposits (formations) within which fossils are buried. These direct impacts are in the form of

physical destruction of fossil remains. Since fossils are the remains of prehistoric animal and

plant life they are considered to be nonrenewable. Such impacts have the potential to be

significant and, under CEQA guidelines, may require mitigation. Assessment of impacts to

paleontological resources is typically evaluated based on the relative abundance of vertebrate

fossils and significant non-vertebrate fossils. The County of Imperial does not have specific

guidelines for addressing impacts to paleontological resources, but instead relies on standard

practices employed in other jurisdictions (personal communication, County of Imperial,

Department of Planning & Development Services). The Bureau of Land Management, as well as

other agencies (e.g., Department of Planning, County of Riverside), employ a three tiered scale

of fossil potential that consists of the following categories: no, low, and high paleontological

resource potential.

No Paleontological Resource Potential

This category includes rock units of intrusive igneous origin, most extrusive igneous rocks, and

moderately to highly metamorphosed rocks. Such rock units are classified as having no potential

for containing significant paleontological resources. Artificial fill materials are also assigned no

paleontological resource potential. Geologic formations with no paleontological resource

potential do not occur within the Project Area.

Low Paleontological Resource Potential

This category includes sedimentary rock units that: 1) are potentially fossiliferous, but have not

yielded significant fossils in the past; 2) have not yet yielded fossils, but possess a potential for

containing fossil remains; or 3) contain common and/or widespread invertebrate fossils if the

taxonomy, phylogeny, and ecology of the species contained in the rock are well understood.

Sedimentary rocks expected to contain vertebrate fossils are not placed in this category because

vertebrates are generally rare and found in more localized strata. Rock units designated as low

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potential generally do not require monitoring and mitigation. However, as excavation for

construction gets underway it is possible that new and unanticipated paleontological resources

might be encountered. In such instances, monitoring and mitigation would be required. Geologic

formations that have been assigned a low paleontological resource potential do not occur within

the project area.

High Paleontological Resource Potential

This category includes rock units which, based on previous studies, contain or are likely to

contain significant vertebrate, significant invertebrate, or significant plant fossils. Units with high

paleontological resource potential include, but are not limited to, sedimentary formations that

contain significant nonrenewable paleontological resources anywhere within their geographical

extent, and sedimentary rock units temporally or lithologically suitable for the preservation of

fossils. These high potential units may also include some volcanic and low-grade metamorphic

(i.e., metasedimentary) rock units. Fossiliferous deposits with very limited geographic extent or

an uncommon origin (e.g., tar pits and caves) are given special consideration and ranked as

highly sensitive. These units include the potential for containing: 1) abundant vertebrate fossils;

2) a few significant fossils (large or small vertebrate, invertebrate, or plant fossils) that may

provide new and significant taxonomic, phylogenetic, ecologic, and/or stratigraphic data; 3)

areas that may contain datable organic remains older than Recent, including Neotoma sp.

(packrat) middens; or 4) areas that may contain unique new vertebrate deposits, traces, and/or

trackways. Areas with a high potential for containing significant paleontological resources

require monitoring and mitigation. Geologic formations that have been assigned a high

paleontological resource potential that crop out within the Project Area include the Quaternary

Lake Cahuilla sediments.

PROJECT RELATED IMPACTS Previously undisturbed Lake Cahuilla sediments underlie the entire Hudson Ranch II Geothermal

Project Area and contain proven and significant paleontological resources that likely will be

negatively impacted by the proposed construction activities associated with the Project. These

construction activities include excavation operations to construct containment basins, access

roads, well pads, surface water drainage ditches, and related storm water pollution prevention

structures. In addition, drilling operations for the proposed geothermal exploration and

production wells will extend to a depth of ~9000 feet and will penetrate not only Lake Cahuilla

sediments, but also older and more deeply buried geologic deposits with high paleontological

resource potentials (e.g., Brawley Formation, Borrego Formation, and Imperial Group).

Near surface excavation operations typically involve large scale earthmoving equipment (e.g.,

bulldozers, scrapers, excavators, and/or back hoes) and generally generate spoil materials that are

large enough to contain identifiable fossil remains. In contrast, well drilling operations typically

involve small diameter drill bits and generally produce finely pulverized spoils that are often

saturated with circulating drilling mud.

NEED FOR IMPACT MINIMIZATION Introduction

The proposed near surface excavation-related negative impacts discussed above (especially

construction of the containment basins) can, and should be minimized, while the deep well

drilling-related negative impacts cannot be minimized. There are several management strategies

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available to avoid or minimize impacts to paleontological resources with high resource potential.

These strategies include avoidance, establishment of Environmentally Sensitive Areas (ESAs),

and traditional mitigation

Avoidance

Avoidance of project impacts can, in some instances, be achieved by project redesign so that

paleontological resources are completely outside the project’s impact area (e.g., a different

alignment route that misses the resource or a construction approach that does not entail

construction excavation that would impact fossiliferous strata). In the context of the Hudson

Ranch II Geothermal Project, avoidance is not a practical strategy to minimize impacts because

existing geologic conditions are uniform across the entire project site.

Environmentally Sensitive Areas

Establishment of ESAs is a rather standard procedure for minimizing impacts on certain types of

projects (e.g., Caltrans roadway projects) and is designed to protect resources within or

immediately adjacent to a project while concurrently allowing the project to proceed. Generally,

ESAs involve some combination of exclusionary fencing or monitoring as an alternative to

excavation. If viable and properly implemented, ESAs can reduce costs and time associated with

more extensive traditional mitigation approaches. For the same reasons given above under

avoidance, establishment of ESAs around paleontologically locales within the Hudson Ranch II

Geothermal Project Area is also not a practical strategy for minimizing impacts.

Development of a Paleontological Mitigation Plan

The most practical strategy for minimizing impacts to paleontological resources with high

potential within the Hudson Ranch II Geothermal Project Area is development and

implementation of a paleontological mitigation plan. A PMP can be implemented prior to,

and/or during, construction, however, the latter is more common on most construction projects.

Although there is no single PMP that is applicable to all construction projects, there are some

standard paleontological mitigation requirements that such a plan should contain, including the

following:

• A qualified paleontologist should attend the pre-construction meeting to consult with the

grading and excavation contractors concerning excavation schedules, paleontological

field techniques, and safety issues. (A qualified paleontologist is defined as an individual

with a MS or Ph.D. in paleontology or geology that is familiar with paleontological

procedures and techniques, who is knowledgeable in the geology and paleontology of

project area, and who has worked as a paleontological mitigation project supervisor in the

area for at least one year.)

• A paleontological monitor should be on-site on a full-time basis during the original

cutting of previously undisturbed deposits of high paleontological resource potential

(e.g., Lake Cahuilla sediments) to inspect exposures for contained fossils. (A

paleontological monitor is defined as an individual who has experience in the collection

and salvage of fossil materials. The paleontological monitor should work under the

direction of a qualified paleontologist.)

• When fossils are discovered, the paleontologist (or paleontological monitor) should

recover them. In most cases this fossil salvage can be completed in a short period of

time. However, some fossil specimens (such as a complete large mammal skeleton) may

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require an extended salvage period. In these instances the paleontologist (or

paleontological monitor) should be allowed to temporarily direct, divert, or halt grading

to allow recovery of fossil remains in a timely manner. Because of the potential for the

recovering of small fossil remains, such as isolated mammal teeth, it may be necessary to

set up a screen-washing operation on the site.

• Fossil remains collected during monitoring and salvage should be cleaned, repaired,

sorted, and cataloged as part of the mitigation program.

• Prepared fossils, along with copies of all pertinent field notes, photos, and maps, should

be deposited (as a donation) in a scientific institution with permanent paleontological

collections, such as the San Diego Natural History Museum. Donation of the fossils

should be accompanied by financial support for initial specimen storage.

• A final summary report should be completed that outlines the results of the mitigation

program. This report should include discussions of the methods used, stratigraphic

section(s) exposed, fossils collected, and significance of recovered fossils.

CONCLUSIONS

1. The Hudson Ranch II Geothermal Project Area is underlain by geologic rock units assigned a

high paleontological resource potential (Lake Cahuilla sediments).

2. Proposed construction related excavation activities associated with the Hudson Ranch II

Geothermal Project likely will impact paleontological resources preserved in those rock units

with high paleontological resource potential.

3. Potential impacts to paleontological resources can be mitigated through development and

implementation of a Paleontological Mitigation Plan.

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