Soils in the Project area are mainly derived from the basalt, andesite tuff, pyroclastic pumice, cinders and ash of various geologic ages from these volcanic sources (USFS et al. 1983).

3.1.1 Regulatory Framework

3.1.1.1 Modoc National Forest

The Modoc National Forest Land and Resource Management Plan (LRMP) (USFS 1991) provides direction for implementing the Forest’s management practices and activities (see Section 1.7). The LRMP management direction specifically applicable to this Geology and Soils analysis, and potentially relevant to the proposed Project, includes the Geology, Soils, Minerals, Special Interest Areas and National Natural Landmarks resource guidance provided in the Forest Standards and Guidelines, Management Prescriptions, and Management Area Direction sections of the LRMP.

The USFS has not yet developed a geologic resource inventory of potential seismic hazard areas. As such, the USFS recommends constructing permanent facilities away from active fault traces to minimize hazards associated with seismic activity. When planning a project, the USFS recommends the following in the Final EIS prepared for the LRMP (USFS 1991a): Establishing the proximity of the site to known faults and epicenters. Reviewing geologic conditions at or near the site that might indicate recent fault or seismic activity. After accumulating all data, determine potential hazards relative to the intended land use or development.

The USFS does not have a formalized multi-agency emergency response plan associated with a volcanic eruption in the Medicine Lake Highlands. In the event of an eruption, the U.S. Geological Survey (USGS) would be the lead agency, establishing all potential hazard zones. The USFS would provide support to the USGS (USFS 1991a).

For areas of high risk for landslides, the USFS requires a site-specific inventory to be completed during the Project planning phase. The inventory should accurately delineate potential areas of mass wasting and identify means to reduce potential impacts (USFS 1991a).

3.1.1.2 Bureau of Land Management

The BLM is the federal agency responsible for authorizing geothermal fluid production and injection operations on federal geothermal leases. Geothermal operations regulations published at 43 CFR Part 3260, as well as the Geothermal Resource Operational (GRO) Orders established under the Geothermal Steam Act, in part regulate geothermal fluid production and injection operations to prevent significant geologic impacts, such as seismicity or subsidence.

3.1.1.3 State of California

The State of California has General Plan Guidelines which can be used by counties and cities as a standard in developing their own General Plans (California Office of Planning and Research 1990). The General Plan Guidelines include a safety element section for the protection of the community from any unreasonable risks. Included in these risks are seismically induced surface rupture, ground shaking, ground failure, slope stability leading to mudslides and landslides, subsidence and other geologic hazards.

The State of California, through the California Division of Oil, Gas, and Geothermal Resources (CDOGGR) has established regulatory requirements for injection wells under the California Code of Regulations, Title 14, Article 6 which set forth specific requirements for initiating and maintaining a geothermal injection project, including limitations on injection pressures.

The Alquist-Priolo Special Studies Zones Act, which was enacted by the State of California in 1972 and renamed the Alquist-Priolo Earthquake Fault Zoning Act in 1993, was passed to prevent the construction of buildings used for human occupancy on the surface trace of active faults. The act requires the State Geologist to delineate earthquake fault zones by regulation along active faults within the state and to issue appropriate maps. For the purposes of this Act, an active fault is one that has moved in the last 11,000 years (Holocene time) (California Department of Conservation 1996).

3.1.1.4 Siskiyou County

Siskiyou County has a General Plan which is used to determine land use planning on lands subject to Siskiyou County jurisdiction. Overlay maps have been developed as part of the General Plan process to address various planning concerns and are used to evaluate a site for potential development. Specific maps have been created with regard to geologic and soil hazards. However, the overlay maps do not provide data for the Project area.

Siskiyou County also has a General Plan element specific to geothermal development, although the applicability of the Geothermal Element of the General Plan to lands managed by federal agencies, including the Project area is limited. Geological issues discussed in the Geothermal Element of the General Plan include subsidence and induced seismicity. The plan states that precise leveling data must be obtained at geothermal facilities to measure actual rates of subsidence. Additionally, detailed microseismic studies should be conducted at prospective geothermal sites prior to large-scale development. The Geothermal Element states that gathering baseline data on the frequency of seismic events and the depth at which they occur will provide valuable information in distinguishing between naturally occurring and induced events (Siskiyou County 1984).

3.1.1.5 Modoc County

The Safety Element of the Modoc County General Plan contains discussions of geologic hazards and seismic hazards, although geologic and seismic hazards raise very limited issues for the General Plan. The General Plan does include policies limiting development on land which has been identified as environmentally unsound to support the development. An action program has been developed which zones areas within potential hazard areas to ensure safe development or appropriate mitigation measures (Mintier Harnish and Associates 1988). However, the County has limited jurisdiction on the federally managed lands of the Project area.

3.1.2 Affected Environment

3.1.2.1 Study Area

Like most geothermal operations, the Project is located in a geologic environment where seismic activity, faulting, and volcanism has occurred. The geologic processes associated with this type of environment, in general, create the geothermal resource. However, as a result, there are also numerous potential geologic hazards associated with this environment. Additionally, soils located in geothermal environments usually originate from fairly recent volcanic materials and tend to not be well developed. As such, soils in the Project area may require extra protection from excessive disturbance and erosion.

The majority of Project impacts to area geology and soils would probably occur during construction and well drilling activities.

3.1.2.2 Study Methods

Preparation of the geology and soil analysis of this document was based on review and evaluation of the following materials: General geologic reference material on Medicine Lake Volcano, as discussed in Section 3.1.2.3 and Section 3.1.2.4 ; The soil survey of Modoc National Forest, as discussed in Section 3.1.2.5 ; The Modoc National Forest LRMP, as discussed in Section 3.1.2.3 , Section 3.1.2.4 , and Section 3.1.2.5 ; Environmental analysis documents prepared for earlier geothermal proposals within the Glass Mountain KGRA, as discussed in Section 3.1.2.3 , Section 3.1.2.4 , Section 3.1.2.5 , and Section 3.1.2.6 ; and U.S. Geological Survey (USGS) topographic maps.

3.1.2.3 Geology and Unique Geologic Features

3.1.2.3.1 Geology

The Project area is located in an area identified as the Medicine Lake Highlands (MLH), which is located about 30 miles (50 kilometers) northeast of Mount Shasta. The MLH is composed primarily of volcanic formations, including cinder cones, caldera basins, craters, irregular rhyolite lava flows, steep volcanic side slopes, recent lava flows, and a basalt capped plateau (BLM 1995).

The MLH originally formed as the Medicine Lake volcano, which was a gently sloping shield volcano measuring approximately 20 miles (32 kilometers) across and approximately 2,500 feet (762 meters) high above the surrounding lands (BLM 1995). The volcanic shield collapsed approximately 500 feet (152 meters) early in the history of the volcano, probably as a result of repeated extrusions of mostly mafic lava (Dzurisin, et al. 1991). The resulting elliptical basin measures approximately 6 miles (12 kilometers) long by 4 miles (7 kilometers) wide (BLM 1995; Donnelly-Nolan 1990).

After the collapse of the caldera, numerous volcanic eruptions resulted in the formation of eight (8) separate rim volcanos which completely hide the former caldera boundaries (California Division of Mines and Geology 1966). Recent volcanic activity in the MLH has included the eruption of basalt flows, obsidian flows and domes, and pyroclastic pumice (BLM 1995).

3.1.2.3.2 Unique Geologic Features

The USFS recognizes areas with unique characteristics as Special Interest Areas (SIAs), which are to be protected for recreational, scientific, cultural, or educational use. Each formally designated SIA is managed with its own set of guidelines and standards. The following SIAs have been established in the vicinity of the Project area on the basis of their unique geologic features (see Figure 3.1.1):

Burnt Lava Flow: The Burnt Lava Flow encompasses about 8,760 acres and is located in Siskiyou County. The flow consists of three (3) separate recent basalt flows: a highly oxidized lava, a fairly smooth pahoehoe lava, and a broken pahoehoe lava. At the time of eruption, the flows were most likely very viscous and merged together without forming easily observable boundaries. The lava surrounded three (3) older cinder cones as it flowed to the surface. The three (3) older cinder cones now appear as “islands” in the flow area, and are covered with conifer vegetation (USFS 1991a). The Burnt Lava Flow is believed to be between 2,660±60 years and 2,800±60 years old (Donnelly-Nolan et al. 1990).

Medicine Lake Glass Flow: The Medicine Lake Glass Flow encompasses 570 acres and is located in Siskiyou County. The flow is a recent stony to glassy black dacite flow which formed on the floor of the Medicine Lake caldera. The flow varies in thickness from 50 to 150 feet and is very blocky (USFS 1991a).

Glass Mountain Glass Flow: The Glass Mountain Glass Flow encompasses about 4,210 acres and is located in Siskiyou County with a small extension located in Modoc County. The formation is a very recent example of multi-stage volcanic activity which has not been modified by weathering, erosion, or vegetative cover (USFS 1991a). This flow is estimated to be 885±40 years old (Donnelly-Nolan et al. 1990). The steep-sided rhyolite and dacite obsidian flow erupted just outside of the eastern caldera rim and flowed down the steep eastern side of Medicine Lake volcano (Donnelly-Nolan et al. 1990). The initial eruption created steep-sided cones and was followed by a pumice eruption of lava extrusions. The lava extrusions started as a stoney to blocky dacite, followed by a glassy dacite and rhyolite, and then a rhyolite obsidian. As the lava extruded from the walls of the pumice cones, the cones were destroyed except for those located at the extreme southern edge of the flow (USFS 1991a).

Little Glass Mountain: Little Glass Mountain is predominantly located within the neighboring Shasta-Trinity National Forest in Siskiyou County. This flow has been recommended for SIA designation and will be evaluated for suitability during the current Shasta-Trinity Forest LRMP period (USFS 1994c). The flow encompasses about 1,440 acres and it is estimated to be about 1,065±90 years old (Donnelly-Nolan et al. 1990). It was formed by repeated overflows of lava, and it is composed of obsidian and rhyolite inter-layered with pumice (USFS 1994b). The Little Glass Mountain rhyolite is believed to have erupted during a single event which occurred during the late Holocene time (Donnelly-Nolan 1988). Thick pumice deposits which surround Little Glass Mountain were derived from this flow (USFS 1994b).

3.1.2.4 Geologic Hazards and Mass Failure

3.1.2.4.1 Seismicity

Fault activity associated with Cascade volcanos and the Modoc Plateau has the potential to produce surface rupture and ground shaking. Although in late 1988 the Medicine Lake Basin area experienced a swarm of small earthquakes, most were microseismic (i.e., earthquakes less than magnitude 3.0) (Dzurisin et al. 1991), and the Project area is not located in an area considered to be of historically high seismic activity. There are no active faults located within close proximity of the proposed Project (Personal Communication - Mark Hoffner, Planner, Siskiyou County, Yreka, California; December 3, 1997). Only two (2) faults located in the vicinity are known which could produce a seismic event of a magnitude 5.0 or greater on the Richter scale; the Likely Fault and the Surprise Valley Fault (BLM 1995). The Likely Fault is located approximately 50 miles (80 kilometers) from the Project wellfield area and is a dip-slip fault. The Surprise Valley Fault, located approximately 75 miles (121 kilometers) from the Project wellfield area, is a normal fault from which there has been an estimated 5,000 feet (1,524 meters) of vertical displacement (USFS 1991a).

Liquefaction may occur as a result of seismic activity. During liquefaction, water-saturated granular material is transformed from a solid state to a semi-liquid state as a result of an increase in the pore-water pressure caused by intense shaking. Three (3) types of liquefaction may occur: flow landslides may occur even on moderate slopes; laterally spreading landslides may occur on gentle or nearly flat slopes when the ground pulsates with the quaking and is accompanied by cracks, fissures and differential settling; and quick condition failure may occur, which is characterized by a complete loss of shear strength (Keller 1982). Liquefaction is not known to have occurred in the Telephone Flat Project wellfield area.

Active faults present two (2) basic hazards to people and structures: ground shaking and surface rupture. Ground shaking occurs when there is sudden movement along a deep portion of a fault. Surface rupture occurs along a fault or fault zone and is characterized by fault motion which may be instantaneous or be by slow creep. Three (3) types of active faults related to ground shaking and/or surface rupture have been described: faults with both ground rupture and seismic shaking hazards; faults with seismic shaking but slight or no ground rupture hazard; and faults with ground rupture but minor to no seismic shaking hazard (Keller 1982).

Induced microseismicity can be caused by both the production and injection of geothermal fluids, which can upset the natural stresses and/or rock strength under some geologic conditions, and can result in numerous microearthquakes. Geothermal fluid production is most commonly the primary cause of shallow microseismicity in geothermal wellfields, while injection may cause deeper microseismic activity in these areas. Withdrawal of geothermal fluid can induce brittle failure by reducing normal stress across fractures, or it may increase shear strength by closing fractures. Fluid injection can cause increased seismicity in a number of ways, including: when fluid is injected at a pressure that locally exceeds ambient fluid pressure, the increase in pore pressure can cause a decrease of effective normal stress across fractures; injected fluid may cool rock adjacent to fractures which reduces the normal stress across them; or the mass loading of the injected fluid may increase vertical stress and the shear stress across dipping fractures in underlying rocks (Greensfelder 1993).

3.1.2.4.2 Volcanism

The MLH has had at least three (3) eruptive volcanic cycles in the last 1,500 years, and the area has been identified by the USGS as one of the four (4) most probable sites in California where a volcanic eruption may occur (USFS 1991a).

An eruption in the MLH area would most likely be similar to previous eruptions and be fairly non-catastrophic. An eruption would be accompanied by gases and deposits of ash, pumice and cinders (USFS 1991a). Based on previous eruptions, eruptions of silicic magma are likely from vents within and just outside of the summit basin. Surface flows of hot molten lava and mud would not be expected to be extensive. However, silicic eruptions may end with the eruption of dacite to rhyolite flows or domes that could reach several miles from their vents. Additionally, clastic volcanic materials could fall several hundred miles downwind (Hoblitt 1987).

Mud flows generally pick up water as they melt snow, slide through lakes, and eventually flow down existing drainages. Since there are relatively few drainages in the MLH, mud flows would not be extensive (USFS 1991a).

3.1.2.4.3 Lava Tube Systems

A large portion of the north and south sides of the Medicine Lake shield volcano was built from molten lava transmitted through lava tubes. As molten lava emerges from a vent and flows downslope, congealing lava from the top and sides of the central channel often form a bridge over the lava stream. If the liquid lava stops rising from its source within the earth, the molten lava moving beneath the crusted-over top of the lava flow will continue to drain downhill and may ultimately leave an open lava-tube cave (Waters 1990).

3.1.2.4.4 Landslides

The Project area has a low risk of slope movement due to the gentle slopes (less than 30 percent), stable parent material (volcanic bedrock), and a large percentage of cohesive soils. Because of the low risk for landslides in the Project area, little monitoring is done (USFS 1991).

3.1.2.4.5 Subsidence

Land subsidence may occur in areas where large amounts of groundwater are withdrawn. Generally, subsidence occurs in areas where there are sedimentary basins filled with unconsolidated sands, silts, clays, and gravels. Localized subsidence in the Project area is unlikely due to the strength of the underlying volcanic bedrock (USFS 1994a).

General subsidence in the area related to the ongoing volcanic activity of the region has occurred in the recent past (Dzurisin et al. 1991).

3.1.2.5 Soils

Soils in the Project area were formed by weathering and mechanical breakdown of extruded volcanic rocks. Generally, the soils consist of 2-12 inches of pumice overburden on slopes of from 1-15 percent. The soils show relatively high forest productivity with moderate to low potential for erosion (USFS et al. 1983).

The primary soil types in the Project area are the Divers-Lapine-Kinzel, Kinzel-Lapine-Divers, Lapine-Wuksi-Divers, and Stonewall-Yallani families. Detailed information for these soil types can be found in the soil survey for the Modoc National Forest Area (USFS et al. 1983). Soils in the Modoc National Forest have been classified according to the water runoff potential, the erosion hazard, and erosion factor (USFS et al. 1983). Five qualitative values were defined for runoff potential, ranging from very rapid (very little water enters the soil and runoff is high) to very slow (water enters the soil almost immediately and runoff is low). Descriptions of these major soil types are provided in Table 3.1.1. A Soils Resources Inventory (SRI) Order 2 survey of the Project wellfield and proposed transmission line route to the Northern Utility Corridor (Route 1) was completed (Alexander 1998).

Table 3.1.1: Characteristics of Major Soil Units in the Proposed Project Area

Soil Unit Name	Runoff Potential	Erosion Hazard Rating (EHR)	Erosion Factor (K)
Divers-Lapine-Kinzel (2-30% slopes)	Very Slow	Moderate	0.17
Kinzel-Lapine-Divers (1-15% slopes)	Very Slow	Low to Moderate	0.17
Lapine-Wuksi-Divers (5-30% slopes)	Very Slow	Low to Moderate	0.20
Stonewall-Yallani (35-70% slopes)	Moderate	Moderate to High	0.20

3.1.2.6 Mineral Resources

The only mineral resources known to exist in the vicinity of the Project area, other than the geothermal resources (see Section 3.2.2.3), are a number of small rock quarries used by the USFS as a source of road-building materials and several existing and former pumice mine operations located north of Glass Mountain (BLM 1995).

3.1.3 Environmental Consequences

This section describes the expected environmental effects of the Project as it relates to geology and soils. The section focuses on those adverse effects that could potentially be significant and/or were identified during the public scoping. For each effect, the significance of the effect is discussed, and any mitigation measures which may be applied to reduce the adverse effects and/or ensure that the adverse effects would not be significant are identified.

3.1.3.1 Significance Criteria

Neither NEPA, CEQ regulations or guidance on NEPA, nor the BLM’s Handbook to implement NEPA or the USFS Environmental Policy and Procedures Handbook provide specific guidance regarding the assessment of geologic or soil hazards.

Appendix G of the CEQA Guidelines indicates that a Project would normally have a significant effect on the environment if it: Exposed people or structures to major geologic hazards; or Caused substantial erosion or siltation.

Geology-related impacts that would be considered significant include: Exposing people or structures to major geologic hazards; Causing substantial erosion or siltation; Creating topographic changes which lead to other adverse impacts; Creating adverse affects to unique geologic features; Creating substantial subsidence; or Preventing the recovery of significant mineral resources.

3.1.3.2 Assessment Methodology

The environmental consequences to geology and soils of the Project were evaluated using earthquake fault location maps, historical volcanic eruption information, soil mapping data, and Project information. The potential impacts to Project workers and infrastructure from geologic hazards were determined from Project work force projections and proposed infrastructure.

3.1.3.3 Proposed Action

During construction, operation, and decommissioning of the Proposed Action, the existing geologic and soil conditions in the Project area could be affected. Additionally, the Proposed Action could be affected by geologic hazards, including seismic activity, volcanic eruption, and ground subsidence. The sections below discuss the potential impacts of the Proposed Action on erosion, topography, unique geologic features and mineral resources, and the effects of geologic hazards on Project facilities and personnel.

3.1.3.3.1 Environmental Consequences of the Proposed Action

The geology and soils-related effects of the Proposed Action are generally relevant to each of the construction, operations and decommissioning phases of the Project.

Geologic Effects: Seismicity The Proposed Action is not located in an area of historically high seismic activity, there are no recently active faults within close proximity to the Project area, and the closest faults which are believed capable of producing a seismic event of a magnitude 5.0 or greater on the Richter scale are located approximately 50 miles (80 kilometers) and 75 miles (121 kilometers) from the Project area. As a result, a significant seismic event which could result in liquefaction, ground shaking and/or surface rupture in the vicinity of the Project area is not likely.

Consistent with requirements of the Uniform Building Code (UBC), Project structures would be designed and constructed subject to the current UBC Seismic Zone 3 standards. Implementation of UBC Seismic Zone 3 standards would conform to the current Building Code Requirements of the Siskiyou County Planning/Building Department.

Impact 3.1.3.3-1: A large seismic event could produce liquefaction, ground shaking and/or surface rupture which could damage or destroy Project-related equipment and structures.

Significance of the Impact: The impact is considered less than significant and no mitigation measures are required.

Induced Seismicity The Proposed Action would include the production of approximately 3.3 million lbs/hr of geothermal fluid and the injection of approximately 2.7 million lbs/hr of spent geothermal fluid, condensate fluids, and cooling tower blowdown (see Table 2.2.2). The production and/or injection of geothermal fluids has been correlated with microseismic activity (i.e., earthquakes of magnitude 3 or less) in other developed geothermal areas. At The Geysers geothermal field there appeared to be a correlation between geothermal development and an increase in microseismic events. Subsequent investigations showed that geothermal production appeared to be the primary cause of shallow (above a depth of approximately 2 km) microseismicity and injection was the primary cause of deeper microseismicity (Greensfelder 1993). The injection of geothermal fluids associated with the Project could also result in some induced microseismicity in the Project area. However, because the injection of Project geothermal fluids would be under relatively low pressure, any induced seismicity would be expected to be minor and would not result in any adverse environmental effects (Fabriol and Glowacka 1997, Kirkpatrick, et al. 1996, and Stark 1991). The hard, relatively porous nature of the volcanic rock which forms the geothermal aquifer, and applicable regulations of the BLM and CDOGGR which limit injection pressures (see Sections 3.1.1.2 and 3.1.1.3), both limit the amount of microseismic activity which could be induced by the Project.

Impact 3.1.3.3-2: The production and injection of geothermal fluids may result in microseismic events in the Project area. These events would not be expected to result in any adverse environmental effects. This impact would be limited by the existing geologic conditions and by federal and state regulations which limit the pressure at which fluids may be injected.

Significance of the Impact: This impact is considered less than significant and no mitigation measures are required.

Volcanic Eruption The Proposed Action is located in a potentially active volcanic area, although the likelihood of a volcanic eruption which could adversely affect the Proposed Action occurring during the operating life of the Proposed Action is very low. Construction, operation and decommissioning of the Proposed Action would not increase the potential for volcanic eruption in the area.

Based on interpretations of the geologic history of the area, if a volcanic eruption were to occur in the vicinity of the Project area, the eruption would likely be relatively non-violent (USFS 1991). The eruption would likely include the expulsion of gases and deposition of ash, pumice, and cinders. Surface flows of molten lava and mud would not be expected to be extensive. Monitoring of volcanic activity precursors, such as substantial changes in geothermal gas or fluid emissions or harmonic seismic activity, would be undertaken by the USGS, USFS and other agencies, which should provide substantial advance warnings.

The risk to Project employees from a volcanic eruption is generally considered very low, due principally to the infrequency of volcanic eruptions in the area and the non-violent nature of the historic eruptions. This risk would be essentially the same as that to any other visitor to the area, although the risk to Project workers would be continuous over the life of the Project. Workers could be evacuated on short notice by any number or alternative routes. However, Project facilities cannot be quickly moved, and may be damaged, or even have to be abandoned, in the very remote chance there were a major eruption in the immediate vicinity of the Project facilities. Only if the damage to Project facilities were to result in the loss of well control, with the resulting continuous flow of geothermal flow to the surface, would there be any substantial adverse environmental effects.

Impact 3.1.3.3-3: There is the remote possibility of loss of well control and discharge of geothermal fluid to the surface in the very unlikely chance of a major volcanic eruption directly damaging Project facilities.

Significance of the Impact: The impact is considered less than significant and no mitigation measures are required, but the following other measure is provided to reduce the potential for adverse effects of the impact.

Other Measure 3.1.3.3-3: The Project Applicant shall develop and maintain an Emergency Response Continency Plan (ERCP) which shall specify those measures which may be necessary and appropriate to respond to the immediate threat of a volcanic eruption, including damage to facilities and injury or loss of life. Special consideration shall be given to feasible measures which may be able to reduce the possibility of loss of well control resulting in the discharge of geothermal fluid to the surface.

Lava Tubes Although the Project area is not known to contain substantial lava tubes, the possibility still exists that a shallow (near surface) lava tube could be encountered during well drilling operations. This may result in the possible loss of drilling fluid circulation and/or, in extreme circumstances, well collapse. If a lava tube is encountered during drilling and a loss of drilling fluid circulation occurs, the driller would try to reestablish circulation by adding “lost circulation materials,” such as cottonseed hulls or crushed walnut shells, to the drilling mud. These benign materials act to reestablish circulation by plugging the holes into which the drilling mud and cuttings are lost. However, if drilling mud circulation cannot be reestablished, as may be likely if a lava tube has been encountered because the “holes” are too large to fill, it may be necessary to abandon the borehole by cementing it in and move the borehole location to avoid the projected course of the lava tube. The loss of even a substantial quantity of drilling mud into a lava tube would not be expected to result in significant impacts to shallow groundwater quality, even if the lava tube is connected to a shallow groundwater aquifer (see Impact 3.2.3.3-10).

Impact 3.1.3.3-4: Encountering a lava tube during drilling operations could result in the loss of circulation of the drilling fluids and possibly the need to abandon the hole.

Significance of the Impact: This impact is considered less than significant and no mitigation measures are required.

Subsidence Generalized, regional land subsidence (or inflation [e.g., an increase in the land surface elevation]) related to volcanic activity could occur if a volcanic eruption or an increase in volcanic activity were to occur. However, land surface changes occurring as a result of volcanic activity would not be likely to significantly adversely affect geothermal facilities or operations.

Land subsidence generally occurs in areas where large quantities of fluids are removed from a subsurface reservoir, which results in compaction of the reservoir, which may translate to surface subsidence. Surface subsidence could potentially result from shallow groundwater withdrawals to meet water demands for drilling operations; for dust control during construction, operations, and decommissioning; and for periodic charging of the cooling tower system and domestic needs during operations. However, the amount of groundwater which would be withdrawn for these activities is a relatively small percentage of the groundwater assumed to be in the aquifer in the vicinity of the Project area (see Section 3.2.3.3.1), and the aquifer rock is composed of volcanic rock, which is very resistive to compaction. As such, land subsidence caused by withdrawal of shallow groundwater would not be expected to occur.

Subsidence could also result from geothermal fluid withdrawals during the operational phase of the Project. However, surface subsidence from geothermal resource production is also unlikely to occur because the production of geothermal fluids would be primarily from deep subsurface fracture systems; and the strength of the volcanic bedrock would prevent compaction of the aquifer; and approximately 82.5 percent of the produced geothermal fluid withdrawn from the reservoir would be injected back into the geothermal system, thus minimizing the pressure reduction in the aquifer, which would minimize the potential for compaction of the aquifer.

Impact 3.1.3.3-5: There is a remote possibility that land subsidence could occur as a result of shallow groundwater or geothermal fluid production from the subsurface reservoirs.

Significance of the Impact: The impact is considered less than significant and no mitigation measures are required. Baseline subsidence surveys, and operational monitoring, reporting, mitigation, and remedial action, if necessary, is a requirement of GRO Order No. 4.

Unique Geologic Features Four (4) unique geologic features, or SIAs, exist in the vicinity of the Project area: the Burnt Lava Flow, located to the south; the Medicine Lake Glass Flow, located to the northwest; the Glass Mountain Glass Flow, located to the northeast; and Little Glass Mountain, located to west. The construction, operation, and decommissioning of the Proposed Action would have no impact on these SIAs.

Landslides The Proposed Action power plant site, geothermal wellfield, and the alternative transmission line route segments D1 and D2 have gentle slopes, a stable bedrock material, and cohesive soils. As such, there is negligible potential for the occurrence of landslides which could adversely affect, or be affected by, the Proposed Action.

Soil Effects: Soil Erosion During construction, operations, and decommissioning of the Proposed Action, the potential exists for an increase in soil erosion. Measures are proposed by the Project which would reduce the potential for adverse effects, including:

Construction of a storm drainage system designed for the 100-year storm. Runoff from the power plant equipment areas and equipment drains would flow to the plant water storage and dump pond, or to natural drainage channels. Runoff from production and injection well pads would be channeled into the well pad sump and then periodically be injected. Runoff from cut-and-fill slopes and other areas outside of the containment areas would be collected in culverts and ditches and carried to natural drainage channels.

New access roads would be designed and constructed in accordance with USFS specifications and good engineering practices.

Geothermal pipeline leaks or ruptures or well blowouts which could cause soil erosion would be quickly controlled and repaired by facility operators.

During decommissioning, disturbed areas would be recontoured, if necessary, and revegetated.

Implementation of these proposed Project measures would largely prevent adverse soil erosion effects.

Impact 3.1.3.3-6: Construction, operation and decommissioning activities could result in soil erosion.

Significance of the Impact: This impact is considered less than significant and no mitigation measures are required, but the following other measure would reduce the adverse effects of the impact:

Other Measure 3.1.3.3-6: The Project Applicant shall develop a comprehensive Soil Conservation and Erosion Control (SCEC) Plan for the Project in consultation with the USFS and BLM prior to issuance of a permit. This SCEC Plan shall explicitly describe those measures to be undertaken to reduce soil erosion during construction, operation and decommissioning of the Project.

Topography The Proposed Action power plant site would encompass a total area of approximately 18.5 acres. The power plant site would be cleared of vegetation and graded to balance cut and fill requirements. Surface gradients on the plant pad would normally be not less than one (1) percent. Cut and fill slopes would be two (2) horizontal to one (1) vertical (2H:1V).

Up to eighteen (18) previously approved or new well pads are planned under the Proposed Action. These well pads would be multiple well pads, and each would measure up to a nominal 400 feet by 600 feet (5.5 acres). Some of the well pads would require cut and fill slopes which would be engineered, terraced, compacted and maintained to minimize erosion and provide slope stability.

Existing roads would be used wherever practical. New roads would be limited in size to that needed to allow safe passage to the power plant site and well pads.

At the end of the life span of the Project, the Project area would be restored to conditions acceptable to the responsible agencies.

Given the limited extent of proposed grading activities and the proposed site restoration activities, the impacts of the Proposed Action on topography would not be significant.

Impact 3.1.3.3-7: Construction of the Project would result in minor but permanent changes to the topography of the Project area.

Significance of the Impact: The impact is considered less than significant and no mitigation measures are required.

Mineral Resource Effects: Mineral Resources There are no aspects of the Proposed Action which would occupy areas of known surface or subsurface mineral resources (other than geothermal resources). The only mineral resources other than geothermal resources in the vicinity of the Project area which are actively being extracted are a number of small rock quarries which are used by the USFS to supply road-building materials such as the gravel pit which formerly occupied existing well pad 87-13 and the existing gravel pit located southeast of well pad 87-13 and Primary Forest Route 97 (see Figure 2.2.1) and a pumice mining operation north of the Glass Mountain flow near the terminus of proposed transmission line segment A2. As such, the Proposed Action would not prevent the recovery of any known mineral resources.

3.1.3.3.2 Unavoidable Adverse Impacts of the Proposed Action

After implementation of the other measures to reduce the adverse effects of the Proposed Action, there are no unavoidable adverse impacts of the Proposed Action related to geology or soils.

3.1.3.4 Alternative Power Plant Site A

3.1.3.4.1 Environmental Consequences of Alternative Power Plant Site A