This appendix describes the environmental setting for the proposed Tank Waste Remediation System (TWRS) activities at the Hanford Site. By describing the environmental conditions that could be potentially impacted by TWRS activities, this appendix provides the context and basis for analyzing the impacts of the Environmental Impact Statement (EIS) alternatives. Data to support comparisons between the potential impacts of the various EIS alternatives are also provided within this appendix. Existing conditions are discussed for all aspects of the environment (soil, groundwater, air, plant and animal species habitats, socioeconomic conditions, biological and ecological resources, cultural resources, land use, visual resources, noise, and transportation). Additional details on existing environmental conditions can be found in the Hanford Site National Environmental Policy Act (NEPA) Characterization Report (Cushing 1994 and 1995 , Neitzel 1996 ), the Hanford Environmental Report for Calendar Year s 1994 and 1995 (PNL 1995 and 1996 ), and in other references cited within the text. Information on the potential TWRS borrow sites was obtained largely from the Site Evaluation Report for Candidate Basalt Quarry Sites (Duranceau 1995).
The Hanford Site is in the semi-arid region of the Columbia Plateau in southeastern Washington State (Figure I.1.0.1). The Hanford Site occupies about 1,450 square kilometers (km2) (560 square miles [mi2]) of shrub and grasslands just north of Richland, Washington. The majority of this large land area, with restricted public access, provides a buffer to the smaller areas within the Hanford Site historically used for producing nuclear materials, waste storage, and waste disposal. About 6 percent of the land has been disturbed and is actively used. The Hanford Site extends approximately 77 kilometers (km) (48 miles [mi]) north to south and 61 km (38 mi) east to west.
The Columbia River flows through the northern part of the Hanford Site, turning south to form part of its eastern boundary. The Yakima River runs along part of the southern boundary and joins the Columbia River at the city of Richland. Adjoining lands to the west, north, and east are principally range and agricultural land. The cities of Richland, Kennewick, and Pasco (also known as the Tri-Cities) comprise the nearest population centers and are located southeast of the Site.
I.1.1 GEOLOGY AND SOIL
Geologic information on the Hanford Site (Figure I.1.1.1) has been collected in connection with a variety of Site activities. Reports by Delaney (Delaney et al. 1991), Reidel (Reidel et al. 1992), and Cushing (Cushing 1994), summarizing the information collected during many of these activities, are the primary basis for the following overview of the Hanford Site's subsurface environment.
The geology of the Hanford Site forms the framework for the Site's groundwater and surface water resources. Of particular relevance are 1) the topography, which impacts surface water flows and infiltration; 2) the vadose zone, because of potential impacts associated with releases during proposed TWRS activities; and 3) the saturated sediments beneath the vadose zone that form the unconfined aquifer, because of potential impacts from releases that pass through the vadose zone from proposed TWRS activities.
Figure I.1.0.1 Hanford Site and Vicinity
Figure I.1.1.1 Geographic Setting and General Structural Geology of the Pasco Basin and Hanford Site
The geology and water resources sections focus primarily on conditions in the 200 Areas, where the tank waste and strontium (Sr) and cesium (Cs) capsules are located and where virtually all TWRS facilities, except for three potential borrow sites, would be located under any of the EIS alternatives. The potential Pit 30 borrow site, a possible source of sand and gravel, is located between the 200 East and 200 West Areas. The geologic setting of the Pit 30 area is the same as is described for the 200 Areas. The potential McGee Ranch and Vernita Quarry borrow sites, possible sources of silt (McGee) and basalt (Vernita), are located approximately 6 km (4 mi) north and west of the 200 West Area. Geologic conditions for the McGee Ranch and Vernita Quarry areas are briefly described in the following sections.
I.1.1.1 Topography and Geomorphology
The existing tank farms are on a broad flat area called the Central Plateau, which overlies an alluvial terrace (Figure I.1.1.1). The Central Plateau is in a portion of the Pasco Basin, a topographic, structural depression in the southwest corner of the Columbia Basin physiographic subprovince. This subprovince is characterized by generally low-relief hills with deeply incised river drainage. The Central Plateau's elevation is approximately 200 meters (m) (650 feet [ft]) to 230 m (750 ft) above sea level. The Plateau decreases in elevation to the north, northwest, and east toward the Columbia River. Plateau escarpments have elevation changes of 15 m (50 ft) to 30 m (100 ft). The proposed Vernita Quarry and McGee Ranch borrow sites are located to the west of the northern portions of the Central Plateau.
The Pasco Basin is an area of generally low relief ranging from 120 m (390 ft) above mean sea level at the Columbia River level, to 230 m (750 ft) above mean sea level in the vicinity of the TWRS sites in the 200 East Area. The Pasco Basin is bounded on the north by the Saddle Mountains; on the west by Umtanum Ridge, Yakima Ridge, and the Rattlesnake Hills; on the south by Rattlesnake Mountain and the Rattlesnake Hills; and on the east by the Palouse Slope (Figure I.1.1.1).
Surface topography at the Hanford Site is the result of the uplift of anticlinal ridges, Pleistocene cataclysmic flooding, Holocene eolian activity, and landslides (Delaney et al. 1991). Uplift of the ridges began in the Miocene Epoch, concurrent with the eruption of the flood basalts and continues to present. Cataclysmic flooding occurred when glacial ice dams in western Montana and northern Idaho were breached, allowing large volumes of water to spill across eastern and central Washington State. Much of the landscape in the path of the floodwater was stripped of sediments and basalt bedrock was scoured, forming scabland topography (elevated areas underlain by flat-lying basalt flows that generally exhibit deep, dry channels scoured into the surface). The last major flood occurred approximately 13,000 years ago during the late Pleistocene Epoch.
Braided flood channels with giant water current ripples, bergmounds (hummocky areas where grounded icebergs melted), and giant flood bars are among the landforms created by flooding that are apparent on the Hanford Site. Since the end of the Pleistocene Epoch, winds have reworked the flood sediments locally, depositing sand dunes in the lower elevations and loess (wind-blown silt) around the margins of the Pasco Basin. Sand dunes generally have been stabilized by anchoring vegetation, except in localized areas where they have been reactivated around disturbed vegetation and within the barchan dune complex in the west-central portion of the Site.
Observed landslide activity in the area is generally limited to the White Bluffs area east of the Hanford Site and the Rattlesnake Hills south of the Hanford Site. No landslide activity has been observed in the vicinity of the tank farms or the TWRS sites in the 200 East Area.
I.1.2 GEOLOGIC STRUCTURE
The Hanford Site lies in the Pasco Basin near the eastern limit of the Yakima Fold Belt. The Pasco Basin is a structural depression bounded by anticlinal ridges on the north, west, and south and a monocline on the east (Figure I.1.1.1). The Pasco Basin is divided by the Gable Mountain anticline in the Wahluke syncline to the north and the Cold Creek syncline to the south. Geologic materials that include basalts and sediments thicken into the Pasco Basin and generally reach maximum thickness in the Cold Creek syncline (Delaney et al.1991).
The 200 Areas are situated between the Gable Mountain anticline and the Cold Creek syncline (Figure I.1.1.1). The Gable Mountain anticline is of particular importance to groundwater flow in the unconfined aquifer. This anticline consists of a series of southeast to northwest trending folds (Trent 1992b). Portions of the Gable Mountain anticline have been uplifted high enough that basalt is above the current water table. These basalts have a low hydraulic conductivity and act as a barrier to horizontal groundwater flow in the unconfined aquifer.
The uppermost basalt underlying the 200 Areas is the Elephant Mountain Member of the Saddle Mountain Basalt Formation (Trent 1992a and b). Two adjacent boreholes north of the 200 East Area (6-53-55 and 6-55-55) encountered the Rattlesnake Ridge interbed of the Ellensburg Formation (Trent 1992b), but the Elephant Mountain Member basalt flow was absent. The absence of the Elephant Mountain Member basalt flow is referred to as a "window" (Trent 1992a and b) and is probably erosional, formed during the Pleistocene cataclysmic flooding. There is no evidence for other substantial erosion into the top of the Elephant Mountain Member and no indication of erosional windows through the basalt into the underlying Rattlesnake Ridge interbed in the 200 West Area (Trent 1992a).
I.1.3 STRATIGRAPHY AND LITHOLOGY
A generalized stratigraphic column illustrating the nomenclature for the formations that underlie the Hanford Site is provided in Figures I.1.3.1 and I.1.3.2.
I.1.3.1 Columbia River Basalt Group
The Columbia River Basalt Group, which is a sequence of basaltic rock found typically on the ocean floor, erupted as basalt flows between 6 and 17 million years ago. These flows cover an area of more than 163,000 km2(63,000 mi2) and have an estimated area of 174,000 km2(40,800 mi2). The thickness of basalt accumulations in the Pasco Basin is in excess of 3,000 m (10,000 ft) (Delaney et al. 1991). The Columbia River Basalt Group is divided into five formations (from oldest to youngest): Imnaha Basalt, Picture Gorge Basalt, Grande Ronde Basalt, Wanapum Basalt, and Saddle Mountains Basalt. Only the Grande Ronde Basalt, Wanapum Basalt, and Saddle Mountains Basalt are exposed on the Hanford Site. The Elephant Mountain member of the Saddle Mountains Basalt forms the uppermost basalt unit beneath most of the Hanford Site, except near the 300 Area where the Ice Harbor member is present, and north of the Central Plateau near Gable Gap where the Saddle Mountains Basalt has been eroded down to the Umatilla member.
Figure I.1.3.1 Generalized Stratigraphy of the Hanford Site
Figure I.1.3.2 Stratigraphic Column for the Hanford Site Showing Nomenclature From Previous Investigations by Various Authors
I.1.3.2 Ellensburg Formation
The Ellensburg Formation consists of a series of sedimentary units that are interbedded between many of the basalt flows of the Columbia River Basalt Group. The Ellensburg Formation generally displays volcanic characteristics produced by volcanic events in the Cascade Range, and silicic characteristics derived from erosion of the Rocky Mountains. At the Hanford Site, the Ellensburg Formation consists of a mix of sediments deposited by the ancestral Clearwater and Columbia Rivers (Delaney et al. 1991). The three uppermost units of the Ellensburg Formation at the Site are the Levey Interbed, confined to the vicinity of the 300 Area, and the Rattlesnake Ridge and Selah interbeds, found beneath most of the Hanford Site (Delaney et al. 1991).
I.1.3.3 Suprabasalt Sediments
The suprabasalt sediments are a sedimentary sequence overlying the basalts at the Site and include the Ringold and Hanford formations. These sediments are up to approximately 230 m (750 ft) thick in the west-central Cold Creek syncline and pinch-out against the Saddle Mountains, Gable Mountain and Umtanum Ridge, Yakima Ridge, and Rattlesnake Hills anticlines. The suprabasalt sediments are dominated by laterally extensive deposits assigned to the late Miocene to Pliocene Ringold Formation and the Pleistocene Hanford formation. The informally defined Plio-Pleistocene unit, early Palouse soil, and pre-Missoula gravels separate the Ringold Formation and Hanford formation locally.
I.1.3.4 Ringold Formation
The Ringold Formation consists of semi-indurated clay, silt, pedogenically altered sediment, fine to coarse grained sand, and gravel. The Ringold Formation at the Site is up to 180 m (600 ft) thick in the deepest part of the Cold Creek syncline south of the 200 West Area, but is largely absent in the northern and northeastern parts of the 200 East Area and adjacent areas to the north (Delaney et al. 1991, Reidel et al. 1992, and Cushing 1994).
Five sediment facies (or differentiation) associations, defined on the basis of lithology, stratification, and pedogenic (formation and development of soil) alteration, are recognized in the Ringold Formation (Delaney et al. 1991). These sediment facies include:
- Fluvial (produced by action of a stream) gravel deposited in wide-shifting river channels;
- Fluvial sand deposited in shallow channels incised into a muddy floodplain,
- Overbank-paleosol deposits that record deposition on a floodplain;
- Lacustrine (in-lake) deposits that record deposition in a lake; and
- Alluvial fan deposits that record deposition of basaltic detritus around the periphery of the Pasco Basin.
The distribution of facies associations within the Ringold Formation forms the basis for stratigraphic subdivision of the formation (Lindsey 1991). The lower half of the Ringold Formation contains five separate stratigraphic intervals dominated by fluvial gravels. These gravels, designated Units A, B, C, D, and E, are separated by intervals containing deposits typical of the overbank-paleosol and lacustrine facies associations (Delaney et al. 1991). The lowermost of the fine-grained sequences overlying Unit A is designated the lower mud sequence. The uppermost gravel unit, Unit E, grades upward into interbedded fluvial sand and overbank deposits that are in turn overlain by lacustrine-dominated strata.
The lower mud sequence (Figure I.1.3.3) consists of overbank and lacustrine deposits. The lower mud sequence is hydrologically substantive in that it is a potential confining layer that may offer some hydraulic separation between the saturated Ringold Formation above and the underlying Unit A gravels. The lower mud sequence is generally absent in the northern part of the 200 East Area and at the main lobe of B Pond (Trent 1992b). In the 200 West Area, the lower mud sequence is generally present throughout, except in the northeast corner (Trent 1992a). In the 200 West Area, the thickness of the lower mud sequence ranges from over 30 m (100 ft) in the south-central portion of the area to nonexistent in the northeast corner.
I.1.3.5 Post-Ringold and Pre-Hanford Units
Thin, laterally discontinuous alluvial deposits separate the Ringold Formation from the Hanford formation in various parts of the Hanford Site. These deposits are referred to informally as the Plio-Pleistocene unit, pre-Missoula gravels, and early Palouse soil (Figure I.1.3.3). The Plio-Pleistocene unit unconformably overlies the Ringold Formation in the western Cold Creek syncline in the vicinity of the 200 West Area. Depending on location, two types of materials may be present within the Plio-Pleistocene unit: 1) interfingering carbonate-cemented silt, locally referred to as the "caliche layer" (Trent 1992a), sand and gravel, carbonate-poor silt, and sand; and/or 2) basaltic detritus consisting of weathered and unweathered basaltic gravels deposited as locally derived slope wash, colluvium, and sidestream alluvium.
Pre-Missoula gravels are composed of quartzose to gneissic pebble-to-cobble gravel with a sand matrix. These gravels are up to 25 m (82 ft) thick, contain less basalt than underlying Ringold gravels and overlying Hanford deposits, have a distinctive white or bleached color, and sharply truncate underlying strata. The early Palouse soil consists of up to 20 m (66 ft) of silt and fine-grained sand. Deposits composing the early Palouse soil are massive, brownish-yellow, and compact.
I.1.3.6 Hanford Formation
The Hanford formation consists of pebble-to-boulder gravel, fine- to coarse-grained sand, and silt. These deposits are divided into three facies; gravel-dominated, sand-dominated, and silt-dominated (Figure I.1.3.3). These facies are referred to as coarse-grained deposits, plane laminated sand facies, and rhythmite facies, respectively (Reidel et al. 1992). The rhythmites also are referred to as the Touchet Beds or slack water deposits. The Hanford formation is thickest in the vicinity of the Central Plateau where it is up to 65 m (210 ft) thick. The Hanford formation was deposited by cataclysmic flood waters that drained out of a glacial lake named Missoula. Hanford Site deposits are absent on ridges more than approximately 385 m (1,260 ft) above sea level, the highest level of cataclysmic flooding in the Pasco Basin (Reidel et al. 1992).
Figure I.1.3.3 General Stratigraphy of the Suprabasalt Sediments of the Hanford Site
The sand-dominated facies was deposited adjacent to the main flood channelways and is found most commonly in the central Cold Creek syncline in the central to southern parts of the Central Plateau and in the vicinity of the Washington Public Power Supply System facilities. The silt-dominated facies was deposited under slack water conditions in back-flooded areas and is found throughout the central, southern, and western Cold Creek syncline within and south of the Central Plateau.
I.1.3.7 Holocene Surficial Deposits
Holocene surficial deposits consist of silt, sand, and gravel that form a thin (less than 10 m [30 ft]) veneer across much of the Hanford Site. These sediments were deposited by a mix of eolian (wind) and alluvial processes.
I.1.4 MINERAL RESOURCES
The geology of the potential Vernita Quarry and McGee Ranch borrow sites contains successions of basalts flows and suprabasalt sediments similar to those found on the Central Plateau and the areas near these sites along the Columbia River. The Vernita Quarry site is located in the Umatilla flow of the Saddle Mountain basalt. The Umatilla Flow at this location is composed of a single collonade characterized by columns 0.9 to 1.2 m (3.0 to 4.0 ft) wide. A bench approximately 12 to 15 m (40 to 50 ft) thick exists at the current quarry site and extends eastward as part of a series of benches that correspond to erode basalt flows along the valley of the Columbia River. The Pomona flow overlies the Umatilla flow and crops out approximately 300 m (1,000 ft) east of the existing quarry. The Pomona flow locally comprises a single colonnade with columns generally less than 0.6 m (2.0 ft) wide (Duranceau 1995).
At the potential McGee Ranch borrow site, a geological evaluation revealed a layer of fine-grained sediments immediately below the surface that range in thickness from 0.5 m to 10.0 m
(1.5 ft to 33 ft). A layer of silty, sandy gravel was identified directly beneath the surficial layer of fine-grained sediments. Hanford formation sediments overlay the Plio-Pleistocene unit and range in thickness from 0.15 to 12 m (0.5 to 40 ft). The ground surface at McGee Ranch is covered with pebbles, some cobble gravels and occasional boulders (DOE 1994h).
Currently no mineral resources other than crushed rock, sand, and gravel are produced from the Pasco Basin. Deep, natural gas production from anticlines in the basalt has been tested by oil exploration companies without commercial success. There are no current indications of any commercial mineral resource potential at any of the TWRS sites.
I.1.5 GEOLOGIC HAZARDS
Geologic processes that alter topography are landslides, floods, and volcanic activity. Each of these processes are briefly discussed in the following text as they relate to proposed TWRS activities.
Landslides in the Ringold Formation sediments are common in areas where these sediments have been oversteepened by erosion, such as the White Bluffs area along the Columbia River. The likelihood of such oversteepening in the TWRS site areas is extremely low because of flat topography, a deep water table, and the absence of any actively eroding streams.
The nearest potential flooding source to the TWRS sites is Cold Creek. Studies of the probable maximum flood show that its effect is limited to the southwestern corner of the 200 West Area only (Cushing 1994). Because of the distance from the river, the probable maximum flood on the Columbia River would not impact the 200 Areas or any of the potential borrow sites. Failure of the upstream dams, either because of natural causes or sabotage, would not likely impact the 200 Areas or the potential borrow sites (Cushing 1994).
I.1.5.3 Volcanic Activity
Two types of volcanic activity have impacted the Pasco Basin in the past: basaltic flood volcanism and cascade-style diacitic volcanism to the west. The basaltic volcanism has been latent for the past eight million years and appears unlikely to resume because of changes in the plate tectonic regime of the region. The only source of volcanic activity that could impact the TWRS sites would be volcanism in the Cascade Mountain Range, more than 100 km (60 mi) west of the Hanford Site. The eruption of Mount St. Helens in 1980 is an example of such a volcanic event. This eruption caused considerable ashfall at the Hanford Site.
Seismicity at the Hanford Site is dominated by the position of the Site within the back-arc terrain of the Cascadia Subduction Zone formed where the Juan de Fuca Plate slides underneath the North American Plate (DOE 1995i). The back arc terrain of Washington occurs east of the Cascade Mountains and is underlain primarily by Jurassic to early Miocene metamorphic and volcanic rocks, which represent the accreted terrains of past collisions and continental deposits eroded from them (Reidel et al. 1989). Overlying a portion of this terrain is the Columbia Basalt Plateau, a region of thick tholeiitic basalt lava flows. The Hanford Site and proposed TWRS project sites lie within a subprovince of this basalt province known as the Yakima Fold Belt (RHO 1979).
The Yakima Fold Belt is characterized by narrow, linear anticlinal ridges of basalt and broad synclinal basins with an east to east southeast orientation. The folds have wavelengths of between 5 and 32 km (3 and 20 mi), amplitudes of less than 1 km (0.6 mi), and are commonly steeper on the northern limb. The faults in the subprovince appear to be associated with the folding and are found on the flanks of the folds. The folds extend eastward up to 113 km (70 mi) from the Cascade Range Province and were growing during the eruption and emplacement of the basalt and probably continue to grow at the present time (DOE 1988). In general, the structures do not impact the sediments that overlie the basalt.
Sources of seismic activity (earthquakes) at the Hanford Site include shallow structures in the Yakima Fold Belt or Columbia River Basalts. The orientation of the structural fabric of the Yakima Fold Belt suggests an origin by north-south compressional forces that operated from the middle Miocene age to the present. Compression during the extrusion of the lavas resulted in the folds propagating upwards through succeeding flows, folding the latest flow, and faulting the underlying flows (Reidel et al. 1989). The Hooper and Convey Model (Reidel et al. 1989) suggests that the compressive stress is horizontal and transmits deformation in a brittle manner only in the Columbia River Basalt Group (WHC 1993). It is believed that the underlying pre-basalt rocks deform in a ductile fashion and thus do not generate seismic activity. One of the most active areas of shallow earthquake activity is along the Saddle Mountain anticline, north of the Hanford Site (RHO 1979). Seismic activity within deep basement structures does not adequately explain the pattern of seismicity recorded in the region. The most recent seismic hazard analysis of the Hanford Site assumes that seismic activity occurs more or less randomly in the crust (WHC 1993). The source of seismic activity in the region that could potentially impact the Hanford Site is the Cascadia Subduction Zone, which lies off the coast of the Pacific Northwest. Two separate sources of seismic activity exist within this zone: an intraplate source where seismic events occur within the subducted Juan de Fuca oceanic plate, and an interplate source where seismic events occur at the interface of the Juan de Fuca and the North American plates. Of the two, the interplate source has the highest probability of generating earthquakes of a magnitude capable of causing ground motion at the TWRS sites that could impact the proposed facilities (WHC 1993).
I.1.6.1 Earthquake History
The Hanford Site lies in an area of relatively low seismic activity (Figures I.1.6.1. and I.1.6.2). Between 1870 and 1980 only five earthquakes occurred in the Columbia Plateau region that had Modified Mercalli Intensities (MMI) of VI or greater. All these events occurred prior to 1937. The largest event was the July 16, 1936 Milton-Freewater, Oregon earthquake (MMI=VII; surface wave magnitude = 5.8) (DOE 1988). The location of this earthquake and its association with known geologic structures are uncertain (DOE 1988).
Other earthquakes with a Richter magnitude of 5.0 or larger have occurred near Lake Chelan, Washington to the northwest, along the boundary of the Columbia Plateau and the Cascade Mountain range, west and north of the Hanford Site, and east of the Hanford Site in Washington State and northern Idaho. In addition, earthquake swarms of small magnitudes occur on and around the Hanford Site. An earthquake swarm is a series of earthquakes closely related in terms of time and space.
Seismicity with the Columbia Plateau can be segregated into three depth zones: 0 to 4 km (0 to 2.5 mi); 4 to 8 km (2.5 to 5 mi); and deeper than 8 km (5 mi). Approximately 70 to 80 percent of the seismic activity occurs in the 0 to 4 km (0 to 2.5 mi) zone, and 90 percent of the activity occurs in the first two zones (0 to 8 km [0 to 5 mi]) (DOE 1988). Most of the earthquakes in the central Columbia plateau are north or northeast of the Columbia River. Most of the earthquakes in the shallowest zone occur as swarms, which are not associated with mapped faults.
Figure I.1.6.1 Historical Seismicity of the Columbia Plateau and Surrounding Areas
Figure I.1.6.2 Recent Seismicity of the Columbia Plateau and Surrounding Areas as Measured by Seismographs
I.1.6.2 Seismic Hazards
Three major structures of the Yakima Fold Belt are found within the Hanford Site: the Umtanum Ridge-Gable Mountain Structure, the Yakima Ridge Structure, and the Rattlesnake Hills Structure. Each is composed of an asymmetrical anticline over-steepened to the north and with associated faults along their flanks. Two types of faults associated with the folds have been identified. Thrust faults occur on the northern, over-steepened limbs of the folds. These folds are sympathetic to the folds with more or less the same strike as the fold axes. Cross faults with a north-northwest trend cut the linear folds into separate segments and show a right lateral strike-slip movement (Reidel et al. 1989). Existing known faults within the Hanford area include wrench (strike-slip) faults, as long as 3 km (2 mi) on Gable Mountain and the Rattlesnake-Wallula Alignment, which has been interpreted as a right-lateral strike-slip fault. The faults in Central Gable Mountain are considered capable faults by Nuclear Regulatory Commission (NRC) criteria in that they have slightly displaced the Hanford formation gravels, but their relatively short lengths give them low seismic potential. No seismicity associated with the Gable Mountain Fault has been observed. The Rattlesnake-Wallula Alignment is interpreted to be capable faults by the NRC (Supply System 1981).
Earthquake sources considered relevant for the purpose of seismic design of TWRS facilities are the Rattlesnake-Wallula Alignment, Gable Mountain, an earthquake anywhere in the tectonic province, and the swarm area. For the Rattlesnake-Wallula Alignment, which passes along the southwest boundary of the Hanford Site, a maximum Richter magnitude of 6.5 has been estimated. For Gable Mountain, an east-west structure that passes through the northern portion of the Hanford Site, a maximum Richter magnitude of 5.0 has been estimated. An earthquake for the tectonic province was developed from the Milton-Freewater earthquake of Richter magnitude 5.75. A Richter magnitude 4.0 event is considered a maximum swarm earthquake for analyzing TWRS alternatives, based on the maximum swarm earthquake in 1973 (Cushing 1994). The Hanford Site current design basis for new facilities is for facilities to withstand a 0.2 gravity earthquake (Richter Magnitude of approximately 6.4) with a recurrence frequency of 2.0E-04.
The surface and near-surface soils in the 200 Areas are not generally well developed and consist of a number of soil types: Rupert sand, Burbank loamy sand, and Ephrata sandy loam. Hezel sand is also present on the western boundary of the 200 West Area. Rupert sand consists of coarse sand and is also known as Quincy sand. Rupert sand covers the majority of the 200 West Area and approximately one-half of the 200 East Area. Burbank sand is coarse-textured sand that covers approximately the northeastern one-third of the 200 West Area, a relatively small portion of the 200 East Area, and the majority of the area between the 200 West and 200 East Areas, where the potential Pit 30 borrow site (sand and gravel source) is located. Ephrata soil is medium-textured soil and covers the northern portion of the 200 East Area. Hezel sand is similar to Rupert sand and covers a portion of the area on, and immediately west of, the boundary of the 200 West Area. The predominant soil types in the general vicinity of the potential Vernita Quarry and McGee Ranch borrow sites are the Rupert sand and Burbank loamy sand.
I.1.7.1 Soil Contamination
Soil monitoring is conducted to detect the potential migration and deposition of radionuclides because of resuspension from other radioactive contaminated areas (wind-blown or water-borne) and waste intrusion by animals (PNL 1993a). The following contaminants have been consistently detectable in soil on the Hanford Site: cobalt-60 (Co-60), Sr-90, Cs-137, plutonium-239 (Pu-239), Pu-240, and uranium (U). Soil concentrations for these radionuclides were higher near and within Hanford Site facilities compared to offsite concentrations. In general, radionuclide concentrations near waste disposal sites are higher than concentrations further away.
Radiological surveys are conducted on Site areas that are known or suspected to contain surface or subsurface contamination. Areas that exceed specified levels are posted as radiologically controlled areas. A total of over 2,500 hectares (ha) ( 6,200 acres [ac]) of surface area and 1,030 ha ( 2,530 ac) of subsurface area were posted at the end of 1994. Ninety percent of the posted surface contamination area and 81 percent of the posted subsurface contamination area are in and near the 200 Areas. The net change in Sitewide surface contaminated areas reduced 44 ha (110 ac) from 1994 to 1995, which includes surface contamination areas, which includes a reduction of 33 ha (82 ac) in the 200 Areas. There was a corresponding net increase in Sitewide posted subsurface contamination areas of 44 ha ( 110 ac) from 1994 to 1995 , which includes an increase of 33 ha ( 82 ac) in the 200 Areas (PNL 1995).
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