Exhibit_D_Appendix_D-3_IWP_for_Lower_Water.pdf

LA-UR-16-24247

June 2016

EP2016-0079

Investigation Work Plan for Lower Water/Indio Canyons Aggregate Area

ESHID-601538

RFP-0000297

Prepared by the Associate Directorate for Environmental Management

Los Alamos National Laboratory, operated by Los Alamos National Security, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC52-06NA253 and under DOE Office of Environmental Management Contract No. DE-EM0003528, has prepared this document pursuant to the Compliance Order on Consent, signed March 1, 2005. The Compliance Order on Consent contains requirements for the investigation and cleanup, including corrective action, of contamination at Los Alamos National Laboratory.

The U.S. government has rights to use, reproduce, and distribute this document. The public may copy and use this document without charge, provided that this notice and any statement of authorship are reproduced on all copies.

EP2016-0079

Investigation Work Plan for Lower Water/Indio Canyons Aggregate Area

June 2016

Responsible project manager:

Kent Rich

Printed Name Signature

Responsible LANS representative:

Randall Erickson

Printed Name

Responsible DOE-EM-LA representative:

David S. Rhodes

Printed Name Signature

Project Manager

Title

Environmental Remediation

Program

Organization

Associate Directorate for

Associate Environmental Director Management

Title

Office Director

Title

Organization

Quality and Regulatory Compliance

Organization

Date

Date

Lower Water/Indio Canyons Aggregate Area IWP v

EXECUTIVE SUMMARY

The Lower Water/Indio Canyons Aggregate Area, located in Technical Area 15 (TA-15) and TA-49 at Los Alamos National Laboratory, includes a total of 3 solid waste management units (SWMUs) and 8 areas of concern (AOCs). Of these 11 sites, 4 sites have been approved for no further action, and 1 site is being addressed under another investigation. This investigation work plan identifies and describes the activities needed to complete the investigation of the remaining 6 SWMUs and AOCs. Details of previous investigations and analytical results for the 6 sites included in this work plan are provided in the historical investigation report for the Lower Water/Indio Canyons Aggregate Area.

The objective of this investigation work plan is to evaluate the historical data and, based on that evaluation, propose sampling to define the nature and extent of contamination associated with the sites within the Lower Water/Indio Canyons Aggregate Area.

vii

CONTENTS

1.0 INTRODUCTION

1.1 Work Plan Overview

1.2 Work Plan Objectives

2.0 BACKGROUND

2.1 General Site Information

2.2 Operational History

2.3 Conceptual Site Model

2.3.1 Potential Contaminant Sources

2.3.2 Potential Contaminant Transport Mechanisms

2.3.3 Potential Receptors and Pathways

2.3.4 Cleanup Standards

2.4 Data Overview

3.0 SITE CONDITIONS

3.1 Surface Conditions

3.1.1 Soil

3.1.2 Surface Water

3.1.3 Land Use

3.2 Subsurface Conditions

3.2.1 Stratigraphic Units

3.2.2 Hydrogeology

4.0 SITE DESCRIPTION AND PROPOSED INVESTIGATION ACTIVITIES

4.1 AOC 15-001, Storage Area

4.1.1 Site Description

4.1.2 Previous Investigations

4.1.3 Proposed Activities

4.2 AOC 15-004(h), Firing Site H

4.2.1 Site Description

4.2.2 Previous Investigations

4.2.3 Proposed Activities

4.3 SWMU 15-009(g), Septic System

4.3.1 Site Description

4.3.2 Previous Investigations

4.3.3 Proposed Activities

4.4 AOC 15-014(d), Drainline and Outfall

4.4.1 Site Description

4.4.2 Previous Investigations

4.4.3 Proposed Activities

4.5 SWMU 15-014(l), Drainline and Outfall from Building 15-185

4.5.1 Site Description

4.5.2 Previous Investigations

4.5.3 Proposed Activities

4.6 AOC C-15-011, Former Underground Tank

4.6.1 Site Description

4.6.2 Previous Investigations

4.6.3 Proposed Activities

viii

5.0 INVESTIGATION METHODS

5.1 Establishing Sampling Locations

5.2 Geodetic Surveys

5.3 Field Screening

5.3.1 Organic Vapors

5.3.2 Radioactivity

5.3.3 Explosive Compounds

5.4 Sampling

5.4.1 Surface Sampling

5.4.2 Subsurface Samples

5.4.3 Borehole Abandonment

5.5 Excavation

5.6 Chain of Custody for Samples

5.7 Quality Assurance/Quality Control Samples

5.8 Laboratory Analytical Methods

5.9 Health and Safety

5.10 Equipment Decontamination

5.11 Investigation-Derived Waste

5.12 Removal Activities

6.0 MONITORING PROGRAMS

6.1 Groundwater

6.2 Sediment and Surface Water

7.0 SCHEDULE

8.0 REFERENCES AND MAP DATA SOURCES

8.1 References

8.2 Map Data Sources

Figures

Figure 1.0-1 Location of Lower Water/Indio Canyons Aggregate Area with respect to Laboratory technical areas and surrounding land holdings

Figure 3.2-1 Generalized stratigraphy of bedrock geologic units of the Pajarito Plateau

Figure 4.1-1 Proposed sampling locations at AOC 15-001

Figure 4.2-1 Proposed sampling locations at AOC 15-004(h)

Figure 4.3-1 Proposed sampling locations at SWMU 15-009(g)

Figure 4.4-1 Proposed sampling locations at AOC 15-014(d)

Figure 4.5-1 Proposed sampling locations at SWMU 15-014(l)

Figure 4.6-1 Proposed sampling locations at AOC C-15-011

Tables

Table 1.1-1 Status of SWMUs and AOC in Lower Water/Indio Canyons Aggregate Area

Table 4.1-1 Proposed Sampling at AOC 15-001

Table 4.2-1 Proposed Sampling at AOC 15-004(h) ix

Table 4.3-1 Proposed Sampling at SWMU 15-009(g)

Table 4.4-1 Proposed Sampling at AOC 15-014(d)

Table 4.5-1 Proposed Sampling at SWMU 15-014(l)

Table 4.6-1 Proposed Sampling at AOC C-15-011

Table 5.0-1 Summary of Investigation Methods

Appendixes

Appendix A Acronyms and Abbreviations, Metric Conversion Table, and Data Qualifier Definitions

Appendix B Management Plan for Investigation-Derived Waste

Appendix C Site Photographs

Plate

Plate 1 Lower Water/Indio Canyons Aggregate Area

1.0 INTRODUCTION

Los Alamos National Laboratory (LANL or the Laboratory) is a multidisciplinary research facility owned by the U.S. Department of Energy (DOE) and managed by Los Alamos National Security, LLC. The Laboratory is located in north-central New Mexico approximately 60 mi northeast of Albuquerque and 20 mi northwest of Santa Fe. The Laboratory site covers approximately 39 mi2 of the Pajarito Plateau, which consists of a series of fingerlike mesas separated by deep canyons containing perennial and intermittent streams running from west to east. Mesa tops range in elevation from approximately 6200 to 7800 ft above mean sea level (amsl). Figure 1.0-1 shows the location of Lower Water/Indio Canyons Aggregate Area with respect to Laboratory technical areas and surrounding land holdings.

The Laboratory is participating in a national effort by DOE to reduce risk to human health and the environment at its facilities. The goal of the Laboratory effort is to ensure that past operations do not threaten human or environmental health and safety in and around Los Alamos County, New Mexico. To achieve this goal, the Laboratory is currently investigating sites potentially contaminated by past Laboratory operations. The sites under investigation are designated as solid waste management units (SWMUs) and areas of concern (AOCs).

The SWMUs and AOCs (the sites) addressed in this investigation work plan are potentially contaminated with hazardous and radioactive components. The New Mexico Environment Department (NMED), pursuant to the New Mexico Hazardous Waste Act, regulates cleanup of hazardous wastes and hazardous constituents. DOE regulates cleanup of radioactive contamination, pursuant to DOE Order 458.1, Administrative Change 3, “Radiation Protection of the Public and the Environment,” and DOE Order 435.1, “Radioactive Waste Management.” Information on radioactive materials and radionuclides, including the results of sampling and analysis of radioactive constituents, is voluntarily provided to NMED in accordance with DOE policy.

Corrective actions at the Laboratory are subject to a Compliance Order on Consent (the Consent Order).

This investigation work plan describes work activities that will be executed and completed in accordance with the Consent Order.

1.1 Work Plan Overview

The Lower Water/Indio Canyons Aggregate Area includes 3 SWMUs and 8 AOCs located in Technical Area 15 (TA-15) and TA-49 (Plate 1). Of these 11 sites, 5 have been previously investigated and 6 require additional characterization. Of the 5 sites previously investigated, 4 sites have been approved for no further action (NFA) status, and 1 site at TA-49 requiring additional characterization is being investigated under another aggregate area investigation (Table 1.1-1). These 5 sites are not discussed further in the work plan. This work plan addresses the 6 remaining sites using the information from previous field investigations to evaluate current conditions at each site.

Section 2 of this investigation work plan presents the general site information, operational history, preliminary conceptual site model, and a data overview. General site conditions at TA-15 are presented in section 3. Section 4 presents summaries of previous investigations and data collected and describes the scope of proposed investigation activities for each site. Section 5 presents investigation methods for proposed field activities. Ongoing monitoring and sampling programs in the Lower Water/Indio Canyons Aggregate Area are presented in section 6. Section 7 is an overview of the anticipated schedule of the investigation and reporting activities. The references cited in this work plan and the map data sources are provided in section 8.

Appendix A of this work plan includes a list of acronyms and abbreviations, a metric conversion table, and a data qualifier definitions table. Appendix B describes the investigation-derived waste (IDW) management plan. Appendix C contains site photographs.

1.2 Work Plan Objectives

The objective of the investigation activities described in this work plan is to finalize determinations of the nature and extent of contamination at the six sites.

To help accomplish this objective, this work plan presents historical and background information on the sites, describes the rationale for proposed data collection activities, and identifies and proposes appropriate methods and protocols for collecting and analyzing data to characterize these sites.

2.0 BACKGROUND

2.1 General Site Information

TA-15, also known as R-Site, occupies portions of Threemile Mesa on the Pajarito Plateau near the southwestern boundary of the Laboratory (Plate 1). TA-15 occupies approximately 1200 acres and is used for high-explosives (HE) research, development, and testing, primarily through hydrodynamic testing and dynamic experimentation. TA-15 contains the Pulsed High-Energy Radiographic Machine Emitting X-rays (PHERMEX) facility and the Dual Axis Radiographic Hydrodynamic Test (DARHT) facility, both of which are or were formerly used for testing weapons under development. Other activities at TA-15 include the investigation of weapons functioning and systems behavior in nonnuclear testing. The sites addressed in this investigation work plan are all at or near the PHERMEX facility.

2.2 Operational History

TA-15 has been used from the mid-1940s to the present for explosives experiments. In that capacity, test explosions ranging from a few kilograms of HE to as much as 650 kg were conducted. These experiments used natural uranium, depleted uranium (DU), lesser quantities of beryllium, and other metals. In most cases, the tests were carried out aboveground, which resulted in the test materials being scattered over areas. Based on estimates from Laboratory records, approximately 75 metric tons of natural uranium and DU have been expended at the TA-15 firing sites since the mid-1940s (LANL 1993, 020946, pp. E2, E9).

Most of the sites in this investigation work plan are located near the PHERMEX facility, which became operational in 1961. During the time period 1961 to 1971, a maximum of 4000 kg of DU was expended at the PHERMEX site. During that same time period, about 150 kg of beryllium, 250 kg of lead, 40 kg mercury, and 40 kg of thorium were expended. Since 1971, less than 1000 kg per year of uranium-238 was expended at the PHERMEX firing site. Beryllium usage decreased from about 10 kg per year in 1971 to about 3 kg per year in 1987 (LANL 1993, 020946, p. 6-5).

2.3 Conceptual Site Model

The sampling proposed in this investigation work plan uses a conceptual site model to predict areas of potential contamination and allow for adequate characterization of these areas. A conceptual site model describes potential contaminant sources, transport mechanisms, and receptors.

2.3.1 Potential Contaminant Sources

Releases at sites within Lower Water/Indio Canyons Aggregate Area may have occurred as a result of normal site operations or potential spills/leaks. Previous sampling results, provided in the historical investigation report (HIR) (LANL 2016, 601539) indicate contamination from uranium. Additional sampling is needed to determine the nature and extent of contamination.

Potential contaminant sources associated with TA-15 include a firing site, septic system, drainlines and outfalls, a former underground tank, and a storage area.

2.3.2 Potential Contaminant Transport Mechanisms

Current potential transport mechanisms that may lead to exposure include airborne transport of contaminated surface soil, dissolution and/or particulate transport of surface contaminants during precipitation and runoff events, disturbance of contaminants in shallow soil and subsurface tuff by Laboratory operations, continued dissolution and advective/dispersive transport of chemical contaminants contained in subsurface soil and tuff as a result of past operations, and disturbance and uptake of contaminants in shallow soil by plants and animals.

2.3.2.1 Surface Processes

Laboratory operations, disturbance and uptake by plants and animals, surface water runoff, and wind can disturb contaminants present in shallow soils. During summer thunderstorms and spring snowmelt, runoff from the mesa top may flow down the hillsides and into the perennial and ephemeral streams present in Lower Water and Indio Canyons. Surface water runoff and erosion of contaminated surface soil may lead to contamination of bench areas on the hillside and contamination of the surface water off-site. Surface water may also access subsurface contaminants exposed by soil erosion. Soil erosion can vary significantly depending on factors that include soil properties, the amount of vegetative cover, the slope of the contaminated area, and the intensity and frequency of precipitation. Surface transport of contaminants represents the dominant potential transport pathway in the Lower Water/Indio Canyons Aggregate Area.

2.3.2.2 Subsurface Processes

Studies have shown that infiltration of natural precipitation is quite low across the mesa tops of the Pajarito Plateau. The average annual potential evapotranspiration rates far exceed precipitation rates.

Under these conditions, infiltration events that propagate beneath the root zone are sporadic and occur only when the short-term infiltration rate exceeds the evapotranspiration rate, such as during summer thunderstorms and spring snowmelt. However, these events more commonly produce runoff into neighboring canyons resulting in infiltration rates below the root zone on the order of a few millimeters or less per year for mesa-top sites (Collins et al. 2005, 092028, pp. 2-84−2-88; Kwicklis et al. 2005, 090069).

This slow infiltration rate generally leads to present-day subsurface contaminant migration of only a few meters deep. Geochemical interactions between the contaminants and the rocks generally act to retard migration further. Therefore, groundwater transport of contaminants through the unsaturated zone to the regional aquifer does not represent a pathway for contaminant transport in the Lower Water/Indio Canyons Aggregate Area.

2.3.3 Potential Receptors and Pathways

The potential human receptors of contaminants are on-site Laboratory workers who could potentially be exposed to contaminants in soil, tuff, and sediment by direct contact, ingestion, or inhalation. Ecological receptors, such as plants and animals, may also be exposed to soil and sediment contaminants.

The current and reasonably foreseeable future land use at TA-15 is industrial. No residential or recreational land use currently exists or is expected to occur at any of the sites in the foreseeable future.

2.3.4 Cleanup Standards

As specified in Section VIII.B.1 of the Consent Order, NMED soil screening levels (SSLs) (NMED 2015, 600915) or Laboratory screening action levels (LANL 2015, 600929) will be used as soil cleanup levels unless they are determined to be impractical (details of the process are outlined in the Consent Order, Section VIII.E, Requests for Variance from Cleanup Goal or Cleanup Level) or unless SSLs do not exist for the current and reasonably foreseeable future land use (i.e., neither NMED nor the U.S. Environmental Protection Agency [EPA] has determined SSLs for some analytes under some land-use scenarios). If NMED SSLs do not exist, EPA regional screening values will be used (http://www.epa.gov/risk/regional-screening-levels-rsls-generic-tables-may-2016).

2.4 Data Overview

The existing data from these sites are approximately 20 yr old and are therefore screening-level data.

Screening-level data are appropriate for applications that require only determination of areas of gross contamination and/or for site characterization. Screening-level data are also often used to specify areas where additional data should be collected. The screening-level data are presented in the Lower Water/Indio Canyons Aggregate Area HIR (LANL 2016, 601539).

3.0 SITE CONDITIONS

3.1 Surface Conditions

The SWMUs and AOCs that comprise the Lower Water/Indio Canyons Aggregate Area under investigation are located on the mesa top in the southeast portion of TA-15. The mesa-top sites in this aggregate area are bounded by steep-walled Water Canyon to the south and east.

TA-15, also known as R-Site, occupies approximately 1200 acres of Threemile Mesa on the Pajarito Plateau near the southwestern boundary of the Laboratory. TA-15 ranges in elevation from 6719 to 7217 ft amsl. Most, if not all, of the disturbances associated with TA-15 activities (site development, open-air explosions, and disposal) have been on this mesa top. Topographically, the area consists of the hill slopes, canyon walls, canyon bottoms, and mesa tops with intermittent streams in the canyon bottoms. Groundcover of unpaved areas include common shrub species including Gambel oak, wavyleaf oak, mountain mahogany, cliffbush, and Colorado barberry, along with grasses including mountain muhly, little bluestem, and blue grama. Some of the most common forbs found are golden aster, bittersweet, and wormwood. Dominant tree species found within TA-15 include juniper, piñon, and ponderosa pine. Most disturbed areas have only a thin soil layer (LANL 1993, 020946, p. 3-5).

3.1.1 Soil

Soil on the Pajarito Plateau was initially mapped and described by Nyhan et al. (1978, 005702). The soil on the slopes between the mesa tops and canyon floors was mapped as mostly steep rock outcrops consisting of approximately 90% bedrock outcrop and patches of shallow, weakly developed colluvial soil.

South-facing canyon walls are generally steep and usually have shallow soil in limited, isolated patches between rock outcrops. In contrast, the north-facing canyon walls generally have more extensive areas of shallow, dark-colored soil under thicker forest vegetation. The canyon floors generally contain poorly developed, deep, well-drained soil on floodplain terraces or small alluvial fans (Nyhan et al. 1978, 005702).

The soil in the bottom of Lower Water and Indio Canyons consists of deep, well-drained soil of the Totavi series, which formed in alluvium in canyon bottoms. The surface soil is a brown gravelly loamy sand, or sandy loam, with 15% to 20% gravel. The permeability of this soil is high, runoff is very slow, and erosion hazard rating is low (Nyhan et al. 1978, 005702, p. 31).

The sites addressed in this investigation work plan are located in the center of PHERMEX Mesa, which is covered by Nyjack loam. Nyjack loam is derived from weathered tuff on level to gently sloping terrains and has medium water capacity and permeability and slight erosion susceptibility. This soil grades to the north to Seaby loam and Hackroy loam on the northeast rim of Potrillo Canyon. Seaby loam forms on gently to moderately sloping mesas and is also formed on weathered tuff. Permeability is moderate in the upper soil and very rapid below. Water capacity is low and erosion rates are moderate. Hackroy loam is a shallow soil formed from tuff. It exhibits low permeability and low water capacities and medium runoff and moderate water erosion hazard. The Seaby grades to the west and east of the PHERMEX site with a small pod of Nyjack located on the extreme eastern edge of TA-15 on the mesa. The northern rim of Water Canyon shows Pogna loam on the west and Hackroy loam on the east. Pogna loam is derived from tuff on gently to strongly sloping mesa tops. It has low water capacity, moderate permeability, medium runoff and moderate erosion rates. A pod of Seaby loam is located in the bottom of Water Canyon at the eastern edge of TA-15.

3.1.2 Surface Water

Most surface water in the Los Alamos area occurs as ephemeral, intermittent, or interrupted streams in canyons cut into the Pajarito Plateau. Springs on the flanks of the Jemez Mountains, west of the Laboratory’s western boundary, supply flow to the upper reaches of Cañon de Valle and to Guaje, Los Alamos, Pajarito, and Water Canyons (Purtymun 1975, 011787; Stoker 1993, 056021). These springs discharge water perched in the Bandelier Tuff and Tschicoma Formation at rates from 2 to 135 gal./min (Abeele et al. 1981, 006273). The volume of flow from the springs maintains natural perennial reaches of varying lengths in each of the canyons.

Mesas of the Pajarito Plateau are generally dry, both on the surface and within the bedrock forming the mesas. The surface water and alluvial groundwater hydrology of the south canyons watersheds is related to several primary factors, including the location and discharge volume of natural and anthropogenic water sources, seasonal events (e.g., snowmelt runoff and storm water runoff), and general regional climatic conditions. Surface water flow in the south canyons system is generally ephemeral and occurs primarily as short-duration storm water runoff. Locally persistent surface water has been observed in bedrock pools or where alluvial groundwater discharges from springs or seeps. Intermittent flow also occurs during snowmelt runoff or is associated with the discharge of alluvial groundwater from stream beds.

Most stream channels that drain the south canyons watersheds are dry for most of the year and are characterized by ephemeral or intermittent flow with only localized areas of perennial flow. In the south canyons watersheds, only Cañon de Valle and Water Canyon support perennial flow. Perennial flow is derived from springs in the eastern Jemez Mountains or the western Pajarito Plateau, but the volume is insufficient to maintain surface flows across the Laboratory before the water is depleted by evaporation, transpiration, and infiltration (LANL 2005, 091523, p. 24). In Water Canyon, snowmelt runoff can extend from the Jemez Mountains to the Rio Grande following heavy winter snowfalls. Storm water runoff also occasionally extends across the Laboratory to the Rio Grande in the south canyons but is transient and associated with heavy rainfall events.

The mesa-top portion of the Lower Water and Indio Canyons Aggregate Area is currently an industrially developed area. No natural surface water is present in this area. During summer thunderstorms and spring snowmelt, runoff flows from the mesa top down the hillsides and into the ephemeral streams in Lower Water and Indio Canyons. Surface runoff from the mesa top enters both canyons by way of several drainages (LANL 1992, 007672).

Indio Canyon has a small drainage area (0.7 mi2) that originates at TA-39 at an elevation of approximately 6860 ft. The canyon extends southeast from TA-39 to Water Canyon at TA-71 for a distance of approximately 6.5 mi. Streamflow in Indio Canyon is ephemeral and results primarily from natural runoff.

The Indio Canyon watershed has no perennial springs or tributaries on Laboratory property (LANL 1997, 055622). Lower Water Canyon has a relatively small drainage area of approximately (2.5 mi2) that originates near the boundary between TA-15 and TA-49 at an elevation of approximately 7100 ft. The canyon extends southeast before it is joined by Potrillo Canyon in TA-71 before entering the Rio Grande in TA-70 at an elevation of approximately 5427 ft amsl. A small perennial spring in Lower Water Canyon, below the confluence with Potrillo Canyon, supports a very short perennial reach (LANL 1997, 055622).

3.1.3 Land Use

Currently, land use within the Lower Water/Indio Canyons Aggregate Area is industrial. The area’s land use is anticipated to remain as industrial through continued use by the Laboratory and will not change in the foreseeable future. Public access to TA-15 is prohibited and is controlled through physical controls, including fencing and limited access via guard stations.

3.2 Subsurface Conditions

3.2.1 Stratigraphic Units

The stratigraphy of the Lower Water/Indio Canyons Aggregate Area is summarized in this section.

Additional information on the geologic setting of the area and information on the Pajarito Plateau can be found in the hydrogeologic conceptual site model for the Laboratory (LANL 2016, 601506).

The bedrock at or near the surface of the mesa top is the Tshirege Member of the Bandelier Tuff (Qbt).

There are approximately 1250 ft of volcanic and sedimentary materials between any potential contaminant-bearing units at the mesa-top surface and the regional aquifer. The upper stratigraphic units present within the Lower Water/Indio Canyons Aggregate Area are described briefly in the following sections. The descriptions begin with the oldest (deepest) and proceed to the youngest (topmost). The generalized stratigraphy is shown in Figure 3.2-1.

3.2.1.1 Guaje Pumice Bed

The Guaje Pumice Bed occurs at the base of the Otowi Member of the Bandelier Tuff, making a significant and extensive marker horizon. The Guaje Pumice Bed (Bailey et al. 1969, 021498; Self et al.

1986, 021579) contains well-sorted pumice fragments whose mean size varies between 0.8 and 1.6 in. Its thickness averages approximately 28 ft below most of the Pajarito Plateau, with local areas of thickening and thinning. The distinctive white color and texture make it easily identifiable in borehole cuttings and core, and it is an important marker bed for the base of the Bandelier Tuff.

3.2.1.2 Otowi Member of the Bandelier Tuff

Griggs and Hem (1964, 092516); Smith and Bailey (1966, 021584); Bailey et al. (1969, 021498); and Smith et al. (1970, 009752) described the Otowi Member. It consists of moderately consolidated (indurated), porous, and nonwelded vitric tuff (ignimbrite) that forms gentle colluvium-covered slopes along the base of canyon walls. The Otowi ignimbrites contain light gray to orange pumice that is supported in a white to tan ash matrix (Broxton et al. 1995, 050121; Broxton et al. 1995, 050119; Goff 1995, 049682). The ash matrix consists of glass shards, broken pumice, crystal fragments, and fragments of perlite.

3.2.1.3 Tephra and Volcaniclastic Sediment of the Cerro Toledo Interval

The Cerro Toledo interval is an informal name given to a sequence of volcaniclastic sediment and tephra of mixed provenance that separates the Otowi and Tshirege Members of the Bandelier Tuff (Broxton et al.

1995, 050121; Broxton and Reneau 1995, 049726; Goff 1995, 049682). Although it is located between the two members of the Bandelier Tuff, it is not considered part of that formation (Bailey et al. 1969, 021498). The unit contains primary volcanic deposits described by Smith et al. (1970, 009752) as well as reworked volcaniclastic sediment. The occurrence of the Cerro Toledo interval is widespread; however, its thickness is variable, ranging between several feet to more than 100 ft.

The predominant rock types in the Cerro Toledo interval are rhyolitic tuffaceous sediment and tephra (Heiken et al. 1986, 048638; Stix et al. 1988, 049680; Broxton et al. 1995, 050121; Goff 1995, 049682).

The tuffaceous sediment is the reworked equivalent of Cerro Toledo rhyolite tephra. Oxidation and clay-rich horizons indicate at least two periods of soil development occurred within the Cerro Toledo deposits.

Because the soil is rich in clay, it may act as a barrier to the movement of vadose zone moisture. Some of the deposits contain both crystal-poor and crystal-rich varieties of pumice. The pumice deposits tend to form porous and permeable horizons within the Cerro Toledo interval and locally may provide important pathways for moisture transport in the vadose zone. A subordinate lithology within the Cerro Toledo interval includes clast-supported gravel, cobble, and boulder deposits derived from the Tschicoma Formation (Broxton et al. 1995, 050121; Goff 1995, 049682; Broxton and Reneau 1996, 055429).

3.2.1.4 Tshirege Member of the Bandelier Tuff

The Tshirege Member is the upper member of the Bandelier Tuff and is the most widely exposed bedrock unit of the Pajarito Plateau (Griggs and Hem 1964, 092516; Smith and Bailey 1966, 021584; Bailey et al.

1969, 021498; Smith et al. 1970, 009752). Emplacement of this unit occurred during eruptions of the Valles Caldera approximately 1.2 million years ago (Izett and Obradovich 1994, 048817; Spell et al. 1996, 055542). The Tshirege Member is a multiple-flow ignimbrite sheet that forms the prominent cliffs in most of the canyons on the Pajarito Plateau. It is a cooling unit whose physical properties vary vertically and laterally. The consolidation in this member is largely from compaction and welding at high temperatures after the tuff was emplaced. Its light brown, orange-brown, purplish, and white cliffs have numerous, mostly vertical fractures that may extend from several feet up to several tens of feet. The Tshirege Member includes thin but distinctive layers of bedded, sand-sized particles called surge deposits that demark separate flow units within the tuff. The Tshirege Member is generally over 200 ft thick.

The Tshirege Member differs from the Otowi Member most notably in its generally greater degree of welding and compaction. Time breaks between the successive emplacement of flow units caused the tuff to cool as several distinct cooling units. For this reason, the Tshirege Member consists of at least four cooling subunits that exhibit variable physical properties vertically and horizontally (Smith and Bailey 1966, 021584; Crowe et al. 1978, 005720; Broxton et al. 1995, 050121). The welding and crystallization variability in the Tshirege Member produce recognizable vertical variations in its properties, such as density, porosity, hardness, composition, color, and surface-weathering patterns. The subunits are mappable based on a combination of hydrologic properties and lithologic characteristics.

Broxton et al. (1995, 050121) provide extensive descriptions of the Tshirege Member cooling units. The following paragraphs describe, in ascending order, subunits of the Tshirege Member present at the Lower Water/Indio Canyons Aggregate Area.

The Tsankawi Pumice Bed forms the base of the Tshirege Member. Where exposed, it is commonly 20 to 30 in. thick. This pumice-fall deposit contains moderately well-sorted pumice lapilli (diameters reaching about 2.5 in.) in a crystal-rich matrix. Several thin ash beds are interbedded with the pumice-fall deposits.

Subunit Qbt 1g is the lowermost tuff subunit of the Tshirege Member. It consists of porous, nonwelded, and poorly sorted ash-flow tuff. This unit is poorly indurated but nonetheless forms steep cliffs because of a resistant bench near the top of the unit; the bench forms a harder protective cap over the softer underlying tuff. A thin (4- to 10-in.) pumice-poor surge deposit commonly occurs at the base of this unit.

Subunit Qbt 1v forms alternating clifflike and sloping outcrops composed of porous, nonwelded, crystallized tuff. The base of this unit is a thin horizontal zone of preferential weathering that marks the abrupt transition from glassy tuff below (in unit Qbt 1g) to the crystallized tuff above. This feature forms a widespread marker horizon (locally termed the vapor-phase notch) throughout the Pajarito Plateau. The lower part of Qbt 1v is orange-brown, is resistant to weathering, and has distinctive columnar (vertical) joints; hence, the term “colonnade tuff” is appropriate for its description. A distinctive white band of alternating cliff- and slope-forming tuffs overlies the colonnade tuff. The tuff of Qbt 1v is commonly nonwelded (pumices and shards retain their initial equant shapes) and has an open, porous structure.

Unit Qbt 2 forms a distinctive medium-brown vertical cliff that stands out in marked contrast to the slope-forming, lighter-colored tuff above and below. It exhibits the greatest degree of welding in the Tshirege Member. A series of surge beds commonly mark its base. It typically has low porosity and permeability relative to the other units of the Tshirege Member.

Unit Qbt 3 is a nonwelded to partially welded, vapor-phase altered tuff that forms the upper cliffs. Its base consists of a purple-gray, unconsolidated, porous, and crystal-rich nonwelded tuff that forms a broad, gently sloping bench developed on top of Qbt 2. Abundant fractures extend through the upper units of the Bandelier Tuff, including the ignimbrite of Qbt 3 of the Tshirege Member. The origin of the fractures has not been fully determined, but the most probable cause is brittle failure of the tuff caused by cooling contraction soon after initial emplacement (Vaniman 1991, 009995.1; Wohletz 1995, 054404).

Unit Qbt 4 consists of a series of variably welded vitric to devitrified ash-flow tuffs characterized by localized thin, discontinuous, crystal-rich, fine- to coarse-grained, volcanic surge deposits. The lower, more indurated parts of Qbt 4 are also significantly fractured. These fractures and surge beds are potential groundwater pathways (LANL 2006, 091698).

3.2.2 Hydrogeology

The hydrogeology of the Pajarito Plateau is generally separable in terms of mesas and canyons forming the plateau. Mesas are generally devoid of water, both on the surface and within the rock forming the mesa. Canyons range from wet to relatively dry; the wettest canyons contain continuous streams and contain perennial groundwater in the canyon-bottom alluvium. Dry canyons have only occasional streamflow and may lack alluvial groundwater. Intermediate-perched groundwater has been found at certain locations on the plateau at depths ranging between 100 and 700 ft. The regional aquifer is found at depths of about 600 to 1200 ft (Collins et al. 2005, 092028).

In the Los Alamos area, groundwater occurs as (1) water in shallow alluvium in some of the larger canyons, (2) an intermediate-perched groundwater body, which lies above a less permeable layer and is separated from the underlying aquifer by an unsaturated zone, and (3) the regional aquifer.

Contamination of the perched water and/or regional groundwater aquifer can occur only by recharge of infiltrating precipitation from contamination at or near the surface to the underlying groundwater. The hydrogeologic conceptual site model for the Laboratory (Collins et al. 2005, 092028) shows that, under natural conditions, relatively small volumes of water move beneath mesa tops because of low rainfall, high evaporation, and efficient water use by vegetation. Atmospheric evaporation may extend into mesas, further inhibiting downward flow.

3.2.2.1 Vadose Zone

The unsaturated zone from the mesa surface to the top of the regional aquifer is referred to as the vadose zone. The source of moisture for the vadose zone is precipitation, but much of it runs off, evaporates, or is absorbed by plants. The subsurface vertical movement of water is influenced by properties and conditions of the materials that make up the vadose zone.

The Bandelier Tuff is generally dry and does not readily transmit moisture. Most of the pore spaces in the tuff are of capillary size and have a strong tendency to hold water against gravity by surface-tension forces. Vegetation is very effective at removing moisture near the surface. During the summer rainy season when rainfall is highest, near-surface moisture content is variable because of higher rates of evaporation and of transpiration by vegetation, which flourishes during this time.

The various units of the Bandelier Tuff tend to have relatively high porosities. Porosity ranges between 30% and 60% by volume, generally decreasing for more highly welded tuff. Permeability varies for each cooling unit of the Bandelier Tuff. The moisture content of tuff beneath the mesa tops is low, generally less than 5% by volume throughout the profile (Kearl et al. 1986, 015368; Purtymun and Stoker 1990, 007508).

Based on the hydrogeologic conceptual model for mesas (Collins et al. 2005, 092028), moisture movement through the vadose zone in the Lower Water/Indio Canyons Aggregate Area is expected to be very slow because of low precipitation, the lack of surface water on the mesa top (including artificial water sources such as ponds), and the drying effect of air exchange along mesa edges. Net infiltration beneath dry mesas is low, with rates generally believed to be less than tens of millimeters per year and commonly on the order of 1 mm/yr or less. Transport times to the regional aquifer beneath dry canyons are expected to exceed hundreds of years (Birdsell et al. 2005, 092048).

3.2.2.2 Alluvial Groundwater

Intermittent and ephemeral stream flows in the canyons of the Pajarito Plateau have deposited alluvium as thick as 100 ft. The alluvium in canyons that originate from the Jemez Mountains is generally composed of sands, gravels, pebbles, cobbles, and boulders derived from the Tschicoma Formation and Bandelier Tuff on the flank of the mountains. The alluvium in canyons that originate from the plateau (such as Water Canyon) is comparatively more fine grained, consisting of clays, silts, sands, and gravels derived from the Bandelier Tuff (Purtymun 1995, 045344).

In contrast to the underlying volcanic tuff and sediment, alluvium is relatively permeable. Ephemeral runoff in some canyons infiltrates the alluvium until downward movement is impeded by the less permeable tuff and sediment, which results in the buildup of a shallow alluvial groundwater body.

Depletion by evapotranspiration and movement into the underlying rock limit the horizontal and vertical extent of the alluvial water (Purtymun et al. 1977, 011846). The limited saturated thickness and extent of the alluvial groundwater preclude its use as a viable source of water for municipal and industrial needs.

Lateral flow of the alluvial perched groundwater is in an easterly, downcanyon direction (Purtymun et al.

1977, 011846).

There is no alluvial groundwater in the Lower Water/Indio Canyons Aggregate Area. Threemile Mesa lacks well-defined drainages and surface-water flow is ephemeral, occurring as overland runoff, primarily following infrequent, intense thunderstorms or during snowmelt.

3.2.2.3 Intermediate-Perched Water

Identification of perched groundwater systems beneath the Pajarito Plateau comes mostly from direct observation of saturation in boreholes, wells, or piezometers or from borehole geophysics. Perched groundwater is widely distributed across the northern and central part of the Pajarito Plateau with depth-to-water ranging from 118 to 894 ft below ground surface (bgs). The principal occurrences of perched groundwater occur in (1) the relatively wet Los Alamos and Pueblo Canyon watersheds, (2) the smaller watersheds of Sandia and Mortandad Canyons that receive significant volumes of treated effluent from Laboratory operations, and (3) the Cañon de Valle area in the southwestern part of the Laboratory.

Perched water is most often found in Puye fanglomerates, Cerros del Rio basalt, and in units of Bandelier Tuff. Based on a few reported occurrences in the southern part of the Laboratory, a few deep boreholes are located in that area. Additional perched zones probably occur beneath the adjacent watersheds of Pajarito and Water Canyons (Collins et al. 2005, 092028, pp. 2-96–2-97).

3.2.2.4 Regional Aquifer

The regional aquifer of the Los Alamos area is the only aquifer capable of providing a large-scale municipal water supply (Purtymun 1984, 006513). The surface of the regional aquifer rises westward from the Rio Grande within the Santa Fe Group into the lower part of the Puye Formation beneath the central and western part of the Pajarito Plateau. The depths to groundwater below the mesa tops range between about 1200 ft along the western margin of the plateau and about 600 ft at the eastern margin. The locations of wells and the generalized water-level contours on top of the regional aquifer are described in the annual Interim Facility-Wide Groundwater Monitoring Plan (LANL 2016, 601506). The regional aquifer is typically separated from the alluvial groundwater and intermediate-perched groundwater by 350 to 620 ft of tuff, basalt, and sediment (LANL 1993, 023249).

Groundwater in the regional aquifer flows east-southeast toward the Rio Grande. The velocity of groundwater flow ranges from about 20 to 250 ft/yr (LANL 1998, 058841, pp. 2–7). Details of depths to the regional aquifer, flow directions and rates, and well locations are presented in various Laboratory documents (Purtymun 1995, 045344; LANL 1997, 055622; LANL 2000, 066802). Groundwater monitoring is conducted under annual updates to the Interim Facility-Wide Groundwater Monitoring Plan (LANL 2016, 601506).

4.0 SITE DESCRIPTION AND PROPOSED INVESTIGATION ACTIVITIES

The following section presents site descriptions, summaries of previous investigation activities, proposed sampling activities, and proposed remedial activities for each SWMU and AOC addressed in this plan. A variety of resources aided in defining and revising the boundaries of each site; which are shown in figures corresponding to each site description. Existing structures, roads, and other features that could readily be observed in the field were of prime importance in defining site boundaries. If these features could still be observed in the field, site boundaries were then established relative to these landmarks. Other types of data references were also used, particularly for former site locations where significant changes have occurred over time. Historic aerial photographs have been an excellent resource. Drawings and sketches were used, particularly for locating structures and utilities, as well as engineering drawings produced for construction or record purposes. For each site, available information was reviewed, conflicting information was resolved as satisfactorily as possible, and site locations and boundaries were revised accordingly.

4.1 AOC 15-001, Storage Area

4.1.1 Site Description

AOC 15-001 consists of a storage area, referred to as “The Boneyard,” located within TA-15 adjacent to inactive Firing Site G [SWMU 15-004(g)] (Figure 4.1-1). AOC 15-001 is located just northeast of a former firing bunker (structure 15-9). The area was used to store materials associated with activities at the DARHT and PHERMEX facilities. These materials, which included equipment, steel, experimental vessels, and construction debris, were stored in the open, on the ground surface, and within transportainers and small storage sheds. Materials stored at AOC 15-001 before the mid-1990s have been removed, but small pieces of debris (e.g., metal scrap) may still be present. The area is currently being used to store equipment associated with ongoing activities at TA-15 (Appendix C, Figure C-1).

4.1.2 Previous Investigations

Although previous investigations have not specifically addressed AOC 15-001, they have addressed the general area that could have been impacted by Firing Site G [SWMU 15-004(g)], including the area occupied by AOC 15-001. A 1982 aerial radiological survey identified no radionuclides above background levels at Firing Site G or adjacent areas. However, during surface surveys performed at the site in 1991 and 1996, several areas of radiological surface contamination were identified. This contamination is believed to be associated with uranium debris from tests conducted at Firing Site G. Although investigation of AOC 15-001 was proposed in the Operable Unit (OU) 1086 RFI work plan, investigation of this site was deferred during the 1995–1996 RFI because it was an active site (LANL 1996, 054977, p. 5-1). However, three surface samples were collected at three locations within and adjacent to AOC 15-001 from depth intervals of 0–0.25 ft, 0–0.33 ft, and 0–0.5 ft below ground surface (bgs). The samples were submitted to an off-site contract analytical laboratory for analysis of uranium.

The data for this investigation are screening level and are presented in the HIR.

4.1.3 Proposed Activities

The nature and extent of contamination have not been defined at this site. A total of 42 samples will be collected from 14 locations (Figure 4.1-1). At each location, samples will be collected at 0.0–1.0 ft bgs, 2.0–3.0 ft bgs, and 4.0–5.0 ft bgs. Samples will be collected from within the footprint and around the perimeter of the former storage area to define the nature and extent of contamination from historical storage activities at this site.

Because this site is associated with firing site activities, including radiography, all samples will be analyzed for target analyte list (TAL) metals, nitrate, perchlorate, pH, explosive compounds, gamma-emitting radionuclides, and isotopic uranium. Investigation samples collected from this site will not be analyzed for cyanide, dioxins/furans, polychlorinated biphenyls (PCBs), and volatile organic compounds (VOCs) because these constituents are not associated with storage or firing site activities. However, samples will be analyzed for semivolatile organic compounds (SVOCs) because the exact nature of the materials stored at the site is not known. Table 4.1-1 summarizes the proposed sampling locations, depths, and analytical suites.

4.2 AOC 15-004(h), Firing Site H

4.2.1 Site Description

AOC 15-004(h) is inactive Firing Site H located northwest of the PHERMEX facility at TA-15 (Figure 4.2-1). Firing Site H is located approximately 100 ft north of the PHERMEX power control building (structure 15-185). The explosives testing firing site was constructed in 1948 and included a concrete pad, a protective berm, an instrument chamber (former structure 15-17), and a camera chamber (structure 15-92). The exact nature of the materials used during tests is not known but may have included DU, beryllium, lead, and HE. Firing site operations were discontinued in approximately 1953 (LANL 1993, 020946, p. 8-23), and the instrument chamber was demolished in 1967. The camera chamber and the concrete pad remain on-site, but the concrete pad has been partially covered with fill.

4.2.2 Previous Investigations

Although the investigation of AOC 15-004(h) was proposed in the OU 1086 Resource Conservation and Recovery Act facility investigation (RFI) work plan, the investigation of this site was deferred during the 1995–1996 RFI because it was within the active PHERMEX hazard area (LANL 1996, 054977, p. 5-1).

However, 11 surface samples (0.0–0.17 ft bgs to 0.0–0.5 ft bgs) and 7 subsurface samples (1.08–

1.58 ft bgs or 1.5–2.0 ft bgs) were collected from 11 locations. The samples were submitted to an off-site contract analytical laboratory for analysis of uranium.

The data for this investigation are screening level and are presented in the HIR.

4.2.3 Proposed Activities

The nature and extent of contamination have not been defined at this site. A total of 60 samples will be collected from 20 locations (Figure 4.2-1). Sampling locations will be sited around the perimeter of the former firing pad and within the area potentially impacted by the former firing site to define the nature and extent of contamination. Samples will be collected adjacent to each side of the firing pad and at accessible step-out locations 50, 100, and 200 ft from the center of the firing site. At each location, samples will be collected at depth intervals of 0.0–1.0 ft bgs, 2.0–3.0 ft bgs, and 4.0–5.0 ft bgs.

The firing site ceased operation in 1953, before the PHERMEX facility was constructed (1959–1961).

During construction of PHERMEX, the access road to the south side of PHERMEX was constructed directly west of Firing Site H. The road cut was excavated through the tuff bedrock, and the current ground surface adjacent to the road consists of the side slopes of the road cut. The ground surface that existed when the firing site was operational was excavated during construction of the road, and the current ground surface consists primarily of exposed tuff and is not representative of the ground surface that could have been impacted by firing site operations (Appendix C, Figure C-2). Therefore, no samples will be collected from the side slopes adjacent to the road west of the firing site.

Because this site is associated with firing site activities, all samples will be analyzed for TAL metals, nitrate, perchlorate, pH, explosive compounds, and isotopic uranium. Investigation samples collected from this site will not be analyzed for cyanide, dioxins/furans, PCBs, SVOCs, and VOCs because these constituents are not associated with firing site activities. Table 4.2-1 summarizes the proposed sampling locations, depths, and analytical suites.

4.3 SWMU 15-009(g), Septic System

4.3.1 Site Description

SWMU 15-009(g) consists of an inactive septic system at the PHERMEX facility that is located south of the chamber building (structure 15-184) (Figure 4.3-1).