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  • EPA/ROD/R07-05/040 2005

    EPA Superfund

    Record of Decision:

    RAILROAD AVENUE GROUNDWATER CONTAMINATION SITE EPA ID: IA0001610963 OU 01 WEST DES MOINES, IA 09/19/2005

  • Ciher

    RECORD OF DECISION

    for the

    Northern Plume

    Operable Unit 1

    Railroad Avenue Groundwater Contamination SiteWest Des Moines, Iowa

    Prepared by:

    U.S. Environmental Protection Agency

    Region VII

    Kansas City, Kansas

    September 2005

    40221019

  • Contents

    Abbreviations and Acronyms i

    I. Declaration D-1

    II. Decision Summary 1

    1.0 Site Name, Location, and Description 1

    2.0 Site History and Enforcement Activities 2

    3.0 Community Participation .': 3

    4.0 Scope and Role of Interim Response Action 4

    5.0 Site Characteristics 45.1 Physical Characteristics 55.2 Nature and Extent of Contamination 75.3 Contaminant Migration and Conceptual Site Model 8

    6.0 Current and Potential Future Site and Resource Uses 12

    7.0 Summary of Site Risks 137.1 Human Health Risk Assessment 137.2 Ecological Risk Assessment 167.3 Risk Assessment Summary and Conclusion 16

    8.0 Remedial Action Objectives 17

    9.0 Description of Alternatives 189.1 Alternative 1: No Action ; 199.2 Alternative 2: Monitored Natural Attenuation 20

  • 9.3 Alternatives: Extraction with Recovery Wells/On-site Tray Aeration/Surface

    Water Discharge 229.4 Alternative 4: In-Situ Stripping Wells 24

    10.0 Comparative Analysis of Alternatives 2610.1 Overall Protection of Human Health and the Environment 26

    10.2 Compliance with Applicable or Relevant and AppropriateRequirements (ARARs) 27

    10.3 Long-Term Effectiveness and Permanence 2710.4 Reduction of Toxicity, Mobility, and Volume Through Treatment 27

    10.5 Short-Term Effectiveness 28

    10.6 Implementability 2810.7 Cost 2910.8 State/Support Agency Acceptance 2910.9 Community Acceptance 30

    11.0 Principal Threat Wastes 30

    12.0 Selected Remedy 3012.1 Summary of the Rationale of Selected Remedy 3012.2 Description of Selected Remedy 31

    12.3 Summary of Estimated Costs 3512.4 Expected Outcomes of the Selected Remedy 36

    13.0 Statutory Determinations 3613.1 Protection of Human Health and the Environment 37

    13.2 Compliance with ARARs 3713.3 Cost Effectiveness 38

    13.4 Utilization of Permanent Solutions and Innovative Treatment Technologiesto the Maximum Extent Practicable 38

    13.5 Preference for Treatment which Reduces Toxicity, Mobility, orVolume 39

    14.0 Documentation of Significant Changes 39

  • III. Responsiveness Summary RS-1

    Tables

    Table 1 VOC Data Summary (Monitoring Wells)Table 2 VOC Data Summary (Water Supply Wells)Table 3 Medium-Specific Exposure Point Concentration SummaryTable 4 Selection of Exposure PathwaysTable 5 Non-Cancer Toxicity DataTable 6 Cancer Toxicity DataTable 7 Summary of Cancer Risk for Each Population EvaluatedTable 8 Summary of Hazard Indices for Each Population EvaluatedTable 9 Risk SummaryTable 10 Comparative Analysis of AlternativesTable 11 Metals, Geochemical, and Biochemical Data SummaryTable 12 Monitored Natural Attenuation Screening Step 1Table 13 Present Worth Cost EstimateTable 14 Final Cleanup Levels

    Figures

    Figure 1 Site Vicinity MapFigure 2Contaminant Contour Map

    Figure 3 Conceptual Site ModelFigure 4Plot of TCE, DCE, and VC vs. Distance Downgradient from Source Area

    May 2004Figure 5 Plot of TCE, DCE, and VC vs. Distance Downgradient from Source Area

    November 2004

  • Abbreviations and Acronyms

    ARARs applicable or relevant and appropriate requirements

    BCF bioconcentration factor

    bgs below ground surface

    BTEX benzene, toluene, ethylbenzene, and xylene

    CERCLA Comprehensive Environmental Response, Compensation, and

    Liability Act

    cis-1,2-DCE cis-1,2-dichloroethene

    CFR Code of Federal Regulations

    cfs cubic feet per second

    COC contaminant of concern

    COPC contaminant of potential concern

    DNAPL dense non-aqueous phase liquid

    DO dissolved oxygen

    EPA U.S. Environmental Protection Agency

    EPC Exposure Point Concentration

    ESD explanation of significant difference

    ESI expanded site inspection

    FS feasibility study

    ft feet

    gpm gallons per minute

    HI hazard index

    HQ hazard quotient

    IAC Iowa Administrative Code

    IDNR Iowa Department of Natural Resources

    MCL maximum contaminant level

    mg/kg milligram per kilogram

    MNA monitored natural attenuation

    msl mean sea level

    Railroad Avenue Groundwater Contamination SiteNorthern Plume Record of Decision

  • MTBE methyl tert-butyl ether

    MW monitoring well

    NAPL non-aqueous phase liquid

    NCP National Contingency Plan

    NPDES National Pollutant Discharge Elimination System

    NPL National Priorities List

    O&M operation and maintenance

    ORP oxidation-reduction potential

    OSHA Occupational Safety and Health Administration

    OU Operable Unit

    PA/SI preliminary assessment and site investigation

    PCB polychlorinated biphenyl

    PCE Tetrachloroethene

    PCOPEC preliminary contaminant of potential ecological concern

    POTW publicly owned treatment works

    PRP potentially responsible party

    RAC Response Action Contract

    RAO remedial action objective

    RBC risk based concentration

    RCRA Resource Conservation and Recovery Act

    RD remedial design

    RfD reference dose

    RI/FS remedial investigation/feasibility study

    RI Remedial Investigation

    ROD record of decision

    SARA Superfund Amendments and Reauthorization Act

    START Superfund Technical Assistance and Response Team

    SVOC semivolatile organic compounds

    TBC to-be-considered

    Railroad Avenue Groundwater Contamination SiteNorthern Plume Record of Decision

  • TCE Trichloroethene

    TCLP Toxicity Characteristic Leaching Procedure

    Aig/L micrograms per liter

    VC vinyl chloride

    VOC volatile organic compound

    WDM West Des Moines

    WDMW West Des Moines Well

    Railroad Avenue Groundwater Contamination SiteNorthern Plume jjj Record of Decision

  • I. Declaration for the Record of Decision

    Northern Plume

    Operable Unit 1

    Railroad Avenue Groundwater Contamination Site

    West Des Moines, Iowa

    Site Name and Location

    The Railroad Avenue Groundwater Contamination site, Comprehensive

    Environmental Response, Compensation, and Liability Information System (CERCLIS)

    identification number IA0001610963, is located in West Des Moines, Iowa, a suburb of

    Des Moines, Iowa, in southwest Polk County in south central Iowa. Two separate source

    areas and their respective contaminant plumes have been identified at the Railroad

    Avenue site. Because there are two separate and distinct plumes, the site has been

    separated into two operable units (OUs): OU 1 - Northern Plume, and OU 2-Southern

    Plume. This Record of Decision (ROD) addresses the Northern Plume (OU 1) of the

    Railroad Avenue Groundwater Contamination site.

    Statement of Basis and Purpose

    This decision document presents the Selected Remedy for the Northern Plume OU

    of the Railroad Avenue Groundwater Contamination site in West Des Moines, Iowa. The

    Selected Remedy was chosen in accordance with the Comprehensive Environmental

    Response, Compensation, and Liability Act of 1980 (CERCLA), as amended by the

    Superfund Amendments and Reauthorization Act of 1986 (SARA), and to the extent

    practicable, the National Oil and Hazardous Substances Pollution Contingency Plan

    (NCP). This decision is based on the Administrative Record for this site.

    The state of Iowa concurs with the selected remedy.

    Dl

  • Assessment of the Site

    The remedial action selected in this ROD is necessary to protect the public health or

    welfare and the environment from actual or threatened releases of hazardous substances

    into the environment.

    Description of Selected Remedy

    This ROD addresses groundwater contaminants of the Northern Plume. The

    Southern Plume and Northern Plume contaminants that are captured by the West Des

    Moines well field were addressed in a separate ROD approved September 26, 2003. The

    Northern Plume contaminants that are not captured by the West Des Moines well field are

    addressed in this ROD.

    The principal threat at this site is chlorinated volatile organic compounds (VOCs)

    contamination in groundwater. The source materials for the VOCs in the Northern Plume

    are currently unknown despite thorough investigative efforts. However, concentrations of

    VOC contaminants in suspected Northern Plume source areas appear to be diminishing

    through natural attenuation processes. The specific VOCs which have been identified as

    contaminants of concern (COCs) are tetrachloroethene (PCE), trichloroethene (TCE), cis-

    1,2-dichloroethene (cis-l,2-DCE), and vinyl chloride (VC).

    The selected remedy will permanently and significantly reduce the toxiciry, mobility,

    and volume of the site COCs through natural attenuation processes as the principalelement of remediation. The major components of the selected remedy for groundwater

    include the following:

    • Institutional controls including local or state well restrictions and public

    education to prevent use of contaminated groundwater

    • Restoration of the aquifer by reduction of the COCs through natural attenuation

    processes

    • Performance of monitoring at the site to confirm the effectiveness of the

    attenuation processes

    D2

  • Statutory Determinations

    The selected remedy is protective of human health and the environment, complies

    with federal and state requirements that are legally applicable or relevant and appropriate

    to the remedial action, is cost effective, and utilizes permanent solutions and alternative .

    treatment (or resource recovery) technologies to the maximum extent practicable. This

    remedy also satisfies the statutory preference for remedies that employ treatment as a

    principal element (i.e., this remedy reduces the toxicity, mobility, or volume of

    contaminants through treatment). Because hazardous substances above health-based

    levels are expected to be onsite in five years, a review will be conducted within five years

    after completion of the remedial action to ensure that the remedy continues to provide

    adequate protection of human health and the environment. . ;

    Data Certification Checklist

    The following information is included in the Decision Summary Section of the ROD.

    Additional information can be found in the Administrative Record for this site.

    • COCs and their respective concentrations

    • Baseline risk represented by the COCs

    • Cleanup levels established for COCs and the basis for the levels

    • How source materials constituting principal threats are addressed

    • Current and future land use assumptions from the baseline risk assessment

    • Groundwater use that will be available at the site as a result of the selected

    remedy

    • Estimated capital, operation and maintenance (O&M), and total present worth

    costs; discount rate; and the number of years over which the remedy cost

    estimates are projected

    D3

  • Decisive factors that lead to selecting the remedy

    Authorizing Signature

    Date

    D4

  • II. Decision Summary

    1.0 Site Name, Location, and Description

    The Railroad Avenue Groundwater Contamination site is in West Des Moines, Iowa,

    which is a suburb of Des Moines, Iowa, in southwest Polk County in south central Iowa,

    (Comprehensive Environmental Response, Compensation, and Liability Information

    System [CERCLIS] identification number IA0001610963). This Record of Decision

    (ROD) was developed by the U.S. Environmental Protection Agency (EPA), as lead

    agency, with support from the Iowa Department of Natural Resources (IDNR). Costs for

    remedial efforts at the Northern Plume portion of the site are being paid by the Superfund

    trust fund.

    The Railroad Avenue site consists of the West Des Moines water treatment plant

    well field, the areas of groundwater contamination, and the potential source areas of the

    contamination (Figure 1). The site is approximately 1,000 acres in size. The West Des

    Moines well field contains 22 West Des Moines municipal water supply wells

    (WDMWs) that currently supply water to the West Des Moines water treatment plant.

    Historically, five municipal wells had been taken offline as a result of the volatile

    organic compounds (VOCs) contamination: WDMW-6, -7, -12, -13, and -21. However,

    interim response actions completed at the site in November 2004 have allowed these

    wells to be returned to service. Wells WDMW-2, -10, and -11 were abandoned because

    of well production problems.

    Two separate and distinct source areas and respective contaminant plumes have been

    identified through the EPA Expanded Site Inspection (ESI) and remedial investigation

    (RI) sampling efforts. Because of the two plumes, the site has been separated into two

    operable units (OUs): OU 1 - Northern Plume, and OU 2 - Southern Plume.

    The EPA is the lead agency for the Northern Plume. The Northern Plume lies along

    Railroad Avenue in West Des Moines, Iowa, approximately between First and Thirteenth

    Street, east of wells WDMW-5, -6, 12, and -13 (Figure 1). The source area for the

    Northern Plume has not been identified. Several suspected source areas for the Northern

    Plume were investigated during the ESI. The ESI included collecting and analyzing

    sediment, surface water, groundwater, surface soil, and subsurface soil samples from

    targeted industrial business properties and surrounding areas.

  • The Southern Plume is located northwest of wells WDMW-19, -20, and -21 where a

    release of trichloroethylene (TCE) has occurred (Figure 1). Further investigation and

    remediation of the Southern Plume are being performed by the potentially responsible

    party (PRP) with the IDNR as the lead agency.

    2.0 Site History and Enforcement Activities

    The Railroad Avenue Groundwater Contamination site was first identified in 1993

    when a routine water distribution sample collected by the city of West Des Moines was

    found to contain cis-l,2-dichloroethene (cis-l,2-DCE) at 1.2 micrograms per liter (ug/L).

    The drinking water standard for DCE is 70 ug/1. Subsequent sampling of well WDMW-

    13 detected cis-l,2-DCE at significantly higher concentrations than the water distribution

    sample, once at a level higher than the drinking water standard in 1996.

    The contamination in the West Des Moines municipal wells was formally brought to

    the attention of the EPA after a site investigation was conducted by the IDNR at a

    potential source area in 1996.

    The EPA performed a Preliminary Assessment/Site Investigation (PA/SI) under the

    Superfund Technical Assessment and Response Team (START) program for well

    WDMW-13 in October 1997. Results of the PA/SI identified two potential groundwater

    contaminant plumes at locations along Railroad Avenue between West Des Moines wells

    WDMW-12 and WDMW-13 and 10th Street. While contaminants were found in wellsWDMW-6 and -7, a distinct groundwater contaminant plume near these two wells was

    not identified. Soil sampling conducted at five potential source areas could not determine

    a primary source area. However, due to the variable groundwater flow gradients induced

    by the water supply wells adjacent to the site, additional source areas south of the

    investigated areas along Railroad Avenue were proposed for investigation.

    An ESI was conducted by EPA in November and December 1999. Groundwater,

    surface soil, subsurface soil, sediment, sewer, and surface water samples were collected

    to confirm the results of the PA/SI and to investigate additional areas. Results from the

    ESI confirmed that there were two source areas and separate groundwater contaminant

    plumes in the southern and northern portions of the study area. Results also indicated a

  • need for further investigation east of the Northern Plume and south of the Southern Plume

    to further delineate the plume areas and to locate any other potential source 'areas.

    The EPA held a community meeting on October 24,2000, in West Des Moines,

    Iowa, to present a review of the results of the ESI. Questions from the public concerning

    the site were answered.-

    In December 2000, IDNR used direct-push sampling techniques to collect

    ground water samples at the water table and at equipment refusal (i.e., at bedrock) at three

    locations along Railroad Avenue. The samples confirmed results of the ESI.

    The EPA performed the RI and Feasibility Study (FS) under the Remedial Action

    Contracts (RAC) program in several phases. Characterization efforts were typically

    performed on a semi-annual basis beginning in April 2001 and continuing through at least

    May 2005.

    A previous interim remedial action that addressed those portions of the Northern and

    Southern Plumes which is captured by the West Des Moines well field was authorized in

    a ROD dated September 26, 2003. The interim remedial action construction was

    completed in November 2004. The previous interim remedial action included increasing

    the treatment capacity of the West Des Moines water treatment plant by constructing new

    aerators.

    3.0 Community Participation

    The Community Relations Plan, the Administrative Record of Activity, the Baseline

    Risk Assessment, the RI Report, the Focused FS Report, and supporting documentation

    were made available to the public for a public comment period which began on July 11,

    2005, and was continued until August 10, 2005. The documents were available at EPA

    Region VII Headquarters in Kansas City, Kansas, and the West Des Moines Public

    Library, West Des Moines, Iowa. The notice of the availability of these documents and

    the time and location of the public meeting were published in the Des Moines Register on

    July 10, 2005. A fact sheet summarizing the Proposed Plan and preferred alternative was

    mailed to residents and local administrators on July 12,2005. A public meeting was held

    during the public comment period at West Des Moines, Iowa, on July 25, 2005. At this

  • public meeting the Proposed Plan was presented to a broader community audience than

    those that had already been involved at the site. At the public meeting, representatives

    from EPA and the IDNR answered questions about the conclusions of the RI/FS, the

    remedial alternatives, and the proposed remedial action. The EPA's response to the

    comments received at the public meeting, as well as written comments received during

    the comment period, are included in the Responsiveness Summary (Section III), which is> •

    part of this ROD.

    4.0 Scope and Role of the Action

    This ROD addresses groundwater contaminants of the Northern Plume that are not

    captured by the West Des Moines well field. Contaminants of the Southern Plume and

    Northern Plume: that are captured by the West Des Moines well field were addressed in a

    separate ROD approved September 26, 2003. The Southern Plume is being addressed by

    the PRP with the IDNR as the lead agency. This ROD includes previous remedial action

    efforts and presents the final action selected for the Northern Plume.

    Several potential source areas for the Northern Plume were investigated; however, a

    specific source area for the Northern Plume was not identified. During the RI, it was

    determined that the western portion of the Northern Plume has adversely affected nearby

    municipal water supply wells. As a result, the EPA conducted a focused FS to address

    contaminated groundwater in close proximity to the municipal water supply wells,

    specifically wells WDMW-5, -6, -12, and -13 in the northern West Des Moines well field

    area. Therefore, EPA pursued an interim action remedy in September 2003 for theSouthern Plume and the northern West Des Moines well field area of the Northern Plume

    which included increasing the treatment capacity of the West Des Moines water treatment

    plant by constructing new aerators. In this ROD, EPA will address the final remedy for

    the Northern Plume OU.

    5.0 Site Characteristics

    The physical characteristics of the site, the nature and extent of contamination, and

    migration of contaminants are discussed in this section. Physical characteristics

    discussed include topography and surface hydrology, regional hydrogeology and soils,

  • and site geology and hydrogeology. The location of contaminant sources and distribution

    of the contaminants of concern (COCs) are discussed.

    5.1 Physical Characteristics

    The Railroad Avenue site lies within the flood plain of the Raccoon River and glacial

    terrain of the Central Lowland Plains. The topography of the site area is relatively flat

    with a gentle south-southeasterly slope toward the Raccoon River. The local relief of the

    site vicinity is about 150 feet. The elevation ranges from approximately 800 feet mean

    sea level (msl) along the Raccoon River to about 950 feet msl on the higher bluffs.

    Surface drainage in the site vicinity flows south-southeast via storm drains to a retention

    pond or a flooded gravel pit (Gravel Pit Lake) and eventually discharges into the Raccoon

    River, which is located along the southeast edge of the site area (Figure 2). Jordan Creek,

    which flows eastward along the north side of the Raccoon River Park and drains much of

    southeast West Des Moines, also discharges into the Raccoon River. The perennial

    Raccoon River flows easterly and has an annual mean discharge of about 2,770 cubic feet

    per second (cfs) in the West Des Moines area. Approximately 10 miles downstream of

    the site, the Raccoon River discharges into the Des Moines River. The Des Moines River

    flows to the east and has a mean annual discharge of 6,790 cfs. The site is situated within

    the 25-year flood plain of the Raccoon River, which last flooded the site area in July

    1993.

    The site lies above the alluvial aquifer of the Raccoon River. The site is underlain

    with alluvial sediments consisting of unconsolidated clay, silt, sand, and gravel whichoverlay shale and coal of the Cherokee Group of the Pennsylvanian System. Boring logs

    for the twelve monitoring wells (MW-1 through MW-12) and six observation wells (OB-

    1 through OB-6) installed as part of the Phase 3 RI indicate the thickness of the

    unconsolidated material to be thinnest in the western portion of the site, grading thicker

    toward the east. The alluvial aquifer is unconfined and consists of permeable, sorted, and

    stratified sand and gravel deposits. Depth to bedrock in the western portion of the site is

    approximately 37 feet; depth to bedrock at the eastern portion of the site is approximately

    52 feet.

    The upper portion of the unconsolidated sediments is characterized by 0 to 7 feet of

    fill material consisting of clay, silt, sand, and miscellaneous materials. The fill material is

  • underlain by silty clay to depths ranging from 5 to 19 feet below ground surface (bgs).

    Underlying the silty clay are relatively thick units of sand and gravel interbedded with

    localized clay and silt lenses containing natural organics. The natural organics

    encountered include wood fragments and lignite lenses up to four inches thick. The

    thickest silty clay lens encountered was ten feet thick and was encountered in well MW-3.

    Depth to groundwater across the site area ranged from approximately 9 to 14 feet

    bgs. Groundwater table contour maps were developed from the groundwater elevations.

    The groundwater flow direction on November 4, 2004, is illustrated on Figure 2. As

    illustrated, the flow direction for groundwater is to the east-southeast. Water levels

    measured on other dates confirmed this groundwater flow direction. On the basis of the

    groundwater contours, it appears that groundwater is recharged from upgradient flow and

    infiltration of precipitation. Groundwater flows toward, and discharges into; the Gravel

    Pit Lake and Raccoon River south and southeast of the site. The average horizontal

    hydraulic gradient at the site varies seasonally from approximately 0.0006 foot per foot

    (ft/ft) to 0.0015 ft/ft.

    A pump test was performed at West Des Moines Water Supply well WDMW-13 as

    part of the RI efforts. Results of the pump test indicate the hydraulic conductivity of the

    alluvial aquifer to be approximately 193 feet per day. Assuming an effective porosity of

    25 percent for the aquifer, the groundwater seepage velocity across the site is estimated to

    be approximately 222 feet per year (ft/yr) to the east-southeast in the summer and

    approximately 139 ft/yr to the east-southeast in the fall.

    The alluvial aquifer is underlain by the Cherokee Group of the Pennsylvanian

    system, which is approximately 400 feet thick and consists primarily of shale with thin

    layers of clay, siltstone, sandstone, limestone, and coal. Bedrock encountered in

    monitoring wells drilled during the Phase 3 RI was shale and coal. Although the shaleunits of the Cherokee Group will most likely act as an aquitard, preventing further

    downward vertical migration of contaminants, sandstone layers within the Cherokee

    Group provide groundwater to some wells in the southern half of Polk County with yields

    from 5 to 25 gallons per minute (gpm). The thicknesses of these sandstone units are quite

    variable and the depth of wells drilled into them varies between 75 and 100 feet.

  • The bedrock aquifer used as a water supply at the Railroad Avenue site consists of

    the Jordan Aquifer (wells WDMW-1, -3, and -4). The Jordan Aquifer consists of

    fractured and porous sandstone and dolostone of the Cambrian-Ordovician System which

    can yield significant amounts of water. The Jordan Aquifer is approximately 2,500 feet

    bgs. Because of the considerable depth of the Jordan Aquifer, it is extremely unlikely to

    be affected by contaminants in the alluvial aquifer. . . . .

    The site lies within the northeast part of the Forest City Basin bedrock structure. No

    major faults have been mapped at the surface in the site vicinity, and none are known to

    be active within the Holocene Epoch.

    5.2 Nature and Extent of Contamination

    The nature and extent of contamination in the groundwater at the Railroad Avenue

    site was evaluated from the ESI and RI sampling events. The monitoring wells and the

    water supply wells were sampled and analyzed for the presence of VOCs, semi-volatile

    organic compounds (SVOCs), pesticides, and polychlorinated biphenyls (PCBs). The

    majority of the analytical results for all wells were non-detect. Data validation efforts

    qualified results where necessary and screened out contaminants identified as potential

    laboratory contaminants. Because of the frequency of detection and elevated

    concentrations, the contaminants tetrachloroethene (PCE), TCE, cis-l,2-DCE, and vinyl

    chloride (VC) were determined to be the COCs for the Railroad Avenue Groundwater

    Contamination site. The analytical results are summarized in Table 1 for the monitoring

    wells and Table 2 for the West Des Moines water supply wells. Also presented in Table1 and Table 2 are the maximum contaminant levels (MCLs) for the analytes.

    The concentrations of TCE, cis-l,2-DCE, and VC in groundwater have been

    contoured in Figure 2 to illustrate the horizontal extent of contaminants in the aquifer.

    PCE was not included in the figure because PCE was consistently detected in only one

    well (MW-6) during the RI sampling events.

    The surface water samples were only analyzed for VOCs. The analytical results for

    the surface water samples were all non-detect for VOCs.

  • Diffusion bag samplers were used to evaluate the vertical stratification of VOCs at

    the site. Diffusion samplers were located at depths correlating to the five most granular

    zones of wells MW-3, MW-7, and MW-10. The diffusion bag results indicate a slight

    trend for higher concentrations of contaminants in the lower part of the aquifer.

    However, significant vertical stratification of contaminants is not indicated so that

    targeting vertical zones for remedial efforts will not be beneficial.

    5.3 Contaminant Migration and Conceptual Site Model

    Several years of site data have been collected to evaluate site conditions. Results of

    the ESI and RI indicate the primary migration pathway of contaminants at the Railroad

    Avenue site is through groundwater. The conceptual model of the site is illustrated in

    FigureS.

    The source area or source areas for the Northern.Plume contaminants at the Railroad-

    Avenue site have not been specifically identified despite thorough investigation efforts.

    However, the source areas for the site contaminants are most likely a single, or several,

    local businesses which improperly managed production and/or waste solvents. Several

    historical and active local businesses in the site area were evaluated during the ESI.

    The COCs for the site are PCE, TCE, cis-l,2-DCE, and VC which are halogenated

    aliphatic compounds. PCE is widely used in the dry cleaning industry and as a solvent

    for degreasing. TCE is a common solvent used for degreasing of metals, textile

    processing, gas purification, and in the manufacturing of Pharmaceuticals. Cis-l,2-DCEis occasionally used in the production of solvents; however, its presence in the

    environment is usually as a result of the degradation of PCE and TCE. VC is also used to

    a limited extent in manufacturing processes, but as with cis-l,2-DCE, VC in the

    environment is usually a result of the degradation of PCE or TCE to cis-l,2-DCE which

    degrades to VC.

    The migration of the COCs in groundwater is complex and subject to several

    physical and chemical processes including biochemical processes and groundwater

    transport. Initially, the COCs leach vertically downward into groundwater from

    contaminated subsurface soils in the source area where the contaminants were originally

    released. As the COCs enter the groundwater, diffusion and advection processes control

    8

  • the migration of the contaminants. Diffusion causes the contaminants to spread in all

    directions within groundwater; adsorption processes cause the COCs to remain sorbed to

    the aquifer matrix. At this site, advective flow is a dominant migration process and

    causes contaminants to migrate along with groundwater in the direction of groundwater

    flow. Once in the groundwater, biodegradation processes reduce the persistence of the

    site contaminants. The COCs typically degrade into daughter compounds through the -

    loss of chlorine atoms. For example, PCE typically degrades to TCE, TCE typically

    degrades to cis-l,2-DCE, cis-l,2-DCE typically degrades to VC, and VC can degrade to

    ethene. Finally, ethene (which is not chlorinated) can degrade to carbon dioxide and

    water.

    While advective flow processes cause contaminants to migrate along with

    groundwater in the direction of groundwater flow, biodegradation processes, including

    reductive dechlorination, will simultaneously degrade the site contaminants.

    Biodegradation is a preferred natural attenuation process because hydrocarbons are

    eventually reduced to more stable, less toxic compounds. Studies have determined that

    chlorinated aliphatic hydrocarbons including PCE, TCE, and cis-l,2-DCE readily degrade

    in anaerobic environments.

    Biodegradation of PCE and TCE is occurring at the Northern Plume area as

    evidenced by the low concentrations of PCE and TCE and the relatively elevated

    concentrations of cis-l,2-DCE and VC which are degradation products of PCE and TCE.

    The extent of TCE, cis-l,2-DCE, and VC are illustrated in Figure 2. Figure 2 shows that

    the highest concentrations of the suspected source contaminant TCE are located near thesuspected release area, whereas down gradient of the source area, the concentrations of

    TCE are lower and the daughter contaminants (cis-l,2-DCE and VC) are higher.

    Other indicators of biodegradation may be identified by the evaluation of site

    geochemical data. Analytical results show the absence of electron acceptors such as

    dissolved oxygen (DO) and nitrate within the plumes. However, sulfate, another less

    easily consumed electron acceptor, is present indicating the aquifer is not at the optimal

    reducing condition for reduction of the cis-l,2-DCE plume. The presence of VC

    indicates that the cis-l,2-DCE plume is reducing, although at a slower rate than PCE and

    TCE. The presence of electron donors, such as carbon sources, also supports the

    occurrence of reductive dechlorination processes. Natural organic carbon was

  • encountered in borings drilled in the area which indicates the availability of a carbon

    source to support reductive dechlorination. The presence of reductive dechlorination by-

    products such as ferrous iron and methane also indicates the occurrence of reductive

    dechlorination processes.

    -Site data indicate PCE, TCE, and cis-lj2-DCE may be biodegrading at-an optimal

    rate and that expansion of these plumes may not occur. However, site data indicate that

    biodegradation of the VC plume may not be occurring. The aquifer is anaerobic which

    does not easily allow biodegradation of VC. The VC plume continues to expand toward

    the Gravel Pit Lake and Raccoon River; however, the VOC contaminants that eventually

    reach these surface water bodies appear to be attenuating through natural processes as

    indicated by the absence of VOCs above detection limits in the surface water samples.

    The COCs are also adsorbing to the aquifer matrix which will impede contaminant

    : extraction. Studies at other sites contaminated with chlorinated hydrocarbons indicate

    that two to four times the dissolved concentrations can be expected to be sorbed to the

    aquifer matrix. Significant amounts of chlorinated solvents can result in accumulations

    of non-dissolved phase contaminants (i.e., dense nonaqueous phase liquids [DNAPL]).

    However, to date DNAPL has not been identified at the site.

    The groundwater flow direction at the Railroad Avenue site is primarily to the east-

    southeast toward the Gravel Pit Lake and the Raccoon River. The distribution of the

    plumes also shows evidence of an east-southeasterly groundwater flow trend. The

    migration rates and paths for the Northern Plume contaminants have been evaluated usinggroundwater flow and contaminant transport computer models and are discussed in the

    Technical Memorandum: Groundwater Model Northern West Des Moines Well Field,

    July 8, 2003. A MODFLOW groundwater flow model was developed and calibrated to

    site water level measurements and pump test data. A contaminant transport model was

    developed to simulate contaminant migration in groundwater in the Northern West Des

    Moines well field. Assumptions used in development of the model included establishing

    no-flow boundaries where the bedrock forms boundaries of the alluvial aquifer and that

    the WDMWs south of well WDMW-9 would not significantly affect migration of the

    Northern Plume.

    10

  • The model shows that the contaminant plumes will continue to be captured by wells

    in the West Des Moines well field and will also eventually reach Gravel Pit Lake and the

    Raccoon River. The VOC contaminants that eventually reach the surface water bodies

    will most likely attenuate through natural processes.

    Initial concentrations of the TCE, cis-1 j2-DCE, and VC contaminant plumes in the

    model were the levels reported in the RI Report for the November 2002 sampling event in

    combination with the ESI data. Together, these data were used as the starting point for

    predictive modeling of contaminant transport.

    The scenarios that were evaluated in the model include: Scenario 1 - No Action (all

    wells pumping); Scenario 2 - Current Response Action (wells WDMW-5, WDMW-8, and

    WDMW-9 pumping); and Scenario 3 - Potential Response Action (all wells pumping,

    except well WDMW-7).

    Model results for Scenario 1 (before shutting down any of the north pumping wells)

    estimate that the MCL for cis-l,2-DCE is exceeded at well WDMW-13 within

    approximately one year. Although, as with any transport contaminant modeling effort,

    estimating the actual time of plume arrival is uncertain, it does appear that the MCL for

    cis-l,2-DCE will be exceeded fairly quickly if well WDMW-13 was returned to

    operation. The TCE plume is reduced to below the MCL in nearly one year, the VC

    plume is reduced to below the MCL in less than ten years, and the cis-l,2-DCE plume is

    nearly gone at forty years with only a small fraction (within the error of the model)

    remaining.

    Model results for Scenario 2 indicate that MCLs will not be exceeded in the

    extraction wells that are operating under this scenario (i.e., pumping wells WDMW-5,

    WDMW-8, and WDMW-9 without pumping wells WDMW-6, WDMW-7, WDMW-12,

    and WDMW-13). The TCE plume is estimated to be reduced to below the MCL in less

    than one year, the VC plume is reduced to below the MCL in less than ten years, and the

    cis-1,2-DCE plume is reduced to below the MCL in nearly thirty years. However, the

    capture zone is reduced, which would allow migration of the plumes toward Gravel Pit

    Lake and the Raccoon River.

    11

  • Scenario 3 evaluates contaminant capture with aggressive pumping at recovery wells

    WDMW-5, WDMW-6, WDMW-8, WDMW-9, WDMW-12, and WDMW-13. The

    modeled maximum pumping rates under Scenario 3 were 20 percent of the historical

    daily average. Aggressive pumping would accelerate removal of contaminants from the

    capture zone of the extraction wells. The TCE plume is estimated to be reduced to below

    the MCLin less than one. year, the VC plume is reduced to below the MCL in nearly ten

    years, and the cis-l,2-DCE plume is reduced to below the MCL in less than thirty years.

    Bioaccumulation of halogenated aliphatic compounds is not expected to be

    significant, based on the low octanol-water partition coefficients and low

    bioconcentration factor values of the COCs.

    6.0 Current and Potential Future Site and Resource Uses

    The Railroad Avenue Groundwater Contamination site is in West Des Moines, Iowa,

    which is a suburb of Des Moines, Iowa. The Northern Plume site lies along Railroad

    Avenue between First Street and Thirteenth Street. The western portion of the Northern

    Plume site contains a softball field complex, light industrial and commercial areas, and

    single-family dwellings. Further to the east, the site is mostly commercial and residential.

    The Valley Junction shopping district is located in the 100- to 300-block area along Fifth

    Street. The Valley Junction shopping area has been renovated with brick sidewalks and

    ornamental street fixtures. The eastern portion of the site is a residential area consisting

    primarily of single-family dwellings. The southern part of the North Plume site consists

    of light industrial businesses and a surface water detention basin. Future use of the site isanticipated to be similar to current uses.

    Groundwater in the Railroad Avenue site and vicinity is currently used as the

    primary water source for the city of West Des Moines and local industries. It is

    anticipated to continue to be used as the city's water source indefinitely. Water supply

    wells used by the city of West Des Moines have been impacted by the North Plume

    contaminants. Surface water in the Railroad Avenue site is used recreationally.

    12

  • 7.0 Summary of Site Risks

    The baseline risk assessment estimates what risks the site poses if no action was

    taken. It provides the basis for taking action and identifies the contaminants and

    exposure pathways that need to be addressed by the remedial action. This section of the

    ROD summarizes the results of the baseline risk assessment for this site.

    7.1 Human Health Risk Assessment

    A human health baseline risk assessment was prepared for the Railroad Avenue

    Groundwater Contamination site for drinking water. This summary presents an overview

    of the risk assessment prepared for the site. The complete risk assessment may be

    consulted in the Administrative Record file for a more detailed evaluation of the site

    risks. The human health risk assessment qualitatively evaluated soils at the site and

    quantitatively evaluated groundwater at the site. Contaminants identified in the soil were

    found to be at acceptable health risk levels and will require no further action.

    Contaminants identified in the groundwater, however, were found to be at unacceptable

    health risk concentrations.

    7.1.1 Identification of Contaminants of Concern

    Risk assessment is an analysis of the potential adverse health effects that may result

    from human exposure to chemical contaminants present at the site. The risk assessmentidentified several contaminants of potential concern (COPCs) in groundwater. Risk

    management evaluation of the COPCs relative to natural occurrence, prevalence, and site

    history determined the COCs for the Railroad Avenue site. The COCs at the Railroad

    Avenue site are PCE, TCE, cis-l,2-DCE, and VC in groundwater. These VOCs may pose

    adverse health effects at relatively high concentrations or exposures. Tables 3.1 through

    3.8 summarize the COPCs and the Exposure Point Concentrations used in the human

    health risk assessment.

    13

  • 7.1.2 Exposure Assessment

    The exposure pathways evaluated in the risk assessment are presented in Figure 3

    which shows the conceptual site model for the site. Table 4 summarizes all of the

    scenarios and pathways considered in the risk assessment. As shown, health risks to both

    current and future residents and workers from exposure (ingestion, dermal contact, and

    inhalation) to groundwater were evaluated. The exposure pathways are also included in

    Tables 3.1 through 3.8.

    7.1.3 Toxicity Assessment

    The human health risk assessment evaluated exposures to carcinogenic and non-

    carcinogenic contaminants at the site. Tables 5.1 and 5.2 summarize the non-cancer

    toxicity data and Tables 6.1 and 6.2 summarize the cancer toxicity data.

    7.1.4 Risk Characterization

    For carcinogens, risks are generally expressed as the incremental probability of an

    individual's developing cancer over a lifetime as a result of exposure to the carcinogen.

    Excess lifetime cancer risk is calculated from the following equation:

    Risk = GDI x SF

    where: risk = a unitless probability (e.g., 2 x 10"5) of an individual's developing

    cancer

    GDI = chronic daily intake averaged over 70 years, milligrams per

    kilogram-day (mg/kg-day)

    SF = slope factor, expressed as (mg/kg-day)"1

    These risks are probabilities that usually are expressed in scientific notation (e.g., 1 x

    10~6). An excess lifetime cancer risk of 1 x 10~6 indicates that an individual experiencing

    the reasonable maximum exposure estimate has a 1 in 1,000,000 chance of developing

    cancer as a result of site-related exposure. This is referred to as an "excess lifetime

    cancer risk" because it would be in addition to the risks of cancer individuals face from

    14

  • other causes such as smoking or exposure to too much sun. The chance of an individual's

    developing cancer from all other causes has been estimated to be as high as one in three.

    The EPA's generally acceptable risk range for site-related exposures is 10"4 to 10"6.

    The potential for noncarcinogenic effects is evaluated by comparing an exposure

    level over a specified time period (e.g., lifetime) with a reference dose (RfD) derived for a

    similar exposure period. A RfD represents a level that an individual may be exposed to

    that is not expected to cause any deleterious effect. The ratio of exposure to toxicity is

    called a hazard quotient (HQ). A HQ less than 1 indicates that a receptor's dose of a

    single contaminant is less than the RfD and that toxic noncarcinogenic effects from that

    chemical are unlikely. The Hazard Index (HI) is generated by adding the HQs for all

    COCs that affect the same target organ (e.g., liver) or that act through the same/

    mechanism of action within a medium or across all media to which a given individual

    may reasonably be exposed. A HI less than 1 indicates that, based on the sum of all HQs

    from different contaminants and exposure routes, toxic noncarcinogenic effects from all

    contaminants are unlikely. A HI greater than 1 indicates that site-related exposures may

    present a risk to human health.

    The HQ is calculated as follows:

    Non-cancer HQ = CDI/RfD

    where:

    GDI = Chronic daily intake

    RfD = reference dose.

    The GDI and RfD are expressed in the same units and represent the same exposure

    period (i.e., chronic, subchronic, or short term).

    A summary of carcinogenic risks for each population is presented hi Table 7; a

    summary of non-carcinogenic risks for each population is presented in Table 8.

    15

  • As indicated in Table 7, unacceptable cancer risks resulted for the hypothetical future

    resident at locations MW-6, MW-8, and well WDMW-13. There were also unacceptable

    cancer risks for the hypothetical future industrial worker at MW-8. The carcinogenic

    risks are associated with the COCs - TCE and VC, and the COPCs - bis(2-ethylhexyl)

    phthalate and arsenic.

    As indicated in Table 8, six exposure scenarios had total HI values that were above

    1: the hypothetical future child and adult residents at well locations MW-6, MW-8, and

    well WDMW-13. The non-carcinogenic risks are associated with the COCs - TCE and

    VC, and the COPCs - manganese, arsenic, and l,2-dibromo-3-chloropropane.

    Tables 9.1 through 9.11 present risk information for COPCs and media/exposure

    points that could hypothetically trigger the need for remedial action. Risk management

    evaluation of the COPCs relative to natural occurrence, prevalence, and site history

    determined the COCs for the Railroad Avenue site. The COCs at the Railroad Avenue

    site are PCE, TCE, cis-l,2-DCE, and VC in groundwater.

    7.2 Ecological Risk Assessment

    The Screening-Level Ecological risk assessment evaluated analytical data as they

    relate to ecological risks at the Railroad Avenue Groundwater Contamination site. The

    risk assessment identified several preliminary contaminants of potential ecological

    concern (PCOPECs). Risk management evaluation of the PCOPECs relative to natural

    occurrence, prevalence, current and future site use, and site history determined thatcurrent and future ecological risks posed by site contaminants are at acceptable levels.

    7.3 Risk Assessment Summary and Conclusion

    The excess carcinogenic risks to current lifetime residents (adult and child) were

    calculated to be 6 x 10~6 to 2 x 10~5 and excess carcinogenic risk to current industrial

    workers was calculated to range from 1 x 10"6 to 3 x 10"6. The excess carcinogenic risk to

    future lifetime residents (adult and child) was calculated to range from 1 x 10^ to 7 x 10~3

    and excess carcinogenic risks to future industrial workers were calculated to range from

    8 xlO'5 to 3x10" (Table 7).

    16

  • The non-carcinogenic risk (expressed as HQs) to current lifetime resident adults,

    current lifetime resident children, and industrial workers was calculated to be 0.5, 1, and

    0.2, respectively. The maximum noncarcinogenic risks to future lifetime resident adults,

    future lifetime resident children, and industrial workers were calculated to be 4, 11, and 1,

    respectively (Table 8).

    The remedial action selected in this ROD is necessary to protect the public health or

    welfare or the environment from actual or threatened releases of hazardous substances

    from this site. However, no current unacceptable risk exists since no one is drinking

    contaminated water.

    8.0 Remedial Action Objectives

    The Comprehensive Environmental Response, Compensation, and Liability Act

    (CERCLA), as amended by Section 121(b) of the Superfund Amendments and

    Reauthorization Act of 1986 (SARA), requires selection of remedial actions which ensure

    protection of human health and the environment, attain applicable or relevant and

    appropriate requirements (ARARs), are cost effective, use permanent solutions and

    alternative treatment technologies or resource recovery technologies to the maximum

    extent practicable, and satisfy the preference for treatment that reduces the toxicity,

    mobility, or volume of contaminants or provide an explanation as to why they do not. To

    satisfy CERCLA requirements, remedial action objectives (RAOs) were developed for

    the Railroad Avenue site. General response actions were then developed to attain the

    RAOs.

    The RAOs developed for the contaminated groundwater at the Railroad Avenue site

    within the West Des Moines well field area are identified below.

    • Prevent ingestion of groundwater having concentrations of the site COCs in

    excess of current regulatory drinking water standards. The current regulatory

    drinking water standards for the COCs are the MCLs. The MCLs are the

    maximum permissible levels established by the Safe Drinking Water Act

    [40 Code of Federal Regulations (CFR) 141] for a contaminant in water that is

    17

  • delivered to any user of a public water system. The MCLs are discussed in further

    detail in the following section.

    • Comply with Iowa Surface Water Criteria for COCs.

    The primary focus,of the remedial action is to address remediation of the

    contaminated groundwater which is the primary risk posed from the site.

    9.0 Description of Alternatives

    CERCLA requires that the selected site alternative be protective of human health and

    the environment, be cost effective, comply with other environmental laws, and use

    permanent solutions, alternative treatment technologies, and resource recovery

    alternatives to the maximum extent practicable. In addition, the statute includes a

    preference for the use of treatment as a principal element for the reduction of toxicity,

    mobility, or volume of the hazardous substances.

    The Focused FS Report prepared for the Northern Plume dated February 25, 2005,

    evaluated in detail four remedial alternatives (including the no action alternative, which

    EPA is required to consider by law) for addressing the contamination associated with the

    Northern Plume at the Railroad Avenue Groundwater Contamination site. As part of the

    process of choosing a remedy, the remedial alternatives from the FS are compared and

    evaluated using nine criteria that appear in the National Oil and Hazardous Substances

    Pollution Contingency Plan (NCP). (These criteria and evaluations are discussed in

    Section 10 below.)

    For the purpose of analyzing and comparing the remedial alternatives, EPA

    estimated costs of the alternatives by making certain assumptions, such as estimating the

    remediation time for pumping and treating groundwater. The EPA Superfund policy is to

    try to estimate costs with "+50/-30 percent" accuracy.

    The present worth of each alternative was calculated for all alternatives assuming a 7

    percent discount rate for up to 30 years. The present worth is a summary measure of cost

    that, for comparison purposes, turns a stream of payments or costs over a future period of

    18

  • years into the equivalent of a single lump sum in the present. The cost estimates, as

    discussed above, are conceptual, with an estimated +50 percent to -30 percent level of

    accuracy. The alternatives from the Focused FS Report are described in the remainder of

    Section 9. Section 10 compares the alternatives. Section 12 discusses the selected

    alternative; Section 12 also discusses several additional measures that will be taken as

    part of the selected remedy, including-remedial design activities.

    All of the alternatives, except the no further action alternative, include institutional

    controls as a common element. The institutional controls include the following:

    • Implementation of well permitting requirements to limit use of groundwater at the

    site. The permitting requirements would consist of an ordinance passed by the

    city of West Des Moines, Iowa, prohibiting the installation of new wells if city

    water is available. If a local ordinance could not be passed, a protected water

    source designation at the state level would be sought. In a protected water source

    area, new well installation would be restricted.

    • Public education would inform local officials on well drilling restrictions and be

    used to inform citizens of the potential health hazards associated with exposure to

    contaminated groundwater. Public education would be implemented through

    informational meetings and flyers.

    9.1 Alternative 1: No Action

    Estimated Capital Cost: $0

    Estimated Annual Operation & Maintenance (O&M) Cost Range: $0 to $50,500Estimated Present Worth Cost: $111,300

    Estimated Construction Timeframe: 0 months

    Estimated Time to Achieve RAOs: Indeterminate

    The NCP requires that the EPA considers a no further action alternative as a baseline

    against which other remedial alternatives can be compared. Under this alternative, no

    further action would be taken to monitor, control, or remediate groundwater

    contamination. Alternative 1 would not meet the RAOs because it does not minimize any

    future potential exposure at the site. The costs for this alternative are for the required

    five-year review.

    19

  • 9.2 Alternative 2: Monitored Natural Attenuation

    Estimated Capital Cost: $24,000

    Estimated Annual O&M Costs Range: $15,300 to $90,300

    Estimated Present Worth Costs: $430,000

    Estimated Construction Timeframe: 1 month - - - •

    Estimate Time to Achieve RAOs: Greater than 30 years

    Alternative 2 would rely on the aquifer's ability to lower contaminant concentrations

    through monitored natural attenuation (MNA) processes. This alternative includes

    groundwater and surface water monitoring to confirm continued efficiency of the natural

    attenuation of contaminants and institutional controls to minimize potential health risks

    associated with groundwater contaminants still undergoing attenuation.

    As discussed in Section 5.3, several years of site data have been collected to evaluate

    site conditions. Advective flow processes cause contaminants to migrate along with

    groundwater in the direction of groundwater flow while biodegradation processes

    simultaneously degrade the site contaminants. Biochemical processes at sites include

    biodegradation where chlorinated hydrocarbons are eventually reduced to more stable,

    less toxic compounds.

    Biodegradation of PCE and TCE is occurring at the Northern Plume site as

    evidenced by the low concentrations of PCE and TCE and the relatively elevated

    concentrations of cis-l,2-DCE and VC which are degradation products of PCE and TCE.Figure 2 illustrates that the highest concentrations of a potential source contaminant,

    TCE, are located near the suspected release area, whereas downgradient of the suspected

    source area, the concentrations of TCE are lower and the concentrations of daughter

    contaminants (cis-l,2-DCE and VC) are higher.

    The EPA has developed site screening criteria for identifying sites where efficient

    biodegradation is occurring. The site data were evaluated using the screening criteria and

    indicate evidence of an adequate biodegradation rate for PCE, TCE, and cis-1,2-DCE.

    20

  • Site data indicate PCE, TCE, and cis-l,2-DCE may be biodegrading at an optimal

    rate and that expansion of these plumes may not occur. However, site data indicate that

    biodegradation of the VC plume may not be occurring. The aquifer is anaerobic which

    does not easily allow biodegradation of VC. The VC plume continues to expand toward

    the Gravel Pit Lake and Raccoon River; however, the VOC contaminants that eventually

    reach these surface water bodies appear to be attenuating through natural processes as

    indicated by the absence of VOCs, including VC, above detection limits in the surface

    water samples.

    This alternative would include implementation of local or state well permit

    restrictions and public education. In addition, groundwater monitoring would be included

    to evaluate the effectiveness of the natural attenuation processes. A detailed sampling

    and quality assurance plan would be written before the groundwater monitoring activities

    began. The sampling and quality assurance plan would include sample locations,

    sampling frequency, sampling procedures, sample analysis methods, and sample

    documentation procedures.

    For the purpose of developing this alternative, it was assumed that one new

    monitoring well would be installed downgradient of the VC plume along First Street.

    The final location of the well would be determined during remedial design and would be

    contingent upon access agreements with the property owners.

    It was assumed that the groundwater monitoring would consist of sampling the newmonitoring well, the twelve existing monitoring wells, three surface water sample

    locations, and four WDM water supply wells (WDMW-5, WDMW-6, WDMW-12, and

    WDMW-13) quarterly for years one and two; semiannually for years three, four, and five;

    and annually after year five until RAOs are attained. The frequency of the monitoring

    may be re-evaluated and modified after the five-year reviews or after review of the

    monitoring data by the EPA and IDNR. The groundwater samples would be analyzed for

    VOCs.

    It was assumed that the surface water monitoring would consist of collecting two

    samples from the Gravel Pit Lake and one sample from the Raccoon River at the same

    21

  • frequency of the groundwater samples. The surface water samples would also be

    analyzed for VOCs.

    The results of the sample analysis would be used to confirm the rate and direction of

    groundwater contaminant migration. If the monitoring results indicate that the plume is

    •migrating towards new receptors, further.remedial actions would be initiated.

    9.3 Alternative 3: Extraction with Recovery Wells/Onsite Tray Aeration/Surface Water

    Discharge

    Estimated Capital Cost: $532,000

    Estimated Annual O&M Costs Range: $44,500 to $145,000

    Estimated Present Worth Costs: $1,343,000

    Estimated Construction Timeframe: 17 to 18 months

    Estimated Time to Achieve RAOs: Greater than 30 years

    Alternative 3 would include extraction of contaminated groundwater using recovery

    wells followed by onsite tray aeration treatment and discharge of the treated groundwaterto Gravel Pit Lake. The recovery wells would be pumped at a rate to hydraulically

    control the groundwater flow which would provide containment of the groundwater

    contaminant plume.

    Groundwater would be extracted using three new recovery wells. Groundwater flow

    equations were used to estimate the location and extraction rate of the recovery wells that

    would prevent migration of contaminants to surface water bodies. It is estimated that

    each recovery well would be pumped at 70 gpm to achieve a 1,380-foot-wide capturezone for each well. Strategically placing three recovery wells would achieve plume

    containment and prevent migration of contaminants to the Gravel Pit Lake and Raccoon

    River.

    The water would be piped to an onsite treatment plant and treated by air stripping

    with tray aerators. The groundwater would be pumped through a tray aeration system to

    remove the contaminants. In a tray aeration system, the contaminated groundwater enters

    the top of the treatment system and flows across a series of aeration trays. Air passes

    upward through openings in the trays and bubbles up through the water. The bubbles and

    22

  • water form a very turbulent mixture that is excellent for stripping the COCs from the

    water and volatilizing them into the air. The vapor/contaminant mixture is removed from

    the system and released to the atmosphere or treated. The system can be readily expanded

    to accommodate an increase in influent flow or contaminant concentration by addition of

    additional trays.

    Based on available information from air stripping tray aeration vendors and the Iowa

    Air Quality Standards, it is anticipated that the air stripper off-gas would not have to be

    treated. Further evaluation of emission rates would be evaluated during the engineering

    design.

    Once collected and treated, the extracted groundwater from the recovery wells would

    be discharged directly to Gravel Pit Lake through a newly constructed discharge line.

    Treatment plant operation efficiency would be evaluated by collecting and analyzing

    influent, effluent, and emission samples.

    The final location of the recovery wells, onsite treatment system, and discharge

    piping would be determined during the remedial design.

    This alternative would include implementation of local or state well permit

    restrictions and public education. In addition, groundwater monitoring would be included

    to evaluate the effectiveness of plume containment by the recovery well. A detailed

    sampling and quality assurance plan would be written before the groundwater monitoring

    activities began. The sampling and quality assurance plan would include samplelocations, sampling frequency, sampling procedures, sample analysis methods, and

    sample documentation procedures.

    It was assumed that monitoring would consist of sampling the twelve existing

    monitoring wells, three surface water sample locations, the three new recovery wells, and

    four WDM water supply wells (WDMW-5, WDMW-6, WDMW-12, and WDMW-13)

    quarterly for years one and two; semiannually for years three, four, and five; and annually

    after year five until RAOs are attained. The frequency of the monitoring could be re-

    evaluated and modified after the five-year reviews or after review of the monitoring data

    23

  • by the EPA and IDNR. The groundwater and surface water samples would be analyzed

    for VOCs.

    It was assumed that the surface water monitoring would consist of collecting two

    samples from the Gravel Pit Lake and one sample from the Raccoon River at the same

    frequency of the groundwater samples. The surface water samples would also be

    analyzed for VOCs.

    The results of the sample analysis would be used to evaluate the rate and direction of

    groundwater contaminant migration and determine if adjustments in recovery well flow

    ratios are needed. If the monitoring results indicate that the plume is migrating towards

    new receptors, further remedial actions would be initiated.

    9.4 Alternative 4: In-situ Stripping Wells

    Estimated Capital Cost: $670,000

    Estimated Annual O&M Costs Range: $66,900 to $171,900

    Estimated Present Worth Costs: $1,844,000

    Estimated Construction Timeframe: 17 to 18 months

    Estimated Time to Achieve RAOs: Greater than 30 years

    Under Alternative 4, two lines of in-situ stripping wells would be placed along theeastern and southern edges of the plume to treat the contaminated groundwater as it

    migrates towards the Gravel Pit Lake and Raccoon River. Based on information from

    vendors, it is estimated that the in-situ stripping wells would have a radius of influence ofapproximately 150 feet. Therefore, it is estimated that ten in-situ stripping wells would

    be required to act as a barrier between the plume and the surface water bodies.

    In-situ stripping wells are double-screened wells that re-circulate groundwater in the

    aquifer by pulling groundwater in through the lower well screen and recharging the

    aquifer through the upper screen using air lift pumping. The air lift pumping would

    simultaneously strip the VOCs from the contaminated groundwater that is pulled in

    through the lower well screen.

    24

  • The stripped VOCs are then discharged from the in-situ stripping well to the air.

    Based on available information, it is not anticipated that treatment of the VOC-laden air

    would be required. Further evaluation of emission rates would be conducted during the

    engineering design.

    This alternative would include implementation of local or state well permit

    restrictions and public education. In addition, groundwater monitoring would be included

    to evaluate the effectiveness of the in-situ stripping wells protection of the Gravel Pit

    Lake and Raccoon River. A detailed sampling and quality assurance plan would be

    written before the groundwater monitoring activities began. The sampling and quality

    assurance plans would include sample locations, sampling frequency, sampling

    procedures, sample analysis methods, and sample documentation procedures. Wells from

    the existing monitoring well network would be used as much as possible to avoid

    duplication of effort and to minimize the number of new monitoring wells installed. One

    new monitoring well would be added to the existing monitoring well network to allow

    evaluation of the effectiveness of the in-situ stripping wells..

    It was assumed that the groundwater monitoring would consist of sampling the new

    monitoring well, the twelve existing monitoring wells, the shallow and deep piezometer

    at each in-situ treatment well, three surface water sample locations, and four WDM water

    supply wells (WDMW-5, WDMW-6, WDMW-12, and WDMW-13) quarterly for yearsone and two; semiannually for years three, four, and five; and annually after year five

    until RAOs are attained. The frequency of the monitoring could be reevaluated and

    modified after the five-year reviews or after review of the monitoring data by the EPAand IDNR. The groundwater and surface water samples would be analyzed for VOCs.

    It was assumed that the surface water monitoring would consist of collecting two

    samples from the Gravel Pit Lake and one sample from the Raccoon River at the same

    frequency of the groundwater samples. The surface water samples would also be

    analyzed for VOCs.

    The results of the sample analysis would be used to evaluate the rate and direction of

    groundwater contaminant migration. If the monitoring results indicate that the plume is

    migrating towards new receptors, further remedial actions would be initiated.

    25

  • 10.0 Comparative Analysis of Alternatives

    Nine criteria are used to evaluate the different alternatives individually and against

    each other in order to select a remedy. The nine evaluation criteria are: (1) overall

    protection of human health and the environment; (2) compliance with ARARs; (3) long-

    term effectiveness and permanence; (4) reduction of toxicity, mobility, or volume of

    contaminants through treatment; (5) short-term effectiveness; (6) implementability; (7)

    cost; (8) state/support agency acceptance; and (9) community acceptance. This section of

    the ROD profiles the relative performance of each alternative against the nine criteria,

    noting how it compares to the other options under consideration. The nine evaluation

    criteria are discussed below and are summarized in Table 10.

    10.1 Overall Protection of Human Health and the Environment

    Overall protection of human health and the environment determines whether an

    alternative eliminates, reduces, or controls threats to public health and the environmentthrough institutional controls, engineering controls, or treatment.

    Human health and the environment would be adequately protected by Alternatives 2,

    3, and 4. Alternatives 3 and 4 would provide the highest overall protection of human

    health and the environment since groundwater contaminants are contained. Alternative 3

    collects and treats the extracted groundwater before contaminants migrate to

    downgradient surface water bodies. Similarly, Alternative 4 would provide treatment ofthe groundwater that passes through the line of in-situ treatment wells before

    contaminants migrate to downgradient surface water bodies. Alternative 2 would provide

    the next best overall protection of human health and the environment through observed

    and documented natural attenuation processes. Alternative 1 would provide the least

    overall protection of human health and the environment since Alternative 1 provides no

    means of control to prevent public exposure to contamination and no monitoring of

    plume changes.

    26

  • 10.2 Compliance with Applicable or Relevant and Appropriate Requirements (ARARs)

    All the alternatives except Alternative 1 would comply with chemical-specific and

    action-specific ARARs. No location-specific ARARs were identified for any alternative.

    Alternatives 2, 3, and 4 would meet chemical-specific ARARs in the long term through

    •MNA processes. Alternatives 3 and 4 contain the groundwater plume above cleanup

    levels (i.e., MCLs) in addition to MNA processes reducing onsite contaminant levels.

    10.3 Long-Term Effectiveness and Permanence

    Long-term effectiveness and permanence consider the ability of an alternative to

    maintain protection of human health and the environment over time.

    Biodegradation of COCs at the site would be effective and permanent under

    Alternatives 2, 3, and 4. Alternatives 3 and 4 would provide best long-term effectiveness

    and permanence because the groundwater plume containing COCs above cleanup levels

    (i.e., MCLs) would be contained. Because no remedial actions would occur,

    Alternative 1 would provide the lowest long-term effectiveness and permanence.

    10.4 Reduction of Toxicity, Mobility, or Volume of Contaminants Through Treatment

    Reduction of toxicity, mobility, or volume of contaminants through treatment

    evaluates an alternative's use of treatment to reduce the harmful effects of principal

    contaminants, their ability to move in the environment, and the amount of contaminationpresent.

    Alternatives 2, 3, and 4 would be effective in reducing the toxicity and volume of

    contaminants at the completion of the remedial actions. However, under Alternative 2

    the reduction of the toxicity of contaminants would occur through natural processes rather

    than treatment efforts in the last phases of the chemical reduction process as the COCs

    eventually degrade to non-toxic compounds. Alternatives 3 and 4 meet the statutory

    preference for treatment as a principal element of the alternative and also reduce mobility

    of contaminants. Because groundwater monitoring would not be conducted under

    Alternative 1, there would be no mechanism to evaluate or demonstrate the reduction in

    27

  • the toxicity, mobility, or volume of contaminants through natural attenuation processes.

    Therefore, reduction in the toxicity, mobility, and volume of contaminants under

    Alternative 1 cannot be presumed.

    10.5 Short-Term Effectiveness

    Short-term effectiveness considers the length of time needed to implement an

    alternative and the risks the alternative poses to workers, residents, and the environment

    during implementation.

    Alternatives 2, 3, and 4 would reach cleanup goals in a similar timeframe which is

    estimated to be in excess of thirty years. However, Alternative 3 may reach cleanup goals

    slightly sooner because the recovery wells would increase the hydraulic gradient at the

    site, resulting in an increased migration rate of the plume. Short-term risk would exist to

    the community from onsite groundwater contaminants which are relatively slower to

    attenuate. However, the short-term risks would be mitigated through institutional

    controls. Alternatives 3 and 4 would contain the plume. The containment efforts may

    need to operate in excess of thirty years. Alternative 1 would not include any action and

    may not meet RAOs.

    There would be no increase in short-term risks to workers under Alternative 1;

    however, there would be a continued risk to the community because contaminants above

    cleanup levels (i.e., MCLs)s would remain onsite unmanaged and unmonitored.

    Alternative 2 would have minor, controllable increased risk to the community duringconstruction, and worker protection would be required during monitoring well

    installation. Alternatives 3 and 4 would have the greatest short-term risks, although still

    moderately low, from construction of the new onsite treatment plant, associated pipelines,

    and in-situ stripping wells.

    10.6 Implementability

    Implementabiliry considers the technical and administrative feasibility of

    implementing the alternative such as the relative availability of goods and services.

    28

  • Alternative 1 would be the easiest alternative to implement because no construction

    or operation would be required. Alternative 2 would be the next easiest alternative to

    implement because the only construction required would be installation of one

    monitoring well, and only periodic monitoring would be required. Alternative 4 would be

    more difficult to implement because it would require installation and O&M often in-situ

    stripping wells. Alternative 3 would be the most difficult to implement, requiring

    construction of a new onsite treatment plant, installation of collection and discharge

    piping, installation of recovery wells, as well as more extensive O&M.

    10.7 Cost

    Cost includes estimated capital and O&M costs as well as present worth costs. The

    present worth cost is the total cost of an alternative over time in terms of today's dollar

    value. Cost estimates are expected to be accurate within a range of+50 percent to -30

    percent.

    Cost estimates were prepared for each alternative. These estimates are approximate

    and made without detailed engineering data. The actual cost of the project would depend

    on the final scope of the remedial action and other unknowns. The total present worth of

    Alternative 1 would be the lowest at a cost of $111,300. The total present worth cost of

    Alternative 4 would be the greatest at a cost of $1,844,000. The total present worth costs

    of Alternatives 2 and 3 are estimated to be $430,000 and $1,343,000, respectively.

    10.8 State/Support Agency Acceptance

    State agency acceptance considers whether the state agrees with the EPA's analyses

    and recommendations of the RI/FS and the Proposed Plan.

    The IDNR supports the preferred alternative, Alternative 2: Monitored Natural

    Attenuation, as proposed by the EPA.

    29

  • 10.9 Community Acceptance

    Community acceptance considers whether the local community agrees with the

    EPA's analyses and preferred alternative. Comments received on the Proposed Plan are

    important indicators of community acceptance.

    During the public comment period, the community expressed its support for

    Alternative 2: Monitored Natural Attenuation, as proposed by the EPA.

    11.0 Principal Threat Wastes

    The "principal threat" concept is applied to the characterization of "source materials"

    at a Superfund site. A source material is material that includes or contains hazardous

    substances, pollutants, or contaminants that act as a reservoir for migration of

    contamination to groundwater, surface water, or air, or acts as a source for direct

    exposure. Contaminated groundwater generally is not considered to be a source material;

    however, non-aqueous phase liquids (NAPLs) in groundwater may be viewed as source

    material.

    The source materials for the VOCs in the Northern Plume are currently unknown

    despite reasonable investigation efforts. However, the source materials for the Northern

    Plume appear to be diminishing.

    12.0 Selected Remedy

    This section expands upon the details of the selected remedy from that which is

    presented in the Description of Alternatives Section (Section 9) of this ROD.

    12.1 Summary of the Rationale for the Selected Remedy

    The selected remedy for the Northern Plume of the Railroad Avenue Groundwater

    Contamination site will consist of Alternative 2: Monitored Natural Attenuation to be

    performed simultaneously with the previous interim action that included expansion of the

    stripping capacity at the West Des Moines drinking water plant as documented in the

    30

  • September 26, 2003, ROD. This previously implemented interim remedial action is

    intended to protect the West Des Moines water supply from contamination, leaving the

    selected remedy to address only the remaining contaminants that do not impact the water

    supply system. This alternative will provide the best balance of trade-offs among

    alternatives with respect to the evaluating criteria. The EPA believes Alternative 2 in

    conjunction with the previous interim action will be protective of human health-and the ,

    environment, will comply with ARARs, will be cost effective, and will utilize permanent

    solutions and alternative treatment technologies or resource recovery technologies to the

    maximum extent practicable.

    The main factors influencing EPA in its selection of Alternative 2 as the remedy

    include:

    • Institutional controls will eliminate or minimize the chance of a receptor being

    exposed to the contaminated groundwater at OU 1.

    • Site data indicate that significant amounts of source material or NAPLs no longer

    remain at OU 1; hence, there is no evidence of principal threat wastes at OU 1.

    • Monitoring of OU 1 is warranted because of the levels of COCs detected in the

    groundwater at OU 1.

    • Current monitoring data at the site indicate that natural attenuation is actively

    occurring at the site.

    12.2 Description of Selected Remedy

    Alternative 2 will rely on the aquifer's ability to lower contaminant concentrations

    through MNA processes. The alternative will include groundwater and surface water

    monitoring to confirm continued efficiency of the natural attenuation of contaminants and

    institutional controls to minimize potential health risks associated with groundwater

    contaminants still undergoing attenuation. The estimated timeframe required to attain

    cleanup levels is greater than thirty years, which is similar to the other proposed

    alternative timeframes. The long timeframe is appropriate for the site since the

    31

  • previously implemented interim action remedy addresses contaminated groundwater that

    is collected by the West Des Moines drinking water plant.

    Several years of site data have been collected to evaluate site conditions and are

    summarized in Tables 1, 2, and 11. Results of the evaluation indicate that migration of

    contaminants in groundwater is complex and subject to several physical and chemical

    processes. However, contaminant migration at the Railroad Avenue site is strongly

    affected by two primary site conditions: groundwater transport, and biochemical

    processes.

    The groundwater flow direction and current extent of the COCs at the Northern

    Plume are illustrated in Figure 2. The groundwater flow direction at the Northern Plume

    is primarily to the east-southeast toward Gravel Pit Lake and the Raccoon River. The

    distribution of the contaminant plumes also shows evidence of an east-southeasterly

    groundwater flow trend. However, distribution of the contaminants indicates that the

    groundwater capture zone created from historical periodic pumping of WDM water wells

    WDMW-5, WDMW-6, WDMW-12, and WDMW-13 has caused contaminants to migratewest toward these water supply wells.

    While advective flow processes cause contaminants to migrate along with

    groundwater in the direction of groundwater flow, biodegradation processes

    simultaneously degrade the site contaminants. Biochemical processes at sites include

    biodegradation which is where hydrocarbons are eventually reduced to more stable, less

    toxic compounds. Studies show that chlorinated hydrocarbons such as PCE and TCEand, to a lesser degree, cis-l,2-DCE, can degrade naturally via reductive dechlorination

    under anaerobic conditions. Biodegradation of PCE and TCE is occurring at the Northern

    Plume site as evidenced by the low concentrations of PCE and TCE and the relatively

    elevated concentrations of cis-l,2-DCE and VC, which are degradation products of PCE

    and TCE. The extent of TCE, cis-l,2-DCE, and VC are illustrated in Figure 2. Figures 4

    and 5 illustrate that the highest concentrations of the source contaminant TCE are located

    near the suspected release area, whereas downgradient of the source area, the

    concentrations of TCE are lower and the daughter contaminants (cis-l,2-DCE and VC)

    are higher.

    32

  • Other indicators of biodegradation include evaluation of materials within the plumes

    which act as electron acceptors and electron donors in the reductive dechlorination

    process and the presence or absence of by-products of the process within the plumes.

    The absence of electron acceptors indicates the occurrence of reductive

    dechlorination.processes. -The site geochemical data show the absence of electron

    acceptors such as dissolved oxygen and nitrate within the plumes. However, sulfate,

    another less easily consumed electron acceptor, is also present indicating the aquifer is

    not at the optimal reducing condition for reduction of the cis-l,2-DCE plume. The

    presence of VC indicates that the cis-l,2-DCE plume is reducing, albeit at a slower rate

    than PCE and TCE.

    The presence of electron donors, such as carbon sources, also supports the

    occurrence of reductive dechlorination processes. Natural organic carbon was

    encountered in borings drilled in the area which indicates the availability of a carbon

    source to support reductive dechlorination.

    The presence of reductive dechlorination by-products also indicates the occurrence

    of reductive dechlorination processes. Site geochemical data show the presence of

    reductive dechlorination by-products within the plumes including ferrous iron and

    methane.

    The EPA has developed site screening criteria for identifying sites where reductive

    dechJorination is occurring (Technical Protocol for Evaluating Natural Attenuation ofChlorinated Solvents in Ground Water, EPA /600/R-98/128, USEPA, September 1998).

    The site data from May and November 2004 were evaluated using the screening criteria.

    Results of the screening evaluation are presented in Table 12 and indicate adequate

    evidence for reductive dechlorination of PCE, TCE, and cis-l,2-DCE.

    Site data indicate PCE, TCE, and cis-l,2-DCE may be biodegrading at an optimal

    rate and that expansion of these plumes may not occur. However, site data indicate that

    biodegradation of the VC plume may not be occurring. The aquifer is anaerobic which

    does not easily allow biodegradation of VC. The VC plume continues to expand toward

    the Gravel Pit Lake and Raccoon River; however, the VOC contaminants that eventually

    33

  • reach these surface water bodies appear to be attenuating through natural processes as

    indicated by the absence of VOCs above detection limits in the surface water samples.

    The migration rates and paths for site contaminants were further evaluated by

    groundwater flow and contaminant transport computer models and are discussed in the

    Groundwater Model Technical Memorandum prepared for the site. Results of the

    computer groundwater modeling show that the contaminant plumes will continue to be

    captured by wells in the West Des Moines well field and will also eventually reach the

    Gravel Pit Lake and Raccoon River.

    This alternative will include implementation of local or state well permit restrictions

    and public education. The alternative will include implementation of well permitting

    requirements to limit use of groundwater at the site. The permitting requirements will

    consist of an ordinance passed by the city of West Des Moines, Iowa, prohibiting the

    installation of new wells if city water is available. If a local ordinance could not be

    passed, a protected water source designation at the state level will be sought. In a

    protected water source area new well installation will be restricted.

    Public education efforts will be performed which will include informing local

    officials on well drilling restrictions and informing citizens of the potential health hazards

    associated with exposure to contaminated groundwater. Public education will be

    implemented through informational meetings and flyers.

    Groundwater monitoring will be included to evaluate the effectiveness of the naturalattenuation processes. A detailed sampling and quality assurance plan will be written

    before the groundwater monitoring activities begin. The sampling and quality assurance

    plan will include sample locations, sampling frequency, sampling procedures, sample

    analysis methods, and sample documentation procedures.

    One new monitoring well will be installed down gradient of the VC plume along

    First Street. The final location of the well will be determined during remedial design and

    will be contingent upon ac

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