Design of Active Sub-Slab Vapor Mitigation Systems Presented by: - - PDF document

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Design of Active Sub-Slab Vapor Mitigation Systems

Presented by: Bob Roth, P.E. (Terracon) Colorado Environmental Management Society March 12, 2019

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What is Vapor Intrusion?

Simply, it is the migration of contaminant vapors from soil, groundwater, or preferential conduit onto the subject property (typically seen as petroleum compounds or chlorinated solvents).

Vapor Intrusion is the migration

• f volatile chemicals from the

subsurface into overlying buildings (USEPA, 2002). Vapors are typically from petroleum compounds, chlorinated solvents, or methane (from landfills). Vapors from soil and/or groundwater flow through pathways such as floor/wall cracks, sewers, or utilities into the indoor air of buildings

Example of Vapor Encroachment/Vapor Intrusion in Denver

In 1994 at the Redfield site located at 5800 E Jewell Avenue in Denver, groundwater was found to be contaminated with chlorinated solvents (TCE and DCE) used for degreasing at the former Redfield Rifle Scope, Inc. The contaminated groundwater moved to the north/northwest under an adjacent neighborhood. Indoor air sampling/analyses showed that 240 homes had concentrations of TCE and DCE above CDPHE’s action levels and required the installation and operation of vapor mitigation systems.

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Why be concerned with vapor intrusion?

Primary Reason

• Protect the health and welfare of building occupants

Secondary Reasons

• Compliance with federal and state regulations.
• Avoid complaints of odors from building occupants.
• Avoid questions from occupants: Is the air safe for us to breathe? Is

the air safe for my children to breathe? Will this affect our property value?

• Avoid relocating occupants (cost and lost revenue).
• Avoid occupants retaining third party to conduct vapor investigations.
• Avoid legal action.
• More cost effective to engineer and install a vapor mitigation system

in new construction than in an existing building.

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Vapor Intrusion Health Risks

Acute and Chronic Effects

• Petroleum vapors (typically benzene, toluene, ethyl

benzene, xylenes, and other gasoline constituents): Cancer, nervous system, liver, and explosive conditions.

• Chlorinated solvent vapors: Cancer, birth defects, nerve

damage, adverse effects on immune system.

• Methane: Explosive, headaches, weakness, nausea.

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Vapor Intrusion Regulations

• CERCLA - authorizes enforcement and cleanup of substances that

present danger to public health and welfare.

• OSWER Directive 9200.2-84 - mandates remedial action to prevent

risks associated with vapor intrusion.

• Subsurface intrusion component added to Hazard Ranking System –

EPA Rule Addition February 8, 2017

• RCRA - authorizes litigation to force cleanup of contamination that

could endanger health or environment

• CDPHE’s Indoor Air Guidance is used to evaluate indoor air exposure

pathway, calculating risk-based air concentrations, indoor air sampling/analysis, and collection of sub-slab vapor samples.

• Department of Labor and Employment, Division of Oil and Public

Safety UST Program.

• Who is liable? Building owner, operator, and real estate

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Passive vs. Active Vapor Mitigation Systems

• Passive VMSs rely on natural venting due to changes in

atmospheric pressure or use a mechanical ventilator powered by the wind which induces a low level vacuum to extract the vapors beneath the floor slab and discharge to the atmosphere.

• Active VMSs use a vacuum blower to extract the vapors

beneath the floor slab and discharge the vapors to the atmosphere.

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Required Components of Passive VMS

• Sub-slab soil gas collectors (slotted or perforated

pipe) installed in gravel

• Soil gas collector headers (solid pipe) to convey

vapors to exhaust piping

• 4-inch minimum layer of sub-slab gravel over soil

gas collectors

• Vapor retarder membrane over sub-slab gravel
• Vertical exhaust piping to convey vapors to

discharge point over roof

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VMS Membranes

• VMS membranes typically made of HDPE
• Usually 40 to 60 mil thickness
• VMS membranes less than 30 mil are not durable

enough to prevent significant damage during placement of reinforcing steel and concrete, and are not recommended in sub-slab applications (EPA, Engineering Issue: Indoor Air Vapor Intrusion Mitigation Approaches).

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Sheet Membrane

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VMS Composite Membrane

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Soil Gas Collector Layout

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VMS Risers

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VMS vertical risers convey collected vapors to roof and discharge the vapors to the exterior of the building.

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Use of Mechanical Ventilators for Passive VMSs

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A wind-powered mechanical ventilator is installed at the outlet of vertical risers to induce a low-level vacuum below the floor slab, extract vapors from beneath the slab, and convey them to the outlet of the vertical riser

Flow Monitoring – Passive VMS with Mechanical Ventilator

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Location of Vertical Risers Relative to Fresh Air Intakes

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Building openings (windows, doors, other), fresh air intakes, and HVAC intakes should be outside a 15 ft radius of the outlets of VMS vertical risers (ASHRAE Standard 62.1-2004)

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Location of Outlets of Vertical Risers Relative to Electrical Equipment When Vapors are Explosive

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If area around riser outlet is rated as Class 1, Division 1 per NEC, electrical equipment within 5 feet of outlet shall be rated same. If area around riser outlet is classified as Class 1, Division 2, electrical equipment within 10 feet of outlet shall be rated same.

Passive VMS Coffee Shop – 2,000 sf

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Passive VMS for 5-Story Mid Rise Office – 34,000 sf

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Passive VMS for 5-Story Mid Rise Commercial/Residential – 68,000 sf

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Questions often asked by Active VMS designers

• What to do for new and existing buildings with

sub-slab soils with soil gas VOC concentrations exceeding VISLs?

• What guidance documents do I use?
• What vacuum is needed under the slab to capture

vapors in order to protect building occupants?

• What flow rate do I select to extract the soil vapor?
• How do I size a vacuum blower?

ANSWERS?

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Primary Design Criterion for Active Sub-Slab VMS

• Maintain sub-slab vacuum to achieve

depressurization of approximately 4 – 10 Pascals (0.016 – 0.040 inches water) over the building footprint. (EPA, 2008. Indoor Air Vapor Intrusion Mitigation Approaches, EPA/600/R-08-115, October 2008)

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State Requirements for Sub-Slab Vacuum

• Web search showed 10 states with VI Guidance

Documents

• Only four states cite requirement for minimum

sub-slab vacuum:

• Minnesota: 3 to 5 Pa (0.012 – 0.02“ H2O)
• New Jersey: 1 Pa (0.004” H2O)
• North Carolina: 4 Pa (0.016” H2O)
• Massachusetts: no less than 0.5 Pa (0.002”

H2O) (recommendation only)

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Flow Rate Design Criterion for Active Sub- Slab Methane Mitigation Systems

• Capable of ventilating the sub-slab

gravel voids at a rate of three (3) vapor changes per hour. (Methane Seepage Regulations – Division 71, Los Angeles Building Code)

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Sub-slab vapor flow paths induced by vacuum applied to sub-slab gravel. Case 1: Flow from vadose zone with negligible flow from sides.

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• Q = kiA/u
• = k dP/(dx) A/u
• Where:
• Q = flow rate of a fluid through a medium
• A = the area of the medium
• k = permeability of the medium
• u = dynamic viscosity of the fluid
• dP = applied pressure to the fluid
• dx = thickness of the medium

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Sub-slab vapor flow rate vs. sub-slab vacuum for different soil types. Case 1: Flow from vadose zone with negligible flow from sides. Flow per 1000 sf area

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Case 2 – Flow from Surface with Negligible Flow from Vadose Zone

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Sub-slab vapor flow rate vs. applied vacuum for different soil types. Case 2: Flow from soil surface with negligible flow from vadose zone. Flow per 1000 sf

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Case 3 – Flow from Soil Surface and Vadose Zone

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Sub-slab vapor flow rate vs. applied vacuum for different soil types. Case 3: Flow from soil surface and vadose zone. Flow per 1000 sf

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Case Studies – Flow/Area vs. Sub-Slab Vacuum

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Flow/Area vs. Sub-Slab Vacuum

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F/A Ratio to Achieve Sub-Slab Vacuum of 4 Pa

Using data base, find F/A required to achieve sub- slab vacuum of 4 Pa (0.016 inch water) F/A = 22.9 x 0.016 F/A = 0.37 cfm/1000 sf Assume FS of 2 F/A = 0.8 cfm/1000 sf (say 1 cfm/1000 sf)

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Fan Sizing (ANSI/AARST)

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F/A Ratios based on ANSI/AARST Fans

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Flow/Area Ratio Based on Sub-Slab Gravel Void Volume Changeouts/Hour

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Single Horizontal Soil Gas Collector Layout (ROI = 20 feet, Area = 2,400 sf)

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Manifolded Soil Gas Collectors (ROI = 30 feet, Area = 105, 000 sf

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A2FAC1A3.pdf

What do I need to consider when designing a VMS that uses long (> 75 feet) horizontal soil gas collectors?

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Common Design Approach: If I use equal spacing between the slots or perforations in my sub-slab vapor collection piping, the vapor flow will be equal along the axis of the piping. IDEALIZED SUB-SLAB VACUUM ISOPLETHS AROUND VAPOR COLLECTION LA TERAL

Sub-slab vacuum isopleths and vapor streamlines for a vapor collection lateral

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Sub-slab vacuum isopleths with equal spacing between slots based on field data

Sub-slab vacuum isopleths and flow streamlines with equal spacing between slots (based on field data)

Pilot Testing at JFK International Airport (1995)

• 70 acres of jet fuel impacted soil and groundwater

under tarmac

• Long horizontal wells needed for the area of

remediation

• Preliminary SVE pilot test using horizontal well with 100

ft screen with equal spacing between slots showed significant decrease of vacuum along the well axis with negligible vacuum at well end

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Full-Scale Pilot Test at JFK International Airport

• Full-scale pilot with 4” diameter HDPE horizontal SVE

well 660 ft long with 530 ft of screen. Spacing between the slots varied slightly based on results of preliminary pilot test.

• To monitor subsurface vacuum in the vadose zone,

vacuum monitoring probes were located 20, 40, 60 and 100 feet from the horizontal well perpendicular to the well axis, and at the ¼ point, midpoint, and ¾ point of the well screen.

Vadose Zone Vacuum Isopleths - SVE Pilot Test at JFK International Airport

Use of Data from JFK Pilot Test for Full- Scale Design

• Flow Net Theory showed that an open area ratio of 2/1

(end half /first half) would more equally distribute the subsurface vacuum

• Approach used to design screens for 42 horizontal

SVE/AS wells; 7,000 feet of SVE wells and 13,000 feet of AS wells

• Performance specifications: 10” water vacuum at end of

SVE wells, 1 to 2 scfm/ft of screen for SVE wells

• Subsurface vacuum monitoring during full-scale

remediation estimated an equal distribution of vacuum along the axis of the horizontal wells

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Active VMS Case Studies

Equal slot spacing vs. variable slot spacing

Case 1: Commercial Warehouse

• VMS installed to address methane below

the building

• 2 - inch perforated PVC pipe (equal spacing

between perforations) in a manifold configuration for vapor collection laterals to extract methane.

Case 1: Sub-slab vacuum isopleths for VMS manifold and vapor collection laterals with equal spacing between slots. Footprint = 42,000

• sf. Four vapor laterals at 180’ = 720’

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Case 1 Monitoring Results and Conclusions

• Sub-slab vacuum ND at two vacuum monitoring

points and decreased two orders of magnitude (5.3 to 0.06 inches water) over a distance of 150 feet.

• Sub-slab vacuum was not evenly distributed
• Where vacuum was ND, risk of methane

migrating through floor slab and into the building.

Case 2 – Methane Mitigation System in Hospital Addition over Former Municipal Landfill

• VMS installed to address methane below the

proposed 86,000 sf building.

• 1000 feet of 4-inch slotted PVC pipe with variable

slot width for vapor collection laterals

• 12 sub-slab vacuum monitoring points

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Horizontal Vapor Collection Piping Slot Width Variability:

Sub-slab VMS Installation at Medical Center Constructed on Former Landfill in Monmouth County , New Jersey .

Sub-Slab Vacuum Measurements:

Sub-slab VMS Installation at Medical Center Constructed on Former Landfill in Monmouth County , New Jersey .

Active VMS Design Steps and Calculations

• Select Flow/Area Ratio (Start with 1 cfm/1000 sf)
• Multiply F/A by area to estimate sub-slab vapor extraction

flow rate needed

• If multiple soil gas collectors in area, prorate flow into

each soil gas collector based on ROI

• Estimate vacuum required at inlet of blower. Use longest

run from soil gas collector and solid piping to blower and estimate total vacuum loss.

• Add vacuum loss to vacuum required at soil gas collector

to achieve sub-slab vacuum under entire slab footprint (Use minimum vacuum of 4 Pa under slab)

• Use oversize factor of 10% for both flow and vacuum for

blower sizing

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Vacuum Loss Calcs for Selecting Vacuum at Inlet of Blower

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Blower Selection

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Methane Mitigation System – Sport Complex on Former Landfill (65,000 sf)

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Methane Mitigation System - Wholesale Warehouse on Former Landfill (148,000 sf)

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Active VMS Equipment

• Vacuum blower(s)
• Valves to throttle/balance vapor flow in manifold legs
• Blower instrumentation (vacuum gauge, flow monitoring

gauge, vapor sample port)

• Electrical conduit/wire
• Control panel
• Sensors (methane, sub-slab vacuum, sub-slab vapor

probes)

• Alarms (interior/exterior horn-strobes)

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26 Existing Buildings What is recommended method to select flow and vacuum for active VMS for existing buildings? Answer?

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Site History

• One story multi-tenant commercial building constructed in 1973
• 270’ x 200’ (54,000 sf).
• Dry cleaning business used dry cleaning machine used from 1998

to 2014.

• Other tenants included, food storage/distribution, refrigeration

service, wood door manufacturer, siding distributor, and paint/varnish vendor.

• Phase 1 Environmental Site Assessment (ESA) – September
• 2016. Dry cleaning machine considered REC.
• Phase 2 ESA - September 2016. Sub-slab soil gas sampling and

analyses.

• Limited Phase 2 ESA – March 2017. Indoor air sampling and

analysis.

Sub-Slab Vapor PCE Isopleths

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Sub-Slab Vapor TCE Isopleths

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Vacuum vs. Distance (VMPs at 10, 20, and 30 feet) Vacuum vs. Distance (VMPs at 20, 40, 50 feet)

Use of Pilot Test Data to Select Blower Flower Rate to Achieve ROI of 50 feet

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Moral of the Story

• Never underestimate the value of a VMS pilot
• test. It will give you building-specific information

that can be used to design the VMS.

• Without a design rationale, one might under

design the VMS which could prolong vapor mitigation or not extract vapors whose concentrations exceed VISLs.

• Without a design rationale, one might over

design the VMS which could increase the cost for equipment as well as increased costs for electric service.

Soil Gas Collectors for VMS in Existing Buildings

• Small areas – Suction pit(s)
• Large areas (warehouses and

factories with open area) – horizontal soil gas collectors

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Suction Pit for Existing Buildings

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Design Challenges for Larger (> 10,000 sf) Projects

• Need for many vapor collection laterals
• Long (> 75 feet) runs of vapor collection laterals require

variable slot spacing to affect uniform distribution of vacuum/flow along axis

• Complex array of sub-slab features such as grade beams

(one or two pour) and footings

• Elevator are preferential paths for vapors to impact floors

above since shafts are below the ground floor slab. Need for sealing elevator pit with vapor membrane

• Ground floor slabs at different elevations
• Select appropriate pipe chase for routing risers to roof

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Watch for contractor changes in sub-slab gravel that could affect sub-slab vacuum

• Check gravel gradation
• If off spec and too fine, collect sample and

have analyzed for pneumatic permeability using ASTM D6539

• Check for vacuum loss through sub-slab

gravel layer to maintain minimum vacuum required

• Resize blower, if needed

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Estimate dP through Sub-Slab Gravel

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Summary of Steps for Design

• For areas <10,000 sf, use flow cited in

ANSI/AARST. Perform vacuum loss calculations through soil gas collectors, piping, and fittings to

• blower. Add 10% to flow and vacuum, and select

blower.

• For areas >10,000 sf, select flow/area ratio

(cfm/1000 sf), multiply by sub-slab area to calculate flow. Perform vacuum loss calculations through soil gas collectors, piping, and fittings to

• blower. Add 10% to flow and vacuum, and select

blower.

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Recommendations

• New Construction
• Per ANSI/AARST, use horizontal sub-slab soil gas

collectors in gravel lined trenches that intersect with sub-slab gravel.

• Existing Buildings
• Conduct pilot test to estimate radius of vacuum

influence and obtain data for sizing vacuum blower/fan.

• For small area, use suction pits
• For larger areas, use horizontal sub-slab soil gas

collectors in gravel lined trenches.

• Perform cost-benefit analysis comparing suction pits vs.

horizontal soil gas collectors.

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Recommendations (continued)

• To optimize efficiency and energy usage of

system, use variable speed motors for blowers. Decrease speed of blowers to maintain minimum sub-slab vacuum required.

• More site-specific data are needed for

development of relationship for flow/area (cfm/1000 sf) ratio with increased correlation (higher confidence for use by VMS designer).

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References (abridged list)

• ANSI/AARST. 2018. Soil Gas Control Systems in New Construction of Buildings.

ANSI/AARST CC-1000 2018.

• ASHRAE. 2004. ASHRAE Standard – Ventilation for Acceptable Indoor Air Quality.

ANSI/ASHRAE Standard 62.1-2004.

• Colorado Department of Public Health and Environment. 2004. Indoor Air Guidance
• Draft. Denver , Colorado. Hazardous Materials and Waste Management Division.

www.cdphe.state.co.us/hm/indoorair.pdf.

• ITRC. 2007. Vapor Intrusion Pathway: A Practical Guideline. V1-1. Washington, D.C.

The Interstate Technology & Regulatory Council Vapor Intrusion Team. www.itrcweb.org.

• ITRC. 2007. Vapor Intrusion Pathway: Investigative Approaches for Typical

Scenarios.V1-1A. Washington, D.C. The Interstate Technology & Regulatory Council Vapor Intrusion Team. www.itrcweb.org.

• NJDEP. 2016. Vapor Intrusion Technical Guidance. NJDEP, August 2016.
• USEPA, 2015. OSWER Technical Guide for Assessing and Mitigating the Vapor

Intrusion Pathway From Subsurface Vapor Sources. Section 6.5.

• USEPA. 2008. Engineering Issue: Indoor Air Vapor Intrusion Mitigation Approaches.

EPA/600/R-08-115.

• USEPA. 1994b. Radon Prevention in the Design and Construction of Schools and

Other Large Buildings. EPA/625/R-92/016.

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Thank You! Questions?

Bob Roth, P.E. (bob.roth@terracon.com) Office: 303-454-5278

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