Practical Guide to Vapor Intrusion - ASTSWMOastswmo.org/files/policies/Tanks/2005SoilVapor... · Why Do You Care About It? (Risk Often More Perceived Than Real) • Health & Safety

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September 2005September 2005

Practical Guide to Vapor IntrusionPractical Guide to Vapor Intrusion

Dr. Blayne HartmanH&P Mobile Geochemistry

Solana Beach, CA858-793-0401

[email protected]

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Seminar OverviewSeminar OverviewPart 1 - Overview of Vapor IntrusionPart 2 - Principles of Vapor MovementPart 3 - Review of Federal & State Guidance Part 4 - Methods to Assess Vapor Intrusion Part 5 - Soil Gas Sampling Issues Part 6 – Supplemental ToolsPart 7 - MitigationPart 8 - A PM’s Guide for Assessing VI

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Evidence of residual LNAPL ?

National

Enquirer

Vapor intrusion cases making headlines around the country – even the National Enquirer!

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Newspaper articles slamming the Johnson-Ettinger model.

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Vaporgate!

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Part 1 Part 1 –– Overview of Vapor Overview of Vapor IntrusionIntrusion

• What Is It? • Why Do You Care about It?• When Should You Worry

About It?• What Sites to Worry?

This part of the seminar gives an overview of what the vi pathway is and why regulators, RPs, consultants, & project managers need to be concerned about it.

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What Is Vapor Intrusion?What Is Vapor Intrusion?

Key Assumptions:– Risk level (1 in 10,000? 100,000? 1,000,000?)– Toxicity of Compounds– Exposure Factors (time, rates, ventilation)

Vapor intrusion refers to the upward migration of contaminants in the vapor phase from groundwater, soil, or soil gas contamination sources.

Key assumptions to the risk determination are the risk level, the toxicity of the contaminant, and the exposure factors. These parameters are often much more important than model parameters such as soil porosity and pressure gradients.

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Why Do You Care About It?Why Do You Care About It?(Risk Often More Perceived Than Real)(Risk Often More Perceived Than Real)

• Health & Safety of Occupants• EPA – RCRA/CERCLA:

Draft VI Guidance Exists • EPA – OUST: Guidance in Prep• Individual States Guidance• Attorneys

In some cases, there is a real threat to occupants.

But in the majority of cases, the risk to occupants is exaggerated, hence the perception is greater than the real risk. Nevertheless, you need to worry about it because the EPA has identified it as a risk pathway, numerous states have their own guidance or policies, and citizen groups and of course, attorneys are making it an issue.

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Why Do You Care About It?Why Do You Care About It?(Risk Often More Perceived Than Real)(Risk Often More Perceived Than Real)

Who is the famous attorney in this picture and which one is she?

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Vapors WarsVapors Wars(Newest Box(Newest Box--Office Blockbuster)Office Blockbuster)

• AEHS 3/2005 & 10/2004 Conferences• AWMA 1/2006 & Indy 9/2006• EPA Seminars (2003 & 2004) • Workgroups Everywhere (ITRC,

EPA-OUST, EPA-OSWR, API) • Requests for Training from >20 States

Vapor intrusion is the hot topic right now and, in movie lore, is a box-office blockbuster. Recent conferences on the topic have all been heavily attended and numerous workgroups are currently drafting policy. Many state agencies and EPA regions are requesting training on the topic.

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ITRC Survey ResultsITRC Survey Results• 39 of 43 states say vapor intrusion is a current

concern being actively addressed • VI concerns in every program (RCRA, FUDs

CERCLA, brownfields, UST, dry-cleaning,• Most preferred methods for evaluating vapor

intrusion: shallow soil gas/subslab sampling followed by indoor air measurements

• 9 states allow for biodegradation of petroleum hydrocarbons

International Technology Regulatory Council (ITRC) conducted a survey of all States re vapor intrusion in late 2004. The results showed that nearly all States are worried about this pathway in most of their programs and that soil gas measurements are preferred over indoor air measurements.

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When to Worry About VI?When to Worry About VI?• If VOC Contamination & Structures Exist:

– Laterally within 100’ (EPA, NJ:100/30)– Vertically Within 100’ (EPA, DTSC & NJ)– NY: No Limits!!!

• Complaining Occupants• Structures With Odors, Wet Basements• Sites With Contamination & Future Use• Attorneys & Communities• Even Animals, Fruits, Vegetables

The EPA & many State guidances use the distance criteria listed above to screen sites needing to assess the pathway. At sites with exisitingcontamination but no current buildings, the pathway will need to be assessed when development is proposed. Attorneys and community activist groups can expand these criteria beyond the EPA limits. In some recent cases, concern about the safety of burrowing animals, and fruits & vegetables has been the reason to assess the vapor intrusion pathway.

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What Compounds?What Compounds?

• VOCs: – Hydrocarbons (benzene, aliphatics)– Chlorinated HCs (TCE, TCA, PCE, VC)– Methane

• Semi-VOCs: – Naphthalene & PAHs– PCBs & Pesticides

The list of compounds that are in the EPA VI guidance and some State guidances include VOCs and semi-VOCs. The semi-VOCs are particularly problematic because the risk-based screening levels (RBSL) are typically very low (eg., for PCBs, the RBSL is 10 to 100 times lower than benzene).

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What Types of Sites?What Types of Sites?• Petroleum Hydrocarbons

– Service Stations, USTs, Pipelines– Oil Furnaces (naphthalene)

• Chlorinated Hydrocarbons– Vapor Degreasers (TCE, TCA,)– Dry Cleaners (PCE, DCE)– Circuit Boards (VC, TCE, CCl4)

• Semi-Volatiles– MGP Sites (PAHs)– Electrical Power (PCBs)

Common sites that may be susceptible for vapor intrusion problems are any locations containing VOCs. Most commonly, USTs, tanks, piping, and re-fueling operations associated with petroleum hydrocarbons. In the colder climates, homes with internal oil tanks are potential candidates. Common sources for chlorinated compounds are dry cleaners (PCE), engine & parts cleaning areas with vapor degreasers (TCE, TCA) and any circuit board manufacturing facility. Semi-volatile sites include MGP sites and any sites with electrical power facilities. Contamination may exist in the soil, groundwater, or as vapor clouds. Structures overlying or near these sources may be at risk.

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Low Low RBSLsRBSLs Mean More Sites Mean More Sites ((VI Fail Levels 10,000 Times Lower)VI Fail Levels 10,000 Times Lower)• Typical Soil Gas Concentrations

– Benzene near gasoline soil: >100 ug/L– PCE under dry cleaner: >100 ug/L

• Typical Water & Soil Fail Levels:– Water: Benzene: 0.3 ug/L, PCE: 0.3 ug/L– Soil: Benzene: 5 ug/kg, PCE: 5 ug/kg

• Soil Gas Levels “Failing” DTSC VI Criteria– Subslab: Benzene: 0.0084 ug/L, PCE: 0.04 ug/L– At 5’: Benzene: 0.042 ug/L, PCE: 0.20 ug/L

For the vapor intrusion risk pathway, acceptable concentration levels are 10 to 1000 times lower than acceptable levels for soil and water. So essentially, this increases the number of sites that have to be evaluated.

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Part 2 Part 2 –– Principles of Vapor Principles of Vapor MovementMovement

• Fick’s Law • Directions & Rates of Movement• Contaminant Partitioning• Attenuation (alpha) Factors

There are a number of basic principles that need to be understood in order to understand and effectively manage the vapor intrusion pathway. Some of these principles you may not have had in school or have never really used them, so you are rusty. We will be using them throughout the rest of this seminar so we will review them now.

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by bus? freeway? elevator?

How do Vapors Move?How do Vapors Move?

So how do vapors move in the vadose zone? There are no buses, or freeways, or elevators moving vapors around. There’s no wind. Vapors do not exhaust themselves like Old Faithful geyser. The principle mechanism is by molecular diffusion. In molecular diffusion, the vapor itself is stagnant and the contaminants move through the stagnant vapor phase. This concept is crucial to understand because it arises in all facets of the vapor intrusion process including sampling techniques and data interpretation.

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Which Way do Things Move?Which Way do Things Move?Movement (Flux) = K d?/dx

where: K is a proportionality constantd?/dx is a gradient

Property Equation ConstantMomentum: Flux = K dH/dx hydraulic condHeat (Poisson’s): Flux = Φ dT/dx thermal condMass (Fick’s): Flux = D dC/dx diffusivity

Momentum, Heat, Mass ALL Move from High to Low

The fundamental equation describing momentum, heat, and mass movement is the same. Movement or flux is equal to a proportionality constant times a gradient. For momentum (groundwater or balls), the equation is known as Darcy’s Law. For heat, the equation is known as Poisson’s Law. For mass, it is known as Fick’s Law. The proportionality constant is known as the diffusivity or diffusion coefficient (D).

Balls, heat, and mass all move the same way: downhill, hot to cold, high to low concentration. As you will see, people often tend to forget this fundamental concept and make incorrect decisions.

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Common Vapor ProfilesCommon Vapor Profiles

FluxDep

th

Concentration

FluxDep

thConcentration

FluxDep

th

Concentration

Flux

Surface Source

Deep Source

Surface and Deep Sources

Knowledge of Fick’s Law enables one to determine the direction of soil gas movement, and hence the direction of the source, from vertical gradients of the soil gas. Three types of common profiles are shown for sources at different locations in the vadose zone. Note that the flux is down the concentration gradient even when the flux is going “uphill” with respect to depth in the vadose zone.

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How Fast do Things Move?How Fast do Things Move?Distance = (2*De*t) 1/2

where: De is effective diffusivity, t is time

Vapors through the Vadose Zone:De = 0.01 cm2/secDistance = (2*0.01*31,000,000) = 800 cm/yr

Vapors through Liquid (into/out of GW):De = 0.000001 cm2/secDistance = (2*0.000001*31,000,000) = 8 cm/yr

Transport in Vadose Zone 100 times faster than in GW

An estimate of how fast contaminants move in the vadose zone can be obtained by a simple calculation based upon the diffusivity. Contaminants move through the vadose zone by molecular diffusion at a rate of 800 cm/yr, which is 8 m/yr, or approx. 25 ft/yr, or 1 inch a day.

Contaminants move through liquid (into or out of) 100 times slower because the diffusion coefficient for liquids is 10,000 times lower. Thus, volatilization of contaminants out of an undisturbed water interface (e.g., groundwater) is glacially slow and typically orders of magnitude below equilibrium. This is a crucial concept when using groundwater data to calculate soil gas concentrations. Calculated soil gas values will always be over estimated.

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Contaminant PartitioningContaminant PartitioningGroundwater to Soil Gas (Henry’s Constant):

H = Csg/Cw, so, Csg = Cw * H

Example: Hbenzene = 0.25 (dimensionless)For GW Conc = 10 ug/LCsg = 10 * 0.25 = 2.5 ug/L

Assumes Equilibrium. Very Rarely Achieved(no mixers or blenders in the subsurface)

Partitioning refers to the distribution of molecules between different phases. Partition coefficients are determined empirically by laboratory measurement. The partition coefficient for water to air partitioning (e.g., groundwater to soil gas) is called the Henry’s Constant or Henry’s Law. It simply is a ratio of the concentration in the air to the concentration in the water. It is simple to calculate the soil gas concentration from groundwater data or the reverse from the dimensionless Henry’s constant.

Henry’s constants are based upon equilibrium being reached. The container was vigorously mixed. Mixers do not exist in the subsurface so equilibrium not reached and actual soil gas concentrations are far below calculated ones.

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Attenuation (alpha) FactorsAttenuation (alpha) Factorsαsg = Cindoor/Csgαgw = Cindoor/(Cgw*H)

• Lower alpha means higher attenuation • Current VI guidances:

– EPA αsg = 0.002 for 5’, 0.1 for sub-slab– NH αsg = 0.002 for 5’, 0.01 for sub-slab– NY State Data Shows αsg < 0.01 – Hydrocarbon αsg likely <0.0001

A common term in the vapor intrusion “community” is the attenuation factor also called the alpha factor. The soil gas alpha factor is a ratio of the indoor air concentration to the soil gas concentration. The groundwater alpha factor is a ratio of the indoor air concentration to the groundwater concentration times its Henry’s constant. Since indoor air values are lower than subsurface values, alpha factors tend to be less than 1, hence lower numbers mean greater attenuation. Thus, inverse alpha factors are often easier to understand.

The EPA draft guidance uses very stringent alpha factors, determined empirically from a limited data base. More recent and larger data bases (IBM Endicott) are showing that the alphas should be orders of magnitude lower, especially for petroleum hydrocarbons.

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In the draft VI guidance, alpha factors can are summarized vs. depth in Figure 3. As you can see in Figure 3a, the highest soil gas alpha is 0.002 at 5 feet below the structure. The inverse is 500.

For groundwater, Figure 3b shows the highest alpha is ~.001. The inverse is 1000.

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Max 1/Alpha vs. Subslab

0.01

0.1

1

10

100

1000

10000

0.1 1 10 100 1000 10000 100000 1000000

Subslab Concentration

1/A

lpha PCE

TCETCA

The best dataset to date comparing indoor air to soil gas measured at the same time is from the State of NY. It shows an inverse alpha factor of 100 to 1000 for sub-slab soil gas data. The current EPA draft guidance only allows an alpha of 10. Hence, the draft guidance is 10 to 100 times more restrictive than this database suggests.

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Using Alpha Factors to Using Alpha Factors to Calculate Screening LevelsCalculate Screening Levels

For Soil Gas:

Csg = Cindoor/αsgFor Groundwater:

Cgw = Cindoor/(H*αgw)

Example: Cin benzene = 0.3 ug/m3Csg (5’) = 0.3/0.002 = 150 ug/m3Cgw = 0.3/(0.20* 0.0005) = 30 ug/L

By using alpha factors, one can calculate target levels or “fail levels” for soil gas and groundwater by knowledge of the acceptable indoor air concentration.

By “fail level”, I mean the concentration of a contaminant in the indoor air, soil gas, groundwater, or soil that fails the upward vapor risk calculation. It also is referred to as risk based screening level (RBSL), target level, ESL, PRG, acceptable value, look-up value, screening level, etc. This term will be used quite frequently throughout the remainder of this seminar.

Many consultants are not familiar with using alphas and calculate incorrect target values.

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Acceptable Soil Gas LevelsAcceptable Soil Gas Levels(Benzene 1e(Benzene 1e--6 Risk)6 Risk)

1927700.0013CT425000.002CA-DTSC

3.1100.1EPA Q61555000.002EPA Q53.1100.1EPA Q4602000.05NJ (Subslab)

1505000.002NHFail Level (ug/m3)1/AlphaAlphaState

A summary of the alpha factor and corresponding acceptable soil gas levels for various States and the EPA draft guidance shows large variation and illustrates the main point: the levels are variable from State to State.

Notice the difference in fail values for the different EPA VI guidance questions.

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Part 3 Part 3 –– Review of Federal & Review of Federal & State GuidanceState Guidance

• The EPA VI Guidance &Training• Summary of State Policies

In this part or the seminar, we will briefly review the EPA draft guidance and some of the current policies among the States. If you are a consultant or RP, you need to know which agency has jurisdiction and what their vapor intrusion policy is in order to know what approaches are allowed and what the acceptable levels are.

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OSWER Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway from Groundwater and Soils

(Subsurface Vapor Intrusion Guidance)Federal Register Notice - November 29, 2002Fact Sheet: Evaluating the Vapor Intrusion into Indoor AirAdobe PDF File [17 KB]Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway from Groundwater and Soils (Subsurface Vapor Intrusion Guidance) (Complete Document)Adobe PDF File [3019 KB]

•Draft GuidanceAdobe PDF File [516 KB] •TablesAdobe PDF File [353 KB]

•Appendices A-CAdobe PDF File [972 KB]

•Appendices D-FAdobe PDF File [722 KB]

•Appendices G-IAdobe PDF File [475 KB]

E-Docket is an on-line system that allows viewers to search the Agency's major public dockets on-line, view the index listing of the contents for the dockets included in the system, and access those materials that are available on-line. You may also submit comments on-line while this docket is open for public comment.

This is the web page for the EPA draft vapor intrusion guidance. From here, the guidance and other supporting materials can be downloaded. The document is a lengthy and difficult read. It is available at: (http://www.clu-in.org/conf/tio/vapor/resource.cfm)

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I. Today's Presentation and References

Download Today's Seminar For Future Reference

Two-Day, In-Person Seminars on Intrusion of Vapors into Indoor Air

Overview of the Science behind USEPA’s Guidance for the Vapor Intrusion to Indoor Air Pathway (presentation by Henry J. Schuver at Jan. 14, 2003 Indoor Air Vapor Intrusion Seminar in Dallas, TX)

II. Draft Guidance and Related LinksDraft Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway from Groundwater and Soils (Subsurface Vapor Intrusion Guidance)

Federal Register Notice - November 29, 2002

Submit comments via EDOCKET

Fact Sheet: Evaluating the Vapor Intrusion into Indoor Air

Links to Additional Resources

This is the web page for the EPA vapor intrusion seminars and other vapor intrusion links, including the draft guidance. The one-day seminar (top bullet) gives a good overview of the EPA’s approach to the pathway and is an easier read that the guidance document itself. The website is: (http://www.clu-in.org/conf/tio/vapor/resource.cfm)

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This page is from one of the presentations in the one-day seminar. Notice that it clearly states that the guidance is not regulation and that other approaches are allowed. Yet, attorneys are already using the guidance in the courtroom as law and because the EPA name is on it, judges & juries seem inclined to enforce it as law. Hence, it is important that the draft guidance be understood even if not followed.

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The current EPA draft VI guidance consists of 3 tiers, consisting of 6 questions. Tier 1 is essentially a screening survey asking basic questions such as whether volatile compound contamination exists and whether buildings exist.

Tier 2 consists of 2 questions/steps: Q4 & Q5. Question 4 is so restrictive (i.e., very low fail levels) that just about every site fails, similar to a vortex or hopper. Question 5 allows more sampling options, is not as conservative, and may be the best tier/question to work within.

Tier 3, question 6, allows for only two investigatory methods, indoor air or sub-slab soil gas, and has very restrictive (i.e., very low fail levels) criteria. Once at this level, it is extremely hard to get out and requires expensive and repeated sampling.

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This slide shows just how cumbersome and confusing the flowcharts for questions 4 through 6 are. The multiple steps in Q5 & Q6 can create confusion, causing one to go the wrong way or to go in circles.

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Overview of Proposed ChangesOverview of Proposed ChangesEPA EPA DraftDraft VI GuidanceVI Guidance

• Tier 1: Primary Screening– Q1: VOCs present?– Q2: Near buildings?– Q3: Immediate concern?

• Tier 2: Secondary Screening– Q4: Generic screening

– Q5: Semi-site specific screening (alphas from charts & tables)

• Tier 3: Site-Specific Pathway Assessment

– Q6: Indoor air (and/or subslab)

• Tier 1: Preliminary Screening• Q1: VOCs present?

– Q2: Near buildings?– Q3: Immediate concern?

• Tier 2: Generic Screening– Q4: Generic residential and non-residential screening

levels

• Tier 3: External Site-Specific Screening– Q5: More site-specific parameters (alpha from

spreadsheet)

• Tier 4: Internal Site-Specific Assessment– Q6: Indoor air or subslab or both– Multiple lines of evidence

Nov. 2002 Fall 2005

The EPA has proposed changes to the draft guidance that are supposed to go into effect in the fall of 2005. The same 6 questions exist, but they have been divided into 4 tiers instead of 3 tiers (Q4, Q5, & Q6 all get their own tier). This was primarily done to allow more flexibility under Q5. Q5 now allows more sampling options external to the structure and more freedom in using the J-E model before having to go inside. This will allow the more versed consultants to be creative.

Slide courtesy of Helen Dawson, USEPA.

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Preliminary Screening

Generic Screening

External Site-

Specific Screening

Internal Site-

Specific Assessment

OSWER VAPOR INTRUSION GUIDANCETable 1. Chemicals Sufficiently Toxic to Pose an Inhalation Risk via Vapor Intrusion

Target Indo or Air Co nc . Bas is

Saturated Vapor Conc .

Suffic iently To xic?

Check (x) Cia, target C = cance r Cv Cv.Cia,target?If Pres ent CASN Chemical (ug/m3) NC = noncance r (ug/m3) (yes/No)

x 83329 Acenaphthene 2.10E+02 NC 2.07E+04 Nox 75070 Acetaldehyde 1.11E+00 C 2.14E+09 Yesx 67641 Acetone 3.50E+02 NC 7.19E+08 Yesx 75058 Acetonitrile 6.00E+01 NC 2.01E+08 Yesx 156592 cis-1,2-Dichloroethylene 3.50E+01 NC 1.06E+09 Yesx 75343 1,1-Dichloroethane 5.00E+02 NC 1.21E+09 Yesx 107062 1,2-Dichloroethane 9.36E-02 C 4.20E+08 Yesx 75354 1,1-Dichloroethylene 2.00E+02 NC 3.13E+09 Yesx 127184 Tetrachloroethylene 4.12E-01 C 1.66E+08 Yesx 156605 trans-1,2-Dichloroethylene 7.00E+01 NC 1.74E+09 Yesx 79005 1,1,2-Trichloroethane 1.52E-01 C 1.67E+08 Yesx 71556 1,1,1-Trichloroethane 2.20E+03 NC 8.88E+08 Yesx 79016 Trichloroethylene 1.11E+00 C 5.19E+08 Yesx 75014 Vinyl chloride (chloroethene) 2.77E-01 C 1.00E+10 Yes

View Selected Chemicals

< 0.1 TL > 0.01 TL to < TL > TL

< 0.1 TL/AF No Action No Action or Investigate IA Sources Investigate IA Sources No Action

> 0.1 TL/AF to < TL/AF Resample SSor No Action

Investigate IA Sourcesor No Action

Investigate IA Sources and Monitor IA

Resample SS or Sample IA

> TL/AF Monitor IA or Mitigate

Monitor IA or Mitigate Mitigate Monitor IA

or Mitigate

No Action** Resample IA orSample SS

Investigate IA Sources or Mitigate Sample IA and/or SSNo Subslab Data

No Indoor Air Data

Sub-

Slab

Con

c.

Indoor Air Conc.

In-Building Concentrations

By Helen Dawson (minor mod. 8/3/05)

Considering: NJ

- Min. 1 boring / house

- 75% soil “ as fine as”

- Near-slab 3 x 10 ft

- I.C. if not Unrestricted

- Delineate for future

This summarizes the series of steps in the new guidance and the way the data from each step will be interpreted.

Slide courtesy of Henry Schuver of EPA

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Potential Changes to Q4?Potential Changes to Q4?(Moderate)(Moderate)

• Eliminate some precluding factors?• Recommend less conservative attenuation

factors for some media?• Simplify decision process?• Provide spreadsheet table for residential and

commercial settings?

The changes in Q4 are moderate compared to the earlier version. The biggest change, still unannounced, is what the sub-slab alpha factor will be dropped to. Currently it is 0.1 which is very restrictive. Most states are using 0.01 to 0.02. The EPA is expected to pick one of these two.


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Potential Changes to Q5?Potential Changes to Q5?(Extensive)(Extensive)

• External site-specific screening approach?• Provide “constrained” spreadsheet to estimate

attenuation factors (rather than graphs)?• Allow more site specific parameters provided

screening is performed with near-building data?– More soil types (with default properties)? – Soil layers?– Non-residential building and exposure parameters?

• Simplify decision process?

Q5 is being changed extensively. In the current guidance, alpha factors are taken from Figure 3 and there is no flexibility to modify them except based upon depth and soil type. In the proposed version, Figure 3 is replaced by a new EPA spreadsheet that allows many more parameters to be varied. This is good news for consultants & RPs, but more of a task for regulators.


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Potential Changes to Q6?Potential Changes to Q6?(Moderate)(Moderate)

• Parallel internal site-specific investigation?

– In-building subslab or indoor air or both– Decision matrix if both subslab and indoor air

• Provide guidance on how to evaluate indoor air data?

– Using multiple lines of evidence

The proposed changes to Q6 are moderate. As in the current guidance, the emphasis will be on sub-slab sampling and indoor air as investigatory approaches. It is still unclear if other approaches will be allowed.


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The Draft VI Guidance “Dead Zone”The Draft VI Guidance “Dead Zone”

• No Guidance for samples from sub-slab to 5’• Most likely zone for Bio & Reaeration

Subslab

DEAD ZONE 5’

The EPA draft VI guidance allows for soil gas sampling at or greater than 5 feet below the structure (Q5) or immediately below the structure (Q6). A “dead zone” in the guidance exists between these sampling depths (from 5 feet to 0.5 feet). Yet, it is within this dead zone that many processes are active influencing soil gas concentrations, such as bioattenuation, reaeration from barometric pumping, and infiltration of precipitation. Many States are allowing soil gas data to be collected within this zone so that the influence of these processes can be ascertained. It is not clear if the revised EPA guidance will allow it also.

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Status of State VI GuidanceStatus of State VI Guidance

• Written:– MA, CT, PA, SC, LA, CO, WI, OR,

ID, NJ, IN, MI, CA, NY, NH, MN • Working on a Draft:

– FL, AZ, UT, WA, ITRC• Pondering:

– ITRC survey implies at least 40 states

Many States have developed or are developing their own VI guidance because they don’t agree with EPA’s or they find it too confusing. Some States currently have written guidance, others have drafts, and others are pondering. Some States are still trying to figure it all out and have asked for training on the subject. ITRC is currently writing a vapor intrusion document for States to adopt. It is due out by the end of 2005.

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Data Preference Data Preference • Indoor Air: MA, CO • Soil Gas: CA, SC, CT, NY, LA, NY, NJ,

NH, IN, WA• GW: Used to Filter Sites

(but once in, can’t use to get out)

• Use of Soil Data: Not Many (PA?)• Flux Chambers – Unfamiliar Audience

At the time of this writing, this is a summary of the preferences of various States. Most prefer active soil gas. A few prefer indoor air measurements. Groundwater data can be used to filter which sites need to be evaluated for the VI pathway, but most States won’t allow you to use additional GW data to get out once you are in. Few States are allowing soil matrix data to be used. Flux chambers are not readily accepted because of unfamiliarity.

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Part 4 Part 4 –– Methods to Assess VIMethods to Assess VI

• Indoor Air Sampling • Predictive Modeling• Measure Flux Directly• Soil Gas Sampling

In this part of the seminar, we will discuss the primary techniques used to assess the vapor intrusion pathway, including the pros & cons of each.

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Indoor Air MeasurementIndoor Air Measurement• Pros:

– Actual Indoor Concentration• Cons:

– Where From?• Inside sources (smoke, cleaners)• Outside sources (exhaust, cleaners)• People activities

– Laborious Protocols– Snapshot, limited data points – Expensive!!

Measuring indoor air might seem to be the most direct and simplest approach, but it has its share of problems. The biggest problem is background sources of contaminants. Many commonly used household products contain some of the target compounds of concern. For example, benzene from gasoline, PCE from dry cleaned clothes, TCA from degreasing cleaners. In addition, the protocols are laborious, intrusive, offer little control, and are expensive. For these reasons, the EPA and many States shy away from this method. However, this method may still be the method of choice if the contaminant of concern is not one commonly found in household products (e.g., 1,1 DCE).

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VOCs with Highest Indoor Median VOCs with Highest Indoor Median ConcentrationsConcentrations

From: Girman, J. Air Toxics Exposure in Indoor Environments, EPA Workshop on Air Toxics Exposure Assessment, 2002. http://www.epa.gov/osp/regions/airtox.htm

From USEPA BASE study

Minimum, maximum, 5, 25, 50, 75, 95th percentiles

This plot shows the most common VOCs detected in indoor air and their concentration levels.

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Modeling Modeling • Pros:

– Can Use GW, Soil (?), Soil Gas Data– Relatively Easy

• Cons:– Which Version to Use? – No Validation – Erroneous Conclusions– Often Too Restrictive– Can Tweek to Your Pleasure

The use of models to calculate an indoor air concentration, and in turn a health risk, is commonplace. Existing models use groundwater, soil, or soil gas data and are relatively easy to use. In general, if default parameters are used, they tend to overcalculate the risk for most situations.

Several versions currently exist with different default values for various parameters, so one must be careful to know what version they are using. Regulators must be careful with using modeling results for a number of reasons, one being that it is easy to try and change the values of some of the variables to get a passing value. As a result, some States will let you use the models as a screening tool, but not give complete closure.

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Johnson and Ettinger ModelJohnson and Ettinger Model

−

+

+

==

1expexp

exp

Beffcrack

cracksoil

Tsoil

BeffT

TB

BeffT

Beffcrack

cracksoil

Beffcrack

cracksoil

TB

BeffT

vs

indoor

ADLQ

LQAD

LQAD

ADLQ

ADLQ

LQAD

CC

ηη

ηα

DTeff = “overall” effective diffusion coefficient [cm2/s]

Deffcrack = effective diffusion coefficient through cracks in foundation [cm2/s]

Cvs = vapor concentration at the source [g/cm3]Cindoor = indoor air concentration [g/cm3]LT = distance from source to basement [cm]AB = cross-sectional area of foundation available for vapor flux [cm2]Qsoil = volumetric flow rate of soil gas into the building [cm3/s]Lcrack = thickness of the foundation [cm]Acrack = area of cracks or openings through which vapors enter building [cm2]QB = building ventilation rate [m3/s]η = the “crack factor”: Acrack/AB so that 0 =< η =< 1

While this equation is in terms of an alpha factor, in reality, what the the J-E model is trying to do is estimate a value for the effective diffusivity, the proportionality constant in Fick’s Law that we talked about previously. There is no easy way to do this because too many factors influence this term. As you can see, there are many variables, some of which can not be directly measured or easily measured.

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Which One & Version to Use? Which One & Version to Use? • Johnson-Ettinger Most Common

– GW, soil, soil gas spreadsheets– Screen & advanced versions– 2003 and 2001 versions differ in some defaults– Hard to compare defaults vs actual values used

• New Version Being Written (2005) – One spreadsheet for all matrices– Summarizes defaults vs actual values used

• Others: Jury

The most common model currently being used in the Johnson & Ettinger (J-E) model. The EPA has written a number of spreadsheets that are based upon the model parameters and allow the use of groundwater, or soil, or soil gas data. The spreadsheets were updated in 2003 and are available from the EPA website referenced previously. Many agencies have constrained many of the model parameters so they can’t be “abused”.

The EPA is currently rewriting the spreadsheets that will be very different in style from the existing ones. One spreadsheet will handle all matrices (groundwater & soil gas, but not soil phase data) rather than separate ones. The spreadsheet will show both default values and the user inputted values to facilitate recognition of what values were changed and by how much.

Other models do exist, but the J-E model is by far the primary one being used around the country.

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This on-line calculator is a handy way to get a feel for “fail levels” without getting into the J-E spreadsheets. It uses EPA Federal default parameters for toxicity info, ventilation rates, etc. It can be found at http://www.epa.gov/athens/learn2model/index.html.

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How Well Does JHow Well Does J--E Predict?E Predict?(From GW & Soil Data)(From GW & Soil Data)

• Hydrocarbons– Calculated SG value too high by 10-1000x – No bioattenuation (10 to 1000x reduction)OVER PREDICTS IN ALMOST ALL CASES

• Chlorinated Solvents – Deep Source– Calculated SG value too high by 10-1000x OVER PREDICTS IN MOST CASES

• Chlorinated Solvents – Surface Source– Calculated SG value too low by 10-1000x UNDER PREDICTS IN MOST CASES

If you are going to use the models/spreadsheets, you need to be aware of the limitations. If the groundwater or soil spreadsheets are being used, the spreadsheet calculates the soil gas concentration assuming equilibrium partitioning. This is likely to give a soil gas concentration orders of magnitude higher than actual values. Also, for hydrocarbons, there is no allowance for bioattenuation. So, the result is that the spreadsheet is likely to over predict risk for hydrocarbons in almost all cases, unless right at the surface. For chlorinated compounds, the spreadsheet is also likely to over predict, but not as much since bioattenuation is not as prevalent. But, if surface sources exist, then vapor clouds might exist and the actual soil gas concentration might be higher than calculated, so in turn, the risk will be underestimated.

Because these models tend to over-predict the risk, especially from groundwater and soil data, you should be careful to verify the predicted risk if they show you are failing.

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Direct Flux Measurement Direct Flux Measurement (Flux Chambers)(Flux Chambers)

• Pros:– Direct Measurement of Intrusion

• Cons:– Proper Location?– Protocols Debated– How to Use Data?– Unsophisticated Audience– Regulatory Acceptance Limited

Surface flux chambers are attractive because they give a direct measurement of the flux into the structure or out of the soil. This eliminates the need to know the effective diffusivity and the uncertainty inherent in the models. The biggest drawback with chambers is whether they can be placed in the proper locations in an existing structure. Also, few regulators, consultants, or vendors have used them, so they are unfamiliar of the protocols to use and how to interpret the data. As a result, regulatory acceptance is limited. In slab-on-grade structures or undeveloped lots or crawl spaces, surface flux chambers may be the best method to use. Chambers will also prove useful to support the presence of bioattenuation.

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Surface Flux Chamber IssuesSurface Flux Chamber Issues

• Two Basic Types:– Static Chamber: Non-continuous (batch) sample

taken after a period of time. No sweep gas required. Allows on-site (real time) analysis.

– Dynamic Chamber: Sample taken over incubation period. Sweep gas required. On-site analysis more difficult.

Both methods have pros and cons.

Two types of chambers exist: Static and dynamic. Dynamic are best suited for high fluxes, such as from landfills, compost piles, etc. Static are best suited for lower fluxes that are more likely at sites with low concentrations of contamination which are likely the majority of vapor intrusion sites.

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Flux Chamber Issues Cont.Flux Chamber Issues Cont.

Sample Collection Issues (San Diego County Protocols):

• Minimum of three (3) chambers to ensure representativeness• Minimum incubation time of 8 hours; 24 hours preferred.• Multiple samples over incubation period preferred to ensure

representativeness and data reliability.• Sample should be taken immediately after purge at start of

incubation.

These are some of the required criteria for flux chamber surveys as currently outlined in the San Diego County Site Assessment & Mitigation manual.

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Flux Chamber Issues Cont.Flux Chamber Issues Cont.Analysis Issues

• Need to use an analytical method that can reach required PRGs. Chamber PRG = (Room PRG* Room height/chamber height)

• Method 8021B & 8260B SIM preferred over TO methods because they can reach required detection levels with small volume (<100 cc) samples. Both can be done on-site.

• If Summas and off-site TO methods used, each Summa must be clean tested before use and Summa sample rate matched to sweep gas inflow.

Additional considerations for flux chamber surveys.

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Soil Gas MeasurementSoil Gas Measurement• Pros:

– Representative of Subsurface Processes– Higher Fail Levels– Relatively Inexpensive – Can Give Real-time Results

• Cons:– Mass Transfer Coefficient Unknown– Overly Restrictive Default Criteria– Protocols still debated

CURRENTLY MOST PREFERRED APPROACH

Measurement of soil gas is by far the most preferred approach around the country. Actual soil gas data are reflective of subsurface properties, are less expensive than indoor air measurements, and allow real-time results. The fail levels are also higher so there is less chance to be chasing blanks.

There are some drawbacks, including the lack of knowledge of the effective diffusivity, very restrictive fail levels for sub-slab data, and debate over how & where to collect samples.

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Part 5 Part 5 –– Soil Gas Sampling Soil Gas Sampling Methods & IssuesMethods & Issues

• Soil Gas Methods• Sampling & Analysis Issues• Where to Collect Samples• Chlorinated-HCs vs. Hydrocarbons• Other Tools/Approaches

This part of the training will focus specifically on soil gas sampling since it is currently the most preferred approach around the country and, if you have a vapor intrusion problem, the chances are high you will be required to oversee or work with soil gas data.

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Which Soil Gas Method?Which Soil Gas Method?

• Active?• Passive? (limited use)• Flux Chambers? (limited use)

Active method most often employed for VI

There are three types of soil gas methods. Active refers to actively withdrawing vapor out of the ground. It gives quantitative values. Passive refers to burying an adsorbent in the ground and letting the vapors passively contact and adsorb onto the collector. It does not give quantitative data and hence can not be used for risk applications, except for screening. Surface flux chambers were discussed previously.

The active method is the one most applicable to risk assessments.

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VI Requires Much Lower DLsVI Requires Much Lower DLs• Typical Soil Gas Concentrations

– MTBE & Benzene near gasoline soil: >100 ug/L– PCE under dry cleaner: >100 ug/L

• Soil Gas Levels a Threat to GW: – MTBE: >10 ug/L– BTEX/PCE: >100 ug/L

• Soil Gas Levels “Failing” VI Criteria– Subslab: Benzene: 0.03 ug/L, PCE: 0.07 ug/L– At 5’: Benzene: 0.15 ug/L, PCE: 0.35 ug/L

The biggest difference between sampling soil gas for site assessments and for vapor intrusion is that we are measuring at concentration levels 1,000 to 10,000 times lower. So, the protocols require much greater care. At such low levels, the chances for false positives from equipment blanks are much greater.

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Probe Installation MethodsProbe Installation Methods• Driven Probe/Rod Methods

– Hand Equipment, Direct-Push– Collect sample while probe in ground

• Vapor Mini-Wells/Implants– Inexpensive & easy to install/remove– Allow repeated sampling– Near surface & deep (down auger flights)– Can “nest” in same bore hole

There are two common ways to collect active soil gas samples: collection through a probe or rod driven into the ground or collection through a vapor well buried into the ground. Both methods give reliable data.

The vapor wells consist of small diameter, inert tubing and offer advantages when vertical profiles are desired or when repeated sampling events are likely. Multiple tubes can be “nested” in the same borehole.

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Soil Gas Key Sampling IssuesSoil Gas Key Sampling Issues(VI Requires More Care & Experience)(VI Requires More Care & Experience)

• How Much to Collect?– Greater the volume, greater the uncertainty

• Containers– Tedlars, Summas, Syringes … Don’t Chill!

• Temporal Variation/Stability– Closer to surface or bldg, greater the effect

• Tracer/Leak Compound– Crucial for sub-slab & large sample volumes

As mentioned previously, lower detection levels requires more experience to ensure valid results. Key sampling considerations include sample volume, container type, repeatability of soil gas data, and leak testing to ensure valid samples are collected.

Smaller volumes create less headaches and minimize the chances for leakage from the surface and desorption off soil. Sample containers must be inert, tested clean, and handled properly (no cooling or heat). If samples are collected closer to the surface or building(<3 feet), temporal effects must be considered. Tracer/leak compounds are required to ensure sample integrity because small leaks can create significant effects at such low concentrations.

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Soil Gas Analysis IssuesSoil Gas Analysis Issues(TO(TO--14/15 or 8260 or 8021)14/15 or 8260 or 8021)

• Both Methods Give Reliable Results• Some Regulators Have Preference• Detection Level Discriminator:

– TO Methods: 1 to 10 ug/m3

– 8260 SIM: >10 ug/m3

– 8260 >100 ug/m3

• If Cumulative Risk:– Benzene & naphthalene fail level 20 ug/m3– PCE & TCE: 100 ug/m3 but <50 if VC

A variety of analytical methods are available to measure soil gas samples, but no federal guidance document exists specifying any one. Methods 8021 and 8260 are soil & water methods but give accurate results for soil gas samples at detection levels above 10 ug/m3. The toxic organic methods (TO) are designed for ambient air samples, so they give accurate results for soil gas samples at much lower detection levels. The TO methods require extensive hardware and are far more expensive.

Neither sets of methods were designed for soil gas samples. Both sets have pros and cons. The first criteria for selection should be which method(s) reach the required detection limits.

DTSC allows all of these methods, but has shied away from 8021 over the years.

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OnOn--Site 8021 Analysis vs.Site 8021 Analysis vs.OffOff--site TOsite TO--15 Analysis 15 Analysis

Summa Canister - TO-15

1 10 100 1000

Tedl

ar B

ag -

On-

Site

GC

1

10

100

1000

This slide shows a comparison of on-site analysis of TCE by 8021 out of a tedlar vs. off-site analysis by TO-15 out of a Summa canister. Correlation is excellent down to values as low as 2 ug/m3.

Slide courtesy of Dr. Dominic DiGuilio, EPA-ORD

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Concentration RangeConcentration Range(( 20 cc sample 20 cc sample volvol injected)injected)

10-2 10-1 1 10 102 103 104 105

ug/m3

TO-15/14A SIM

8260 SCAN

TO-15/14A SCAN

8260 SIM

Normal Range

Extended by dilution

This slide shows the concentration ranges for methods 8260 and TO-15. Note that the TO methods can only reach a maximum concentration of approximately 1000 ug/m3 (1 ug/L) before the sample needs to be diluted. Soil gas concentrations can exceed 1000 ug/L, requiring large dilutions of the sample if analyzed by TO methods. The larger the dilution factor the greater the possibility that low-level blanks get reported as false positives. The 8260 method concentration range is a better match for most soil gas samples, especially at sites where previous data do not exist.

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SG Levels vs. Risk LevelsSG Levels vs. Risk Levels• Typical Soil Gas Concentrations

– Benzene near gasoline soil: >100,000 ug/m3– TPH vapor: >1,000,000 ug/m3– PCE under dry cleaner: >100,000 ug/m3

• Soil Gas Levels “Failing” VI Criteria– Subslab: Benzene: 3 ug/m3, PCE: 4 ug/m3– At 5’: Benzene: 150 ug/m3, PCE: 200 ug/m3

TO Method & Hardware Not Designed For This

Typical soil gas concentrations at leaky UST, dry cleaner, and industrial solvent sites are in the 100,000s to 1,000,000 of ug/m3. But, for 1 in 1 million risk , the risk-based screening levels are less than 10 to 100 ug/m3. This large concentration range creates a number of analytical headaches.

The TO-methods and hardware (canisters, flow chokes) are not designed for such high concentrations. System carryover, large dilutions, and contaminated canisters increase the potential for false positives, raises reporting levels, and gives air labs logistical fits which limits the utility of these methods.

The 8260 and 8021 methods can’t get lower than 10 to 100 ug/m3 so they may not reach required DLs.

In practice, a combination of these methods is the best approach. If expected values are high, then the 8021 & 8260 are advantageous to use than the ultra-sensitive TO methods. If expected values are low, then the TO methods offer advantages.

The primary criteria for selection should be which method(s) reach the required detection limits and the expected concentration range.

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OnOn--Site/OffSite/Off--Site AnalysisSite Analysis(8021(8021--82608260--TO15 Combo)TO15 Combo)

• Allows Purge/Sample Volume Test• Allows Measure of Leak Compound

– Ensures valid samples• Real-Time Results to Guide Program• Validates Off-site Data

– Minimizes false positives• Allows Optimal Method to Be Used

– 8260 if > 1 ug/L, TO-15 if < 1 ug/L

On-site data are extremely useful to ensure that the samples do not have tracer/leak levels above acceptable levels, allow the DTSC required purge volume test, provide real-time data for decision making, and to validate detections seen in the off-site data. If measured values are high, then the on-site methods (8021, 8260) are more appropriate to use than the ultra-sensitive TO methods. If on-site values are low or below detection, then the samples can be measured off-site by the TO methods.

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Beware of the HardwareBeware of the Hardware

The tackle box on the left shows the required hardware to collect soil gas samples in Summas.

The syringe to the right is the only collection device required for on-site analysis of soil gas.

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Beware of the HardwareBeware of the Hardware

The system on the left shows a sampling train to collect soil gas samples in mini-cans or Summas.

The syringe to the right is the only collection device required for on-site analysis of soil gas.

The more hardware, the greater the potential for sampling problems such as blanks and loose fittings.

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Where to Sample SpatiallyWhere to Sample Spatially• Source Not Immediately Below

– Collect on side towards source– Collect on other sides of structure– Preferential pathways at edges & conduits

• Source Below– Collect around structure before sub-slab– Get decent coverage

• Representativeness– Need enough points.– What algorithm?

There is currently much debate on where to collect samples and no existing protocols or guidance. So, common sense comes into play. If source not directly below, collect samples between the structure and the source at a depth that is deep enough to give repeatable results. Collect in any known preferential pathways, such as utility lines. If the source is below, collect around the structure before going inside the structure.

Spatial averaging allows a better representation of what’s below the structure. One approach is to collect samples on all sides of the house and use an averaging method to get a value under the structure footprint.

If real-time data exist, add additional points depending upon the results. If not, collect extra samples spatially and analyze if necessary.

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Where to Sample VerticallyWhere to Sample Vertically• Initially Deep Enough for “Stable Data”

– If contamination not deeper, how can it be shallower?

– For HCs, 3’ to 5’ below structure– For Cl-HCs, at GW or mid-way to GW

• Work Within the VI “Dead Zone”– Zone of Most Bioattenuation– Zone of Surface Reaeration

• Vertical Profiles– Determine direction of source– Can aid in documenting bioattenuation

The closer you get to the surface, the greater the chance that surface processes such as atmospheric pumping, precipitation, & advective flow from structures will affect the soil gas. So, initially, sample 5’ bgs to get below this zone if the source is below.

Collect samples within the draft VI guidance’s dead zone, from 5 feet to the surface to document that the concentrations are attenuating. Vertical profiles also will aid in determining the direction of the source.

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SubSub--Slab vs. NearSlab vs. Near--Slab Slab ProfilesProfiles

?

Are sub-slab samples the best to collect?

Slide courtesy of Dr. Paul Lundegard, Unocal/Chevron

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SubSub--Slab vs. NearSlab vs. Near--SlabSlab• EPA & Some States Prefer Sub-slab

– “Ponding” effect under slab?– Balls don’t run uphill

• Good Comparison Database Lacking• Very Intrusive. Attorney Time.• If O2 High Around Slab, Near-slab OK• For Cl-HCs, at GW or mid-way to GW

The draft VI guidance strongly advocates sub-slab samples and some State agencies agree. Some are fearful that the contaminants build-up under the slab (“ponding effect”). But, sub-slab sampling is intrusive and often leads to legal complications. By Fick’s law, the sub-slab concentration can be no higher than the source concentration, so if the source below, collection of samples at the source depth or midway to the source will give useful data and not create as many legal headaches.

If high oxygen levels exist all around the slab at a shallow depth, and the slab small, there is a good chance that reaeration under the slab is occurring and sweeping contaminants clear.

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SubSub--slab vs. Near Slabslab vs. Near SlabHouse #3House #3

0 25O2 (%)

10

8

6

4

2

0D

epth

(ft)

0 5 10CO2 (%)

0 10 20CH4 (%)

sub-slabbare soil

Sub-slab versus near-slab soil gas profiles from a house in Santa Maria, CA.


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SubSub--Slab vs. Near Slab Slab vs. Near Slab SamplingSampling

0 2 4 6 8 10 12 14 16 18 20 22 24x (m)

-8

-6

-4

-2

0

Dept

h BG

S (m

)

0 2 4 6 8 10 12 14 16 18 20 22 24x (m)

-8

-6

-4

-2

0

Dept

h BG

S (m

)

-- from API, 2004

0 2 4 6 8 10 12 14 16 18 20 22 24

x (m)

-18

-16

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S (m

)0 2 4 6 8 10 12 14 16 18 20 22 24

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h BG

S (m

)0.001

1E-05

NO OXYGEN BELOW SLAB OXYGEN BELOW SLAB

©2003 Shell Oil Company. All rights reserved. Do not reproduce without the express written permission of copyright owner.

The 2 figures to the left show model results for situations with high subsurface contamination and no oxygen replenishment. You can see the oxygen concentrations fall off quickly with depth near the house and there is no oxygen under the house for the organisms. Hence, sub-slab concentrations are much higher than near-slab. In such situations, sample sub-slab or outside the slab at deeper depths.

The 2 figures to the right show the situation where oxygen is being replenished. In this case, oxygen levels near-slab are greater than 10% and there is plenty of oxygen sub-slab for bioattenuation to occur. Hence, near-slab concentrations of the hydrocarbons are a good reflection of sub-slab concentrations.

Thus oxygen data can be used to get a handle on whether near-slab data might be representative of sub-slab data.Modeling performed by Dr. Lillian Abreau & Dr. Paul Johnson, ASU

72

0.01 20 40 60 80 100

= field data COA-3 under asphalt Model Case 1 = No slab= field data COA-2 no asphalt Model Case 2 = With slab

No Slab

Case 2-Slab

0

10

8

2

4

6

Dis

tanc

e (f

eet)

Benzene content (ppm soil gas)

Unocal Tank Farm - Soil Gas Profiles vs model projections- Linear X axis

This slide shows the theoretical concentration profiles for benzene if there was a slab with contaminants building up underneath it to the source concentrations and if there was no slab and concentrations decreased by molecular diffusion only. Then measured data are plotted to compare. You can see that for the two profiles (one under asphalt and one not under asphalt), the measured values are far below either theoretical profile indicating that bioattenuation is present. These type of data can be used to demonstrate the effectiveness of bioattenuation at a site.

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SubSub--Slab OSlab O22 Transport Transport ExperimentExperiment

3 m

N2 injectionO2 sensor

0 100 200Injection Time (min.)

05

101520

Oxy

gen

(%)

Oxygen Displacement with N2

Oxygen replenishment study conducted at a house in Santa Maria, CA. The sub-slab was flooded with nitrogen to remove all oxygen.


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SubSub--Slab OSlab O22 Transport Transport ExperimentExperiment

3 m

N2 injectionO2 sensor

0 20 40 60 80 100Time (days)

05

101520

Oxy

gen

(%)

Sub-slab O2 Recovery

Oxygen replenishment study conducted at a house in Santa Maria, CA. Oxygen replenishment only took several days implying sub-slab lateral vapor movement is quite fast. Hence, contaminants won’t build-up under slab.


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How Often to Sample?How Often to Sample?• Closer to Surface, More Variability

– 3’ to 5’ bgs generally considered stable– Upper few feet likely variable

• Seasonal Effects– Extreme temperature variations– Heating/cooling of structure– Heavy periods of rain

Vapor Implants Allow Easy Resampling

The closer to the surface, the more the potential temporal variation. Depths of 3’ to 5’ below the surface are generally considered stable and repeatable.

Larger variations can be expected in areas of extreme temperature variation (northern climates), during heavy periods of precipitation, and when the structure’s heating or ventilation systems are operative.

If conditions suggest that temporal variations may be significant and if the measured values are close to the fail level, then repeated sampling may be appropriate and vapor implants are a good approach.

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Bioattenuation of HCsBioattenuation of HCs

• Existing data suggest O2 effective barrier• Attenuation > 10,000 times• Vertical profiles of COC & O2

• DNA can confirm presence of tropic bugs

A vast number of studies have been performed clearly demonstrating that the bioattenuation of hydrocarbon vapors occurs in aerobic soils. In general, the studies show that when oxygen levels are 10% or greater (a published study by NJDEP suggested oxygen levels as low as 6% are sufficient), and a couple feet of vadose zone exist between the source and receptor, that the hydrocarbons aren’t escaping into the receptor. Attenuation factors can be as high as 10,000 times (alpha = 0.0001).

Documention that this process is occurring is done by collecting vertical profiles of the soil gas for the hydrocarbons, oxygen, and carbon dioxide. If shown to occur, many agencies are conservatively allowing a factor of 10 to 100 reduction in the alpha factor.

77

This is the theoretical profile for hydrocarbon VOCs, CO2, and oxygen in the soil gas with depth where bioattenuation is active.

ASTSWMO, 9/05 77

Theoretical Bio ProfileTheoretical Bio Profile

soil surface

O2

CO2

VOCscleansoil

petroleumproduct

increasingdepth

VOCs

O2

flux

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Static Flux Chamber Static Flux Chamber

Photo of a static flux chamber equipped with a LandTech GEM 2000 real-time oxygen, carbon dioxide, and methane analyzer used to collect data continuously for 3 days over a site with high near-surface (1 to 2 feet bgs) methane and carbon dioxide concentrations.

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CO2 & Methane in Flux ChamberCO2 & Methane in Flux ChamberBY092503Tech

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Carbon dioxide and methane data collected every 15 minutes for 24 hours. Note increase in carbon dioxide but no corresponding increase in methane. How can this be possible since soil gas concentrations were the same and the gas transport mechanisms for both would be the same?

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Part 6 Part 6 -- Supplemental Supplemental Tools/Data Tools/Data

• Site Specific Alpha Using Radon– Factor of 10 to 100. $100/sample

• Indoor Air Ventilation Rate– Factor of 2 to 10. <$1,000 per determination.

• Real-Time, Continuous Analyzers – Can sort out noise/scatter

• Pressure Measurements – Can held interpret indoor air results

There are some other inexpensive tools/data that can be applied to better evaluate some of the default model parameters and the vapor intrusion pathway. These tools/data have much more influence on the resulting risk than measurement of soil porosity and cost about the same.

Radon can be used to determine a site-specific alpha that may be 10 to 100 times lower than the default alpha allowed. Tracers can be used to measure the room ventilation rates and may give values 2 to 10 times higher than the default value, especially for commercial sites.

For HCs, vertical profiles of the soil gas can demonstrate bioattenuation. In some States, the agencies allow for a 10 times reduction in risk if bioattenuation is demonstrated. Although the DTSC guidance recognizes bioattenuation, it currently gives no indication as to how the data will be interpreted.

Real-time analyzers can be used to locate problem houses, preferential pathways into structures, or sort out background scatter. Pressure measurements are helpful with indoor air data to possibly show abackground source.

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VI Investigation Methods: VI Investigation Methods: NonNon--VOC MeasurementsVOC Measurements

Non-VOC measurements used to evaluate vapor intrusion while avoiding background VOC issues.

Non-VOC measurements used to evaluate vapor intrusion while avoiding background VOC issues.

KEY POINT:

RadonRadon

Building PressureBuilding Pressure

Naturally occurring tracer gas used to measure attenuation through building foundation.

Magnitude and duration of building pressure fluctuations: negative vs. positive building pressure.Building ventilation rates: design standards, HVAC operation, indoor tracer gas.

Air ExchangeAir Exchange ER

Slide courtesy of Dr. Thomas McHugh, GSI, Houston, TX

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Rate at which indoor air is replaced by ambient (fresh) air.

What

ASHRAE Std.

62.1-2004

SF 6SF 6

Air Exchange: What ‘n How

KEYPOINT:

Multiple methods available to estimate or measure site-specific air exchange rate.

Recommended ventilation rates for commercial building.

Ventilation Standards

Fresh air intake rateEconomizer function

HVAC SystemTracer

GasMeasure dilution of tracer gas to determine air exchange rate

Air Exchange

BUILDING

ESTIMATION METHODS


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Recommended Building Ventilation Rates

KEYPOINT:

Buildings designed for high density use will have high air exchange rates.

ANSI / ASHRAE Standard 62.1 – 2004Ventilation for Acceptable Indoor Air Quality

Building Type

Air Exchange Rate (# / day)

USEPA Default (Residential)

Office Space

SupermarketClassroomRestaurant

6

12

1768

102

High Building

Ventilation


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KEY POINT:

Results of tracer gas study matched expected effect of HVAC system.

Altus Results: Building Air ExchangeAltus AFB Building 418What:

Release SF6 Tracer gas into building to measure from air exchange.

Results:

MorningEvening

19/day16/day

1200 CFM1000 CFM

> 1140 CFM1140 CFM

Air Exchange: Measured Values

Sample TimeSample Time

Building Air ExchangeBuilding Air Exchange

Fresh Air Entry RateFresh Air Entry Rate

Expected Fresh Air Entry Rate from HVAC system

Expected Fresh Air Entry Rate from HVAC system

Tracer Gas StudyTracer Gas Study


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Building Building UnderpressurizationUnderpressurization

∆P

0-50 Pa - range (Nazaroff et al., 1985)0-5 Pa - typical values (Robinson et al., 1997)

Is building pressure important?


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Measure pressure gradient across building foundation

Pressure Gradient: Wazzat ?

How

What

Understand driving force for vapor transport across foundation: positive vs. negative pressure gradient.Document impact of ambient weather on sampling program

Why LowPressure

High Pressure

Pressure TransducerPressure transducer with ports open to building and below foundation


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DATA INTERPERATION:Measured pressure gradient likely reflects typical conditions (i.e., no extreme weather observed).Bi-directions flow expected across building foundation.

Pressure Gradient Measurements:School Building

Diff

eren

tial P

ress

ure

(Pas

casl

)

Time (July 14-15, 2005)

-40

-30

-20

-10

0

10

20

30

40

6:00 9:00 12:0015:0018:0021:00 0:00 3:00 6:00 9:00 12:0015:00

EPA Default Gradient Neg. Pressure

Pos. Pressure Key Site Conditions

Atmospheric Pressure: Steady (7430 - 7460 Pascals)

Wind Speed: 0-17 mph (Avg. = 8)

Precipitation: About 1” rain

Temperature: 74 - 91 ˚F

HVAC System Operation: Normal


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House House UnderpressureUnderpressure TestsTests

0 50 100 150 200 250 300Elapsed Time (minutes)

-5

0

5

10

15∆

P (P

a)O

utsi

de -

Insi

de

window fan“on”

Pressure variations in a house are typically not that large and unlikely to have a major impact on flux.


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HV

AC

fan

bath

room

fan

kitc

hen

fan

win

dow

fan

(1)-

win

dow

fan

(2)-

0

2

4

6

8

Pou

t-Pin (P

a)

UnderpressureUnderpressure Test ResultsTest Results

win

dow

s cl

osed

win

dow

s op

enPressure variations due to common household fixtures/appliances. The pressures/vacuums created are not very large.


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Continuous Monitoring DataContinuous Monitoring DataHUNTINGTON BEACH SITE - SOIL GAS

0

5

10

15

20

25

30

35

6:18:0

0 PM

7:09:0

0 PM

8:00:0

0 PM

8:50:0

0 PM

9:41:0

0 PM

10:32

:00 P

M

11:23

:00 P

M

12:13

:00 A

M

1:04:0

0 AM

1:55:0

0 AM

2:45:0

0 AM

3:36:0

0 AM

4:27:0

0 AM

5:18:0

0 AM

6:08:0

0 AM

6:59:0

0 AM

7:50:0

0 AM

8:41:0

0 AM

9:31:0

0 AM

10:22

:00 A

M

11:13

:00 A

M

12:03

:00 P

M

12:54

:00 P

M

1:45:0

0 PM

2:36:0

0 PM

3:26:0

0 PM

4:17:0

0 PM

5:08:0

0 PM

5:59:0

0 PM

6:23:0

0 PM

Time

Val

ue (%

) CH4CO2O2

This is an example of continuous monitoring of soil gas data, in this case methane, oxygen, and carbon dioxide in a vapor well. Similar data collected inside a structure could be useful in differentiating between vapor intrusion and ambient/background scatter. Or it can be used to demonstrate the repeatability of shallow soil gas data.

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Importance of Enough DataImportance of Enough Data

• Background Variable• Lots of Potential Scatter • Structures are “Mixing Containers”• Need to Separate Sources

As with any type of site investigation, it is difficult to reach any conclusions with any degree of confidence with only a handful of data points. Vapor intrusion assessments with 1 soil gas and/or indoor air analysis per residence are very difficult to interpret, but the data are so expensive to collect that this is quite commonly the case.

92

COC Correlations in Redfield PreRemediation Indoor Air (w/o 3 TCE outliers)

TCE vs. 1,1,1-TCA1,1,1-TCA = 3.920 + 2.389 * TCE

Correlation: r = .301 (N=723)

TCE (ug/m3)

1,1,

1-TC

A (u

g/m

3)

-500

50100150200250300350400

-2 2 6 10 14 18 22 26

TCE vs. PCEPCE = 3.13 + .742 * TCE

Correlation: r = .227 (N=723)

TCE (ug/m3)

PCE

(ug/

m3)

-200

20406080

100120140160

-2 2 6 10 14 18 22 26

Regression95% confid.

TCE vs. 1,1-DCE1,1-DCE = -.049 + 3.977 * TCECorrelation: r = .694 (N=723)

TCE (ug/m3)

1,1-

DC

E (u

g/m

3)

-200

20406080

100120140160

-2 2 6 10 14 18 22 26

Redfield CO DataRedfield CO Data

These plots are indoor air data from the Redfield, CO site. The large data set allows trends in the data to be recognized and forensic approaches to be successful. This would not be possible with only a handful of data points.(Slide compliments of David Folkes of Envirogroup)

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BY092503Tech

18.4

18.6

18.8

19.0

19.2

19.4

19.6

19.8

20.0

10:02

:00 AM

10:48

:00 AM

11:38

:00 AM

12:29

:00 P

M

1:20:0

0 PM

2:10:0

0 PM

3:01:0

0 PM

3:52:0

0 PM

4:42:0

0 PM

5:33:0

0 PM

6:24:0

0 PM

7:14:0

0 PM

8:05:0

0 PM

8:56:0

0 PM

9:47:0

0 PM

10:37

:00 P

M

11:28

:00 P

M

12:19

:00 AM

1:09:0

0 AM

2:00:0

0 AM

2:51:0

0 AM

3:42:0

0 AM

4:32:0

0 AM

5:23:0

0 AM

6:14:0

0 AM

7:04:0

0 AM

7:55:0

0 AM

8:46:0

0 AM

9:36:0

0 AM

Time

Valu

e (%

)

O2

Oxygen in Flux Chamber Oxygen in Flux Chamber –– 1 Day1 Day

Oxygen data in the flux chamber for the same 24 hour period. Note variation (~5%) over the period. Real or instrument variation?

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Oxygen in Flux Chamber Oxygen in Flux Chamber –– 3 3 DaysDays

GeoSoils - O2

18.4

18.6

18.8

19

19.2

19.4

19.6

19.8

20

20.2

10:4

1:00

AM

12:

57:0

0 P

M

3:1

3:00

PM

5:3

0:00

PM

7:4

6:00

PM

10:

02:0

0 P

M

12:

18:0

0 A

M

2:3

5:00

AM

4:5

1:00

AM

7:0

7:00

AM

9:2

3:00

AM

11:

40:0

0 A

M

1:5

6:00

PM

4:1

2:00

PM

6:2

9:00

PM

8:4

5:00

PM

11:

01:0

0 P

M

1:1

7:00

AM

3:3

4:00

AM

5:5

0:00

AM

8:0

6:00

AM

10:

22:0

0 A

M

12:

39:0

0 P

M

2:5

5:00

PM

5:1

1:00

PM

7:2

7:00

PM

9:4

4:00

PM

12:

00:0

0 A

M

2:1

7:00

AM

4:3

3:00

AM

6:5

0:00

AM

Time

Val

ue (%

)

O2

Oxygen data for 3 consecutive days. Note that the variation is repeatable each day proving that the variation is reflective of a real process (barometric pumping). This conclusion could not have been reached with a limited number of data points.

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Helium Distribution- 2 weeks after injection @ 5D- permanent probes below concrete

SF6 Distribution- 1 month after injection @ 5C- probes 6” below 6” slab

Tracer TestTracer Test

Tracers added underneath a house with a slab show rapid horizontal dispersion with little vertical dispersion. This implies that horizontal transport, likely from barometric pumping, is relatively rapid underneath small slabs and will bring oxygen in and flush the contaminant out.

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Part 7 Part 7 -- Mitigation Options Mitigation Options

• Passive Soil Gas Venting (<$1,000)• Active Soil Gas Venting ($2,000-$3,000)• Liquid Boot• Increase Room Ventilation Rate• Indoor Air Filter • Institutional Controls

Some of the more common mitigation methods for subsurface vaporsinclude passive and active venting of the soil gas below the home, increased circulation in the structure, or institutional controls. Liquid boot is a sealer that is injected underneath structures.

Simple soil gas venting systems can cost less than $1,000 and rarely are less than several thousand.

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Indoor Air FiltrationIndoor Air Filtration

• Size of shop vac

• 22 pounds carbon

• Effective up to 1500 ft2

• 3-Speed, 300 CFM fan

• Electrical usage ~ 60 watt light bulb

www.allerair.com

An inexpensive ($500) and simple indoor air filtration system.

98

Results of Active VentingResults of Active Venting

From: Folkes, D.: Vapor Intrusion Assessment and Mitigation - Practical Issues and Lessons Learned, EPA Office of Solid Waste RCRA Corrective Action EI Forum, August 15-17, 2000

This slide demonstrates how the concentrations of compounds being sourced from below (1,1 DCE) dropped when a venting system was installed, but the other indoor contaminants did not, suggesting the other contaminants were from background sources, not from below.

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Typical Home Protection SystemTypical Home Protection System• Below-Slab Soil Gas Collector & Membrane

– 4-in perf. pipe in 12-in square rock-filled trench.– All foundation sections influenced by collector.– Multiple, redundant, vertical vent risers.– 80-mil membrane under entire foundation and

sealed to all utility penetrations and vent risers.–Tested twice for integrity.– Monolithic concrete pour.

• Utility seals in all electrical connections, including low-voltage systems.

• Utility dams throughout project site to limit horizontal migration in utility bedding.

This slide was prepared by a local environmental consultant and summarizes the typical protection system used at new houses to mitigate methane gas intrusion.

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Soil Gas Collector Soil Gas Collector --Perforated pipe below each Perforated pipe below each

foundation.foundation.

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Perforated pipe bedded in gravel.Perforated pipe bedded in gravel.

102

ASTSWMO, 9/05 102A completed subA completed sub--slab collector and 80slab collector and 80--mil barrier system.mil barrier system.

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Mitigation ExpertsMitigation Experts• Radon

– Ron Mosley EPA/ORD/NRMRL [email protected]

• VOCs– David Folkes, Envirogroup

[email protected]

• Methane– Michael Geyer, SCS Engineers

[email protected]

Mitigation contacts

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Part 8 Part 8 -- A PM’ StepA PM’ Step--byby--Step Guide Step Guide to Assess the VI Pathwayto Assess the VI Pathway

Part 8 is a recommended step-by-step guide for a PM to use when investigating a potential vapor intrusion problem. This guide assumes that you already have a volatile organic compound (VOC) contamination issue either in the groundwater, soil, or soil gas and that structures either directly overlie or are close to the contaminated area.

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1. Do 1. Do VOCsVOCs & Receptors Exist Within & Receptors Exist Within Allowable Distances?Allowable Distances?

• EPA: 100’ lateral & vertical • NH: 30’ lateral & 15’ vertical• NJ: 100’for Cl-HC, 30’ for HCs• NY: No LimitsPresence of Odors, Occupant Complaints,

May Supercede These Limits

Step 1. Check distances between receptors and contamination source to see if receptors lie within regulatory limits. For undeveloped sites with contamination, the site will likely need to be assessed when development is proposed. Certain situations such as the presence of odors, complaints from residents, proximity to a landfill, or attorneys may require assessment of the pathway even if outside the regulatory limits.

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2. Is the Situation Acute?2. Is the Situation Acute?

• Presence of Odors • Occupant Complaints • Gas Alarms• Attorneys

Step 2. Do you have a situation that requires immediate or emergency action. Certain situations such as the presence of odors, complaints from residents, proximity to a landfill, or attorneys may require immediate action.

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3. Have Site Conceptual Model3. Have Site Conceptual Model

• Know Target Compounds• Know Sources

– GW, Soil, Vapor Cloud• Preferential Pathways?

– Utilities, Fractured Bedrock, Clay Layers• Structure Type

– Slab, Basement, Crawl-space, Attached Garage

Step 3. Have an idea of the contaminant type and source, the potential pathways to the receptors, and the type of receptors (type, residential, commercial, children). This is often referred to as a site conceptual model. It need not be overly detailed, but enough should be known to get an idea of whether you have a real problem or a perceived, but not real, problem. This knowledge is also necessary to design an optimum sampling plan if required.

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4. Use Existing Data to Determine if 4. Use Existing Data to Determine if Potential Risk ExistsPotential Risk Exists

Compare GW or Soil Gas To Screening Values• Using Default Alphas or• Lookup Tables• EPA Calculator or Spreadsheets

Step 4. If groundwater, soil gas, and soil phase data already exist, compare values to target/screening values. The target values might be determined from look-up tables, calculated from default alphas, or from the EPA spreadsheets. If measured values are below target values, then no further work may be required.

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5. Review 5. Review WorkplanWorkplan to Ensure Proper to Ensure Proper Method UsedMethod Used

• Indoor Air Okay if: – COC not common in Ambient Air– Ambient air PRGs high

• Active Soil Gas – Most Likely Approach– External or Internal (Sub-slab)

Step 5. Request & review the workplan to see which method is being proposed to determine whether vapor intrusion is an issue. If the contaminant of concern is not in common household products or not in the ambient air, indoor air might be the best choice. Require soil gas data before groundwater or soil phase data since the spreadsheets using these data tend to overestimate the risk.

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6. Collect SG Samples Properly6. Collect SG Samples Properly• Soil Gas VI Requires More Care & Experience• Small-volume Samples (mini-Summas) • Tracer/Leak Compound• Vapor Implants for Profiles & Resampling• Analytical Method: 8021, 8260, TO-14/15:

Select based upon required DL• Good Hardware

Step 6. Review workplan to ensure proper sampling and analysis protocols are being followed. Be sure that the field technician, consultant, or the soil gas collection firm has the proper experience and knowledge to collect the high quality samples required for vapor intrusion, knows what analytical methods are best to use, and has the experience to recognize and correct problems when they occur.

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7. Know Where to Collect SG Samples7. Know Where to Collect SG Samples• Good Spatial Coverage • Lateral Profiles to Document Attenuation• Vertical Profiles for HC Bioattenuation • Vertical Profiles for Max Cl-HC Conc• Use Real Time Analysis When Possible• Allow Sampling in “Dead Zone”

especially for Hydrocarbons• Sub-slab if Risk Still Apparent

Step 7. Be sure that the consultant and the soil gas collector have the proper experience and knowledge to know where to collect soil gas samples both spatially and vertically to efficiently assess the VI risk pathway. Consider if real-time analysis is an option and whether real-time information can improve the chances of assessing the pathway. Collect sub-slab samples if risk still apparent from other data.

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8. Know Other Possible Tools8. Know Other Possible ToolsLines of Evidence ApproachLines of Evidence Approach

• Radon – Can Give Slab-Specific Alpha• Ventilation Rate Measurement • Flux Chambers if Conditions Allow• Real-Time, Continuous Analyzers• Pressure Measurements• Forensics

Step 8. Consider and allow the supplemental tools and when to use them. If sub-slab samples are required, collect samples for radon also to possibly enable a lower alpha to be documented. Building ventilation rates are easy and inexpensive to measure. Flux chambers may be useful in somesituations if other methods not applicable. Real-time analyzers have application if screening of many structures are required (e.g., neighborhoods) and if ambient concentrations are close to fail levels.

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9. Assist/Monitor the Consultant9. Assist/Monitor the ConsultantVapors, vapor intrusion, soil gas, fairly new to many consultants. Ensure the consultant follows the above steps, understandsthe principles, and uses experienced field people (their own or subs)

Step 9. Vapor Intrusion is a new and unfamiliar pathway to manyconsultants and field service companies. Their knowledge of the scientific principles, regulatory criteria, field equipment and methods, and data interpretation is likely less than for more traditional groundwater and soil assessment programs.

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10. If Pathway Complete:10. If Pathway Complete:MitigateMitigate

Mitigation has many forms depending upon whether the structure already exists or is beingbuilt. Land use issues, legal issues all come into play.

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The Most Common GoofThe Most Common Goof

1 ug/L Benzene equals: a) 1 ppbvb) 1 ppmvc) 330 ppbvd) None of the Above

Vapor units is one of the most common mistakes being made by practitioners in this field. Let’s see how you do:

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Previews of the VI FuturePreviews of the VI Future• VI Likely to be a Concern at Your Sites • New EPA Guidance & Spreadsheets to

be Much Easier & User Friendly• Sub-slab RBSL Criteria Likely to Get

Less Restrictive by 10x in Near-term• VI Pathway at Petroleum Hydrocarbon

Sites Likely to Be Incomplete due to Bioattenuation

Here are some predictions & previews of the vapor intrusion pathway for the next few years. Some upcoming changes in regulatory guidance for hydrocarbons and maybe chlorinated hydrocarbons are likely to make the pathway less restrictive in the near future.

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Existing Documents & TrainingExisting Documents & Training• Overview of SV Methods

– LustLine Part 1 - Active Soil Gas Method, 2002– LustLine Part 2 - Flux Chamber Method, 2003– LustLine Part 3 - FAQs October, 2004

• Regulatory Guidance– CA-EPA Collection Protocols– San Diego County Collection/Analytical

ProtocolsAvailable at www.HandPmg.com

A summary of existing documents and training on the VI pathway and soil gas methods can be found at these locations:

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Existing Documents & TrainingExisting Documents & Training• SOPs

– Soil Gas Sampling, Sub-slab Sampling, Vapor Monitoring Wells/Implants, Flux Chambers (www.HandPmg.com)

– EPA-ORD Sub-slab SOP–Draft, Dr. Dom DiGuilio (www.iavi.rti.org/resources)

• Other – API Soil Gas Document www.api.org/bulletins– Vapor Intrusion Web Site: http:iavi.rti.org

More documents: