Nevada Division of Environmental Protection Bureau of Corrective

NDEP, Petroleum in Soils Closure Checklists, July 2022 Update Page 1 of 6

Nevada Division of Environmental Protection

Bureau of Corrective Actions

Petroleum in Soils Closure Checklists

July 2022 Update

Statement of Purpose

“The purpose of this document and the attached checklists is to clearly define all options

and requirements for closure under Nevada’s Corrective Action regulations for sites that

have soil contamination resulting from releases of petroleum. It has been drafted to assist

property owners, environmental consultants, and NDEP case officers in making informed

decisions about petroleum cleanups, to promote consistency in closure decisions, and to

make petroleum cleanups efficient and protective.”

Introduction

In 2009, the Nevada Division of Environmental Protection, Bureau of Corrective Actions (NDEP)

amended its site cleanup regulations at NAC 445A.226 to 445A.22755 and removed its longstanding

numeric soil action level for total petroleum hydrocarbons (TPH). The soil action level for TPH of 100

mg/kg was originally established as part of the leaking underground storage tank program and was later

included for all sites under corrective action. No other hazardous substances were given specific,

numeric action levels in regulation. Hazardous substances other than petroleum relied on risk-based

decision making to address these substances on a site- and chemical-specific basis. The removal of the

numeric standard in 2009 was intended to place petroleum in the same risk-based decision making

framework as all other hazardous substances.

Having a single action level for TPH did not accurately reflect the wide range of petroleum

products that may require site cleanup, and the use of the action level as a default cleanup standard

often led to costly cleanups that did not consider site risks. As defined in statute, “petroleum” refers to a

range of petroleum mixtures and formulations which are liquid at standard temperature and pressure

and includes crude oil, diesel, heating oil, gasoline, mineral oil, or any other formulation. Each

formulation may behave differently when released to the environment based on their composition,

mobility, volatility, and persistence. Each formulation also consists of constituents with different

toxicities and exposure risks. The 100 mg/kg action level was established to ensure protectiveness for

the petroleum formulations that presented the greatest risk to human health. This resulted in releases

of less mobile and less toxic formulations being held to a restrictive standard.

Not only is the use of a single action level hindered by the range of formulations that falls under

the regulatory definition of “petroleum” but also by the fact that each formulation of petroleum is itself

a mixture of many constituent hydrocarbon molecules. The 100 mg/kg petroleum action level in

regulation was based on analysis of Total Petroleum Hydrocarbons (TPH). This approach does not

differentiate between individual petroleum constituents. Analysis of total petroleum hydrocarbons may

not provide an accurate reflection of site risks because it does not differentiate between the

constituents that serve as risk-drivers based on the toxicity and mobility of constituents with known

health effects.

NDEP, Petroleum in Soils Closure Checklists, July 2022 Update Page 2 of 6

The NDEP decided to deemphasize the 100 mg/kg TPH in soil action level at corrective action

sites by eliminating that numeric value from the regulations; however, the NDEP has not eliminated it as

a pathway to closure for sites with petroleum contamination. Rather, the use of a single numeric action

level has been placed alongside a number of equally valid pathways to closure that can be considered by

owners or operators of property where a petroleum release has occurred. All closure options discussed

in this document are fully supported by the corrective action regulations at Nevada Administrative Code

445A.226 to 445A.22755. This document is intended to identify and describe all options for closure so

that an owner or operator may select an approach that best meets their available resources,

timeframes, and intentions.

At this time, this document only addresses corrective actions involving petroleum. Corrective

actions involving other hazardous substances follow the same regulations and many of the same

considerations, but this document is specific to petroleum and to the issues related to petroleum

hydrocarbons. Some assumptions about the mobility and biodegradation of petroleum may not be

applicable to other hazardous substances. While the general precepts about corrective action and site

closure under Nevada regulations have wide applicability and can be used as a guide for all soil cleanups,

specific determinations for petroleum cleanups should not be applied to other cleanups without

chemical-specific consideration by property owners, consultants, and NDEP case officers.

Additionally, this document is intended for use at sites with soil contamination only. The NDEP

will not provide closure of a site until all contaminated media have been adequately addressed;

however, the soil-specific focus of this document may not be applicable at sites with both soil and

groundwater contamination.

Basis for Update

This July 2022 update supersedes the prior September 2014 version of this document and

provides an update to the “Analyte-Specific Closure Levels” found in Table 1 of Appendix B. The

concentrations shown in Table 1 of Appendix B represent those published in the May 2022 version of

the US Environmental Protection Agency (US EPA) “Regional Screening Levels – Generic Tables”. The US

EPA Regional Screening Levels are periodically updated and published on the US EPA’s website

(https://www.epa.gov/risk/regional-screening-levels-rsls). The most current version of the Regional

Screening Levels should always be consulted prior to use.

Document Overview

The substance of this document is the checklists, tables, and technical papers located in the

appendices. These are tools to help NDEP case officers make consistent, defensible closure

determinations for petroleum releases. They are also intended to communicate requirements for soil

closure to property owners and environmental consultants. The next few sections of this document

describe some of the decision-making for determining the appropriate closure method and provide

justification for the general requirements applicable to all sites.

NDEP, Petroleum in Soils Closure Checklists, July 2022 Update Page 3 of 6

The Four Closure Options

For any Corrective Action project it is critical to identify an endpoint or goal that can guide site

cleanup decisions. To assist owners and operators the NDEP has identified four endpoints in its

corrective action regulations for sites with soil contaminated by petroleum. Each of the four closure

options relies on a few different regulatory citations and has different requirements for demonstration,

but all options are available and equally valid for use at all sites.

This section is intended to introduce the four closure options and to provide general

comparisons between them. The comparisons are intended to assist property owners and operators

make a determination about their preferred approach to site cleanup since each closure option may

have its own benefits or drawbacks. The NDEP does not dictate which closure option is appropriate for

sites but determines whether requirements have been satisfied. The four closure options are:

1) Clean Closure: all confirmation samples (using analytical EPA Method 8015 modified) at the site

are below 100 mg/kg for TPH. This is the reportable concentration that has been set for TPH by

the NDEP as generally posing an acceptable level of risk for all exposure scenarios.

2) Analyte-Specific Closure: all confirmation samples show that petroleum hydrocarbon

constituents are below default action levels established by the NDEP to be protective of direct

contact exposure and leaching to groundwater. Constituents to be screened are selected based

on their toxicity (both cancer and non-cancer hazards) and presence in petroleum formulations.

3) A-thru-K Closure: concentrations of petroleum hydrocarbons and petroleum constituents above

default action levels may be left in place based on a site-specific analysis of the A-thru-K factors

listed in the corrective action regulations to show that known residual concentrations are still

protective even if they are above action levels.

4) ASTM RBCA Closure: all concentrations of petroleum hydrocarbons and constituents are below

Site Specific Target Levels established through a Tier II or Tier III analysis conducted consistent

with the ASTM Risk-Based Corrective Action method E1739-95 or equivalent.

The ordering of the closure options is not intended to convey agency preference. Rather, the closure

options are presented as a continuum, from options that rely more on cleanup and removal of soil to

options that rely more on modeling or calculation of site risks.

There are many factors that weigh into an owner or operator’s decision about the best pathway

to closure. Among the many considerations are costs for assessment and cleanup; project turnaround

time; long-term liability and continuing obligations; future site uses; technical knowledge and

sophistication required to achieve closure; level of regulatory involvement and the likelihood of findings

of deficiencies by case officers; and property ownership issues. Each closure option represents a balance

of these factors: some closure options may have a lower cleanup cost that comes at the expense of

longer project times or greater continuing obligations, while others may favor swift resolution of issues

through excavation with little or no requirement for regulatory review of protectiveness determinations.

The most fundamental balancing factor amongst the closure options is the level of effort for

assessment versus cleanup necessary to achieve regulatory concurrence. Generally, sites that achieve

closure through excavation or treatment will require fewer resources for assessment and vice versa.

Excavation and/or treatment of contaminated soils can represent a significant portion of project costs;

however, the costs of assessment, calculations, and modeling should not be discounted. Making a

defensible argument that residual petroleum contamination can remain in place above action levels

NDEP, Petroleum in Soils Closure Checklists, July 2022 Update Page 4 of 6

involves variable costs depending on the size of the project, regulatory scrutiny, land use assumptions,

and site conditions. In most instances, at small sites the costs of excavation and disposal will generally

be lower than the cost to make a defensible argument to leave the contamination in place. As the

volume and depth of contamination increases, the cost of excavation, disposal and/or treatment will

begin to exceed the costs of leaving it in place.

There may be other factors besides cost which influence an owner or operator’s decision about

an appropriate closure approach. Closure approaches that rely on excavation and disposal are generally

quicker than other approaches. If a site cleanup is under a time deadline because of property transfer or

construction, then additional costs for excavation and disposal of soils may be less important than the

time it takes to obtain a no further action determination from the NDEP. An owner’s long-term liability

or continuing obligations to maintain the protectiveness of leaving residual contamination in place may

be another consideration that is prioritized above cleanup cost. Because determinations to leave

residual contamination in place usually involve assumptions about land use and nearby populations,

these closures might be reevaluated in the future if assumptions change, or they may need to rely on

the protection of an environmental covenant; this can represent an unacceptable continuing obligation

or liability for a property owner, who may elect to avoid these obligations or liabilities by undertaking

additional cleanup. Also, individuals who are undertaking corrective action on property that they do not

own may be constrained by the desires of the property owner for a specific level of cleanup.

1) Clean Closure: cleanup proceeds until confirmation samples show

that TPH is below 100 mg/kg. Cleanup costs are higher because less

residual contamination is left behind, but the assessment and

analytical costs are lower for TPH analyses and there is less likelihood

of regulatory determinations requiring additional information for

cl os ure.

2) Analyte-Specific Closure: cleanup proceeds until confirmation

samples show that constituents are below action levels even while

residual TPH contamination remains. Assessment costs are higher

beca us e the laboratory a na lyses a re more costly, res i denti al vs.

industrial exposures must be addressed, and accurate volatile results

must be obtained.

3) A thru K Closure: residual petroleum contamination above action

levels may be left in place through a site-specific demonstration of

protectiveness. Protectiveness must be demonstrated through site

characterization and defensible arguments of fate and transport.

Assumptions may need to be controlled by land use restrictions.

4) ASTM RBCA Closure: s i te-s pecific target l evels a re developed for a

site through risk-based calculations of fate, transport, and toxicity. The

process is fully laid out in ASTM standards but requires site-specific

inputs for calculations. Assumptions may need to be controlled by land

us e res tri cti on.

General relationship between closure types and the level of effort for both assessment

and cleanup to achieve closure

Assessment &

Calculations

Excavation & Treatment

It should be noted that once an owner or operator selects a closure option and a corrective

action approach, he or she is not irreversibly tied to that closure option or approach, since corrective

action sites might evolve as more information is collected. For instance, a property owner who decides

NDEP, Petroleum in Soils Closure Checklists, July 2022 Update Page 5 of 6

that clean closure is the appropriate goal may make a different decision later in the project if

significantly more contamination is discovered or if excavation becomes difficult because of utilities or

overlying structures. Corrective Action Plans will need to be amended to incorporate revised

approaches, but the NDEP will not hold a property owner or operator to a specific cleanup as long as

another viable approach to closure is presented.

General Provisions for All Closures

Each of the closure options has different requirements that must be demonstrated to allow the

NDEP to provide a “No Further Action” letter to a property owner or operator; however, there are some

requirements that are common to all closure decisions made by the NDEP. These requirements have

been identified as general provisions that apply to all soil corrective action cases regardless of which

closure option is being pursued:

General Provision #1 – Ongoing Releases Addressed

All continuing inputs of petroleum contamination to the soil being addressed by corrective

action shall be identified and eliminated prior to closure. This includes ongoing releases from

underground storage tanks and associated piping; discharges from drains and washouts; leaks from

barrels or aboveground tanks; etc. This provision does not, in itself, create an obligation for an owner or

operator to address any other potential sources of petroleum that may result in future releases or to

address any sources not directly related to the contamination being addressed by corrective action.

General Provision #2 – Abatement

Closure decisions are made on residual concentrations that remain in soil after actions are taken

to respond to the release of petroleum. None of the closure options are intended to relieve an owner or

operator from the obligation to take abatement actions in response to releases of petroleum, such as

the removal of released product and the excavation of grossly impacted soil. In instances where

historical contamination is discovered, there may no longer be any product or grossly impacted soil

present, and this provision for abatement may not apply.

General Provision #3 – Complete Characterization

Closure shall be provided only with a full understanding of contaminant delineation. This

includes knowledge of the depth and lateral extent of soil contamination as well as a general

understanding of the location and volume of the areas with the highest remaining concentrations of

petroleum (and petroleum constituents, as appropriate). Closure will also be dependent on a full

understanding of exposure pathways relevant to the site including pathways and rates of migration for

contaminants and awareness of any potential receptors (although these factors may be more fully or

appropriately addressed as part of a specific closure type, rather than as a general provision).

General Provision #4 – Aesthetics

While site closure is granted by the NDEP based on health-based standards and/or

determinations in accordance with Nevada statutes and regulations, the consideration of aesthetic

factors will assist in ensuring closure decisions are not reopened. Any visible staining or odoriferous soil

NDEP, Petroleum in Soils Closure Checklists, July 2022 Update Page 6 of 6

left at or near the surface, even if the release was closed using health-based standards, may result in

future notification by the public, adjacent property owners, or future operators to the NDEP as an

apparent release, which could result in the reopening of a case. The NDEP recommends a minimum of

one (1) foot of clean material be present, where feasible, over any underlying residual contamination

that remains in-place at closure to address potential aesthetic concerns. The NDEP may also recommend

additional actions to address aesthetic concerns.

General Provision #5 – Imported Fill Is Consistent With Closure Conditions

Any fill material that is imported to the site as a component of corrective action to fill

excavations, restore grades, or cap residual contamination must meet closure criteria for the site, which

may be required to be demonstrated at NDEP’s discretion. Property owners/operators and CEMs are

also cautioned that use of fill containing hazardous substances above reporting limits established in NAC

445A.347 can be considered a separate reportable release for the site.

Using the Closure Checklists

Closure checklists (Appendix A) have been developed to summarize and condense the general

provisions and closure-specific requirements for cases with petroleum contamination in soil. The general

closure provisions, applicable for all cases, need to be satisfied to allow the case officer to advance

towards closure. If any of these general closure provisions is not satisfied for a given site, the case officer

is instructed to work with an owner or operator to resolve the deficiency prior to providing closure. For

the closure-specific checklists, only one of the separate checklists must be completed. If an owner or

operator cannot satisfy the requirements of a particular checklist, one of the other closure options may

be applicable.

Appendix B contains tables developed by the NDEP that provide greater detail for specific

closure types as referenced in the checklists in Appendix A.

Appendix C contains a technical paper published by the American Petroleum Institute that is

referenced in several of the closure-specific checklists.

Nevada Division of Environmental Protection

Bureau of Corrective Actions

Petroleum in Soils Closure Checklists

Appendix A

Checklists

NDEP, Petroleum in Soils Closure Checklists, General Closure Provisions Page 1 of 3

Nevada Division of Environmental Protection

Petroleum in Soils Closure Checklists

General Closure Provisions

Facility Name:

Case Officer:

Facility ID:

Date:

In order to achieve closure for a site where petroleum has been released to the soil or where petroleum

has been discovered in soils as a result of subsurface investigation, a facility owner/operator must be

able to demonstrate that these five general closure conditions have been satisfied.

A case officer should use his or her best professional judgment to determine whether information

provided by the facility owner/operator is sufficient to make a determination and to check off the

“Satisfied” box.

A set of general guidelines to help a case officer make a decision is included for each provision. A case

officer may fill out the “Notes:” section to provide a brief justification for the determination, if

warranted.

General Closure Provision#1—Ongoing Releases Satisfied

□ Not Satisfied □

Requirement: All continuing inputs of contamination to the soil (directly related to the corrective action

case) have been identified and controlled to prevent redeposition of contaminants after closure.

Some things to look for when determining that the “Ongoing Releases” provision is Satisfied

• The release was a one-time, accidental event such as mobile source releases, gas station overfills,

etc. that does not represent a potential on-going source.

• If leak is from an active regulated UST, all compliance issues related to release prevention and leak

detection have been resolved or are being actively overseen by the appropriate UST compliance

group.

• If the release occurred at a facility with a water pollution control permit or a RCRA waste

generation permit, the facility owner or operator has resolved all permit compliance issues arising

from the release.

• If the release was from a fixed source container that is not regulated by another program (i.e.,

aboveground storage tanks), the facility owner or operator has taken steps to prevent future releases.

• If the release source is not known, the facility owner or operator has provided adequate

documentation to demonstrate that there are no on-going sources to the contamination.

Notes: ______________________________________________________________________________

___________________________________________________________________________________

Recommended next steps: ______________________________________________________________

___________________________________________________________________________________

NDEP, Petroleum in Soils Closure Checklists, General Closure Provisions Page 2 of 3

General Closure Provision #2—Abatement Satisfied

□ Not Satisfied □

Requirement: Appropriate abatement actions were taken in response to the release to prevent further

degradation, and closure decisions are being made on residual soil contamination only.

Some things to look for when determining that the “Abatement” provision is Satisfied

• If the case was the result of a release to the ground, the owner or operator conducted abatement

and/or cleanup prior to applying action levels or pursuing a risk-based closure.

• The case was the result of discovery of subsurface contamination, and the owner or operator

pursues a risk-based closure after reducing the volume or concentration of contaminants remaining in

place.

• If the NDEP used EMAR or other sources of funding to address an imminent and substantial hazard

at the site or another government agency expended funds to address hazards that are recoverable

through regulation, cost recovery has been resolved with the owner or operator.

Notes: ______________________________________________________________________________

___________________________________________________________________________________

Recommended next steps: ______________________________________________________________

___________________________________________________________________________________

General Closure Provision #3—Characterization Satisfied

□ Not Satisfied □

Requirement: Site decisions were made with an understanding of contaminant makeup, location,

concentrations, and exposure pathways.

Some things to look for when determining that the “Characterization” provision is Satisfied

• Vertical delineation of soil contamination has been determined with either non-detect samples at

depths below the contamination, a data set showing clear decreasing trends of contaminants at

depth (approaching action levels or other acceptable concentrations under a risk-based closure), or

samples from the soil column all the way to the top of the water table.

• Lateral delineation of soil contamination has been determined by surface sampling, sidewall

sampling, or other exploratory sampling (drilling, boreholes, etc.).

• For sites with multiple release sources or commingled soil contamination, vertical and lateral

delineation has been achieved for all sources or areas of contamination.

• Density of confirmation samples is adequate to make an informed decision (e.g., equal to or greater

than the regulatory minimum established for a UST excavation or an equivalent density for larger

excavations).

Notes: ______________________________________________________________________________

___________________________________________________________________________________

Recommended next steps: ______________________________________________________________

___________________________________________________________________________________

NDEP, Petroleum in Soils Closure Checklists, General Closure Provisions Page 3 of 3

General Closure Provision #4—Aesthetics Satisfied

□ Not Satisfied □

Requirement: The release has been addressed such that site conditions will not likely result in the re-

reporting of residual contamination by occupants, neighbors, or future owners based on visual or other

aesthetic conditions.

Some things to look for when determining that the “Aesthetics” provision is Satisfied

• Surface staining associated with remaining contaminants is not present in an area exceeding 4’ by

4’ after corrective action.

• Soils with contaminants near saturation are not present within 2 feet of ground surface after

corrective action, as this would likely cause surface staining in the future as a result of capillary action.

• Odors from volatile constituents are not noticeable and persistent in adjacent structures after

corrective actions.

• Excavations have been filled in, and the site is free of depressions that would promote ponding.

• Solid waste indirectly related to the release (such as vehicle wreckage at mobile source cases, solid

wastes co-disposed with hazardous substances at illegal dumps, etc.) have been removed or

addressed to end perception of release and to not promote future illegal disposal.

Notes: ______________________________________________________________________________

___________________________________________________________________________________

Recommended next steps: ______________________________________________________________

___________________________________________________________________________________

General Closure Provision #5—Imported Fill Satisfied

□ Not Satisfied □

Requirement: Any material imported to the site as a component of corrective action to fill excavations,

restore grades, or cap residual contaminants has either been demonstrated to be consistent with closure

criteria or is not likely to be inconsistent with closure criteria.

Some things to look for when determining that the “Imported Fill” provision is Satisfied

• Source of imported fill is discussed in the closure request and reasonably rules out the presence of

hazardous substances, regulated substances, or hazardous waste.

• Sampling results are provided to reasonably rule out the presence of hazardous substances,

regulated substances, or hazardous waste at concentrations inconsistent with site closure.

Notes: ______________________________________________________________________________

___________________________________________________________________________________

Recommended next steps: ______________________________________________________________

___________________________________________________________________________________

NDEP, Petroleum in Soils Closure Checklists, “Clean Closure” Checklist Page 1 of 1

Nevada Division of Environmental Protection

Petroleum in Soils Closure Checklists

“Clean Closure” Checklist

Facility Name:

Case Officer:

Facility ID:

Date:

Clean Closure: all confirmation samples (using analytical EPA Method 8015 modified) at the site are

below 100 mg/kg for TPH. This is the reportable concentration that has been set for TPH by the NDEP as

generally posing an acceptable level of risk for all exposure scenarios.

Clean Closure Checklist

All Requirements In Grey Must Be Met

____

Proper field sample collection procedures used

Confirmation samples are taken as discrete samples and are collected and preserved using

appropriate procedures to minimize loss of volatile constituents prior to analysis.

____

Proper laboratory analytical method used

All confirmation samples are analyzed using EPA Method 8015 Modified for Petroleum Hydrocarbons

____

Proper laboratory sample preparation procedure used

The laboratory preparation procedure is appropriate for the type of petroleum product released:

Gasoline—Purge and Trap

Diesel and other mid-range products—Purge and Trap + Solvent Extraction

Oil and other high-range products—Solvent Extraction

Unknown—Purge and Trap + Solvent Extraction

____

Appropriate detection limit achieved

The reported detection limit from the laboratory is less than 100 mg/kg for total petroleum

hydrocarbons for all confirmation samples.

____

Action level for clean closure met in all confirmation samples

All confirmation samples are below 100 mg/kg for Total Petroleum Hydrocarbons.

____

Destruction certificates or disposal certificates provided for all excavated soil

All soil above 100 mg/kg TPH that was excavated as a result of corrective action or abatement actions

and that has been taken off-site for treatment or disposal has been accounted for with disposal or

destruction certificates. If soil has been treated on-site and remains on-site in accordance with an

approved corrective action plan, this requirement may be marked N/A and considered satisfied.

NDEP, Petroleum in Soils Closure Checklists, “Analyte-Specific Closure” Checklist Page 1 of 1

Nevada Division of Environmental Protection

Petroleum in Soils Closure Checklists

“Analyte-Specific Closure” Checklist

Facility Name:

Case Officer:

Facility ID:

Date:

Analyte-Specific Closure: all confirmation samples show that petroleum hydrocarbon constituents are

below default screening levels established by the NDEP to be protective of direct contact exposure and

leaching to groundwater. Constituents to be screened are selected based on their toxicity (both cancer

and non-cancer hazards) and presence in petroleum formulations.

Analyte-Specific Closure Checklist

All Requirements In Grey Must Be Met

____

Proper field sample collection procedures used

Confirmation samples are taken as discrete samples and are collected and preserved using

appropriate procedures to minimize loss of volatile constituents prior to analysis.

____

All contaminants of potential concern have been analyzed

Confirmation samples contain analytical results for all contaminants of potential concern associated

with the petroleum product released. The contaminants of potential concern are identified on Table 1

of Appendix B. If the petroleum product has not been identified, all constituents on the Table should

be analyzed.

____

Proper laboratory analytical methods used

All confirmation samples are analyzed using the appropriate laboratory method identified on Table 1

of Appendix B, and the laboratory has employed an appropriate sample preparation for the analytical

method.

____

Appropriate detection limit achieved

The reported detection limit from the laboratory is below the screening level for all constituents. (This

may require the use of Selected Ion Monitoring for polynuclear aromatic hydrocarbons for sites

where they are a contaminant of potential concern.)

____

Action levels for Analyte-Specific Closure have been met

All concentrations are below the action levels for analyte-specific closure in all confirmation samples.

____

Residual TPH concentrations are not indicative of NAPL migration

All concentrations of TPH are below the levels indicative of NAPL migration for the soil type at the site

as published by the American Petroleum Institute in Appendix C

____

Land use assumptions are supported and protective

If the higher action levels for industrial or commercial exposure scenarios are used at the site,

information presented by the facility owner or operator should demonstrate that future land use will

remain industrial/commercial or is controlled through an environmental covenant.

____

Environmental Covenant discussed when residual petroleum contamination exceeds 100 yds

If greater than 100 yds

of petroleum impacted soil is to remain on the site, an environmental

covenant should be considered and discussed with a supervisor to determine whether future

management of petroleum contaminated soils needs to be controlled.

____

Destruction certificates or disposal certificates provided for all excavated soil

All soil above 100 mg/kg TPH that was excavated as a result of corrective action or abatement actions

and that has been taken off-site for treatment or disposal has been accounted for with disposal or

destruction certificates. If soil has been treated on-site and remains on-site in accordance with an

approved corrective action plan, this requirement may be marked N/A and considered satisfied.

NDEP, Petroleum in Soils Closure Checklists, “A-thru-K Closure” Checklist Page 1 of 1

Nevada Division of Environmental Protection

Petroleum in Soils Closure Checklists

“A-thru-K Closure” Checklist

Facility Name:

Case Officer:

Facility ID:

Date:

A-thru-K Closure: concentrations of petroleum hydrocarbons and petroleum constituents above

screening levels may be left in place based on a site-specific analysis of the A-thru-K factors listed in the

corrective action regulations to show that known residual concentrations are still protective even if they

are above default action levels.

A-thru-K Closure Checklist

All Requirements In Grey Must Be Met

____

“A-thru-K” closure request presented in an acceptable format

The “A-thru-K” presents a coherent, defensible argument for closing the site with contamination

above action levels, and it includes all supporting data, figures, and calculations relied on in the

argument.

____

Data quality is sufficient to make defensible determinations about protectiveness

The “A-thru-K” analysis is based on data of sufficient quality as determined either by adherence to an

approved quality assurance project plan or to generally accepted standard operating procedures for

data collection and analysis.

____

All constituents of concern have been identified and properly addressed

The “A-thru-K” closure request addresses all constituents of concern at the site. Constituents of

potential concern include all the constituents associated with the petroleum product that has been

released; constituents of concern include all the constituents of potential concern that exceed health-

based standards (Table 1 of Appendix B).

____

All exposure pathways have been examined and properly addressed

The “A-thru-K” closure request examines all exposure pathways and determine whether they are

incomplete, potentially complete, or complete at the site.

____

The direct contact exposure pathway is demonstrated to be incomplete

Contamination in the top 6 feet at a site must be below analyte-specific action levels (Table 1 of

Appendix B) or demonstrated to be inaccessible both to excavation/treatment and to direct contact

by receptors.

____

Petroleum saturated soils have been remediated or removed to a reasonable extent

The facility owner or operator must make reasonable efforts to treat or remove soils that are

indicative of NAPL formation or migration (API, Appendix C) as a step to minimize further degradation

of subsurface soils or potential impacts to groundwater. The reasonableness of efforts may consider

the vicinity of structures, depths of contamination, or remoteness of the location. If petroleum

concentrations above screening levels for NAPL migration remain at the site, vadose zone modeling or

calculations must demonstrate that groundwater impacts will not occur or will be sufficiently

controlled.

____

Environmental Covenant discussed when residual petroleum contamination exceeds 100 yds

If greater than 100 yds

of petroleum impacted soil is to remain on the site, an environmental

covenant should be considered and discussed with a supervisor to determine whether future

management of petroleum contaminated soils needs to be controlled through a covenant. The

covenant may also stipulate specific land use practices, engineering controls, and periodic review and

reporting to NDEP to affirm maintenance of the engineering and institutional controls.

NDEP, Petroleum in Soils Closure Checklists, “ASTM RBCA Closure” Checklist Page 1 of 1

Nevada Division of Environmental Protection

Petroleum in Soils Closure Checklists

“ASTM RBCA Closure” Checklist

Facility Name:

Case Officer:

Facility ID:

Date:

ASTM RBCA Closure: all concentrations of petroleum hydrocarbons and constituents are below Site

Specific Target Levels established through a Tier II or Tier III analysis conducted consistent with the

ASTM Risk-Based Corrective Action method E1739-95 or equivalent.

ASTM RBCA Closure Checklist

All Requirements In Grey Must Be Met

____

ASTM RBCA conducted in accordance with Method E1739-95

The facility owner/operator and their consultant have submitted sufficient information to the NDEP

and in a format that allows the NDEP to determine whether the Method was followed appropriately.

____

Data quality is sufficient to make defensible determinations about protectiveness

The analyses in the ASTM RBCA are based on data of sufficient quality as determined either by

adherence to an approved quality assurance project plan or to generally accepted standard operating

procedures for data collection and analysis.

____

All constituents of concern have been properly addressed in the RBCA analysis

Procedures in the ASTM RBCA method are followed for the identification of contaminants of concern.

Site Specific Target Levels are developed for all contaminants of concern.

____

All exposure pathways have been examined and properly addressed

Procedures in the ASTM RBCA method are followed for the identification of completed exposure

pathways and the Site Specific Target Levels are established based on the most conservative exposure

pathway calculation for the site.

____

Confirmation sampling shows constituents of concern to be below Site Specific Target Levels

Samples show that residual contamination is below Site Specific Target Levels developed for the site.

The density and quality of samples is sufficient to demonstrate achievement of Site Specific Target

Levels.

____

Residual TPH contamination addressed either directly or indirectly in the ASTM RBCA

Residual TPH contamination is shown to be unlikely to further degrade subsurface soils or

groundwater through either the development of SSTLs for TPH or through the excavation and

treatment of soils above screening levels for NAPL migration published by the API in their June 2000

“Soil and Groundwater Research Bulletin” (Appendix C).

____

Environmental Covenant discussed when residual petroleum contamination exceeds 100 yds

If greater than 100 yds

of petroleum impacted soil is to remain on the site, an environmental

covenant should be considered and discussed with a supervisor to determine whether future

management of petroleum contaminated soils needs to be controlled through a covenant. The

covenant may also stipulate specific land use practices, engineering controls, and periodic review and

reporting to NDEP to affirm maintenance of the engineering and institutional controls.

Nevada Division of Environmental Protection

Bureau of Corrective Actions

Petroleum in Soils Closure Checklists

Appendix B

Tables

NDEP, Petroleum in Soils Closure Checklists, Table 1: “Analyte-Specific Closure” Page 1 of 1

NDEP Petroleum In Soils Closure

Table 1: “Analyte-Specific Closure” Levels

Analyte Name

Preparation/

Analytical Method

Gasoline

Diesel

Heating oil

Jet Fuel

Residential

(mg/kg)

Industrial/

Commercial

(mg/kg)

Acenaphthene

3540

/8270C or D

3600

45000

Anthracene

3540/8270C or D

18000

230000

Benzene

5035/8260B

1.2

5.1

Benzo(a)anthracene

3540/8270C or D

1.1

Benzo(a)pyrene

3540/8270C or D

0.11

2.1

Benzo(b)fluoranthene

3540/8270C or D

1.1

Benzo(k)fluoranthene

3540/8270C or D

210

Chrysene

3540/8270C or D

110

2100

Dibenz(a,h)anthracene

3540/8270C or D

0.11

2.1

Ethylbenzene

5035/8260B

5.8

Fluoranthene

3540/8270C or D

2400

30000

Fluorene

3540/8270C or D

2400

30000

Ideno(1,2,3-c,d)pyrene

3540/8270C or D

1.1

Methyl t-butyl ether (MTBE)

5035/8260B

210

1-Methylnaphthalene

3540/8270C or D

2-Methylnaphthalene

3540/8270C or D

240

3000

Naphthalene

5035/8260B or 3540/8270C or D

2.0

8.6

Pyrene

3540/8270C or D

1800

23000

Styrene

5035/8260B

6000

35000

Toluene

5035/8260B

4900

47000

1,2,4-Trimethylbenzene

5035/8260B or 3540/8270C or D

300

1800

1,3,5-Trimethylbenzene

5035/8260B or 3540/8270C or D

270

1500

Xylene (mixture)

5035/8260B

580

2500

Notes:

The use of this table is subject to the general and specific provisions listed in the Petroleum in Soils Closure Checklists. The values shown

in this table represent those published in the May 2022 version of the US EPA “Regional Screening Levels – Generic Tables”. The US EPA

Regional Screening Levels are periodically updated and published on the US EPA’s website (https://www.epa.gov/risk/regional-screening-

levels-rsls). The most current version of the Regional Screening Levels should always be consulted prior to use.

The EPA Methods listed for each constituent are accepted for use by this Division. Other Laboratory methods may be acceptable to the

Division but must be pre-approved prior to use.

Residential screening levels for all constituents are based on numbers developed by the US EPA as the “Regional Screening Levels for

Chemical Contaminants at Superfund Sites” for a default residential exposure scenario with a target cancer risk (TR) of 1E-06 and a target

hazard quotient (THQ) of 1.0.

Industrial screening levels for all constituents are based on numbers developed by the US EPA as the “Regional Screening Levels for

Chemical Contaminants at Superfund Sites” for a default exposure scenario with a target cancer risk (TR) of 1E-06 and a target hazard

quotient (THQ) of 1.0. Industrial screening levels can be allowed at industrial and commercial sites where a case officer is comfortable

that the reasonable expectation of future land use is industrial or commercial; if there is any doubt about future land use or surrounding

land use, an environmental covenant should be considered in order to control the land-use assumption or additional corrective action

should be undertaken to meet residential screening levels.

EPA Method 3550 for ultrasonic extraction is also an acceptable method with any 8270C or D analysis in this table. When using method

3550 it is necessary to determine whether the low-concentration procedure or moderate/high-concentration procedure is appropriate.

The low-concentration procedure is appropriate if anticipated concentrations are below 20 mg/kg.

A differentiation is made between JP-4 and other formulations of jet fuel. Where a constituent is marked “4”, that constituent should only

be sampled if the source of contamination is suspected to be JP-4. An “X” indicates that the constituent should be sampled for any

formulation of jet fuel.

An optional procedure in EPA Method 8270 for lower detection limits on polynuclear aromatic hydrocarbons (PAH) is Selected Ion

Monitoring (SIM). In order to achieve detection limits below residential screening levels and some industrial screening levels, the NDEP is

currently recommending the use of SIM on confirmation samples for PAHs.

A facility owner/operator or CEM may exclude MTBE from gasoline sampling if the NDEP case officer agrees with the argument for

exclusion based on the timeframe of the release or otherwise conclusive proof that the additive was not present at the time of release.

NDEP, Petroleum in Soils Closure Checklists, Table 2: “A-thru-K Closure” Page 1 of 3

NDEP Petroleum in Soils Closure

Table 2: Types of information that may be appropriate for an A-thru-K Closure

Simple Sites

Small volume of residual soils

Low concentrations

Incomplete exposure pathways

Qualitative analysis

Complex Sites

Large volume of residual soils

High concentrations

Complete exposure pathways

Quantitative analysis

(a) The depth of any groundwater

A narrative of contaminant locations in relation

to the top-most water table from site-specific

investigations or reasonably accurate,

published sources

An understanding of all productive saturated

zones underlying the site

First-order fate and transport calculations

Site comparisons of contaminant migration to

groundwater from similar sites in the vicinity

Multi-phase vadose zone modeling of

contaminant transport to the top-most water

table

Identification of all nearby sources of hydraulic

head such as areas of heavy irrigation,

infiltration basins, leaking utilities, etc.

Analysis of historic water table fluctuations

(b) The distance to irrigation wells or wells for drinking water

Inventory of all wells within a ¼-mile, ½-mile, and

1-mile radius

Narrative of general groundwater consumption in

the vicinity of the site, i.e. prevalence of

irrigation, domestic use, and municipal supply

General depths of drinking water and irrigation

extraction from area groundwater

Construction details of wells in the vicinity of the

site

Saturated zone modeling of potential

groundwater contaminant transport with

established wells as target points

Environmental covenant on groundwater use to

preclude future well installations

A discussion of soil type and its relation to

migration of contaminants to groundwater or

exposure of residual contaminants due to

erosion or runoff

Site lithology and the presence or absence of

aquitards

Quality of imported fill

Derivation or collection of soil properties for

vadose zone modeling including effective

porosity, bulk density, soil organic carbon, and

water content

Biological activity of the soil and biodegradation

of residual contaminants

Environmental covenant which precludes export

of site soils for use as fill in other locations

(d) The annual precipitation

A narrative of precipitation and storm events

typical for the site and its relation to vadose

zone migration and surface runoff

Infiltration calculations

Presence of site features that may act

equivalently to engineered controls or caps to

prevent run-off or infiltration, such as paved

surfaces

Identification of any major sources of additional

hydraulic head in the vicinity of the site

Use of site-specific precipitation regime in vadose

zone modeling

Drainage analysis for a 24-hour 25-year storm

event

Engineered controls for prevention of run-off or

infiltration

Use of designed and maintained evapo-

transpirative covers

NDEP, Petroleum in Soils Closure Checklists, Table 2: “A-thru-K Closure” Page 1 of 3

(e) The type of waste or substance that was released

For petroleum releases, the specific petroleum

product involved in the release should be

known or deduced based on site sampling

The age of the release and the amount of

weathering of petroleum contamination should

be determined

While the NDEP is already aware of the mobility,

toxicity, and amenability to treatment of most

petroleum products, it should be shown how

these factors were considered as part of a

determination to leave contamination in-place

TPH fractional analysis for use to model total

petroleum hydrocarbon behavior in the

subsurface

Leach tests of site samples as an input to vadose

and saturated zone modeling

(f) The extent of the contamination

As a general condition of closure, all sites should

have an understanding of the lateral and

vertical extent of remaining contaminants

Simple mass calculations for remaining

contaminants, contaminants removed through

corrective action, and contaminants released to

the environment, if known

Identification and extent of any contaminants at

concentrations that indicate potential soil

saturation

Stratigraphic analysis of subsurface

concentrations for a conceptual model of

historic fate and transport of petroleum

constituents in the soil

Modeling of potential LNAPL formation and

movement in the vadose zone

Mass flux potential to groundwater

(g) The present and potential use for the land

An operational site history including present use

and recently past uses

A discussion of property uses in the vicinity of the

site

Property zoning

Identification or location of overlying or adjacent

Redevelopment Zones

Deed restrictions

Environmental covenant containing specific land

use restrictions

Durable engineering controls intended to prevent

future exposures regardless of land use

(h) The preferred routes of migration

A complete Conceptual Site Model including

migration pathways and receptors

Vapor intrusion screening

Identification of preferential paths for migration

for any and all pathways, such as utility

corridors, vaults, unsealed well bores,

discontinuities in aquitards, sand lenses, etc.

Detailed vapor intrusion studies and risk

assessment

(i) The location of structures or impediments

If petroleum constituents above health-based

action levels are left in-place in the top six feet

of soil, a fully detailed rationale should be given

as to why the location of structures or

impediments prevents corrective action

An updated, correctly scaled and labeled site map

showing locations of residual contamination in

relation to all surface structures and subsurface

utilities, to the extent known

Structural analysis to support termination of

excavation of contaminants at depth, including

both the integrity of surrounding structure

foundations and excavation shoring

requirements

NDEP, Petroleum in Soils Closure Checklists, Table 2: “A-thru-K Closure” Page 1 of 3

(j) The potential for a hazard related to fire, vapor or an explosion

A professional judgment as to whether the

volume and concentrations of residual

contamination would be able to produce

flammable vapors

Identification of all adjacent subsurface, enclosed

spaces, particularly any utility vaults or any

other space with potential ignition sources

PID readings from any adjacent subsurface vaults,

basements, crawlspaces, or sub-basements

Installation of vapor barrier, ventilation or

recovery systems

(k) Any other information specifically related to the site which the director determines is appropriate

Recalcitrance of site contaminants to past

corrective actions

Data Quality Assessment in accordance with

federal guidelines

Nevada Division of Environmental Protection

Bureau of Corrective Actions

Petroleum in Soils Closure Checklists

Appendix C

American Petroleum Institute

“Soil & Groundwater Research Bulletin” June 2000

Non-Aqueous Phase Liquid (NAPL) Mobility Limits in Soil

assessment. This paper is confined to discussion of the mobility

of non-aqueous phase liquids, either as pure chemicals or as

chemical mixtures.

Many organic chemicals, including hydrocarbons, are nearly

immiscible in water. Release of a non-aqueous phase liquid

(NAPL) to near-surface unsaturated soil can result in downward

gravity-driven migration of the NAPL towards the water table.

At the water table, light nonaqueous phase liquids (LNAPL),

including petroleum, which are less dense than water, will

mound and spread horizontally. LNAPL may also move with

the groundwater gradient. Dense nonaqueous phase liquids

(DNAPL) will migrate downward, mound, and spread

horizontally, until a path of least resistance further downward

into the saturated region is found. This could be when the

accumulation is great enough to exceed the capillary entry

pressure into the saturated zone, or when the DNAPL mound

reaches a region of high vertical permeability, or when it reaches

a fracture.

The volume of mobile NAPL depletes as immobile residual

chemical is left behind through the soil column in which the

NAPL is descending. NAPL migration may be limited by this

depletion, or by physical barriers, such as low permeability

layers. Our intent in this paper is to determine conservative

NAPL concentrations in unsaturated soil, below which the NAPL

will be immobile. By "conservative" we mean under-predicting

the concentration at which mobility would actually occur.

PRESENCE OF A

NAPL IN SOIL

For a pure chemical, NAPL will not be present at concentrations

below the soil saturation limit (USEPA, 1996; ASTM E1739,

PS104-98), defined as:

[1]

with

sat,soil,i

soil saturation limit for chemical i (mg/kg)

pure chemical aqueous solubility limit for

chemical i (mg/L)

soil water content (cm

-water/cm

-soil)

SOIL & GROUNDWATER

RESEARCH BULLETIN

ABSTRACT

Conservative screening concentrations for non-aqueous phase

liquids (NAPL) that could be considered immobile in unsaturat-

ed zone soils are presented. Total concentrations measured at a

crude oil or petroleum product release site (using total petrole-

um hydrocarbon [TPH] or a similar analysis method) can be

compared to the screening concentrations to determine the

potential for NAPL to migrate in soil. The screening values are

based on an analysis of published data for a range of soil texture

classifications and a range of NAPL density from 0.7 to 1.5

g/cm3.

The paper includes summary tables and histograms of residual

NAPL void fraction, Sr, as a function of soil type. These provide

a basis for selecting conservative values used in calculating

screening concentrations for immobile NAPL. For example, in

medium to coarse sands, with Sr = 0.06 cm3-oil/cm3-void, one

would expect that NAPL would be immobile in 90% of samples

with equivalent NAPL concentration levels for this soil type.

Measured concentrations of immobile NAPL reported in the lit-

erature vary considerably with soil type, chemical composition,

and the measurement method. The proposed screening levels

are conservative (lower range) estimates within the range of

measured residual NAPL concentration values. Higher values

could be applicable in many cases, both in unsaturated and sat-

urated soil conditions.

This paper addresses immobile bulk NAPL in soils at concen-

trations up to the threshold of mobility. This document does not

address the movement and flow of NAPL, the dissolution of

NAPL chemical into soil pore water solution, nor NAPL

volatilization into soil pore air. Transport by these mechanisms

may be estimated using other published and accepted methods.

INTRODUCTION

Organic chemicals released to soil may migrate as vapors in soil

gas, as dissolved constituents in soil pore water, or as a bulk

phase liquid which is immiscible in water. Assessment of poten-

tial migration pathways for chemical releases into the

environment are discussed in several related documents

(USEPA 1996, 1991; ASTM E1739, PS104-98). These

migration pathways are important in a general risk-based site

American

Petroleum

Institute

A summary of research

results from the American

Petroleum Institute & GRI.

June 2000 No. 9

NON-AQUEOUS PHASE

LIQUID (NAPL) M

OBILITY LIMITS IN SOIL

EDWARD

J. BROST

✦

GEORGE E. D

EVAULL

✦

EQUILON E

NTERPRISES LLC

✦

WESTHOLLOW

TECHNOLOGY CENTER

✦

HOUSTON

, TEXAS

oc,i

organic carbon/water partition coefficient

for chemical i (L-water/kg-oc)

mass fraction of organic carbon in soil (g-oc/g-soil)

dry soil bulk density (g/cm

)

Henry's law coefficient for chemical i

(cm

-water/cm

-air)

soil air content (cm

-air/cm

-soil)

For a pure chemical, C

sat,soil

is a value above which the chemical

is present in soil pore water at its aqueous solubility limit, and is

present in soil pore air at its saturated vapor concentration.

Equilibrium partitioning of the chemical between soil (sorbed),

pore water, and pore vapors at concentrations below C

sat,soil,i

presumed.

For mixtures of miscible chemicals that are fractionally soluble

in water, including petroleum, the concentration at which NAPL

will be present is a function of the mixture composition. The soil

saturation limit for the mixture, using methods presented in

Johnson et al., (1990), Mott (1995), and Mariner (1997), is:

[2]

with

sat,soil,T

soil saturation limit for the NAPL mixture,

total concentration (mg/kg)

mass fraction of each chemical i in the NAPL

mixture (kg/kg)

N the number of individual chemicals in the mixture

Note that Eq. [2] simplifies to Eq. [1] for a single chemical. The

component concentration of a chemical i at the soil saturation

limit in a mixture is (C

sat,soil,T

). The soil saturation limit

calculated for a pure chemical, in every case, will be greater

than the chemical component concentration (C

sat,soil,T

) calcu-

lated for a mixture, that is:

sat-soil,i

sat,soil,T

Eq. [1] overstates C

sat-soil,i

for components in a mixture because it

does not consider effective vapor pressure and solubility limits

(Rault's law) for the mixture components (USEPA, 1996). The

soil saturation limits for mixtures (and pure chemicals) tabulated

in this paper were calculated with computer codes included with

DeVaull et. al., (1999). This method is consistent with the

references cited above.

RESIDUAL NAPL CONCENTRATION

Our intent in this paper is to define a soil concentration, C

res,soil

below which the NAPL, if present, will not migrate due to

convection or gravity. This refers to a pure chemical concentration

or a total chemical mixture concentration, as applicable. This

residual NAPL concentration in soil is specified as:

[3]

with

= S

and

res,soil

residual NAPL concentration in soil (mg-res/kg-soil)

residual non-aqueous phase volume fraction

(cm

-res/cm

-soil)

density of chemical residual non-aqueous phase

liquid (g-res/cm

-res)

dry soil bulk density (g-soil/cm

-soil)

soil porosity (cm

-void/cm

-soil)

fraction of residual non-aqueous phase filled void

(cm

-res/cm

-void)

Residual non-aqueous phase volume fraction (θ

, or retention

capacity) is similarly defined by Cohen and Mercer (1990) and

Zytner et. al., (1993), but in dimensional units of (cm

-res/L-soil).

The value of C

res,soil

is generally much larger than the soil

saturation limit, C

sat,soil

. Eq. [3] includes only the residual NAPL

volume. Additional chemical mass within the soil matrix is

contained in soil pore water and soil pore air, and is sorbed onto

soil. These volumes may be included in a slightly more compli-

cated equation consistent with the assumptions in Eqs. [1] and

[2]; these terms may generally be neglected. This leaves the

residual NAPL concentration in soil, C

res,soil

, directly related to

the residual NAPL volume fraction in soil, θ

, or the residual

NAPL fraction in the voids, S

Below the residual NAPL concentration in soil, C

res,soil

, capillary

retention forces are greater than the gravitational forces which

tend to mobilize the NAPL. These capillary forces (in this

context, including surface tension effects, van der Waals, and

Coulombic forces), particularly at low residual non-aqueous

phase levels, may exceed the gravitational force by several

orders of magnitude. The residual NAPL concentration in soil,

res,soil

, may depend on NAPL properties including liquid density,

surface tension, and viscosity. It also may depend on soil

properties including porosity, organic carbon fraction, moisture

content, relative permeability, moisture wetting history, and soil

heterogeneity.

For concentrations greater than the threshold C

res,soil

level,

capillary retention forces are less than the gravitational forces,

and the NAPL is mobile. Movement of NAPL in soil is beyond

the scope of this paper. It is covered in a number of references,

however, including Charbeneau (1999), Huntley and Beckett

(1999), USEPA (1991), Cohen and Mercer (1990), and

Pfannkuch (1983).

This paper describes the determination of screening values for

NAPL immobility in soil. Screening values are expressed as the

residual NAPL concentration in soil, C

res-soil

, the non-aqueous

phase volume fraction in soil, θ

, and the residual non-aqueous

phase fraction in the soil voids. Our study included a review of

existing measured data on residual NAPL concentration in soil,

published empirical models, and methods of field measurement.

The calculated value, C

sat,soil

, as previously defined in Eqs. [1]

and [2] predicts the presence or absence of a residual NAPL.

Since a NAPL must be present to be mobile, it also represents a

conceivable screening concentration for NAPL mobility.

However, observed residual NAPL concentrations based either

on laboratory measurement or physical removal of NAPL from

impacted sites are typically several orders of magnitude higher

than C

sat,soil

. The value C

sat,soil

specifies the presence or absence of

a residual phase; it does not address mobility. In this effort, we

have used available data to define values for C

res,soil

which can be

conservatively used to screen sites for NAPL mobility. A

comparison of calculated C

sat,soil

values with measured values

of C

res,soil

is shown in Table 1 for selected chemicals and

hydrocarbon mixtures.

The trend of C

sat,soil

in Table 1 decreases with decreasing chemical

(or mixture) solubility and vapor pressure. The measured

values of residual NAPL concentration in soil and residual

NAPL fraction in voids do not show a similar decreasing trend.

Therefore, using a calculated C

sat,soil

value as a screening level for

the mobility of a residual phase becomes increasingly and

significantly more conservative for less soluble, less volatile

chemicals and chemical mixtures.

Screening levels for NAPL mobility consistent with the

definition of residual NAPL concentration n soil, C

res,soil

, have

already been implemented in a number of programs. The State

of Ohio [OAC 3745-300-08 Generic Numerical Standards] has

promulgated rules, including values of residual NAPL concen-

tration in soil, for several combinations of specified soil types

and petroleum composition ranges. The State of Washington

[WAC 173-340-747 Part VII Cleanup Standards] has proposed

values based on a similar methodology. CONCAWE (1979,

1981) provides residual NAPL concentration in soil values for a

range of petroleum products and soil types.

EXISTING MODELS AND METHODS

Monographs are available which detail the movement of NAPL

in soils (Charbeneau, 1999; Huntley and Beckett, 1999; USEPA,

1991; Cohen and Mercer, 1993; and Pfannkuch, 1983). Several

investigators have specifically developed empirical models for

predicting immobile NAPL, as a residual NAPL concentration

in soil, C

res,soil

, for a limited number of NAPL types in various

soil matrices. Summaries of two published approaches follow.

Hoag and Marley (1986) proposed an empirical method to

estimate residual NAPL saturation values for gasoline in dry

sand and in sand matrices containing moisture at field capacity.

Their equations, which relate measured gasoline retention at

residual saturation with soil particle surface area, are:

[4a]

[4b]

with

res,soil

residual NAPL concentration in soil (mg-res/kg-soil)

average sand particle diameter (cm)

density of water (g/cm

) = 1

Eqs. [4a] and [4b] refer, respectively, to residual NAPL

concentration in dry soil and soil initially at field moisture

capacity. An assumption in these equations is that the soil

particles and soil surface area can be defined by an average soil

particle diameter (Sauter mean diameter). These authors found

that changes in soil surface area adequately predicted changes in

residual NAPL saturation. Smaller soil particles have greater

available surface area in a given volume or weight of soil, and

the associated narrower pores will result in greater capillary

forces. Residual NAPL concentration in soil therefore decreases

with increasing particle size. At field capacity moisture content,

measured C

res,soil

was reduced. At field capacity moisture, many

of the smaller pore spaces are saturated with water. This

reduces the overall pore volume available for trapping NAPL.

Eqs. [4a] and [4b] were developed using Connecticut sands

sieved into three classifications; fine (d

= 0.0225 cm), medium

= 0.0890 cm) and coarse (d

= 0.2189 cm ). A fourth set of

experiments was conducted using mixed sands with the mixture

being made from equal portions of each of the above three clas-

sifications. Effectively, Eqs. [4a] and [4b] have been developed

for data in the range of:

0.02 cm < d

< 0.22cm

Zytner et. al., (1993) correlated measured soil retention capacity

with soil porosity, soil bulk density, and NAPL density. Their

experiments included several NAPL types in a variety of natural

soils. The soils were air dried (less than 1.5% moisture),

saturated with NAPL, and then allowed to drain. Their empirical

equation, for dry soils is:

[5]

Table 1. Residual NAPL Concentration in Soil Compared to Soil Saturation Limit.

with

res,soil

residual NAPL concentration in soil (mg-res/kg-soil)

soil porosity (cm

-void/cm

-soil)

density of chemical residual NAPL (g-res/cm

-res)

dry soil bulk density (g-soil/cm

-soil)

This study was limited to air dried soils and did not specifically

include sand. It does, however, show a dependence of C

res,soil

soil porosity, θ

, and chemical density, ρ

A wide range of natural soils was used in the development of

Eq. [5], including sandy loam (θ

= 0.45), clay (θ

= 0.466),

organic top soil (θ

= 0.555), two different peat mosses (θ

0.8), as well as mixtures of these soils. Three NAPL types were

included in their work to assess the influence of NAPL density

on retention capacity: tetrachloroethene (ρ

= 1.622 g/cm

trichloroethene (ρ

= 1.456 g/cm

), and gasoline (ρ

= 0.75 g/cm

res,soil

values obtained in their study ranged from 414,000 to

6,894,000 mg/kg for PCE, 329,000 to 5,219,000 mg/kg for

TCE, and 94,000 to 2,738,000 mg/kg for gasoline. Effectively,

Eq. [5] has been developed for data in the range of:

[6]

The broad range of values for C

res,soil

can be attributed to the

range in soil densities, from 0.2 g/cm

(peat moss) to 1.5 g/cm

(sandy loam).

Although the C

res,soil

measurements used in developing Eqs. [4]

and [5] were conducted by different researchers using different

soils, a comparison of dry fine sand data (Hoag and Marley,

1986; θ

= 0.4, and ρ

= 1.6 g/cm

) with dry sandy loam data

(Zytner et. al., 1993; θ

= 0.45, ρ

= 1.5 g/ cm

) show very good

agreement of C

res,soil

of 104,000 and 115,000 mg/kg, respectively,

for gasoline.

MEASURED

DATA AND C

OMPARISON WITH

MODELS

Cohen and Mercer (1990) compiled measured residual NAPL

saturation data from several investigators, including residual

NAPL fraction in the voids, S

, or residual NAPL volume

fraction, θ

, for a number of organic liquids and soil types. These

values represent the residual amount of hydrocarbon remaining

in soil pore volume after the soil was saturated with hydrocarbon

and then allowed to drain. Values from Cohen and Mercer, with

additional tabulated data from other references, are included

in Table 2 (see pages 5 and 6). This table also includes

additional values derived from the experimental data, including

the residual NAPL concentration in soil, C

res,soil

The values in Table 2 vary considerably between experiments,

soil types, and chemicals. While this may be due to differences

in laboratory test methods, it may also indicate the reasonable

range in measured residual NAPL concentration in soils encoun-

tered between different soil types, chemical types, and measure-

ment observations.

Calculated values for the soil saturation limit, C

sat,soil

, for the

indicated chemicals or chemical mixtures, are included in Table

2. These values are plotted in Figure 1. In all cases, C

res,soil

greater than C

sat,soil

. As a measure of immobile NAPL, C

sat,soil

Figure 1. Comparison of data for residual NAPL concentration

in soil, C

res,soil

, to the calculated soil saturation limit, C

sat,soil

. All

plotted values are from Table 2. The solid diagonal line marks a

direct correspondence between residual NAPL concentration in

soil and soil saturation limit. For ranges of residual NAPL

concentration in soil data in the same test series (Table 2), the

upper and lower values are joined by a horizontal line. In all

cases the calculated soil saturation limit is much less than the

measured residual NAPL concentration in soil.

Figure 2. Comparison of data for residual NAPL concentration in

soil, C

res,soil

, from Table 2 to the models of Eq. [4a] Hoag and

Marley (1986), zero soil moisture; Eq. [4b] Hoag and Marley

(1986), field capacity soil moisture; and Eq. [5] Zytner et al.,

(1993). Filled points indicate the data value is within the

intended range of model applicability. For ranges of residual

NAPL concentration in soil data (Table 2), both the upper and

lower values are shown as points. The solid diagonal line marks

a direct correspondence between measured and modeled residual

NAPL concentration in soil. The plot indicates that the empirical

models generally predict higher residual NAPL concentration in

soil than the measured values given in Table 2.

Table 2. Summary values of residual NAPL concentration in soil, C

res,soil

, residual NAPL volume fraction, θ

, and residual NAPL

fraction in the voids, S

. Calculated values for soil saturation limit, C

sat,soil

, are also shown. Parameters for the calculations are shown

in the second part of the table.

Table 2. (continued) Values for soil properties used in the calculations.

Several histograms of measured residual NAPL void fraction,

, as a function of soil type, are shown in Figure 3. These his-

tograms are based on the relevant data in Table 2 and provide a

basis for estimating conservative values of S

within a specified

statistical tolerance limit. Numerical values are given in Table

3. For example, with a medium to coarse sand, in specifying a

screening level of S

= 0.06, we would expect 90% of individ-

ual samples with equivalent NAPL concentrations below this

level to be immobile in this soil type.

We expect that the tolerance limits in Table 3 and Figure 3 are biased

conservatively, given that the Table 2 data showed lower residual

NAPL concentration in soils than the empirical correlations of Eqs.

[4] or [5]. The data in Table 2 is for NAPLs with densities ranging

from about 0.7 to 1.5 g/cm

. The screening values for residual

NAPL fraction in the voids, S

, in Table 3, should be valid and rea-

sonably conservative for this range in NAPL density.

Consolidated minimum values for S

are shown in Table 4 for

the various NAPL types in Table 2 listed as "medium sands".

Again, these should be reasonably conservative screening

values for NAPL mobility, for the indicated pure chemicals and

hydrocarbon mixtures. No tolerance limits are specified for the

Table 4 values, given the sparse data available when the screening

values are qualified by both soil type and NAPL composition. If

a tolerance limit is needed, or for chemicals not listed in Table

4 (with densities in the range of 0.7 to 1.5 g/cm

including

petroleum and crude oil), we suggest the use of the S

parameters

in Table 3 as screening values. A tolerance limit of 90% is

reasonable in most cases.

These screening values are intended to be worst-case estimates

for mobility. Higher values may be applicable on a site-specific

basis. For example, with an adequate distance in unsaturated

Figure 3. Cumulative distribution for measured residual NAPL

void fraction, S

, as a function of soil type. These cumulative

histograms are based on the data in Table 2. Values for the

"medium to course sand" and the "fine to medium sand" are very

similar over the distribution. The "coarse sand and gravel" shows

much lower values and narrower distribution of S

over the range

of different experiments. Tolerance limits for these distributions

are given in Table 3.

Table 3. Screening values for residual phase void fraction

as a function of soil type. The tabulated values are based on

distributions of data from Table 2 for each soil type. The 95%

statistical tolerance limit indicates that 5% of individual measure-

ments showed lower values for S

; the 50% tolerance limit is

the median value for the soil type. The 90% tolerance limit is

sufficiently conservative for most screening applications. The

distribution of values is plotted in Figure 3.

Table 4. Residual Saturation Screening Values. Values are

tabulated for medium to coarse sand and represent lower limits

from Table 2. If a tolerance limit is needed, or for chemicals

not listed (but with densities in the range of 0.7 to 1.5 g/cm

including petroleum products and crude oil), we suggest the

use of the S

parameters in Table 3 as screening values.

underpredicts measured values of C

res,soil

by a factor ranging

from 5 to over 50,000. As was noted in Table 1, the difference

between C

sat,soil

and C

res,soil

increases with decreasing NAPL

volatility and decreasing aqueous solubility.

A comparison of the data in Table 2 for residual NAPL concen-

tration in soil, C

res,soil

to the models of Eq. [4a], [4b], and [5] is

shown in Figure 2. Within the applicable range of values in the

original references, both models predict values of C

res,soil

which

are, on average, biased high relative to the comparable values

listed in Table 2. In all cases, excepting point 38 (tetra-

chloroethene) in Table 2, for Eq. [4a], the model to data ratio

ranges from 0.7 to 69; for Eq. [4b], the ratio ranges from 0.3 to

27; for Eq. [5], the model to data ratio ranges from 0.3 to 11.

Point 38 has an exceptionally broad range of measured C

res,soil

values in the same soil.

Both the models of Zytner et. al., (1993) and Hoag and Marley

(1986) are correlations based on measured data. The indicated

bias between the models and data of Table 2 could be due to

differences in data measurements methods, or may indicate the

reasonable range in variability for this type of measurement.

SCREENING VALUES FOR RESIDUAL NAPL

CONCENTRATION

Based on the model to data comparisons of the last section, it is

possible to specify conservative screening values for NAPL

mobility based on a range of qualifying information. In many

cases the screening levels will be very conservative estimates of

mobility. In such cases, site-specific measurements may be used

to refine the estimate, if necessary. Such measurements, for

example, could include observation (or lack thereof) of floating

and migrating hydrocarbon in shallow groundwater wells

surrounding a known NAPL source area.

ASTM E 1739-95: Guide for Risk-Based Corrective Action

Applied at Petroleum Release Sites, American Society for

Testing and Materials, West Conshohocken, PA.

ASTM PS 104-98: Standard Provisional Guide for Risk-Based

Corrective Action, American Society for Testing and

Materials, West Conshohocken, PA.

Boley, Troy M. and Thomas J. Overcamp, 1998.

Displacement of Non-wetting Liquids from Unsaturated Sands

by Water Infiltration, Ground Water Journal of the Association

of Groundwater Scientists and Engineers, September -

October, 1998.

Carsel, R.F. and R.S. Parrish, 1988. Developing Joint

Probability Distributions of Soil and Water Retention

Characteristics, Water Resources Research, 24(5): 755-769.

Cary J.W., J.F. McBride and C.S. Simmons, 1989a.

Trichlorethelene Residuals in the Capillary Fringe as Affected

by Air-entry Pressure, Journal of Environmental Quality,

18:72-77.

Charbeneau, R.J., R.T. Johns, L.W. Lake and M.J.

McAdams, Free-Product Recovery of Petroleum Hydrocarbon

Liquids, American Petroleum Institute Publication No. 4682,

Washington, D.C., June 1999.

Cohen, Robert M. and James W. Mercer, 1993. DNAPL

Site Evaluation; CRC Press, Inc., Boca Raton, FL.

CONCAWE (Conservation of Clean Air and Water -

Europe), DePastrovich, T.L., Baradat Y., Barthel R.,

Chiarelli A., and Fussell D.R., 1979. Protection of

Groundwater from Oil Pollution, CONCAWE Report No.

3/79.

CONCAWE, Fussell, D.R., H. Godjen, P. Hayward, R.H.

Lilie, A Macro and C. Panisi, 1981. Revised Inland Oil Spill

Clean-up Manual, CONCAWE Report No. 7/81, Den Haag,

150 pp.

Converly, M.P., 1979. The Behavior and Movement of

Petroleum Products in Un-consolidated Surficial Deposits,

M.S. thesis, University of Minnesota.

Huntley D. and G. D. Beckett, 1999. Assessment of LNAPL

Sources: Distribution, Mobility, Risk and Risk Reduction,

(American Petroleum Institute, Review Draft).

DeVaull, G. E., G. M. Deeley, C. C. Stanley, and W.

Hamilton, 1999. Development of Tier 1 Risk-Based

Corrective Action (RBCA) Tools for Application at

Exploration and Production Facilities, report prepared for Gas

Research Institute under GRI Contract No. 5097-210-3874,

Review Draft.

Hoag, G.E. and M.C. Marley, 1986. Gasoline Residual

Saturation in Unsaturated Uniform Aquifer Materials, Journal

of Environmental Engineering, ASCE, 112(3):586-604.

Johnson, P. C., Hertz, M.B., Byers, D. L., Estimates for

Hydrocarbon Vapor Emissions Resulting from Service Station

soil between the lower depth of a mobile NAPLand groundwater,

it may also be reasonable to account for potential NAPL redis-

tribution in the unsaturated soil layer. This redistribution would

decrease the concentrations of mobile NAPL to concentrations

in soil equivalent. to S

. After this redistribution, an acceptable

distance between the deepest expected NAPL penetration and

the historical top boundary of the water table capillary fringe

must still remain.

These screening values, as already discussed, are intended for

use in estimating conservative limits of NAPL mobility. The

data of Table 2 may be used for other purposes, such as relating

a known released volume of NAPL to an equivalent soil volume

at the residual concentration level. While it is not the purpose of

this paper to detail this type of calculation, the variability of an

estimated residual concentration level, as illustrated in Figure 3,

clearly needed to be considered.

SUMMARY AND CONCLUSIONS

Screening values describing residual saturation of NAPLs in

unconsolidated vadose zone soils have been tabulated. These

values are proposed for use in estimating concentrations of

immobile NAPL in soil. The values, in Tables 3 and 4, are based

on measured, published values for residual NAPL concentra-

tions in soil, C

res,soil

, in the unsaturated soil zone.

Another value, the soil saturation limit, C

sat,soil

, has already found

use as a screening level for NAPL mobility. C

sat,soil

is a calculat-

ed value estimating the presence of a residual NAPL. Data in

this paper shows C

sat,soil

, is a factor up to 50,000 times less than

the residual NAPL concentration in soil, C

res,soil

. For screening

immobile NAPL concentrations the soil saturation limit is

exceptionally conservative. We would instead recommend use

of the values in Tables 3 and 4.

A complete site assessment, in addition, would also include

evaluation of other potential transport mechanisms, including

soluble dissolution into mobile soil pore water, and volatiliza-

tion into soil pore air. These transport mechanisms, as noted

previously, are discussed elsewhere.

Use of residual NAPL concentration in soil values for screening

immobile NAPL presumes homogenous soils and soil properties.

Consolidated soil matrices, macropores, and fractures will

greatly affect the flow and movement of NAPL and must be

recognized when these screening values are applied. Further, we

note that the values have been developed using a limited data

set, from multiple authors, and no attempt has been made to

judge bias or error in the individual measurement techniques.

ACKNOWLEDGEMENT

Support for preparing this document from American Petroleum

and GRI (formerly the Gas Research Instutite) under GRI

Contract No. 5097-210-3874 is gratefully acknowledged.

REFERENCES

API, 1996. A guide to the Assessment and Remediation of

Underground Petroleum Releases, American Petroleum

Institute, Publication No. 1628, third edition, Washington,

D.C., July 1996.

Technical Meeting, Tucson, AZ (March 20-22), M.F. Hult, ed.,

USGS Water-Resources Investigation Report 84-4188.

Poulsen, M.M. and B.H. Kueper, 1992. A Field Experiment

to Study the Behavior of Tetrachloroethylene in Unsaturated

Porous Media, Environmental Science and Technology,

26(5):889-895, ACS.

USEPA, 1991. Assessing UST Corrective Action Technology -

A Scientific Evaluation of the Mobility and Degradability of

Organic Contaminants in Subsurface Environments, United

States Environmental Protection Agency, Office of Research

and Development, Washington, DC, EPA/600/2-91/053,

September, NTIS Accession No. PB92-114552.

USEPA, 1996. Soil Screening Guidance: Technical

Background Document, United States Environmental

Protection Agency, Office of Solid Waste and Emergency

Response, Washington, D.C., EPA/540/R-95/128, NTIS

Accession No. PB96963502.

Washington Administrative Code 173-340-747 Part VII

Cleanup Standards, State of Washington. Proposed draft regulation

http://www.wa.gov/ECOLOGY/tcp/regs/rule/340vii.doc.

Wiedemeier, T.H., Rifai, S.H., Newell, J.C., and Wilson,

J.T., 1999. Natural Attenuation of Fuels and Chlorinated

Solvents in the Subsurface, John Wiley and Sons, Inc., 617pp.

Zytner, R. G., N. Biswas and J. K. Bewtra, 1993. Retention

Capacity of Dry Soils for NAPLS, Environmental Technology,

14, 1073-1080.

Remediations and Buried Gasoline-Contaminated Soils,

Petroleum Contaminated Soils, Vol 3, P.T. Kostecki and E.J.

Calabrese, eds., (Lewis Publishers, MI), 1990, pp 295-326.

Lin, C., G.F. Pinder and E.F. Wood, 1982. Water Resources

Program Report 83-WR-2, October, Water Resources Program,

Princeton University, Princeton, New Jersey.

Mariner, P. E., M. Jin, and R. E. Jackson, 1997. An

Algorithm for the Estimation of NAPL Saturation and

Composition from Yypical Soil Chemical Analyses, Ground

Water Monitoring and Remediation, 17, 1:122-129.

Mercer, James W., and Robert M. Cohen, 1990. A Review

of Immiscible Fluids in the Subsurface: Properties, Models,

Characterization and Remediation, Journal of Contaminant

Hydrology, 6 107-163.

Mott, H.V., A Model for Determination of the Phase

Distribution of Petroleum Hydrocarbon Release Sites, Ground

Water Monitoring and Remediation, 15, 3, 1995, pp. 157-167.

Ohio Administrative Code 3745-300-08 Generic numerical

Standards, State of Ohio. http://www.epa.ohio.gov/derr/vap/

rules/rule_8.pdf.

Pfannkuch, H. O., 1983. Hydrocarbon Spills, Their Retention

in the Subsurface and Propagation into Shallow Aquifers,

Water Resources Research Center, United States Department

of the Interior, NTIS Accession No. PB83-196477.

Pfannkuch, H., 1984. Mass-exchange Processes at the

Petroleum-water Interface, Paper presented at the Toxic-Waste

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