low-risk site closure - gsi-net.com

Microsoft Word - FA890310C8114_LowRiskManual_final

S.K. Farhat C.J. Newell

M. Vanderford T.E. McHugh N.T. Mahler

GSI ENVIRONMENTAL INC. HOUSTON, TEXAS

J.L. Gillespie P.N. Jurena A.A. Bodour

AIR FORCE CENTER FOR ENGINEERING & THE ENVIRONMENT LACKLAND AFB, TEXAS

JULY 2012

Low-Risk Site Closure

Guidance Manual to Accelerate Closure of Conventional and Performance Based Contract Sites

L O W - R I S K S I T E C L O S U R E G U I D A N C E M A N U A L

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DISCLAIMER The Low-Risk Closure Guidance Manual is made available on an as-is basis without guarantee or warranty of any kind, expressed or implied. The United States Government, GSI Environmental Inc., the authors, and reviewers accept no liability resulting from the use of this documentation. Implementation and interpretation of the predictions of the manual are the sole responsibility of the user. Cover Art: Cover photograph courtesy of Dr. Thomas Sale, Colorado State University, Fort Collins, Colorado. For Citation: Farhat, S.K., C.J. Newell, M. Vanderford, T.E. McHugh, N.T. Mahler, J.L. Gillespie, P.N. Jurena, and A.A. Bodour, Low-Risk Site Closure Guidance Manual to Accelerate Closure of Conventional and Performance Based Contract Sites, developed for the Air Force Center for Engineering and the Environment by GSI Environmental Inc., Houston., Texas, July 2012. Contacts: Dr. Shahla Farhat [email protected] Dr. Chuck Newell [email protected]


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Low-Risk Site Closure Guidance Manual to Accelerate

Closure of Conventional and PBC Sites

AIR FORCE CENTER FOR ENGINEERING AND THE ENVIRONMENT

TABLE OF CONTENTS

Section Page No.

EXECUTIVE SUMMARY ............................................................................................ ES-1

1.0 INTRODUCTION ...................................................................................................... 1

1.1 What is a Low-Risk Site? .................................................................................. 11.2 What is Site Closure? ....................................................................................... 11.3 What is Low-Risk Site Closure? ........................................................................ 21.4 Goals of this Document ..................................................................................... 2

2.0 NEW THINKING ABOUT CLOSING GROUNDWATER SITES .............................. 4

2.1 New Technical Concepts .................................................................................. 42.2 New Low-Risk Closure Regulatory Approaches ............................................... 5

3.0 DO I HAVE A LOW-RISK SITE? ............................................................................. 8

3.1 QUESTION I. Do You Have a Complete CSM That Reflects Key Low-Risk Closure Concepts? ........................................................................................... 83.1.1. Question I.1. Have all of the components of the CSM been

evaluated? ............................................................................................. 83.2 QUESTION II. Are Sources Controlled? ........................................................ 13

3.2.1. Question II.1. Are there any significantly mobile source materials? ............................................................................................ 13

3.2.2. Question II.2. Is the source zone free of any environmentally significant quantity of NAPL? .............................................................. 14

3.2.3. Question II.3. Is it possible that any further source zone cleanup will be constrained by matrix diffusion processes? ............................. 16

3.2.4. Question II.4. Are sources relatively small? ......................................... 183.2.5. Question II.5. Are source zone concentrations stable or

decreasing? ......................................................................................... 193.2.6. Question II.6. Is there evidence of on-going source attenuation

processes? .......................................................................................... 213.2.7. Question II.7. Will future source remediation only marginally

improve site conditions? ...................................................................... 23

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3.3. QUESTION III. Will Residual Contamination Have No Adverse Effect on Present and Future Land and Water Uses? ................................................... 263.3.1. Question III.1. Is the groundwater plume stable, decreasing, or

probably decreasing? .......................................................................... 263.3.2. Question III.2. Is there evidence of on-going natural attenuation

processes in the plume? ..................................................................... 263.3.3. Question III.3. Are conditions protective of potential and future

receptors? ........................................................................................... 293.3.4. Question III.4. Is there a near-term need for the impacted

groundwater resource or any impacted land uses? ............................ 31

4.0 REDUCING LONG-TERM MONITORING INTENSITY ......................................... 34

5.0 REFERENCES ....................................................................................................... 36

6.0 CASE STUDIES FIELD APPLICATION OF LoRSC MANUAL ......................... 39

APPENDICES

Appendix A.Summary Of State Programs For Site Exit/Closure ................................ 101

Appendix B.Low-Risk Site Quick Reference Checklist and Blank Forms ................... 102

Appendix C.Conceptual Site Model ............................................................................ 109

Appendix D.14 Compartment Model Step-by-Step Guide and Template ................... 110

TABLES Table ES.1. LoRSC Manual Decision Logic Table 1. Application of the Plume Magnitude Classification System Table 2. Summary of Natural Attenuation Footprints at MNA Case Study Sites

(NRC, 2000) FIGURES Figure ES-1. LoRSC Manual Decision Logic Flow Chart Figure 1. Example of site with source excavation, but where groundwater plume

remains Figure 2. Criteria for low-threat closure of chlorinated solvent sites, San Francisco

Bay California Regional Water Quality Board (Figure from CRWQCB, 2009)

Figure 3. Example of a CSM for a monitored natural attenuation remedy (USEPA, 2004)

Figure 4. Depiction of a low-risk site using Sales 14 Compartment Model Figure 5a. Example of DNAPL mobility Figure 5b. Example of LNAPL mobility Figure 6a. Example of significant quantity of DNAPL Figure 6b. Example of significant quantity of LNAPL Figure 7. Conceptual model of matrix diffusion effects as part of plume response

(AFCEE, 2007)

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Figure 8. MAROS plume trend classification system Figure 9. Method for assessing the geochemical environment for groundwater

chlorinated solvent MNA (Truex et al., 2006) Figure 10. Qualitative Decision Chart on the merits of source depletion (Sale et al.,

2008; Kavanaugh et al., 2003) Figure 11. RBCA analyses for both potential and actual receptors (Figure A.3 from

GSI, 2007) Figure 12. Economic value normalization methodology of groundwater in the SRT

(Newell et al., 2008) Figure 13. Architecture of the Groundwater Sensitivity Toolkit (GSI, 2002)

LIST OF ABBREVIATIONS


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AFCEE Air Force Center for Engineering and the Environment CDPHE Colorado Department of Public Health and Environment CF Confidence Factor cis-DCE cis-Dichloroethene COV Coefficient of Variation CRWQCB California Regional Water Quality Control Boards CSM Conceptual Site Model DNAPL Dense Non-Aqueous Phase Liquid ESTCP Environmental Security Technology Certification Program ITRC Interstate Technology and Regulatory Council GSI GSI Environmental Inc. GTS Geostatistical Temporal/Spatial LIF Laser Induced Fluorescence LNAPL Light Non-Aqueous Phase Liquid LoRSC Low-Risk Site Closure LTMO Long-Term Monitoring Optimization LUFT Leaking Underground Fuel Tank MAROS Monitoring and Remediation Optimization System MCL Maximum Contaminant Level MNA Monitored Natural Attenuation NAPL Non-Aqueous Phase Liquid NRC National Research Council NSZD Natural Source Zone Depletion PBC Performance Based Contracting PCE Tetrachloroethene POE Point of Exposure PWS Public Water Supply RBCA Risk Based Corrective Action SERDP Strategic Environmental Research and Development Program SRT Sustainable Remediation Toolkit TCE Trichloroethene TCEQ Texas Commission on Environmental Quality TDS Total Dissolved Solids USAF U.S. Air Force USEPA U.S. Environmental Protection Agency UST Underground Storage Tank WQP Water Quality Protection

EXECUTIVE SUMMARY


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EXECUTIVE SUMMARY To help provide United States Air Force (USAF) site managers and site consultants a roadmap for effective exit strategies, the Air Force Center for Engineering and the Environment (AFCEE) has funded the development of a comprehensive decision support tool, the Low-Risk Site Closure (LoRSC) Manual. While the LoRSC Manual can be applicable to any type of groundwater contaminant, such as petroleum fuels, chlorinated solvents, pesticides, and metals, some of the decision logic is based on key processes at hydrocarbon and chlorinated solvent sites. The Manual is also designed for sites managed under Performance Based Contracting (PBC) approaches, as well as other contracting methods. This guide was developed to help site managers determine if they have a low-risk site by combining key concepts, information, and experience into one dynamic decision support tool. This information can then be used to assist site managers build effective exit strategies for closing low-risk sites and/or reducing long-term monitoring intensity. An exit strategy for a given site can be strengthened by using multiple lines of evidence; therefore, this guide provides weight-of-evidence decision logic to build consensus between site stakeholders. The LoRSC Manual was developed to provide site stakeholders with a specific, focused, technology transfer roadmap that can be used to support regulatory decision making by outlining:

1) how low-risk sites work, 2) why they wont cause a future environmental problem, 3) why they should be closed, or at a minimum, should be monitored only on a very

limited basis,

The Manual is intended to provide a methodology that can be used by site personnel to identify the type of USAF site and its probability for potential closure (e.g. gasoline spill on shallow soil only, TCE under 500 feet of fractured rock), and evaluate and prioritize sites based on threat criteria grouping sites as LoRSC Type A, B, or C: LoRSC Type A Sites: Strongest case for low-risk closure or reduced monitoring; LoRSC Type B Sites: Moderately good case for low-risk closure or reduced

monitoring; LoRSC Type C Sites: More difficult for low-risk closure or reduced monitoring.

The decision logic is based on identifying and examining three main categories of data: a comprehensive Conceptual Site Model (CSM), control of sources, and adverse effects of residual contamination. The low-risk site decision logic is presented below.

EXECUTIVE SUMMARY


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Low-Risk Sites FAQ 1. Is this concept just for solvents and fuels or are there any other types of

contaminants for which this Manual can be used?

The AFCEE LoRSC Manual can be applied to any type of groundwater contaminant including chlorinated solvents, petroleum fuels, metals, and pesticides.

2. What are the key types of data I need to apply the LoRSC Approach?

Almost all of the data collected to characterize and remediate a site are used: the Conceptual Site Model, presence and mobility of NAPL, site history, hydrogeology, trends in groundwater concentration data, lines of evidence for natural attenuation processes, receptor information, and need for the impacted groundwater.

3. Can this occur just for residential or also for industrial levels? Does this change the concept of low-risk?

Low-risk closure will apply for both residential and industrial levels as long as the conditions are met.

4. This sounds too risky, what if I dont want to apply any risky technology to my site?

The AFCEE LoRSC methodology is not a risky technology. It is a methodology for examining and analyzing data that should have already been collected. Additionally, the methodology provides a pathway for identifying critical missing gaps in the data.

5. Can this be applied to metals sites?

Low-risk closure will apply to metals as long as the conditions are met.

6. Can the Manual be used to identify what additional work needs to be done to close a site?

The AFCEE LoRSC methodology provides a pathway for identifying critical missing gaps in the data.

Table ES.1 LoRSC Manual Decision Logic


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Low-Risk Decision Questions Key Low-Risk Decision Criteria Answers For Must Have Questions Answers For Supporting

QuestionsManual Reference

I. Do You Have A Complete Conceptual Site Model (CSM) That Reflects Key Low-Risk Closure Concepts?

1. Have all of the components of the Conceptual Site Model been evaluated?

Conceptual Site Model checklist. Yes No Section 3.1.1

II. Are Sources Controlled?

1. Are there any significant mobile source materials? DNAPL sites: no mobile DNAPL observed. LNAPL sites: no expanding LNAPL zone and zero or low LNAPL transmissivity.

Yes No Section 3.2.1

2. Is the source zone free of any environmentally significant quantity of NAPL?

Little or no DNAPL observed in transmissive zones, and no significant LNAPL accumulation based on specific volume calculations.


3. Is it possible that any further source zone cleanup will be constrained by matrix diffusion processes?

Qualitative evaluation of matrix diffusion processes based on geology, chemical properties, timing of initial release, and remediation efforts.


4. Are sources relatively small? Plume is classified as a Mag 4 Plume Magnitude Category or less based on mass discharge estimates, OR maximum source concentrations are < 20x Maximum Contaminant Level (MCL).


5. Are source zone concentrations stable or decreasing? Representative source zone concentrations over time are shown to be stable, decreasing, or probably decreasing.


6. Is there evidence of on-going natural attenuation processes in the source zone?

Footprints of source zone attenuation are seen (such as generation of daughter products or consumption of electron acceptors).


7. Will future source remediation only marginally improve site conditions?

There is Less Need For Source Treatment based on the Qualitative Decision Chart. Yes No Section 3.2.7

III. Will Residual Contamination Have No Adverse Effect on Present and Future Land and Water Uses?

1. Is the groundwater plume stable or shrinking? Plume trend analyses showing decreasing plume over time. Yes No Section 3.3.1

2. Is there evidence of on-going natural attenuation processes in the plume?

Analyses of natural attenuation processes and footprints of natural attenuation in the plume. Yes No Section 3.3.2

3. Are conditions protective of potential and future receptors?

Analyses showing all exposure pathways for receptors are incomplete or present acceptable risks, and that future exposure will not occur.


4. Is there a near-term need for the impacted groundwater resource or any impacted land uses?

Evaluation of future needs for groundwater resource and associated overlying land uses. Yes No Section 3.3.4

MUST HAVE: All Yes?

SUPPORTING: How Many Yes?

Yes (Type A or B) No (Type C)

Type A if 3-4 Yes Type B if 0-2 Yes

WHAT IT MEANS LoRSC Site Type A (Strongest case for low-risk closure or reduced monitoring) = All Must Have Questions = Yes AND 3 or 4 of the Supporting Questions = Yes LoRSC Site Type B (Moderately good case for low-risk closure or reduced monitoring) = All Must Have Questions = Yes AND 0 to 2 of the Supporting Questions = Yes LoRSC Site Type C (More difficult for low-risk closure or reduced monitoring) = Any Must Have Questions = No

KEY: Must Have Data: Critical Line of evidence for low-risk site closure - necessary to demonstrate these criteria at almost all sites if applicable. Supporting Data: Supporting line of evidence, with 0-4 of the supporting lines recommended for low-risk site closure.

FIGURE ES.1 LoRSC Manual Decision Logic Flow Chart


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INTRODUCTION


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1.0 INTRODUCTION

1.1 What is a Low-Risk Site? Complete cleanup of contaminated groundwater sites is often difficult, and consequently, clean closure in the immediate future is unattainable at many sites. This problem is particularly acute at sites with releases of chlorinated solvents, but hydrocarbon and other releases can also result in persistent groundwater concentrations in excess of closure criteria. While contaminant concentrations at such sites may decrease significantly due to remediation and/or natural attenuation, persistent low-levels of groundwater contamination above closure criteria can preclude objectives such as reaching background concentrations or drinking water standards. However, this type of contamination, when combined with other key factors, can mean that the site actually poses very little risk to human health and the environment. Such a Low-Risk site may be amenable for complete closure in some regulatory jurisdictions, or a conditional closure where limited monitoring is required while the site attenuates.

Figure 1. Example of site with source excavation, but where groundwater plume remains

(photograph courtesy of Dr. Thomas Sale, Colorado State University).

1.2 What is Site Closure? Site closure has different meanings under different regulatory programs. For example, hydrocarbon sites regulated by the Texas Commission on Environmental Quality (TCEQ) can be closed by meeting the following criteria that indicate a low-risk site:

No impacted or threatened water wells are present within 0.5 mile radius of the site.

The affected groundwater zone is not considered part of a state designated major/minor aquifer.

The affected groundwater is unlikely to be used in the future. There is no discharge of the affected groundwater to a surface water body used

for human drinking water, contact recreation, or habitat to a protected or listed endangered plant and animal species located within 0.25 mile radius of the site.

A depth to water greater than 15 feet (or depth to utilities that is greater than 15 feet) and an affected aquifer that is not part of a karst or fractured bedrock geology.

INTRODUCTION


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Other sites in Texas can obtain a reduction in natural attenuation monitoring under the Texas Risk Reduction Rules if the site is shown to have a stable plume in a designated Plume Management Zone institutional control. Furthermore, groundwater monitoring may be terminated if the plume is shown to be shrinking and there is no threat of future impact on downgradient locations. Likewise, the California Regional Water Quality Board allows:

No Further Action closure of Underground Storage Tanks (UST). Under specific conditions, concentrations at the site may be greater than the water quality objectives at the time of closure.

The San Francisco Bay Region allows low-risk/low-threat closure at both petroleum fuel and chlorinated solvent sites under specific conditions. Under this guidance, concentrations at the site may be greater than the water quality objectives at the time of site closure.

Seven different state regulatory programs were found that currently have some type of program that appears to address low-risk sites. Section 2.2 below discusses these in detail.

1.3 What is Low-Risk Site Closure? For certain sites, the risk posed by residual, hard-to-remove groundwater contamination is very low. Depending on the particular regulatory program, this class of site might be suitable for:

Complete Closure: No further action; or Conditional Closure: Some type of conditional closure where future site

maintenance requirements (such as long-term monitoring) are greatly reduced. In theory, low-risk closure could apply to either residential or industrial land uses, although applying a low-risk type closure would likely be easier for industrial land uses.

1.4 Goals of this Document The goal of this document is to provide site consultants, site managers, and regulators tools and new information to:

Better understand the lifecycle of sites with groundwater contamination (for example, sites contaminated with chlorinated solvents, petroleum hydrocarbons, metals, pesticides, etc.) and how low-risk sites work.

Learn about previously under-appreciated groundwater fate and transport processes.

Balance what can and what cannot be achieved with existing groundwater remediation technologies.

Bring together key pieces of site information to build a comprehensive CSM and determine if the site can be categorized as a low-risk site.

Explain why low-risk sites wont cause future environmental problems.

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At sites with the right characteristics and at the right stage of the plume lifecycle, build a technically sound, science-based case for some type of Low-Risk Closure.

Provide a framework useful for both PBC projects and other approaches for closing sites.

It should be emphasized that this document is not regulatory guidance, does not establish policy, nor does it replace any existing state or federally mandated programs or requirements. This guide is intended to help site managers determine if they have a low-risk site by providing key concepts, information, and experience in one dynamic decision support tool. This information can then be used to assist site managers build effective exit strategies for closing low-risk sites and/or reducing long-term monitoring intensity. The exit strategy for a given site can be effectively strengthened by using multiple lines of evidence; therefore, this guide provides weight-of-evidence decision logic to build consensus between site stakeholders.

NEW THINKING ABOUT CLOSING GROUNDWATER SITES


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2.0 NEW THINKING ABOUT CLOSING GROUNDWATER SITES Over the past several years, there has been an increased focus on the end game of site remediation projects, and how to get sites to closure, both from a technical and regulatory perspective.

2.1 New Technical Concepts The difficulties experienced at hundreds of these sites has led to a more detailed look at the performance of remediation technologies and on previously-underappreciated environmental processes that now appear to be a major constraint in our ability to close sites. A series of scientific and engineering studies, many of them funded by the Air Force Center for Engineering and the Environment (AFCEE) and the Department of Defenses Strategic Environmental Research and Development Program (SERDP) and Environmental Security Technology Certification Program (ESTCP) have shed new light on:

Expectations for in-situ remediation technology performance such as thermal remediation, chemical oxidation, bioremediation, and chemical reduction.

The importance of matrix diffusion: stored contaminant mass resulting from

diffusion of dissolved groundwater contaminants into low-permeability zones.

Understanding that many groundwater source zones naturally attenuate over time, even ones containing light non-aqueous phase liquids (LNAPL) and dense non-aqueous phase liquids (DNAPL), due to natural flushing and degradation processes within the source zone itself.

How simple groundwater tools and models can help understand and account

for key source and plume processes for site closure purposes. Examples include AFCEEs SourceDK model (Farhat et al., 2004), the U.S. Environmental Protection Agencys (USEPA) REMChlor model (Falta et al., 2007), ESTCPs Mass Flux Toolkit (Farhat et al., 2006), and the upcoming ESTCP Matrix Diffusion Toolkit (Farhat et al., 2012).

The 14 Compartment Model, which guides users to consider where the mass

and mass fluxes are located at the site and in what phase (i.e., vapor, NAPL, aqueous, and sorbed). This model is centered on matrix-diffusion effects, where 7 of the 14 different compartments are low permeability compartments.

Each one of these major points is discussed in the Low-Risk Closure approach presented in the following pages.



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2.2 New Low-Risk Closure Regulatory Approaches There is increasing emphasis on managing and closing low-risk sites. Several state regulatory programs currently allow the closure of low-risk sites. Example programs are listed below with details of documents currently available in Appendix A.

California (both chlorinated solvent and petroleum fuel sites, http://www.swrcb.ca.gov/),

Alaska (petroleum fuel sites, www.dec.alaska.gov), Florida (low yield/poor quality sites, www.dep.state.fl.us), Texas (petroleum fuel sites, www.tceq.state.tx.us), North Carolina (underground storage tank sites, www.ncdenr.gov), Wisconsin (petroleum fuel sites, www.dnr.wi.gov), and Wyoming (storage tanks, www.deq.state.wy.us).

Several states (California, Florida, North Carolina, and Wisconsin) also allow groundwater site closure with contaminants in place under specific conditions. Two programs that are particularly relevant were developed in California and Colorado. In 2009, the San Francisco Bay Region of the California Regional Water Quality Control Board (CRWQCB), developed the Assessment Tool for Closure of Low-Threat Chlorinated Solvent Sites built upon their 1996 guidance for low-risk closure of fuel-impacted sites. In this document,

The Groundwater Committee, a staff committee of the San Francisco Bay Regional Water Quality Control Board (S.F. Bay Water Board) embarked on a project to develop criteria for evaluating if and when chlorinated solvent sites that pose little threat to human and ecological health, water quality, and beneficial uses but do not yet meet cleanup standards at all locations, could be closed. This process is referred to as low-threat closure.

Under this system, nine separate criteria1 must be met for a low-threat closure (CRWQCB, 2009) (Figure 2). On May 1, 2012, the California State Water Board adopted the statewide Low-Threat Underground Storage Tank Case Closure Policy:

With the knowledge and experience gained over the last 25 years of investigating and remediating petroleum UST releases, site conditions and characteristics have been identified that if met, will generally ensure the protection of human health, safety and the environment. This Policy identifies those standardized criteria. The Policy is necessary to establish consistent, statewide case closure criteria for low-threat petroleum UST sites in California.



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Figure 2. Criteria for low-threat closure of chlorinated solvent sites, San Francisco Bay California Regional Water Quality Board (CRWQCB, 2009, Table ES-1).

The Colorado Department of Public Health and Environment (CDPHE) also issued a draft Guidance for the Closure of Low-Threat Sites with Residual Ground Water Contamination in August 2010. This methodology is based on six lines of evidence that must be met for a low-threat closure:

1. Adequate characterization of the site. 2. Remediation of source areas. 3. No exposure to contaminants. 4. Demonstration of natural attenuation processes. 5. Definition of the timeframe for achieving remediation goals. 6. Ability to enact, implement and maintain institutional controls over time.

The Colorado guidance does not have specific criteria for determining what is needed to have adequate site characterization; it is site and regulatory program specific (CDPHE, 2010). The document states:

Division personnel will apply professional judgment in each case, factoring in such elements as: the cause of the suspected release, the chemicals of concern, the complexity of the site hydrology and hydrogeology, the magnitude of the problem, and the potential for future exposures.



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The CRWQCB (2009) specifies this type of characterization work is needed to apply their low-threat closure guidance:

Site characterization work should be designed to minimize uncertainty and maximize accuracy to 1) effectively characterize pollutant distribution and migration pathways in all media, including soil, soil-gas, and groundwater, and 2) identify potential migration pathways to allow for appropriate decision-making pertaining to risk, monitoring and remediation.

DO I HAVE A LOW-RISK SITE?


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3.0 DO I HAVE A LOW-RISK SITE? This section provides a variety of methodologies, calculations, graphics, scientific literature, and modeling approaches/tools that can be used to determine if a site could be considered low-risk, and be a candidate for closure and/or reduction in intensity of long-term site care. The methodology relies partially on key concepts presented in the Assessment Tool for Closure of Low-Threat Chlorinated Solvent Sites (CRWQCB, 2009) and the Draft Guidance for the Closure of Low-Threat Sites with Residual Ground Water Contamination (CDPHE, 2010). A quick reference checklist for the LoRSC Manual decision logic is provided in Appendix B.

3.1 QUESTION I. DO YOU HAVE A COMPLETE CSM THAT REFLECTS KEY LOW-RISK CLOSURE CONCEPTS?

3.1.1. Question I.1. Have all of the components of the CSM been evaluated? Criteria: CSM checklist is complete. Development of the CSM should start in the early stages of site investigation and it should be updated and refined continuously as additional information becomes available. The CSM is typically supported by visual aids such as tables, diagrams, maps, figures, and hydrogeologic cross-sections. Critical information contained in a comprehensive CSM should include, where available:

1. Site Information including historical, current, and future property use or industrial activities.

2. Site Investigations including dates of investigations, soil borings, geophysical investigations, site geochemistry, presence of off-site affected groundwater, evidence of NAPL, and dates of most recent NAPL observation.

3. Source Characterization including primary (e.g., tank, drum, sump, etc.) and secondary (e.g., NAPL, contaminated soil, etc.) source locations, release mechanisms (e.g. spills, landfill), size and boundary, substance(s) released, date(s) of release, volume and mass of substance(s) released, and source control measures taken.

4. Constituents of Concern including chemical constituents of regulatory concern; identification of those most likely to pose some risk due to their presence, toxicity and mobility at the site.

5. Nature and Extent of Contamination including the horizontal and vertical distribution of the contamination and concentrations.

6. Hydrogeology including stratigraphy, vadose (unsaturated) and saturated zone types, aquifer properties such as hydraulic conductivity, gradient and porosity, confining unit soil type, depth to top of aquifer, depth to groundwater, direction of



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groundwater flow including preferential pathways, recharge, proximity to surface waters, and interaction between groundwater and surface water.

7. Geochemistry geochemical parameters and conditions such as oxygen concentrations, nitrate, sulfate, and iron.

8. Migration and Exposure Pathways including groundwater, surface water, soil, air, sediment, and biota, and identification of both complete and incomplete exposure pathways.

9. Contaminant Attenuation Pathways including advection, dispersion, chemical and biological transformation mechanisms, sorption, and dilution.

10. Receptors including identification of and mitigation activities protecting actual and potential a) human receptors (e.g., well locations, groundwater-to-surface water discharge locations, underground utilities, etc.), b) ecological receptors, c) sensitive receptors (e.g., day-care centers, schools, residences, hospitals, etc.), and d) current and future groundwater and surface water resources. Identification of any potential adverse effects should also be included.

11. Soil Remediation including date(s) of soil remediation initiated and completed, remediation technology employed, soil volume treated (or removed), results of treatment/removal, and adequacy of treatment in meeting regulatory standards.

12. Groundwater Remediation - including date(s) of groundwater remediation initiated and completed, remediation technology employed, information on NAPL recovery, results of treatment, and adequacy of treatment in meeting regulatory standards.

13. 14 Compartment Model diagram of different contaminant phases/compartments at the site. The 14 Compartment Model is discussed below.

14. Stakeholders including regulatory agencies, property owners, developers, municipalities, and adjacent communities.

If possible these elements should be described in a graphic, such as a block diagram showing key parts of a CSM. Figure 3 shows an example CSM for a Monitored Natural Attenuation (MNA) remedy (USEPA, 2004). A template for a CSM is provided in Appendix C.



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Figure 3. Example of a CSM for a monitored natural attenuation remedy (USEPA, 2004).



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A very useful conceptual framework is the 14 Compartment Model developed by Sale et al. (2008) (Figure 4). The model provides a means for 1) accounting for the relative distribution of contaminant mass at a site, 2) assessing the stage of plume maturity, and 3) evaluating site response to remedial treatment.

Source Zone Plume

Zone/ Phases

Low Permeability Transmissive Transmissive

Low Permeability

Vapor IP IP IP IP DNAPL 0 0 NA NA

Aqueous 2 1 1 2 Sorbed 2 1 1 2

Legend:

Figure 4. Depiction of a low-risk site using Sales 14 Compartment Model. Solid arrows

represent reversible mass transport between compartments, while dashed arrows represent irreversible transport. See Appendix D for instructions on completing the 14 Compartment Model for a specific site. (NA = Not Applicable; IP = Incomplete Pathway.)

As described by Sale and Newell (2011): It is important to realize that the 14 Compartment Model is a useful tool, but it is only part of a conceptual site model. Explicitly considering the 14 Compartment Model helps ensure that all of the different phases and transmissive zones are considered when making management decisions. But it is also important that a conceptual site model include a mass balance that addresses the spatial distribution of the mass of contaminants, and the fluxes of contaminants within the site, as well as the hydrogeologic and biogeochemical information needed to evaluate fate and transport. The use of the 14 Compartment Model is designed to encourage the development of integrated strategies, in conjunction with the other aspects of a quantitative conceptual site model. The quantitative application of the 14 Compartment Model is discussed in detail in the Decision Guide document prepared by Sale and Newell (2011). In general, the user puts in an order-of-magnitude estimate of the pre-remediation concentration in each box (for example, 1000 mg/L would be a 3 as shown in 103 mg/l). Some compartments may not be measured directly, but the concentrations can be inferred by evaluating



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concentrations in adjacent compartments. The change in concentration due to remediation would then be applied using an order of magnitude approach. If a remediation technology is thought to reduce concentrations in a particular compartment by two orders of magnitude (to 0.01 of the pre-remediation concentration, or a 99% reduction), then the post-remediation version of the 14 Compartment Model would have a 1 in that box. Similar before-and-after versions can be developed for the mass discharge (sometimes called mass flux) between compartments. The overall goal is to show order-of-magnitude changes on a semi-quantitative basis. Detailed measurements of concentration in each compartment are helpful, but not necessary (Sale and Newell, 2011). Note there is an increasing interest in the regulatory community to use this model for regulatory decision making. The Interstate Technology and Regulatory Councils (ITRCs) Integrated DNAPL Site Strategy Technology and Regulatory Guidance (2011), depends heavily on the 14 Compartment Model to guide accurate decision making about remediation and management of chlorinated solvent sites. A template and step-by-step guide for the 14 Compartment Model is provided in Appendix D. A qualitative application of the model includes identifying all phases/zones that could potentially contain the contaminants. If the CSM includes all of items 1-14 (Section 3.1.1) relevant to the Site, and includes a qualitative 14 Compartment Model, then Question I.1 is answered YES.



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3.2 QUESTION II. ARE SOURCES CONTROLLED?

3.2.1. Question II.1. Are there any significant mobile source materials? Criteria: No mobile DNAPL. No expanding LNAPL zone. A controlled chlorinated solvent source zone should have no consistently observed mobile DNAPL in monitoring wells, should not discharge to surface water, nor exhibit any other evidence of an expanding DNAPL zone (e.g., Figures 5a and 5b). For hydrocarbon sites, any LNAPL accumulation must not be expanding spatially. For other types of NAPL, the NAPL accumulation must not be expanding spatially.

Figure 5a. Example of DNAPL mobility. Top panel depicts no mobility of DNAPL while the

lower panel shows the mobility of DNAPL (brown color). Light red color indicates dissolved phase plume.

Figure 5b. Example of LNAPL mobility. Top panel depicts no mobility of LNAPL while the

lower panel shows the overall LNAPL footprint increasing in size, indicating mobility of LNAPL.

Chlorinated solvent sites: DNAPL is extremely mobile, and in most cases any consistent, observed DNAPL accumulations in wells should be remediated before a site can be



AFCEE 14

considered a low-risk site. However, most chlorinated solvent DNAPLs have high density and low viscosity, so that migration in relatively permeable media can cease within a few months to a few years following the time of release (USEPA, 2009). Therefore, it is unlikely that there is a currently expanding DNAPL zone at a chlorinated solvent site if: 1) the most recent release is more than a few years old; 2) the site is a smaller release; 3) DNAPL has not been observed in any monitoring wells; and 4) the site is adequately characterized. Hydrocarbon sites: LNAPL presence in monitoring wells can, in certain circumstances, be caused by low-mobility, low volume LNAPL accumulations. Therefore, the key criterion is to confirm that the LNAPL zone is not increasing in size by comparing maps of the observed LNAPL accumulations over time. If consistently LNAPL-free wells on the periphery of the LNAPL zone change so that LNAPL is consistently observed, then the LNAPL zone may be expanding. If the overall LNAPL areal footprint is not expanding over time, then there are no significantly mobile source materials. (Note that the size of the LNAPL footprint is considered at the scale of the entire site, and that very small scale changes at the pore level or hypothesized changes between monitoring points would not be considered as indicating the presence of mobile LNAPL). If LNAPL is found in monitoring wells, LNAPL transmissivity calculations can be made. The ITRC LNAPL technical guidance (ITRC, 2009) states that Beckett and Lundegard (1997) proposed that appreciable quantities of LNAPL cannot be recovered and that there is little migration risk associated with a well with an LNAPL transmissivity (Tn) of 0.015 ft2/day. However, ITRC LNAPL Team members experience indicates that hydraulic or pneumatic recovery systems can practically reduce Tn to values between 0.01 and 0.8 ft2/day. Alternatively, if the LNAPL transmissivity is 0.01 ft2/day then there are no significantly mobile source materials. Apparent thickness (the thickness of the LNAPL in monitoring wells) should not be used as an indicator of mobile LNAPL or of significant LNAPL accumulation because formation effects in fine-grained soils can greatly magnify the amount of LNAPL in the well compared to the specific volume of LNAPL in the formation. Adamski et al. (2005) provide a detailed description of the conceptual model of LNAPL behavior in fine-grained soils. LNAPL mobility tracer technology and companion calculations, developed by Colorado State University, can be used to determine if the LNAPL zone is expanding. The LNAPL tracer technology utilizes a fluorescent dye that is only visible in an LNAPL. The dye is injected into a well containing LNAPL and intermittently agitated to obtain uniformly mixed tracer concentrations at the time of measurement (Smith et al., 2012). The rate of disappearance of the dye is then used to estimate the LNAPL migration rate (LNAPL velocity). More importantly, corresponding calculations and simple modeling can be used to determine if the rate of Natural Source Zone Depletion (NSZD) is enough to keep the LNAPL body from expanding (Mahler et al., 2012). If tracer tests and NSZD calculations indicate no expansion for the LNAPL body, then there are no significantly mobile source materials. If there is no significantly mobile NAPL in the source zone, then Question II.1 is answered YES.



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3.2.2. Question II.2. Is the source zone free of any environmentally significant quantity of NAPL?

Criteria: Little or no DNAPL observed in transmissive zones, and no significant LNAPL accumulation based on specific volume calculations. Chlorinated solvent sites: To answer yes, DNAPL is either not directly observed in core samples; or the average saturation (percent of pore spaced filled with DNAPL) of the DNAPL observed in cores collected from the transmissive zone is less than 1% (e.g., see Figures 6a and 6b). For core analyses, dye testing or other enhanced DNAPL evaluation techniques are preferred to help reduce the occurrence of false negatives. General indirect rules about DNAPL occurrence (such as the 1% rule) should not be used by themselves to indicate the presence of DNAPL, but only with other, converging lines of evidence (see USEPA, 2009 for a discussion of the 1% rule). This is because the indirect methods have a considerable uncertainty (the USEPA says the 1% rule is a generality, and that DNAPL may be present), and some matrix diffusion experts are now suggesting that 1% of solubility could be generated by matrix diffusion processes alone, resulting in false positives. In summary, if DNAPL has never been observed in core samples, and/or if DNAPL has been observed, but has an average saturation of less than 1% in the source zone, then there is no environmentally significant quantity of DNAPL in the source zone.

Figure 6a. Example of significant quantity of DNAPL.



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Figure 6b. Example of significant quantity of LNAPL. Hydrocarbon sites: For sites with LNAPL observed in monitoring wells, specific volume (the volume of LNAPL divided by the area) can be used to evaluate the actual amount of LNAPL present. Average LNAPL specific volumes less than 0.1 feet can be considered to be relatively low accumulations of LNAPL (this is equivalent to 33,000 gallons of LNAPL per acre). This value is based on recent research that indicates that NSZD in LNAPL zones are degrading at the rate of thousands of gallons per year (or a potential degradation time of about 30 years) (Adamski et al., 2005; Mahler et al., 2012). If LNAPL has never been observed in core samples, and/or if LNAPL has been observed, but has a specific volume of less than 0.1 foot in the source zone, then there is no environmentally significant quantity of LNAPL in the source zone. Apparent thickness (the thickness of the LNAPL in monitoring wells) should not be used as an indicator of mobile LNAPL or of significant LNAPL accumulation because formation effects in fine-grained soils can greatly magnify the amount of LNAPL in the well compared to the specific volume of LNAPL in the formation. Adamski et al. (2005) provide a detailed description of the conceptual model of LNAPL behavior in fine-grained soils. If there are no environmentally significant quantities of NAPL in the source zone, then Question II.2 is answered YES.

3.2.3. Question II.3. Is it possible that any further source zone cleanup will be constrained by matrix diffusion processes?

Criteria: Qualitative evaluation of matrix diffusion processes based on geology, chemical properties, timing of initial release, and remediation efforts. Most remediation programs specify that source control actions should use treatment to address "Principal Threat" wastes (or products) wherever practicable (USEPA, 1999). Principal threat wastes are those source materials that are highly toxic or highly mobile that generally cannot be reliably contained or would present a significant risk to human health or the environment should exposure occur. They include liquids and other highly mobile materials (e.g., solvents) or materials having high concentrations of toxic



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compounds (USEPA, 1991). Low-level threat wastes are source materials that generally can be reliably contained and that would present only a low risk in the event of release (USEPA, 1991). Since contaminated groundwater is not source material, it is neither a principal nor a low-level threat waste (USEPA, 1991). Furthermore, matrix diffusion sources are neither highly toxic (low-strength) nor mobile, and can be reliably contained at most sites via MNA (because they are typically low strength sources). Consequently, because matrix diffusion sources are not a Principal Threat Waste, there is no need for immediate or near-term treatment. However, future remediation of matrix diffusion-dominated sources would be more difficult (i.e., likely more difficult than removing NAPL from transmissive zones). Therefore, matrix diffusion sources are a supporting line of evidence for Low-Risk Site designation (it is a Supporting question, not a "Must Have" question). The potential for matrix diffusion effects can be seen at virtually any site with heterogeneity in the subsurface, NAPL, and/or where persistent groundwater contaminant concentrations after source-zone remediation have been observed (Figure 7). Key factors favoring matrix diffusion (adapted from Sale et al., 2008), ordered from more important to potentially less important, include:

Presence of Low-Permeability lenses or strata in an affected aquifer in contact with transmissive zones containing plumes.

High concentrations of contaminants. Older release sites (i.e., significant elapsed time since contaminant release). Geologic settings where transmissive zones are only a small fraction of the total

volume of the aquifer. Aquifers with relatively slow groundwater flow rates. Sediments with high fraction organic carbon content. Presence of contaminants that exhibit stability in their physical setting. Release of large amounts of contaminants.

Figure 7. Conceptual model of matrix diffusion effects as part of plume response (AFCEE, 2007).

Transmissive sand Low permeability silts

Expanding diffusion halo in stagnant zone

Simultaneous inward and outward diffusion in stagnant zones

Advancing solvent plume



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Site factors can be evaluated to qualitatively estimate if matrix diffusion effects are expected to be significant. In general, there is potential for significant matrix diffusion effects if NAPL, or the aqueous phase contaminant plume in the transmissive unit, has been in direct contact with Low-Permeability material (i.e. fine-grained sands, silts, or clays) or sedimentary rock for 20 years or more. Simple planning-level models such as the square root model or the Dandy Sale model in the ESTCP Matrix Diffusion Toolkit develop by GSI Environmental (Farhat et al., 2012), can be used to quantitatively determine if matrix diffusion could be an important component at a site. Although most groundwater research to date related to matrix diffusion has focused on chlorinated solvent sites, other research has indicated potential matrix diffusion effects for MTBE releases (Rasa, 2011). Interestingly, one of the earliest multiple-site research studies, the 1995 California Leaking Underground Fuel Tank (LUFT) Historical Case Analysis (Rice et al., 1995) may have found evidence of matrix diffusion effects when they identified a category of exhausted plumes with low concentration, stable benzene/toluene/xylenes/ethylbenzene plumes. The cause of the exhausted plumes was never identified, but is consistent with matrix diffusion effects at old, weathered hydrocarbon site plume zones. If it is possible that any further source zone cleanup will be constrained by matrix diffusion processes, then Question II.3 is answered YES.

3.2.4. Question II.4. Are sources relatively small? Criteria: Plume is classified as a Mag 4 Plume Magnitude Category or less based on mass discharge estimates, OR maximum source concentrations are < 20x MCL. Estimates of mass discharge (mass per time, also called mass flux) have become increasingly valuable at sites with contaminated groundwater plumes (ITRC, 2010). However, understanding the broader implication of flux measurements is not always intuitive. Specifically, because mass discharge values lack context, it can be difficult to communicate the magnitude and significance of mass flux/mass discharge to stakeholders and decision makers. New classification methodology has been developed (Newell et al., 2011) that bases mass discharge on a plume magnitude (Mag) scale (see Table 1). Based on 10 different categories of mass discharge ranges, the system provides a simple contextual method for understanding plume strengths. The classification system can assist site managers in using site specific mass discharge to refine CSMs, prioritize sites, determine potential impacts, and evaluate plumes both temporally and spatially. For example, with this approach, a Mag 4 Plume was used to define a low-risk plume because a Mag 4 plume cannot cause an exceedance of a 5 g/L MCL in a drinking water well pumping 100 gallons per minute due to mixing of clean water and the plume.



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Table 1. Application of the Plume Magnitude Classification System. Mass

Discharge (g/day)

Plume Classification

Low-Risk Plume? Impact*

< 0.001 Mag 1 YES Limited impact 0.001 to 0.01 Mag 2 YES

Could impact a domestic well, pumping at 150 gallons per day (gpd) or less

0.01 to 0.1 Mag 3 YES Could impact a well pumping at 1 gallons per minute (gpm) or less 0.1 to 1 Mag 4 YES Could impact a well pumping at 10 gallons per minute (gpm) or less 1 to 10 Mag 5 MAYBE Could impact a well pumping at 100 gpm or less

10 to 100 Mag 6 MAYBE Could impact a stream with a mixing zone base flow of 1 cubic feet per second (cfs) or less

100 to 1,000 Mag 7 LIKELY NOT

Could impact a stream with a mixing zone base flow of 10 cfs or less

1,000 to 10,000 Mag 8 LIKELY NOT

Could impact a stream with a mixing zone base flow of 100 cfs or less

10,000 to 100,000 Mag 9 LIKELY NOT

Could impact a stream with a mixing zone base flow of 1,000 cfs or less

>100,000 Mag 10 LIKELY NOT Could impact a stream with a mixing zone base flow of >10,000 cfs * Impact based on a drinking water standard in pumped water or mixing zone of 5 g/L.

Natural attenuation (both biotic and abiotic) is assumed to be the main mechanism for residual pollutant concentrations achieving cleanup standards within a reasonable time-frame. Based on a study of low-risk closures in the California San Francisco Bay area, sites with residual concentrations less than or equal to 20 times the site cleanup standard (e.g., the MCL), have a greater probability of achieving these standards in a reasonable timeframe via natural attenuation (CRWQCB, 2009). Using these two data sources, the LoRSC Manual defines a source as a small source if the key constituent being discharged from the source is either:

1. A Mag 4 plume or less based on the Plume Magnitude Classification System in Table 1, AND/OR

2. The current maximum concentrations of key groundwater constituents in the source zone are all less than ~20x their MCLs.

If the source is small, then Question II.4 is answered YES.

3.2.5. Question II.5. Are source zone concentrations stable or decreasing? Criterion: Representative source zone concentrations over time are shown to be stable, decreasing, or probably decreasing. Representative concentrations could be average, geometric mean, or maximum observed concentrations from each sampling event. Decreasing trends in source zone wells can be demonstrated by:



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Graphing natural log concentration (typically in mg/L or g/L, or molar concentration for parents+daughter compounds) vs. time for several source zone wells (see Newell et al., 2002 for a discussion of this method for MNA analysis). At least five years of temporal data are preferred to ensure enough time to determine source zone trends. Spatial and temporal trends for both parent compounds and key breakdown products (if any) should be evaluated.

Data can be analyzed using linear regression or non-parametric tests (such as the Mann-Kendall test) and the methodology shown in Figure 8 (Aziz et al., 2003). Several software tools, such as AFCEEs Monitoring and Remediation Optimization System (MAROS) program (Aziz et al., 2003) and the GSI Mann-Kendall spreadsheet (Connor et al., 2012) are available to help site managers make these types of computations.

Note that it is easy to confuse different types of rates in natural attenuation analysis. A USEPA document is available that describes different types of rates used in MNA evaluations and how to calculate them (Newell et al., 2002).

Figure 8. MAROS plume trend classification system. CF = Confidence Factor, S = Mann-Kendall Statistic, and COV = Coefficient of Variation (Aziz et al., 2003).

If the average trend in all source zones wells (using a method such as the one employed in MAROS and shown in Figure 8) is either Probably Decreasing or Decreasing or Stable then the source zone concentrations indicate natural attenuation processes are active. If the source zone concentrations are stable, decreasing or probably decreasing, then Question II. 5 is answered YES.



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3.2.6. Question II.6. Is there evidence of on-going source attenuation processes? Criteria: Footprints of source zone attenuation are seen (such as generation of daughter products or consumption of electron acceptors). Since 2004, there has been increased emphasis on MNA as a remediation technology for source zones, with the development of field studies, process information, models, and protocols designed specifically for source zone attenuation. For this low-risk criterion, footprints of natural attenuation describe indicators of change in groundwater other than a decline in the concentration of the original contaminant in or near the source zone. Examples include:

Depletion of oxygen, nitrate and sulfate indicate hydrocarbon degradation. Low oxygen, nitrate, and sulfate concentrations indicate more anaerobic

geochemical conditions that support reductive dechlorination of many chlorinated solvents such as trichloroethene (TCE) and tetrachloroethene (PCE).

Generation of cis-1,2-dichloroethene (cis-1,2-DCE) and other daughter products indicates that biodegradation of TCE is occurring in groundwater.

Generation of 1,1-dichloroethene (1,1-DCE) indicates that abiotic degradation of 1,1,1-trichloroethane is occurring in groundwater. Abiotic degradation is the chemical transformation that degrades contaminants without microbial facilitation. This can result in partial or complete degradation of contaminants. Typically, only halogenated compounds are subject to these mechanisms in the groundwater environment.

Presence of reactive minerals and soils that can abiotically degrade chlorinated solvents, e.g., magnetite.

Changes in compound specific isotope ratios can provide supporting evidence documenting that biodegradation or abiotic transformation processes are actually destroying contaminants at the site (USEPA, 2008).

Genetic analyses of microbial populations can provide an optional line of evidence supporting MNA. Members of the Dehalococcoides group of bacteria are the only organisms known to date to completely degrade chlorinated ethenes to harmless products. Therefore, for chlorinated solvent sites, the presence or absence of these organisms can provide information on whether MNA is an appropriate approach at a specific site (USEPA, 2006). Other metabolic and genetic indicators can also demonstrate the presence of microbes capable of cometabolic degradation of compounds.

Key references that discuss footprints and indicators of MNA include: The National Research Councils (NRC) book on Natural Attenuation for

Groundwater Remediation (NRC, 2000). Scenarios Evaluation Tool for Chlorinated Solvent MNA (Truex et al., 2006) (e.g.,

see Figure 9). USEPA Technical Protocol for Evaluating Natural Attenuation of Chlorinated

Solvents in Groundwater (USEPA, 1998). Technical Protocol for Implementing Intrinsic Remediation with Long-Term

Monitoring for Natural Attenuation of Fuel Contamination Dissolved in Groundwater (Wiedemeier et al. 1999a).



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Natural Attenuation of Fuels and Chlorinated Solvents (Wiedemeier et al., 1999b).

Frequently Asked Questions about MNA in the 21st Century (Adamson et al, 2012).

Figure 9. Method for assessing the geochemical environment for groundwater chlorinated

solvent MNA (Truex et al., 2006). Note the authors of this document use the term anaerobic for sites with conditions known to support reductive dechlorination, and the

term anoxic for more border-line but still low-oxygen conditions. Note that while most of these protocols have focused on evaluating MNA in the plume, it is the intent of this low-risk document to apply these specific MNA criteria (footprints of natural attenuation) in or near the source zone. If there is evidence of on-going natural attenuation processes in the source zone, AND there are key footprints of natural attenuation in the source zone, then Question II.6 is answered YES.



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3.2.7. Question II.7. Will future source remediation only marginally improve site conditions?

Criteria: There is Less Need for Source Treatment based on weight of evidence from the Qualitative Decision Chart (Figure 10). In 2003, the USEPA convened an Expert Panel to evaluate the state of DNAPL site remediation. The outcome from the Expert Panel was later modified for use as a chart in the Frequently Asked Questions Regarding Management of Chlorinated Solvents in Soil and Groundwater document (Sale et al., 2008, p. 26). The chart uses a weight-of-evidence logic to resolve the relative need for source treatment. Primary reasons for considering source treatment include reducing the potential for DNAPL migration, decreasing source longevity, reducing loading to downgradient plumes, attainment of MCLs, complying with regulations, and achieving intangible benefits. While developed for chlorinated solvents, the chart can easily be adapted for hydrocarbon sites by changing DNAPL to LNAPL in the first row. One key consideration is that one must have realistic expectations for what source remediation can provide and at what cost. This topic was covered as Frequently Asked Question 13 in Sale et al., (2008) for chlorinated solvent sites where the results from several multiple-site remediation performance studies have indicated that chemical oxidation, bioremediation, and thermal treatment projects have, as a very general rule, reduced source concentrations by one, and sometimes two orders of magnitude (i.e., 90% to 99%) (Sale et al., 2008). These studies include a 59-site study that included four different types of in-situ remediation technologies (McGuire et al., 2006), a detailed state-of-the-practice review of thermal treatment (Johnson et al., 2009), and a comprehensive survey of chemical oxidation performance (Krembs et al., 2010). The cost of treatment can be estimated using general unit costs (i.e., see Sale et al., 2008) or by getting quotes from technology vendors. Another tool that can be used to evaluate the benefits of source treatment is the USEPAs REMChlor model (Falta et al., 2007; Falta et al., 2005a and 2005b). This simple analytical model can be used to estimate the impact of source zone remediation, plume remediation, or combined source and plume remediation projects, plume concentrations, and mass discharge rates.



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Figure 10. Qualitative Decision Chart on the merits of source depletion (Sale et al., 2008; Adapted from USEPAs The DNAPL Remediation Challenge: Is There a Case for Source

Depletion? (Kavanaugh et al., 2003).



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These same general concepts, as shown in Figure 10, and remediation performance and cost information, as shown in Sale et al.s FAQ 13 (2008), can often be applied to other types of sites besides chlorinated solvent sites. If the Qualitative Decision Chart in Figure 10, when used with a weight of evidence approach, indicates Less Need for Source Treatment, then Question II.7 is answered YES.



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3.3. QUESTION III. WILL RESIDUAL CONTAMINATION HAVE NO ADVERSE EFFECT ON PRESENT AND FUTURE LAND AND WATER USES?

3.3.1. Question III.1. Is the groundwater plume stable, decreasing, or probably decreasing?

Criterion: Plume trend analysis showing stable, decreasing, or probably decreasing plume over time using method in Figure 8. Decreasing trends in plume wells can be demonstrated in a similar fashion as the techniques for the source zone described in Section 3.2.5:

Graphing natural log concentration vs. time for several plume zone wells. At least five years of temporal data are preferred to ensure enough time to determine source zone trends. Spatial and temporal trends for both parent compounds and key breakdown products (if any) should be evaluated.

Total mass and center of mass can be evaluated for plumes over time. Trend analysis for total mass in the plume can demonstrate overall decreasing trends, providing strong evidence for a shrinking plume. Data can be analyzed using linear regression or non-parametric tests (such as the Mann-Kendall test). Several software tools, such as AFCEEs MAROS program (Aziz et al., 2003) and the GSI Mann-Kendall spreadsheet (Connor et al., 2012) are available to help site managers make these types of computations.

Note that it is easy to confuse different types of rates in natural attenuation analysis. A USEPA document is available that describes different types of rates used in MNA evaluation and how to calculate them (Newell et al., 2002; Wilson 2011).

If the average trend in all plume wells (using a method such as the one employed in MAROS and shown in Figure 8) is either Probably Decreasing or Decreasing or Stable then the plume concentrations indicate natural attenuation processes are active.

If the groundwater plume is stable, decreasing, or probably decreasing, then Question III.1 is answered YES.

3.3.2. Question III.2. Is there evidence of on-going natural attenuation processes in the plume?

Criteria: Analysis of natural attenuation processes and footprints of natural attenuation in the plume. Footprints of natural attenuation describe indicators of change in groundwater other than a decline in the concentration of the original contaminant in or near the plume. For example:

Depletion of oxygen, nitrate and sulfate indicate hydrocarbon degradation. Low oxygen, nitrate, and sulfate concentrations indicate more anaerobic

geochemical conditions that support reductive dechlorination of many chlorinated solvents such as TCE and PCE.



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Generation of cis-1,2-DCE indicates that biodegradation of TCE is occurring in groundwater.

Generation of 1,1-DCE indicates that abiotic degradation of 1,1,1-trichloroethane is occurring in groundwater. Abiotic degradation is the chemical transformation that degrades contaminants without microbial facilitation. This can result in partial or complete degradation of contaminants. Typically, only halogenated compounds are subject to these mechanisms in the groundwater environment.

Presence of reactive minerals and soils that can abiotically degrade chlorinated solvents, e.g., magnetite.

Changes in compound specific isotope ratios can provide supporting evidence documenting that biodegradation or abiotic transformation processes are actually destroying contaminants at the site (USEPA, 2008).

Genetic analyses of microbial populations can provide an optional line of evidence supporting MNA. Members of the Dehalococcoides group of bacteria are the only organisms known to date to completely degrade chlorinated ethenes to harmless products. Therefore, for chlorinated solvent sites, the presence or absence of these organisms can provide information on whether MNA is an appropriate approach at a specific site (USEPA, 2006). Other metabolic and genetic indicators can also demonstrate the presence of microbes capable of cometabolic degradation of compounds.

A reduction in mass flux/mass discharge along the flow path (in both time and space) can be used to indicate natural attenuation of the plume (USEPA, 1998).

Key references that discuss footprints and indicators of MNA include:

The NRCs book on Natural Attenuation for Groundwater Remediation (NRC, 2000) (e.g., see Table 2).

Scenarios Evaluation Tool for Chlorinated Solvent MNA (Truex et al., 2006). USEPA Technical Protocol for Evaluating Natural Attenuation of Chlorinated

Solvents in Groundwater (USEPA, 1998). Technical Protocol for Implementing Intrinsic Remediation with Long-Term

Monitoring for Natural Attenuation of Fuel Contamination Dissolved in Groundwater (Wiedemeier et al. 1999a).

Natural Attenuation of Fuels and Chlorinated Solvents (Wiedemeier et al., 1999b).

An Approach for Evaluating the Progress of Natural Attenuation in Groundwater (Wilson, 2011).

Frequently Asked Questions about MNA in the 21st Century (Adamson et al, 2012).



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Table 2. Summary of Natural Attenuation Footprints at MNA Case Study Sites (NRC, 2000).



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The NRC (2000) compiled a list of sites where the footprint approach was employed, and this summary is reproduced as Table 2 above. If there is evidence of footprints of natural attenuation in the plume, then Question III.2 is answered YES.

3.3.3. Question III.3. Are conditions protective of potential and future receptors? Criteria: Analysis showing all exposure pathways for actual receptors are incomplete or do not present excess risk, and that future exposure will not occur at levels above risk criteria. An assessment of actual receptors in the area needs to be developed to determine if any exposure pathways are complete. Key exposure pathways are:

Groundwater ingestion through existing water supply wells. Discharge to sensitive receiving (surface) waters. Indoor air impacts results from the groundwater to indoor air pathway for existing

receptors. Demonstration of protective conditions will usually include an evaluation of current and possible future property use. The presence of deed notices, zoning restrictions and other institutional controls limits potential exposure and can be used as one line of evidence supporting low threat conditions. Additionally, an assessment of future hypothetical receptors and exposure pathways, or an analysis that demonstrates that there will be no future complete exposure pathways, supports a conclusion of no adverse effects from the residual contamination. Overall, this assessment should ensure that unacceptable risks to water quality, human health, ecological, and sensitive receptors, both current and future, are identified and mitigated. It should be demonstrated that the residual contamination present at the site will not adversely impact current and future receptors. Evaluation of potential impacts to current and future receptors should include (CRWQCB, 2009):

Human health. Ecological exposure, e.g., aquatic life, wildlife, wetlands, crops, vegetation, and

habitats. Sensitive receptors. Downgradient groundwater. Downgradient surface water. Anticipated cross-media transfer exposures, e.g., non-domestic or agricultural

uses, indoor air vapor intrusion through volatilization, surface water or other aquifer contamination through hydraulic connections.

Changes in use or potential use of site or surrounding properties. Sensitive or vulnerable groundwater basins. Discussion of feasibility of existing or future engineering or institutional controls

applied to limit or prevent exposures. Vapor intrusion.



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Estimating mass discharge reaching a potential Point of Exposure (POE) such as a water well or surface water body can be used to demonstrate protective conditions (Einarson and Mackay, 2001; Newell et al., 2011). Tools that can be used to estimate risks to receptors include the RBCA Tool Kit (GSI, 2007, see Figure 11 below). Off-site plumes will need special consideration to ensure no illegal or uncontrolled access to residual contaminated media.

Figure 11. RBCA analyses for both potential and actual receptors (Figure A.3 from GSI, 2007).

Sites where residual contaminant concentrations are above groundwater cleanup standards pose potentially unacceptable threats or risks, based on current or anticipated use of land or water resources, and often necessitate risk management measures. Such measures include institutional controls (e.g., land-use covenants, deed restrictions, and soil management plans) and engineering controls (e.g., soil capping, fencing, sub slab venting, and vapor barriers). Measures such as these are necessary to protect human health and safety, and the environment. Sites with risk management measures may be eligible for low-risk site closure provided:

The risk management measure is appropriate for the site circumstances, both current and future;

The site meets all other closure criteria; Continued oversight from a regulatory agency is not required for the risk

management measure(s); and The risk management measures are robust, durable, and sustainable over time.

Placement of any institutional control on a property may require:

Providing appropriate agencies with property information, e.g., title insurance and survey of affected property.

Verification with local municipalities that proposed use prohibitions are consistent with local zoning requirements.



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Providing all site stakeholders with notification of intent to install institutional control. Documentation must be provided to appropriate agencies verifying the notification of all site stakeholders.

Examples of risk management controls eligible for low-risk site closures include:

Deed restrictions, for sites where only the drinking water standards have not been met, prohibiting use of groundwater for drinking water.

Zoning restrictions limiting property use to commercial or industrial. Voluntary protective measures, e.g., vapor barriers preventing potential indoor air

exposures due to soil gas intrusion. However, these protective measures cannot be required to prevent an existing, eminent, or potential threat.

Conversely, some risk management measures may make a site ineligible for low-risk closure, e.g.:

Containment zones or other required waste-containment measure for high concentration/high risk sites.

Institutional controls prohibiting sensitive land use or restricting excavation or soil trenching.

Active engineering controls required to mitigate exposure to or prevent the spread of the constituent residual concentrations.

All exposure pathways for actual receptors should be incomplete or present acceptable risk, and an analysis should show there will be no unacceptable risks in the future.

If current and future risks are zero or acceptable, then Question III.3 is answered YES.

3.3.4. Question III. 4. Is there a near-term need for the impacted groundwater resource or any impacted land uses?

Criteria: Evaluation of future needs for groundwater resource and associated overlying land uses. Evaluation of potential impacts to current and future water resources should be based on best professional judgment and include review and documentation of (adapted from CRWQCB, 2009):

All relevant water resources publications. Local groundwater and surface water management plans. Groundwater protection and beneficial use evaluations. Domestic and agricultural water well locations. Municipal water supply and monitoring well locations. Consultations with local water agencies. Future beneficial use timeframes.

The current yield and water quality can be important factors in determining the near- term uses for the groundwater resources. For example, the State of Texas defines



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groundwater with yield 3000 mg/L as not being usable groundwater. The AFCEE Sustainable Remediation Toolkit (SRT) (AFCEE, 2010) (Figure 12) provides a method for estimating the economic value of water in a contaminated groundwater unit, based on the estimated yield of the formation, volume of affected groundwater, and other factors. Plumes in groundwater with little to no economic value may be considered at low risk for future exploitation. The hydraulic communication between a plume in a shallow unit with a deeper unit, when aquifer data (e.g., pumping tests) are not available or not substantial, needs to be considered. This can be done by:

Evaluating the local and regional stratigraphy and establishing if a competent aquitard is present.

Employing tools such as the American Petroleum Institutes Groundwater Sensitivity Toolkit (Figure 13) (GSI, 2002) that account for vertical flow across an aquitard or vertical flow in an artificial penetration such as an abandoned well.

Using indicators such as geochemistry, groundwater age, local discharge points, and other factors.

Note that in some regions the beneficial uses assigned to groundwater basins and surface water bodies do not differentiate between shallow and deeper groundwater aquifers (CRWQCB, 2009). Typically in such cases, the shallow aquifer will have the same beneficial use designation as the deeper aquifer unless an exception, as allowed by state regulations, can be demonstrated. In certain instances, resource degradation, such as pre-existing poor quality or salt-water intrusion due to excessive pumping, may deem the use of a deeper aquifer impractical as a drinking water source.

Figure 12. Economic value normalization methodology of groundwater in the SRT (Newell

et al., 2008). PWS = Public Water Supply; WQP = Water Quality Protection; TDS = Total Dissolved Solids.



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Figure 13. Architecture of the Groundwater Sensitivity Toolkit (GSI, 2002).

The residual contamination should show no large adverse economic impact or denial of large-scale beneficial land or water use if residual contamination remains at the site. If there are no adverse effects to land and water uses from the residual contamination, then Question III.4 is answered YES.

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4.0 REDUCING LONG-TERM MONITORING INTENSITY An alternative available when a complete low-risk/low-threat site closure is not justified is the reduction in the long-term monitoring intensity at the site. Groundwater characterization and remediation efforts at most sites result in large data sets and a number of monitoring locations that may or may not be useful in long-term plume management. Depending on the size and complexity of the plume, several tools and techniques are available to improve the efficiency of groundwater monitoring networks. When negotiating reduction in monitoring intensity, results of several qualitative and quantitative evaluation strategies should be assembled to clarify the evidence for streamlined data collection. Qualitative evaluation strategies rely on expert professional opinion and understanding of site-specific conditions. Tools for qualitative evaluation include decision logic trees, assembling and comparing site data, and forming a lines of evidence approach to site data. Most long-term monitoring optimization (LTMO) techniques involve either an initial or final qualitative evaluation of the monitoring program. Quantitative techniques include statistical, geo-statistical and mathematical optimization methods. Common steps in developing an optimized long-term monitoring network include:

Identification of site goals and objectives: Sampling strategies should provide sufficient data to support site reporting and regulatory goals. Each sample should address one or more monitoring objectives such as demonstrating containment of the plume, attenuation of mass and protectiveness of the remedy. Sampling frequency should be proportional to the rate of change of concentrations in the plume and sufficient to satisfy the reporting frequency of regulatory programs.

CSM: A thorough CSM is essential to developing appropriate monitoring strategies. LTMO techniques can be beneficial for sites where characterization efforts and active remedial work are largely complete. Aspects of the CSM that are of particular importance in LTMO strategies include source control, delineation of the plume, consistent hydrogeological environment, contaminant attenuation mechanisms and location of potential receptors.

Minimum data requirements: As a general rule, sites where minimum data requirements are met (e.g., four to six separate sample events, between six and 15 monitoring locations) and where concentrations have largely stabilized are good candidates for reduction in monitoring effort.

Plume stability: The determination that a groundwater plume is stable can be the prelude to a reduction in monitoring effort and frequency (USEPA, 1999; USEPA, 2004; ITRC, 2007). Most methods recommended to demonstrate plume stability include analyzing historic groundwater data from individual well locations, preparation of contaminant concentration contour maps, concentration vs. time, and concentration vs. distance graphs. Quantitative statistical approaches to stability assessment include a review of summary statistics, data distributions, and appropriate trend analyses for both individual monitoring locations and plume-wide measures (Vanderford, 2010).

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Attenuation Mechanisms: Qualitative approaches to demonstrating the appropriateness of reduced monitoring efforts can include analyses of geochemical biodegradation indicators and evaluating the sustainability of mass destruction mechanisms (Chapelle et al., 2003; TNRCC, 1997).

Institutional and Land Use Controls: Restriction of access to groundwater and elimination of potential exposure factors can be strong support for a reduction in monitoring effort.

Software tools, such as the peer-reviewed and nationally recognized AFCEE Geostatistical Temporal/Spatial (GTS) Optimization Algorithm (Cameron and Hunter, No Date; Hunter, 2011) and MAROS (Aziz et al., 2003), are available to assist site managers optimize LTMO analyses. A new AFCEE software tool, the 3-Tiered Monitoring Optimization (3TMO) tool, will be available in the near future (Hunter, 2011). AFCEE has also developed a comprehensive Long-Term Monitoring Optimization Guide (AFCEE, 2006) for effective identification and application of appropriate LTMO strategies and optimization.

REFERENCES


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5.0 REFERENCES Adamski, M., V. Kremesec, R. Kolhatkar, C. Pearson, and B. Rowan, 2005. LNAPL in Fine-

Grained Soils: Conceptualization of Saturation, Distribution, Recovery, and Their Modeling, Ground Water Monitoring and Remediation 25(1):100-112.

Adamson, D.T. and C.J. Newell, 2012. Frequently Asked Questions About Monitored Natural Attenuation in the 21st Century. Environmental Security and Technology Certification Program(ESTCP) Project ER-201211, In Preparation.

AFCEE, 2006. Long-Term Monitoring Optimization Guide. HQ Air Force Center for Environmental Excellence, Brooks City-Base, TX.

AFCEE, 2007. AFCEE Source Zone Initiative. Prepared by T.C. Sale, T.H. Illangasekare, J. Zimbron, D. Rodriguez, B. Wilking, F. Marinelli for AFCEE, Brooks City-Base, San Antonio, TX.

AFCEE, 2010. Sustainable Remediation Tool (SRT), version 2.1, Brooks City-Base, San Antonio, TX.

Aziz, J.J.; M. Ling, H.S. Rifai, C.J. Newell, and J.R. Gonzales, 2003. MAROS: a Decision Support System for Optimizing Monitoring Plans, Journal of Ground Water, Vol. 41, No. 3.

Beckett, G. D., and P. Lundegard. 1997. Practically ImpracticalThe Limits of LNAPL Recovery and Relationship to Risk, pp. 44245 in Proceedings, Petroleum Hydrocarbons and Organic Chemicals in Ground Water: Prevention, Detection, and Remediation Conference. Houston: Ground Water Publishing Company.

Connor, J.A. S.K. Farhat, M. Vanderford, and C.J. Newell, 2012. GSI Mann-Kendall Toolkit for Constituent Trend Analysis, GSI Environmental Inc, Houston, TX, July 2012.

Cameron, K. and P. Hunter, No Date. Optimization of LTM Networks Using GTS: Statistical Approaches to Spatial and Temporal Redundancy, AFCEE. http://www.afcee.af.mil/shared/media/document/AFD-070831-023.pdf. Accessed July 25, 2011.

Chapelle, F. H., M. A. Widdowson, J.S. Brauner, E. Mendez, and C.C. Casey, 2003. Methodology for Estimating Times of Remediation Associated with Monitored Natural Attenuation. Columbia, S.C., U. S. Geological Survey (USGS): 58.

CDPHE, 2010. Draft Guidance for the Closure of Low-Threat Sites with Residual Ground Water Contamination. Colorado Department of Public Health and Environment, August 13, 2010.

CRWQCB, 2009. Assessment Tool for Closure of Low-Threat Chlorinated Solvent Sites. California Regional Water Quality Control Board, San Francisco Bay Region, July 31, 2009.

Einarson, M.D. and D.M. Mackay. 2001. Predicting the Impacts of Groundwater Contamination, Environmental Science and Technology 35, no.

low-risk site closure - gsi-net.com

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