United States Office of Research and EPA/625/K-96/001

Environmental Protection Development May 1996

Agency Washington, DC 20460

1EPA Seminars

Bioremediation of

Hazardous Waste Sites:

Practical Approaches to

Implementation

May 29--30, 1996----Chicago, IL

June 4--5, 1996----Kansas City, MO

June 6--7, 1996----Atlanta, GA

June 18--19, 1996----San Francisco, CA
EPA/625/K-96/001

May 1996

Seminars on

Bioremediation of Hazardous Waste Sites:

Practical Approaches to Implementation

Office of Research and Development

U.S. Environmental Protection Agency

Washington, DC
Notice

Mention of trade names or commercial products does not constitute endorsement or recommendation for use.

ii
Contents

Background Information for Bioremediation Applications ............................... 1-1

Bioventing ................................................................ 2-1

Bioremediation of Sediments ................................................... 3-1

Aerated Lagoons: A Case Study ................................................ 4-1

Oil-Contaminated Shorelines .................................................. 5-1

Land Treatment ............................................................ 6-1

Land Treatment Unit Case Study:

Champion International Superfund Site ........................................... 7-1

Phytoremediation ........................................................... 8-1

Development and Application of Composting Techniques for Treatment of

Soils Contaminated With Hazardous Waste ........................................ 9-1

Biopile Treatment of Soils Contaminated With Hazardous Waste ........................ 10-1

Effective Treatment of Hazardous Waste Constituents in Soil by

Lignin-Degrading Fungi ..................................................... 11-1

Slurry Bioreactors for Treatment of Contaminated Soils,

Sludges, and Sediments ..................................................... 12-1

Fixed Film Bioreactors ...................................................... 13-1

Suspended Growth Bioreactors ................................................ 14-1

Natural Attenuation of Ground Water ........................................... 15-1

Natural Attenuation of Soils .................................................. 16-1

Natural Attenuation of Landfills ................................................ 17-1

Natural Attenuation of Sediments .............................................. 18-1

Source Control: Free Product Recovery and Hydraulic Containment ...................... 19-1

Air Sparging/Air Injection .............................................. 19-5

iii
State Review: Natural Attenuation of Ground Water and Soils .......................... 20-1

Monitoring .............................................................. 21-1

Modeling ............................................................... 22-1

iv
Sources of Information

Recent EPA Bioremediation Publications

http://www.epa.gov/docs/ORD

Bioremediation in the Field Bulletin

Latest edition EPA/540/N-96/500

Bioremediation in the Field Search System: Database on national and some international field

applications 

Version 2.0 EPA/540/R-95/508b

Also on the Internet

Request to be on EPA's bioremediation mailing list or to request specific bioremediation documents

513-569-7562

NRMRL/SPRD Home Page

http://www.epa.gov/ada/kerrlab.html

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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

14-1

Suspended Growth Bioreactors

Dolloff F. Bishop and Richard C. Brenner

Office of Research and Development, National Risk Management Research Laboratory,

U.S. Environmental Protection Agency, Cincinnati, OH

Introduction

Suspended growth bioreactors are standard technology for treating organic contaminants in

aqueous and waste sludge systems. The reactors use microbial metabolism under aerobic,

anaerobic, or sequential anaerobic/aerobic conditions to biosorb organic compounds and

biodegrade them to innocuous residuals. The microbial activity in the systems produces biomass that

is removed by gravity sedimentation, with a portion of the settled biomass recycled to maintain a

desired mixed liquor suspended solids concentration in the bioreactor. The excess biomass is wasted

to a sludge disposal process. Reactor configurations include sequencing batch reactors (SBRs),

completely mixed activated sludge systems, plug flow activated sludge systems, and aerobic and

anaerobic digestors.

The reactor systems used to efficiently treat hazardous wastes in aqueous streams or sludges require

sufficient amounts of organic carbon in the stream or sludge to support a stable microbial culture

in the bioreactor (i.e., at least 5 to 10 pounds influent biochemical oxygen demand [BOD] per day

per 1,000 cubic feet of bioreactor volume and at least 100 pounds influent volatile suspended

solids [VSS] per day per 1,000 cubic feet of aerobic or high-rate anaerobic digester volume) (1).

Conversely, influent concentrations and/or loadings of hazardous wastes high enough to cause

inhibitory effects and process performance disruption must be avoided. Typical loading ranges for

suspended growth processes (1) are shown in Tables 1 and 2.

The restrictions noted above limit application of suspended growth reactors in hazardous waste

biotreatment, although addition of powdered activated carbon to a bioreactor (1) may expand the

application area. Thus, ground water or leachates contaminated with low levels of BOD often will

not be efficiently treated at the contaminated source by onsite suspended growth bioreactors without

the addition of supplemental organic carbon. With this limitation, an alternative approach for

treatment of dilute hazardous waste streams in suspended growth bioreactors can be considered.

The dilute waste stream can be discharged to a central wastewater treatment plant (with plant

management approval) for combined offsite treatment with municipal wastewater.

Representative Reactor Systems

A typical system for onsite treatment (2) of aqueous waste streams (Figure 1) for leachates or highly

contaminated ground water includes an equalization tank, a splitter box, and a contact stabilization

activated sludge process with a secondary clarifier. Ancillary processes include a waste sludge

digester with supernatant return to the equalization tank and a volatile organic compound (VOC)

stripper for unproved management of poorly degradable VOCs in the aqueous effluent. This
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

14-2

relatively complex biosystem may also require tertiary treatment processes such as sand filtration

and/or carbon adsorption to meet effluent discharge standards. Carbon adsorption may also be

applied to VOC stripper air discharges, if required.

The alternative approach of discharging the hazardous waste stream to a central wastewater

treatment plant (3), if available, offers more cost-effective biotreatment. U.S. Environmental

Protection Agency (EPA) evaluated such an approach in two pilot clarification/activated sludge

systems (Table 3) typical of continuous plug flow municipal wastewater treatment plants. One

bioreactor was operated at a sludge retention time (SRT) of 4 days, the other at an SRT of 8 days.

The municipal wastewater fed to the systems was spiked with up to 28 hazardous organic

compounds. The spiked concentrations in the wastewater were less than or equal to 0.25 mg/L and

less than or equal to 0.5 mg/L for the 4- and 8-day SRT systems, respectively. Finally, the sludges

produced in the municipal pilot system receiving wastewater with 0.5 mg/L of spiked contaminants

were treated in pilot anaerobic digesters to evaluate the impact of the hazardous contaminants in

the wastewater sludges on the anaerobic digestion process (4). Three completely mixed pilot-scale

digesters (Figure 2) maintained at 35.5EC with a 30-day solids retention time were used to simulate

typical digester operation. Two of the digesters were fed contaminated primary and secondary

sludges from the pilot study. The third digester (used as a control) was fed similar sludges without

the hazardous organic contaminants.

Performance and Conclusions

The onsite activated sludge system achieved moderate to high removal efficiencies (Table 4) of

benzene, toluene, ethyl benzene, and xylenes (BTEX) and low to high removals (Table 5) of

chlorinated solvents (2). The performance of the complex onsite system suggests that tertiary

treatment may be necessary if stringent effluent discharge standards are required. Alternative fixed

film bioreactors, in general, would provide superior and more cost-effective bioremediation.

The alternative approach, evaluated by EPA, of discharging contaminated ground water or leachates

to a central wastewater treatment plant generally resulted in high removals (Tables 6 and 7) of the

influent hazardous contaminants (3). Removals were superior to those provided by the onsite

activated sludge system. The two treatment systems were not identical, however, and did not treat

the same contaminants. The superior performance at the central plant may have been related to

more effective biomass generated by the large amount of easily degradable organic substrate in the

municipal wastewater. In any event, the complex onsite system will exhibit substantially increased

costs per unit of contaminant removed when compared with costs at central treatment plants.

The performance of anaerobic digestion on the contaminated sludges from the pilot study evaluating

the central treatment plant alternative was compared with that of a control digester (4). Gas

production and solids reduction for digestion of contaminated sludges and control sludges were

nearly identical. Degradation of the hazardous contaminants (Table 8) was apparent. Twelve

chemicals appeared consistently in the digester treating contaminated sludge, and, at steady state,
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

14-3

contaminant degradation or transformation ranged from 93 to 98 percent. Sorption into the

digester solids also was an important removal mechanism, especially for aromatics.

EPA generated an integrated model for predicting the fate of organics in wastewater treatment plants

(5), which includes components for stripping or volatilization, sorption on solids, and

biodegradation. The biodegradation component (6) includes a structural activity group contribution

method for estimating contaminant biodegradation kinetics.

The experimental data generated by the EPA studies described above were used to successfully

validate the integrated model.

References

1. Metcalf & Eddy. 1991. Wastewater engineering: Treatment, disposal, and reuse, 3rd ed.

In: Tchobanoglous, G., and F.L. Burton, eds. New York, NY: McGraw-Hill.

2. Nelson, C., et al. 1993. Reactors for treatment of solid, liquid, and gaseous phases. In:

Proceedings of Seminars on Bioremediation of Hazardous Waste Sites: Practical

Approaches to Implementation. EPA/600/K-93/002. Washington, DC.

3. Bhattacharya, S.K., et al. 1990. Fate and effects of selected RCRA and CERCLA

compounds in activated sludge systems. In: Proceedings of the Fifteenth Annual Research

Symposium—Remedial Action, Treatment, and Disposal of Hazardous Waste.

EPA/600/9-90/006. U.S. EPA, Risk Reduction Engineering Laboratory, Cincinnati, OH.

4. Govind, R., et al. 1991. Fate and effects of semivolatile organic pollutants during anaerobic

digestion of sludge. Water Res. 25:547-556.

5. Govind, R., et al. 1991. Integrated model for predicting the fate of organics in wastewater

treatment plants. Environ. Prog. 10:13-23.

6. Desai, S.M., R. Govind, and H. Tabak. 1990. Development of quantitative structure-activity

relationships for predicting biodegradation kinetics. Environ. Toxicol. Chem.

9:1,092-1,097.
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Suspended

Growth Reactors

Dolloff F. Bishop or Gregory Sayles

Office of Research and Development

National Risk Management Research Laboratory

U.S. Environmental Protection Agency

Cincinnati, OH

Suspended Growth Bioreactor

Configurations

● Completely mixed activated sludge systems

(continuous wastewater feed)

● Plug flow activated sludge systems

(continuous wastewater feed)

● Sequencing batch reactors (batch wastewater

feed)

● Aerobic digesters (batch or continuous

sludge feed)

● Anaerobic digesters (batch or continuous

sludge feed)

Table 1. Activated Sludge

Loading Ranges

Detention Volumetric Loading

Reactor Configuration Time (hr) (lb BOD/day/1,000 ft3)

Plug flow (conventional) 4–8 20–40

Completely mixed 3–5 50–120

Step feed 3–5 40–60

Contact stabilization 1.5–3 60–75

Extended aeration 18–36 10–25

SBR 12–50 5 –15

Table 2. Sludge Digester

Loading Rates

Sludge Digester Retention Solids Loading

Type Time (day) (lb SS/day/1,000 ft3)

Aerobic

 Waste activated sludge (WAS) 10–15 100–300

 Primary + WAS 15–20 100–300

Standard-rate anaerobic 30–60 40–100

High-rate anaerobic 15–20 100–200

Applications of Suspended

Growth Reactors

● Onsite applications limited to moderate or

high strength leachates or ground water

● Inhibitory concentrations of hazardous

wastes can prevent onsite application

● PAC addition to activated sludge reactors can

extend onsite inhibitory waste applications

● Alternatively, ground water and leachates

can be routed to and processed at central

wastewater treatment plants

Figure 1. Onsite Activated Sludge System

VOC Stripper

Splitter Box

Contact

Tank

Clarifier

Holding

Reaeration Tank

Tank

Equalization

Tank

Digester Activated

Sludge

Waste Sludge

Supernatant

Inlet

To VOC

Stripper and

Tertiary Filter for

Further Treatment

F

F

F
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Table 3. Conventional Operating

Performance of Pilot Systems*

 % Removals

 4-day SRT 8-day SRT

Component Continuous Intermittent** Continous Intermittent**

TSS 97 97 95 94

COD 82 81 88 87

NH4-N 76 81 88 98

*Feed to systems was Mill Creek municipal wastewater at the EPA Test

and Evaluation Facility in Cincinnati, OH

**Continous or intermittent hazardous contaminant addition

Figure 2.

Pilot

Digester

System

Table 4. Representative Onsite

Activated Sludge System Performance

for BTEX Compounds

Influent Conc.

Compound (ppb) % Removal

Benzene 120 78

Toluene 1,000 89

Ethylbenzene 270 94

Xylenes (total) 700 95

Table 5. Representative Onsite Activated

Sludge System Performance for Chlorinated

Compounds

Influent

Conc.

Compound (ppb) % Removal

Chlorobenzene 180 78

Methylene chloride 31 100

Trichloroethane 250 80

1,2-Dichloroethane 100 56

1,2-Dichloropropane 21 67

Table 6. Representative Removals in

Acclimated Pilot System Operating at

4-Day SRT

Influent

Conc.

Compound (ppb) % Removal

Toluene 284 99

Xylenes (total) 175 99

Chlorobenzene 255 99

Trichloroethane 201 97

1,2-Dichloropropane 228 77

Table 7. Representative Removals in

Pilot System Operating at 8-Day SRT

Influent

Conc.

Compound (ppb) % Removal

Di-n-bytylphthalate 428 96

1,4-dichlorobenzene 391 95

Lindane 425 56

Naphthalene 431 98

1,2,4-trichlorobenzene 655 85
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Table 8. Fate of Representative

Organics in Digesters

Feed Fate Mechanism (% Distribution)

Compound mg/kg Sol. Vol. Sorpt. Biodeg.

Di-n-bytylphthalate 270 1 0 3 96

1,4-dichlorobenzene 275 4 16 68 13

Lindane 490 0 0 2 98

Naphthalene 230 4 4 65 27

1,2,4-trichlorobenzene 750 3 5 66 26

Model for Predicting Fate of Organics

in Wastewater Treatment

● Primary sedimentation mass balances

● Mass balances in secondary treatment

■ Biodegradation

■ Sorption

■ Volatilization (diffused aeration)

■ Stripping (surface aeration)

● Group contribution method for estimating

biokinetics
15-1

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Natural Attenuation:

Site Characterization

Attenuation of Petroleum

Hydrocarbons and Solvents

in Ground Water

John Wilson

Office of Research and Development

National Risk Management Research Laboratory

U.S. Environmental Protection Agency

Cincinnati, Ohio

Two Basic Questions for

Bioremediation

●When to start?

●When to stop?

When to Stop?

● When proactive remediation is

no longer doing any good

● When proactive remediation is

no faster than intrinsic

remediation or natural

attenuation

After Proactive

Remediation

Is the spread of contamination

contained by natural attenuation?

● Yes? Go into long-term

monitoring

● No? Implement another

approach

Natural Attenuation or

Passive Bioremediation

● The preferred description is

natural attenuation

● All bioremediation is “natural”

● Neither the microorganisms

nor the microbiologists are

“passive”

Natural Attenuation

Usually implemented as a

component of a comprehensive

remedial strategy that includes

source control or source removal

● Free product recovery

● Soil vacuum extraction

● Bioremediation
15-2

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

● Determination is site specific

● Requires extensive site

characterization

● Requires a risk assessment

Natural Attenuation Natural Attenuation

● Burden of proof is on the

proponent, not the regulator

● Not a default technology or

presumptive remedy

● Not complete until goals of the

regulatory agency have been

reached to their satisfaction

Patterns of Natural

Bioremediation

● Limited by supply of a soluble

electron acceptor

■ Aerobic respiration

■ Nitrate reduction

■ Sulfate reduction

● Controlled by mixing processes

(bioplume)

Patterns of Natural

Attenuation

● Limited by biological activity

■ Iron reduction

■ Methanogenesis

■ Sulfate reduction

● First-order kinetics

Patterns of Natural

Attenuation

● Limited by supply of

electron donor

● Reductive dechlorination

● Controlled by supply of

electron donor

Initial Elements of a

Quantitative Assessment of

Natural Attenuation

1. Thoroughly delineate the extent of

contaminated ground water

2. Determine trajectory of groundwater flow

3. Install monitoring wells along

plumes
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

4. Determine apparent attenuation along

plumes

5. Correct apparent attenuation for dilution or

sorption

6. Assume corrected attenuation is

bioattenuation

7. Confirm bioattenuation from stoichiometry

of electron acceptors or donors

Additional Elements of a

Quantitative Assessment of

Natural Attenuation

Lines of Evidence

● Documented loss of

contaminants at the field scale

● Geochemical indicators

● Laboratory microcosm studies,

accumulation of metabolic endproducts, volatile fatty acids,

FAME

Document Occurrence of

Natural Attenuation

● Use geochemical data to support natural

attenuation

● Trends during biodegradation (plume

interior vs. background concentrations)

■ Dissolved oxygen concentrations below background

■ Nitrate concentrations below background

■ Iron II concentrations above background

■ Sulfate concentrations below background

■ Methane concentrations above background
15-4

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

HILL AFB, UTAH AUGUST 1993

HILL AFB, UTAH AUGUST 1993

HILL AFB, UTAH 

AUGUST 1993
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

HILL AFB, UTAH AUGUST 1993

HILL AFB, UTAH 

AUGUST 1993
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

HILL AFB, UTAH AUGUST 1993

HILL AFB, UTAH 

AUGUST 1993

HILL AFB, UTAH AUGUST 1993

HILL AFB, UTAH 

AUGUST 1993
15-7

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Correcting Attenuation for

Dilution or Sorption

Identify a component of

the plume that can serve

as a tracer

Correcting Attenuation for

Dilution or Sorption

To correct apparent

attenuation for dilution or

sorption, divide the

concentration of contaminants

by the concentration of a

conservative tracer

A Good Tracer

Is not biodegradable in the

absence of oxygen
15-8

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

A Good Tracer

Is present in the plume

source area at

concentrations at least

100 times its detection

limit

A Good Tracer

Has the same sorptive

properties as the regulated

compounds
15-9

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Near Source
15-10

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Toe of the Plume

Remediated
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Background
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Natural Attenuation of

Chlorinated Solvents

Mechanism of Chloroethene

Biotransformation

Reductive dehalogenation:

● Oxidation/reduction reaction where electrons are transferred

from donor to chlorinated hydrocarbon acceptor

Co-metabolic process:

● Organisms growing on alternate carbon sources

Primary substrates:

● Potential for natural (soil organic matter) and anthropogenic

sources

Alternative Pathways for

Chloroethene Biotransformation

Oxidative biodegradation:

● Vinyl chloride shown to biodegrade under aerobic conditions

● Fe reducers may also oxidize vinyl chloride

Supporting evidence:

● Transport properties (migration) of DCE and VC relative to TCE

● Aerobic biodegradation of vinyl chloride to CO 2 demonstrated in

microcosms

CO2

DCE

VC

Native

Biotransformations for

Chloroethenes

Patterns of Natural

Attenuation Sites

Type I Low background organic matter

concentrations, dissolved oxygen

and possibly nitrate greater than

1 mg/L

Type II Anthropogenic carbon sources (e.g.,

BTEX, landfill leachate) are present

Type III Native organic carbon drives

dechlorination
15-13

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation
15-15

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Microcosm Studies for

Complex Technical Issues Resources Required

To conduct ground-water

microcosm studies:

● 18–24 months

● $100–$300 K
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation
15-17

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Apparatus

and

Geometry

Associated

with a

Borehole

Flowmeter

Test

Error produced by using the

average hydraulic conductivity as

revealed by a conventional

aquifer test to estimate the

interstitial seepage velocity (and

thus residence time) of the JP-4

plume at George AFB
15-18

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Hydraulic

Average Conductivity

Hydraulic of Most

Monitoring Conductivity Transmissive

Well (cm/sec) Interval (cm/sec)

MW-27 0.0074 0.11

MW-28 0.0046 0.022

MW-29 0.0028 0.062

MW-31 0.013 0.26

MW-45 0.0032 0.0056

MW-46 0.018 0.40

Bioscreen Input Screen

Bioscreen Input Screen

Bioscreen

Bioscreen will be available on the

NRMRL/SPRD Web page:

www.epa.gov/ada/kerrlab.html

A Retrospective Evaluation

of In Situ Bioremediation

Procedure used to estimate the

impact of residual petroleum

hydrocarbons on ground-water

quality at the Public Services site

in Denver, Colorado.
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

In many floodplain landscapes, the

most important transfer of

contaminants from LNAPL to ground

water is through diffusion from the

LNAPL to transmissive layers in the

aquifer, rather than through

dissolution and direct advection.

This suggests an approach to

estimate the impact of spills of

petroleum hydrocarbons on ground

water.
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation
15-21

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Calibration of Aquifer Test

Using a Geoprobe Calibration Factor for

SPRD/NRMRL Geoprobe

Hydraulic Conductivity

(cm/sec)

equals

Yield (mL per sec per cm

drawdown)

multiplied by 0.03

Fuel Derived Organic Compounds

at the Public Services Site
15-22

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Electron Acceptor Supply at the

Public Services Site

1. Determine hydraulic conductivity in

the first transmissive interval below

the LNAPL.

2. Determine hydraulic gradient in that

interval.

3. Assume a porosity, and calculate a

seepage velocity under the LNAPL.

4. Determine the length of the LNAPL in

the direction of ground-water flow.

5. Calculate residence time of water in the

transmissive interval moving under the

LNAPL.

6. Determine the highest concentration of

contaminant dissolved in ground water

in contact with LNAPL (Raoult’s Law

using core samples or direct

measurement on water).

7. Measure the vertical distance between

the bottom of the LNAPL and the top of

the transmissive part of the aquifer.

8. Calculate the diffusion gradient.

9. Look up the diffusion coefficient of the

contaminant in water (Chemical

Engineering).

10. Calculate the diffusive flux from the

LNAPL to the transmissive part of the

aquifer.

11. Use the residence time of ground water

under the NAPL to calculate total loading

by diffusion to the transmissive part of

the aquifer.

12. Determine the volume of water in the

transmissive part of the aquifer.

13. Estimate the concentration of

contaminant in the transmissive part of

the aquifer in the absence of

biodegradation.

14. Measure the supply of oxygen, nitrate,

and sulfate in the uncontaminated

ground water upgradient of the spill.

15. Compare the electron acceptor demand

of the contaminants to the electron

acceptor supply associated with oxygen,

nitrate, and sulfate in ground water

upgradient of the spill.

16. If methane concentrations in the ground

water in contact with the LNAPL are

greater than 0.1 mg/L, include methane

in the calculation of electron acceptor

demand.

Residence time 235 days

Highest conc. BTEX 175 mg/L

Diffusion path length 1.5 meters

Thickness of transmissive

 interval 1.2 meters

Loading BTEX 0.6

mg/liter

BTEX capacity 51 mg/L
15-23

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

What are the prospects that

natural attenuation is

preventing the spread of

BTEX contamination in

ground water? (containment,

not remediation)

Where Should It Work?

● River valley alluvial deposits

● Unglaciated coastal

environments on the Gulf of

Mexico and Atlantic Ocean

What To Watch Out For!

● Glacial outwash

● Upland landscapes

● Fractured bedrock aquifers

● Karst landscapes, limestone

aquifers
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

● How far will a plume move if it

is subject to Natural

Attenuation?

● How far will ground water move

in 10 years?

● How fast is water moving

through the source of groundwater contamination?

● What is the hydraulic

conductivity of the most

transmissive material that has

LNAPL?

● What is the hydraulic gradient?

● Multiply conductivity by

gradient, then divide by

porosity (0.3) to predict plume

velocity, use velocity; to predict

plume length after ten years.

● Hydraulic conductivity >10 feet

per day: Might have a huge plume

● Hydraulic conductivity 10 to 0.1

feet per day: Need more

information

● Hydraulic condictivity <0.1 foot

per day: Natural Attenuation often

will take care of it
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

15-25

Appendix: Procedure Used To Estimate the Impact of Residual Petroleum

Hydrocarbons on Ground-Water Quality at the Public Services Site in

Denver, Colorado

John Wilson

Office of Research and Development, National Risk Management Research Laboratory, 

U.S. Environmental Protection Agency, Cincinnati, OH

1. Determine hydraulic conductivity in the first transmissive interval below the light nonaqueous

phase liquid (LNAPL).

This was done using a Geoprobe to conduct a series of aquifer tests.

2. Determine the hydraulic gradient in that interval.

This was calculated using water elevations in monitoring wells. It also corresponded with the

gradient of the Platte River on a topographic map. Flow in the transmissive layers of the

floodplain was parallel to the river.

3. Assume a porosity, and calculate a seepage velocity under the LNAPL.

The assumed porosity was 0.35. Seepage velocity is the product of the hydraulic

conductivity (0.058 cm/sec) multiplied by the hydraulic gradient (0.0012 meter/meter) and

then divided by the assumed porosity (0.35). In this case, seepage velocity was 0.17 meter

per day.

4. Determine the length of the LNAPL in the direction of ground-water flow.

The length is based on analysis of core samples. It is estimated to be 40 meters.

5. Calculate residence time of water in the transmissive interval moving under the LNAPL.

Residence time is the length of the LNAPL divided by the seepage velocity of the ground

water. In this case, 40 meters divided by 0.17 meters per day or 235 days.
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

15-26

6. Determine the highest concentration of contaminant dissolved in ground water in contact

with LNAPL (Raoult's Law using core samples or direct measurement on water).

Raoult's Law says that the concentration of a particular compound in solution in ground

water should equal the water solubility of that compound multiplied by its mole fraction in

the NAPL. We will make two important conservative assumptions. Because most fuels are

a "boiling cut" at the refinery, we will assume that the molecular weights of the components

are approximately the same, and that mass fraction equals mole fraction. We will also

assume that the solubility of benzene, toluene, ethylbenzene, and xylenes (BTEX) is the

solubility of the most soluble component, benzene. The hot spot contained 206 mg/kg BTEX

in 1,176 mg/kg total petroleum hydrocarbon (TPH), predicting a mole fraction of 0.18.

Multiplying that mole fraction by the solubility of benzene (1,000 mg/liter) predicts a

concentration of BTEX of 180 mg/liter.

Direct measurements often underestimate the true concentrations estimated from analysis

of core samples due to dilution from uncontaminated water.

7. Measure the vertical distance between the bottom of the LNAPL and the top of the

transmissive part of the aquifer. 

This was done by "sniffing" core samples and by analysis of TPH in core samples, and by

close-interval measurement of hydraulic conductivity using the Geoprobe. In this case, the

vertical distance was 1.5 meters. 

8. Calculate the diffusion gradient.

The gradient is the change in concentration divided by the depth interval. The conservative

assumption is that the concentration at the bottom of the gradient is zero. Under this

assumption, the gradient is estimated as the highest concentration in contact with the NAPL

divided by the depth interval to the transmissive layer. In this case, the gradient is 180

mg/liter to zero over 1.5 meters. The gradient is 180 mg/liter per 150 centimeters, or 1.2

E-03 mg/cubic centimeter per centimeter.

9. Look up the diffusion coefficient of the contaminant in water. 

A variety of chemical engineering handbooks are available, such as Chemical Engineering.

In general, diffusivity is inversely proportional to the square root of molecular weight. Of the

BTEX compounds, benzene is the lightest and diffuses the fastest. The diffusion coefficient

of benzene is 0.8 E-05 square centimeters per second.

10. Calculate the diffusive flux from the LNAPL to the transmissive part of the aquifer.

The flux is estimated by multiplying the diffusion gradient by the diffusion coefficient and

then by the porosity. In this case 1.16 mg/cubic centimeter per centimeter multiplied by 0.8
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

15-27

E-05 centimeter squared per second, then by 0.35 cubic centimeters water per cubic

centimeter aquifer material equals 3.2 E-09 mg/square centimeter per second, or 2.8

mg/square meter per day.

11. Use the residence time of ground water under the NAPL to calculate total loading by

diffusion to the transmissive part of the aquifer.

The loading is the flux multiplied by the residence time. In this case, 2.8 mg/square meter

per day multiplied by the residence time of 235 days is 658 mg per square meter.

12. Determine the volume of water in the transmissive part of the aquifer. 

The volume is the thickness of the transmissive interval multiplied by the porosity. Based on

the vertical mapping of hydraulic conductivity using the Geoprobe, the effective thickness

is 1.2 meters. Under each square meter there is 1.2 cubic meters of aquifer material in the

transmissive zone. The assumed porosity is 0.35, equivalent to 0.42 cubic meters or 420

liters of ground water under each square meter.

13. Estimate the concentration of contaminant in the transmissive part of the aquifer in the

absence of biodegradation.

The estimated concentration is the loading due to diffusion divided by the volume of water

in the transmissive interval. In this case, 235 mg per square meter divided by 420 liters

under each square meter equals 0.6 mg/liter BTEX.

14. Compare the electron acceptor demand of the contaminants to the electron acceptor supply

associated with oxygen, nitrate, and sulfate in ground water upgradient of the spill.

In this case, the analysis will be done on water samples at the downgradient edge of the

LNAPL. Based on the stoichiometry of bacterial metabolism, 0.21 mg/liter of BTEX is

consumed for each mg/liter of sulfate, 0.21 mg/liter of BTEX is consumed for each mg/liter

of nitrate, and 0.32 mg/liter of BTEX is consumed for each mg/liter of oxygen.

Concentrations of 0.5, 4.9, and 239 mg/liter of oxygen, nitrate, and sulfate have the

capacity to support microbial metabolism of 0.16, 1.0, and 50 mg/liter of BTEX,

respectively. This compares favorably with an estimated loading of only 0.6 mg/liter BTEX.
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

16-1

Natural Attenuation of Soils

Daniel Pope

Dynamac Corporation, Ada, OK

Generally, the following factors must be considered when evaluating contaminated soil for the use

of natural attenuation as a remedial alternative:

# The mass/concentration, mobility, and toxicity of contaminants.

# The proximity of receptors, including both human and environmental receptors, with

particular emphasis on sensitive human receptors and threatened/endangered

species/habitats.

# The current and planned use of the aquifer underlying or adjacent to the site for

public and private water supplies.

# The applicability and practicality of using of institutional controls to reduce the risk

of exposure of sensitive receptors and ground water to soil contamination.

Site investigation may reveal one of the following scenarios in which natural attenuation of

contaminated soil is a viable option:

1. Contamination is found essentially only in the unsaturated zone, and the

contamination concentration/mass and mobility are low enough that no significant

threat to ground-water quality exists. In this case, natural attenuation may be

considered as a primary remedy.

2. Active remediation has reduced soil contamination to the equivalent of Scenario 1.

3. Active remediation is ongoing, but Scenario 1 is applicable in certain areas of the

site; natural attenuation can be used for those areas while active measures

continue in the areas not suitable for natural attenuation.

Natural attenuation in soils in the unsaturated zone involves a complex interaction among the

chemical, physical, and biological properties of the site and contaminants. As in the saturated zone,

evaluation of natural attenuation involves assessment of site characteristics, including geology, water

flux, and soil chemistry; site microbiology, including microbial populations, microbial ability to

degrade contaminants, and degradation rates; and contaminant characteristics, including solubility,

toxicity, volatility and degradability. 

Contaminants in the unsaturated zone may be dissolved in the soil pore water adsorbed to soil

particles, or retained as residual saturation of free-phase liquid in soil pores or as vapor in the soil

gas. The applicability of natural attenuation depends on the interrelationship between the

contaminant parameters (e.g., mass/concentration, toxicity) and the factors that affect contaminant

mobility and degradation. If mobility of the contaminants is low enough that sensitive receptors are
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

16-2

not at risk and other attenuation mechanisms can operate to reduce contaminant concentration or

mass to the desired levels, then natural attenuation may be applicable as an alternative remedy.

Mobility of contaminants in each compartment of the unsaturated zone varies according to the

contaminant, soil type and chemistry, water flux, and associated factors. Estimates of mobility

should be made using one of the models applicable to contaminants in the unsaturated zone.

Attenuation mechanisms include those that essentially dilute the contaminant concentration, those

that reduce contaminant mobility (adsorption, and for metals a change of oxidation state), and those

that change the contaminant to less harmful forms, such as biodegradation of organics and change

of oxidation state for metals.

In the unsaturated zone, evaluation of natural attenuation of organic contaminants focuses on

biodegradation, because the other significant components of natural attenuation for most

contaminants either transfer the contaminants to another location (leaching, volatilization) or merely

reduce contaminant mobility and perhaps biodegradability (adsorption). The site characteristics

favorable for natural attenuation of soils and sediments are essentially those favorable for aerobic

bioremediation, because in unsaturated zone soils, aerobic bioremediation is usually the most

important factor in bioremediation. Even in an aerobic zone, however, anaerobic degradation may

be occurring. For instance, it has been found that pentachlorophenol (PCP) may degrade better in

soils that are "moderately aerobic" than in soils with high oxygen content or very low oxygen content.

Anaerobic microsites in the soil may favor removal of chlorine from the aromatic ring of PCP, and

then aerobic bioremediation could complete the degradation.

Soil oxygen levels greater than or equal to 2 percent are usually enough to support aerobic

remediation. Earlier workers recommended that soil oxygen be above 10 percent, but experience

indicates that many sites do not seem to show a significant increase in biodegradation as soil oxygen

is raised above 2 percent.

A redox potential (Eh) of 50 millivolts is considered the minimum for oxidizing, aerobic conditions.

An Eh below 50 millivolts (mV) indicates reducing, anaerobic conditions. An Eh of 400 to 800 mV

indicates highly aerated conditions, while 100 to 400 mV indicates less aerated but still aerobic

conditions. Generally, if the redox potential is less than 100 mV, active measures would be

considered if aerobic conditions are desired. Soil color can give a qualitative estimate of redox

conditions: reds, yellows, or browns indicate oxidizing conditions; gray or blue indicates reducing

conditions; and mottled colors indicate spatial variability of redox conditions.

Soil pH strongly influences the microbial activity, availability of nutrients, and chemistry of some

contaminants. Usually a pH of 5 to 9 is acceptable for bioremediation, although pH may affect

bioremediation of varying contaminants differently, and specific types of degradation may not occur

at certain pHs.

Soil moisture is closely associated with soil biological activity. Low soil moisture usually causes low

biological activity. Low soil moisture may decrease contaminant mobility, allowing more time for

bioremediation to work. Generally, soil moisture is optimum for bioremediation at about 50 to 80

percent of field capacity, where the large pores are filled with air and the small soil pores are filled
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

16-3

with water. At least 10 percent air-filled porosity is recommended for oxygen diffusion.

Soil temperature is closely related to biological activity. Biodegradation essentially stops at 0EC.

Most biodegradation rates are determined at about 20 to 25EC. Generally, metabolic activity is

halved by a 10EC drop in temperature, all other conditions staying the same. This does not

necessarily mean that biodegradation is twice as fast at a site where the mean temperature is twice

that of another site. For instance, there is at least some evidence that microbes acclimated to low

temperatures can biodegrade petroleum hydrocarbons at low temperatures about as fast as

microbes acclimated at 20EC can degrade contaminants at 20EC.

Microorganisms require nutrients such as nitrogen and phosphorus for metabolic activity. Soil

nutrient levels are usually considered from a soil concentration perspective or from the perspective

of ratios of the nutrients. For instance, a desirable concentration range for nitrogen and phosphorus

in the soil solution might be 150 to 200 ppm nitrogen and 25 to 35 ppm phosphate, although firm

evidence for recommending particular levels for bioremediation is generally lacking. From a nutrient

ratio perspective, a carbon:nitrogen:phosphorus (C:N:P) ratio of 120-300:10:1 is often

recommended. This ratio was originally based on the ratio of nutrients in microbial cells, with the

assumption that the ratio of nutrients presented to the microorganism in its environment should be

the same as the ratio in the cell. There has been little research conducted in the field to determine

the best soil nutrient concentrations or ratios for bioremediation. Also, there is little information

available on the desirable amount of trace nutrients in soils, although apparently enough trace

nutrients are available in most soils and sediments so that increasing their levels has no discernable

effect on bioremediation. 

For the biological component of natural attenuation to be effective, there must be a suitable

microbial community at the site that can degrade the contaminants. Microorganism communities

can be evaluated in many ways. Unfortunately, most of the evaluation methods do not give clear

answers to the question of most practical importance: Will the indigenous microorganisms degrade

the contaminants quickly enough to levels low enough that the contaminants will be prevented from

reaching sensitive receptors at toxic levels?

Microbial evaluation techniques include measures of microbial presence and activity such as

population counts, community profiles, degradation ability, and metabolic activity. Microbial

population counts ordinarily range from 1 to 10 x 10 counts/g soil, depending on the soil and the 6

method of counting. The correlation between population counts and biodegradation rates is difficult

to determine. Microbial identification techniques include techniques for identification of particular

species, as well as community assessment techniques including FAME profiles and sole carbon

source profiles. Generally, species identification is of limited usefulness for making decisions in field

remediation activities.

Of more interest are techniques to determine microbial ability to degrade the contaminants of

interest under laboratory conditions. Indigenous microorganisms can be grown in culture media

containing the contaminants of interest, or simply in samples of the site soil. Contaminant

degradation rates can be determined from these types of studies, although the laboratory rates may

not be representative of the rates that will be found in the field. In cases where microbial ability to

degrade the contaminants is in question, however, these tests can be helpful to establish the

feasibility of using bioremediation/natural attenuation at the site.
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

16-4

Also useful both in the field and in the laboratory are tests to indicate microbial activity. Respiration

measurements to determine O consumption and CO production are most commonly used. 2 2

Measuring CO2 production alone can be misleading, since CO2 sources and sinks other than

microbial activity may be significant. O depletion in contaminated zones compared with similar 2

"background" zones is strong evidence for biological degradation of contaminants when O2

depletion data parallels contaminant disappearance, daughter product appearance, and secondary

indicators.

Contaminants vary in their biodegradability. Generally, more water soluble compounds are more

degradable. For instance, petroleum hydrocarbons with longer chains or more rings are less water

soluble and less easily degraded. Specific examples include n-alkanes, n-alkylaromatics, and

aromatics from 5-22 carbons, which usually are biodegradable. Petroleum hydrocarbons with more

than 22 carbons tend to have fairly slow biodegradation rates. Fused aromatics and cycloparaffinics

with four rings or more may be very slow to biodegrade. The larger compounds tend to be more

strongly adsorbed to soil or trapped in soil pores, reducing their bioavailability, mobility, and

potential to reach receptors.

Wood preserving contaminants, also often candidates for bioremediation/natural attenuation, vary

widely in biodegradability, since wood preservatives by definition are selected for their toxicity to

microorganisms. Polynuclear aromatics (PAHs) of three rings or less are generally considered to be

readily biodegradable. Chlorinated phenols, such as PCP and tetrachlorophenol, are biodegradable,

but their toxicity to microorganisms is a significant factor in their resistance to biodegradation at high

concentrations. Dibenzodioxins and dibenzofurans appear to be difficult to biodegrade.

Physical and chemical components of natural attenuation in the unsaturated zone include

volatilization and leaching as the most significant factors, although chemical reactions such as

hydrolysis can be significant for some contaminants, such as pesticides. Adsorption significantly

affects contaminant mobility, availability, and potential biodegradability. Volatilization can be a

significant factor for those contaminants with high vapor pressure, such as gasoline and similar

petroleum contaminants, naphthalene, methyl naphthalene, and three-ring PAHs, and chlorinated

aliphatics. Loss of contaminants by volatilization is more likely in the unsaturated zone than in the

saturated zone. Leaching of contaminants must be monitored and controlled, since leaching to

ground water is one of the most important potential impacts of soil contaminants. Lysimeters can

be used so that excessive leaching can be detected before the contaminants enter ground water.

Both the potential for leaching and volatilization can be modeled to estimate the part these play in

attenuation of the contaminants.
16-5

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Natural

Attenuation

of Soils

Daniel Pope

Dynamac Corporation

Ada, OK

What Are the Requirements

for NA To Be Used as the

Primary Remedy for Soils?

● Further impairment to GW

quality not a serious threat

● Receptors not impacted

● Site is controllable through

institutional controls

What Are the Requirements

for NA To Be Used as a

Secondary Remedy for Soils?

● Along with ongoing active

remediation alleviating serious

threats

● After active remediation

alleviated serious threats

Natural Attenuation as a

Remedial Alternative

for Soils

Contaminant Releases

● Migrate from source area

● Area of contamination

expand until equilibrium

reached

● Natural attenuation equals

source output

When/Where Is

Equilibrium Reached?

● Site factors – Soil type,

precipitation influx . . .

● Contaminant factors –

Solubility, concentration,

carrier . . .
16-6

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Equilibrium

● Eventually, natural attenuation

exceeds rate of source output,

and concentration of

contaminant(s) stabilizes or

decreases

● Importance of source control as

the primary remedial alternative

Advantages of Natural

Attenuation

● Actual contaminant degradation in

many cases, rather than just phase

transfer or sequestration

● Nonintrusive – allows continued

use of site

● Less potential for releases due to

site disruption, lack of control of

remedial process

Advantages of Natural

Attenuation

● Works in conjunction with other

technologies

● Generally less costly than

alternatives

● Can be evaluated by site

characterization and monitoring

Advantages of Natural

Attenuation

● Data necessary for proving

applicability of natural attenuation

are readily applicable to other

technologies

● Site accessibility, equipment

limitations are not a problem

● Common contaminants of regulatory

concern (BTEX) are susceptible to NA

 Disadvantages of Natural

Attenuation

Upfront costs may be greater than

other technologies, though longterm costs will probably be lower

Evaluating the Potential

for Natural Attenuation

in Soils
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Site Characterization

● What site characteristics are

favorable or unfavorable for

NA?

● Favorable for aerobic

bioremediation of vadose zone

Soil Oxygen Levels

● Soil oxygen levels >2%?

● May be enough for aerobic

remediation

Redox Potential

● Eh >50 millivolts = oxidizing,

aerobic conditions

● Eh <50 millivolts = reducing,

anaerobic conditions

Redox Potential

● 400–800 mV highly aerated

conditions

● 100–400 mV less aerated, but

still aerobic

Soil Color

● Reds, yellows, browns indicate

oxidizing conditions

● Gray or blue indicates reducing

conditions

● Mottled colors indicate spatial

variability

Soil pH

● Usually 5–9 is acceptable

● High pH may not inhibit

bioremediation
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Soil Moisture

● Low moisture, low biological

activity

● But mobility may be low, so

may have a long time available

for bio

Soil Moisture

● 50–80% of field capacity

● Large pores filled with air,

small pores filled with water

● Air/Water in soil inversely

related

Soil Moisture

● Sandy Soils ~–0.1 – 0.15 Bar

● Loams ~–0.3 – 0.5 Bar

Air-Filled Porosity

>10% recommended

for oxygen diffusion

Soil Permeability

Saturated hydraulic

conductivity >10–5 cm/sec

Soil Temperature

● Biodegradation stops at 0°C

● Most rates determined around

20–25°C

● Metabolic activity halved by

10°C drop
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Soil Nutrient Levels

● Soil concentration

● Concentration ratio

Nutrient Concentrations

TON >1.5%

Nutrient Ratios

● C:N:P 120–300:10:1 often

recommended

● Largely based on ratios in cell

mass

● Little research conducted in

field

Trace Nutrients

● Little specific information for

bioremediation in soils

● Apparently enough available in

most soils

Measures of Microbial

Presence and Activity

● Population counts

● Community profiles

● Degradation ability

● Metabolic activity

Microbial Population Counts

● From 1 to 10 x 10 exp6

counts/g soil

● Relationship to transformation

rates is minimal
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Microbial Identification

● Isolation of specific degraders

● FAME profiles

● Community profiles by

exposure to range of carbon

sources

Microbial Ability To Degrade

Contaminants

● Culture tests

● Microcosm tests

Microbial Activity

● Respiration O2/CO2

● ATP

Biodegradability of Petroleum

Compounds

● More water soluble, more

degradable, usually

● Longer chains, more rings less

water soluble

Biodegradability of Specific

Petroleum Compounds

● n-alkanes, n-alkylaromatics,

aromatics from C5–C22 usually

fairly biodegradable

● above C22 usually are fairly slow

biodegradation rates

● Fused aromatics, cycloparaffinics

>4 rings may be very slow

Biodegradability of Wood

Preserving Contaminants

● Polynuclear aromatics (PAHs)

● Chlorinated phenols

● Dibenzo – dioxins and furans
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Biodegradability of

Chlorinated Solvents

● Methylene chloride

● 1,2-DCA

● Chloroethane

Monitoring Plan

● Soil and possibly GW

● Soil gas, soil borings, pore

water

Case Study Site History

● Waste oil recycling facility

● Oil blended with benzene,

toluene, or xylene

● Two tank farms, with

sludge/water in bermed area

Site History (continued)

● Victoria clay soil: low

permeability, high water-holding

capacity, high to very high shrinkswell potential, poor drainage

● Caliche fill in driveway

● Apparently no GW contamination

Remedial Plan

● Removal of tanks, barrels,

buried piping, debris and

sludges

● 2,200 yd of soil remaining (TPH

up to 50,000 ppm)
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Treatment Goals for Soil

● <1% oil and grease (O&G)

● 10,000 mg/kg TPH

● Land treatment chosen as

remedial technology

Evaluation for Natural

Attenuation

● Contaminant characteristics

● Site characteristics

● Ecological and health receptors

Contaminant

Characteristics

● Are the contaminants of

concern readily biodegradable?

● Suppose they are not readily

biodegradable, but mobility is

low?

Contaminant Distribution

● Contaminants in sludge not

readily biodegradable in situ

● Contaminants in soil or

dissolved probably degradable

Site Characteristics

● Are site conditions favorable?

● Can they be made favorable with

minimum input?

● Will they be favorable after active

remediation is done?

● Receptors

Time Required for Natural

Attenuation

Once contaminants are identified

as biodegradable, time/mobility

are the main factors
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Is the timeframe necessary for

NA reasonable, considering sitespecific circumstances?

Time Required for Natural

Attenuation

What Is a Reasonable

Timeframe?

● Depends on amount of contaminant,

toxicity, and mobility

● Proximity of receptors – humans,

environmental

■ Especially sensitive humans, threatened/endangered

species

■ Public/private water supplies

● Potential use of aquifer

● Reliability/enforceability of institutional

controls

Contaminated Soil

● Free phase residual

● Adsorbed material

● Dissolved contaminant

Contaminated Soil

● Evaluate mobility of

contaminants

● Evaluate means to reduce

mobility
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

17-1

Natural Attenuation of Landfills

Dolloff F. Bishop

Office of Research and Development, National Risk Management Research Laboratory, 

U.S. Environmental Protection Agency, Cincinnati, OH 

Introduction

Evidence is emerging that indicates natural attenuation may play a valuable role in addressing

certain types of landfills. Landfills are usually closed municipal fills that may have received mixed

wastes, including municipal solid wastes as well as a variety of industrial and hazardous wastes.

Some of these landfills may pose a low risk to human health and the environment and, therefore,

be candidates for consideration for use of natural attenuation. This decision must be made on a siteby-site basis. It does not indicate a preference over the Agency's current policy to manage landfill

content, leachate, and gases by use of containment systems including covers and bottom liners. 

The complex mixtures (1) of organic and inorganic nonhazardous and hazardous materials in

landfills are slowly being degraded or transformed through natural attenuation (natural abiotic and

microbial processes). The contaminants are also being leached (2-4), by rainfall or by ground-water

intrusion, from the fill into the ground-water aquifers below. Volatile organic compounds (1) may

also volatilize with the principal landfill gases of methane and carbon dioxide. What needs to be

defined are the types of hazardous waste landfills and the appropriate conditions where natural

attenuation would be considered.

Based on mass balance approaches, municipal landfills also are recognized as globally significant

sources (5) of atmospheric methane, but methane field emission measurements are limited and

extremely variable. There has been no attempt to reconcile national or global estimates of projected

mass balance yields of methane generation with the limited field data on methane emissions (6).

Recent research (7), however, has surprisingly revealed that landfills in the active methanogenic

stage with aerobic soil covers and with gas recovery systems actually act as methane sinks, removing

methane from the atmosphere rather than emitting landfill methane. The effect is attributed to high

capacities for methane oxidation to carbon dioxide by indigenous methanotrophs in aerobic soil

covers.

With aerobic permeable soil covers, uncapped landfills with substantially stabilized organic fill and

limited gas emissions and sites with gas recovery and flaring systems also should develop indigenous

methanotrophic and heterotrophic aerobic bioprocesses in aerobic, permeable soil cover. These

aerobic processes should degrade both methane emissions and most volatile organic chemicals in

the landfill gases. In addition, evidence is evolving that indicates that natural attenuation (intrinsic

bioremediation) can stabilize and even shrink contaminated ground-water plumes below landfills.
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

17-2

Landfill Lysimeter Studies

EPA's National Risk Management Research Laboratory conducted a lysimeter study (1) on the West

KL Landfill in Kalamazoo, Michigan, to assess bioactivity and the fate of the hazardous contaminants

in the fill material under capped and rainfall simulations. The wastes were obtained from an area

of the West KL Landfill with industrial wastes and were transported under nitrogen to EPA's Test and

Evaluation Facility in Cincinnati. The materials were hand mixed, also under nitrogen, to reduce fill

heterogeneity, then placed in lysimeters operated at 35EC. The anaerobic lysimeters, pertinent to

assessment of natural attenuation, included three replicate microcosms of capped systems with two

abiotic controls and three replicate microcosms simulating rainfall with two abiotic controls. The

abiotic controls used sodium azide to minimize anaerobic activity.

The bioactivity in the lysimeters was monitored by measurement of gas production and by assessing

the fate of specific contaminants in the fill. The cumulative gas productions (Figures 1 and 2) of the

capped and rainfall simulators in the 400-day study revealed a long period of approximately 150

days before redevelopment of bioactivity in the disturbed fill in the rainfall simulator and only

marginal bioactivity in the capped simulators. Fill gas analysis on carbon dioxide and methane also

confirmed substantial bioactivity in the rainfall simulators compared with the marginal activity in the

capped simulators.

Analyses of the fate of specific contaminants in the fill was difficult, unfortunately, with significant

variability in the mass balances caused by heterogeneity in the fill and analytical variability

associated with fill material. Trends on dehalogenation of highly chlorinated solvents (Figures 3 and

4) for example, also suggested improved bioactivity in the rainfall lysimeters compared with the

capped lysimeters. Unfortunately, the poor mass balance results and variability from lysimeter to

lysimeter prevented statistically valid assessments of the fate of specific contaminants.

Research Approach

Clearly, with bioactivity in permeable soil covers and with intrinsic bioremediation in ground water,

responsible risk/benefit management requires assessing the applicability of natural attenuation

processes as cost-effective approaches for managing risk in contaminated high-volume landfills,

both as control options when active remediation can be discontinued and as the principal

remediation approach in contaminated areas when risk is acceptably low. These natural attenuation

processes, however, will require appropriate monitoring to ensure acceptable risk management of

the variety of contaminants in landfills. Monitoring methods will include standard individual

contaminant analyses in soils, leachates, and gases, as well as ecological and health effects assays.

The rate of natural attenuation of contaminants in landfills is the sum of the rates of several biotic

and abiotic processes. These processes include intrinsic biodegradation of the contaminants, the

chemical transformation of the contaminant (humification) into the organic matter associated with

landfills, and the rates of mass transport of contaminants to the locations of these reactions. The

development of a protocol for assessing the use of natural attenuation in landfills on a site-specific

basis requires the compilation of a database on rates of pertinent biotic and abiotic processes for

various contaminants and environmental settings, and the development or improvement of fate and

transport models that employ the rates to describe the activity of these processes.
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

17-3

The tasks in the development of the protocol are to:

# Review and summarize pertinent biotic and abiotic degradation and stabilization

(containment) science and engineering in the surface and subsurface of landfills

including bioavailability and alternative endpoints. Develop critical supplemental

attenuation rate data to support protocol development.

# Develop supplemental attenuation rate data using laboratory and field studies. 

# Review, evaluate, improve, and summarize existing fate and transport models for

hazardous compounds in landfills.

# Review and summarize available monitoring and sampling tools for landfill

characterization.

# Prepare a draft protocol and validate with lab, pilot, and field studies.

References

1. U.S. EPA. 1995. Laboratory evaluation of in situ biodegradation of hazardous pollutants

in Superfund landfills. Contract No. 68-C2-0108. National Risk Management Research

Laboratory, Cincinnati, OH. 

2. Schultz, B., and P. Kjeldsen. 1986. Screening of organic matter in leachates from sanitary

landfills using gas chromatography combined with mass spectroscopy. Water Res. 20:965-

970.

3. Dewalle, F.B., and E.S.K. Chiang. 1981. Detection of trace organics in well water near

a solid waste landfill. J. Am. Water Works Assoc. 73:206-211.

4. Dunlap, W.J., et al. 1976. Organic pollutants contributed to ground water by a landfill.

In: Proceedings of the Research Symposium on Gas and Leachates From Landfills, Rutgers

University Cooks Colleges, New Brunswick, NJ, March 24-26, 1975. EPA/600/9-76/004.

pp. 96-110.

5. U.S. EPA. 1995. Estimate of global methane emissions from landfills and open dumps.

EPA/600/R-95/019. Washington, DC. 

6. Bogner, J., and R. Scott. 1995. Landfill methane emissions: Guidance for field

measurements. Final report to International Energy Agency, Expert Working Group on

Landfill Gas.
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

17-4

7. Bogner, J., et al. 1995. Landfills as atmospheric methane sources and sinks.

Chemosphere 31:4,119-4,130.
17-5

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Natural Attenuation of

Landfills

Dolloff F. Bishop

Office of Research and Development

National Risk Management Research Laboratory

U.S. Environmental Protection Agency

Cincinnati, OH

Characteristics of Typical

Hazardous Waste Landfills

● Usually closed municipal landfills with

permeable soil cover

● No impermeable liners to minimize

leachate transport

● Partial anaerobic stabilization of

organic materials

● Gas production often highly variable

● Municipal solid wastes and a variety of

industrial and hazardous wastes

Landfill Emissions

● Leachate with a variety of

contaminants entering groundwater aquifer

● Carbon dioxide and methane gas

emissions

● Variety of VOCs at low

concentrations in gas emission

Natural Attenuation at Landfills

● Anaerobic bioprocesses degrade municipal solid

wastes and many hazardous contaminants in fill

● Intrinsic bioremediation (anaerobic and aerobic

processes) occurs in ground water at varying rates

● Aerobic methanotrophs bioxidize methane in

permeable aerobic soil cover

● Aerobic bioxidation of VOC can occur in aerobic soil

cover

● With aerobic soil cover and gas recovery systems,

landfill can remove methane from atmosphere rather

than emit methane

Landfill Lysimeter Study

● Superfund West KL Landfill in Kalamazoo,

Michigan

● Selected waste from industrial area of the fill

● Hand mixed under nitrogen to reduce

heterogeneity

● Lysimeters operation with 3 replicates and 2

abiotic controls simulating capped and

rainfall conditions at 35°C

● Bioactivity confirmed by measuring gas

production and assessing specific

contaminant fate

Figure 1. Cumulative Gas

Production for Capped Columns

Capped Landfill Lysimeters

CAP 1 No moisture addition

CAP 2 No moisture addition

CAP 3 No moisture addition

CAP 4 Abiotic control without moisture

CAP 5 Abiotic control without moisture
17-6

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Figure 2. Cumulative Gas

Production for Uncapped Columns

Uncapped Landfill Lysimeters

UNC 1 (rainfall)

UNC 2 (rainfall)

UNC 3 (rainfall)

UNC 4 Abiotic control

UNC 5 Abiotic control

Figure 3. Distribution of

Tetrachloroethylene for CAP 3

Mass Biodegraded

78% Mass in Carbon

9%

Mass Remaining in Soil

13%

Figure 4. Distribution of

Tetrachloroethylene for UNC 3

Mass Biodegraded

89%

Mass in Leachate

0%

Mass in Carbon

8%

Mass Remaining in Soil

3%

Natural Attenuation Research

Approach

● Review and extend current science in

natural attenuation of contaminated

landfills

● Review and summarize available natural

attenuation rates at sites

● Develop supplemental attenuation rate

data

● Review and improve fate and transport

models

Natural Attenuation Research

Approach (continued)

● Review available monitoring tools

● Evaluate biological and health assays to

assess cleanup objectives

● Prepare a draft protocol with

information summaries

● Validate and improve protocol with

laboratory, pilot and field studies
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

18-1

Natural Attenuation of Sediments

Dolloff F. Bishop

Office of Research and Development, National Risk Management Research Laboratory,

U.S. Environmental Protection Agency, Cincinnati, OH

Introduction

Contaminants in sediments (1) include a wide variety of organic compounds and metals. Metals

cannot be destroyed but often can be transformed by bioprocesses to less toxic forms. As

representative organic contaminants, high molecular weight polychlorinated biphenyls (PCBs) and

polynuclear aromatic hydrocarbons (PAHs), from widely used multicomponent Arochlors and

creosotes, partition strongly to and persist in sediments (2). They bioaccumulate up the food chain

and thus produce potential human health and environmental risks (3).

Intrinsic bioremediation (natural attenuation), even of these persistent compounds, occurs naturally

but slowly in sediments, using indigenous microorganism and enzymatic pathways of both aerobic

and anaerobic processes (2, 5, 6). In general, increasing the molecular weight of the organic

contaminants (Figures 1 and 2) increases partitioning and reduces the bioavailability of the organic

compounds, thus reducing the biodegradation rate and extent of degradation.

PAHs biodegrade most rapidly through aerobic processes, with the degradation rates usually

decreasing as aromatic ring structure increases from two to six rings (5-7). In PCB biodegradation,

anaerobic processes (8-10) slowly dechlorinate the highly chlorinated PCB congeners to lightly

chlorinated congeners. Aerobic processes (11, 12) then biodegrade the lightly chlorinated

congeners. 

Quiescent sediments with substantial contamination are anaerobic (1) except in the upper layer

adjacent to water. Dissolved oxygen of approximately 8.0 mg/L in water, slow oxygen diffusion into

sediments, and slow diffusion of contaminants to the sites of microbial activity limit the kinetically

more rapid aerobic degradation processes. The mass transport limitations reduce bioavailability and

increase the persistence of PAHs, lightly chlorinated biphenyls, and other aerobically degradable

organic contaminants in sediments. Natural turbulent mixing of sediments with the water column

and slow oxygenation at the surface of quiescent sediments do produce limited and slow

biodegradation of aerobically degradable contaminants (11).

In contrast, highly chlorinated congeners of PCBs and other chlorinated contaminants are gradually

dechlorinated naturally in contaminated sediments, the PCBs (2) to mono-, di-, and trihomologs.

The products of anaerobic dechlorination accumulate, increasing concentrations of lightly

chlorinated PCBs and other partially dechlorinated contaminants in sediments (11-13). Lightly

chlorinated PCBs and other partially dechlorinated organic contaminants, in general, bioaccumulate

less strongly. These PCBs have less potential human toxicity (14, 15) than the highly chlorinated

congeners.
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

18-2

Natural Attenuation Evaluation

With a pattern of slow natural dechlorination of highly chlorinated contaminants and slow aerobic

biodegradation of the less chlorinated residuals and other aerobically biodegradable contaminants

(such as PAHs), the U.S. Environmental Protection Agency's (EPA's) Bioremediation Program plans

to examine natural attenuation as a possible approach for management of contaminated sediments

and will prepare a protocol for assessing the use of natural attenuation as a best management

practice for managing risk at specific sites with contaminated sediments.

These natural attenuation processes will require appropriate monitoring to ensure acceptable risk

management. The initial priority contaminants are PAHs and metals, found at petroleum, wood

preserving, and town gas wastes sites, and PCBs. Monitoring methods will include standard

individual contaminant analyses and ecological and health effects assays (alternative endpoints).

The rates of natural attenuation of contaminants in sediments are the sum of the rates of several

biotic and abiotic processes. These processes include intrinsic biodegradation of the contaminants,

the chemical transformation of the contaminant into organic matter associated sediments

(humification), and the rates of mass transport of electron donors or acceptors, amendments, or

contaminants to locations where the microbial reactions occur. The development of a protocol for

assessing natural attenuation at specific sites requires the compilation of databases on the rates of

the biotic and abiotic processes for various contaminants and environmental conditions, as well as

the improvement and validation of fate and transport models that employ the rates to describe the

integrated action of these processes. Research and development includes:

# Review and summarize pertinent biotic and abiotic degradation and stabilization

(containment) science and engineering in sediments, including contaminant

bioavailability and alternative endpoints. Extend through experimental and field

research.

# Review, evaluate, and improve existing fate and transport models for hazardous

compounds in sediments. 

# Review and summarize available monitoring and sampling tools for sediment site

characterization.

# Prepare a draft protocol, including information summaries.
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

18-3

References

1. U.S. EPA. 1994. Assessment and remediation of contaminants sediments program:

Remediation guidance document. EPA/905/R-94/003. Great Lakes National Program

Offices. 

2. Abramowicz, D.A. 1995. Aerobic and anaerobic PCB degradation in the environment.

Environ. Health Perspec. 103(5):97-99.

3. Safe, S. 1980. Metabolism uptake, storage and bioaccumulation. In: Kimbrough, R., ed.

Halogenated biphenyls, naphthalenes dibenzodioxins and related products. Elsevier, North

Holland: pp. 81-107.

4. Bedard, D.L., and R.J. May. 1996. Characterization of the polychlorinated biphenyls in

sediments of woods pond: Evidence for microbial dechlorination of Arochlor 1260 in situ.

Environ. Sci. Technol. 30:237-245.

5. Cerniglia, C.E. 1992. Biodegradation of polycyclic aromatic hydrocarbons.

Biodegradation 3:351-368.

6. Shuttleworth, K.L., and C.E. Cerniglia. 1995. Environmental aspects of PAH

biodegradation. Appl. Biochem. Biotechnol. 54:291-302.

7. Seech, A., B. O'Neil, and L.A. Comacchio. 1993. Bioremediation of sediments

contaminated with polynuclear aromatic hydrocarbons (PAHs). In: Proceedings of the

Workshop on the Removal and Treatment of Contaminated Sediments. Environment

Canada's Great Lakes Cleanup Fund, Wastewater Technology Centre, Burlington, Ontario.

8. Brown, J.F., et al. 1984. PCB transformations in upper Hudson sediments. Northeast

Environ. Sci. 3:167-179.

9. Brown, J.F., et al. 1987. Environmental dechlorination of PCBs. Environ. Toxicol. Chem.

6:579-593.

10. Quensen, J.F., III, S.A. Boyd, and J.M. Tiedje. 1990. Dechlorination of four commercial

polychlorinated biphenyl mixtures (Arochlor) by anaerobic microorganisms from sediments.

Appl. Environ. Microbiol. 56:2,360-2,369.

11. Flanagan, W.P., and R.J. May. 1993. Metabolic detection as evidence for naturally

occurring aerobic PCB biodegradation in Hudson River sediments. Environ. Sci. Technol.

27:2,207-2,212.

12. Harkness, M.R., et al. 1993. In situ stimulation of aerobic PCB biodegradation in Hudson

River sediments. Science 259:503-507.
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

18-4

13. Liu, S.M., and W.J. Jones. 1995. Biotransformation of dichloromatic compounds in nonadapted and adapted freshwater sediment slurries. Appl. Microbiol. Biotechnol. 43:725-732.

14. Safe, S. 1992. Toxicology structure-function relationship and human environmental health

impacts of polychlorinated biphenyls: Progress and problems. Environ. Health Perspec.

100:259-268.

15. Abramowicz, D.A., and D.R. Olson. 1995. Accelerated biodegradation of PCBs.

Chemtech. 24:36-41.
18-5

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Natural Attenuation

of Sediments

Dolloff F. Bishop

Office of Research and Development

National Risk Management Research Laboratory

U.S. Environmental Protection Agency

Cincinnati, OH

Contaminants in Sediments

● Wide variety of organic compounds and

metals

● Persistent high molecular weight organic

compounds

● Widely distributed contaminants: PCBs

and PAHs

● Bioaccumulation in food chain may cause

health and environmental risk

● Natural attenuation occurring slowly using

aerobic and anaerobic processes

PAH and PCB Natural

Attenuation

● PAHs biodegrade most rapidly through aerobic

processes

● Rates decrease as aromatic ring structure

increases from 2 to 6 rings

● PCBs biodegrade usually through sequential

anaerobic/aerobic processes

● High chlorinated PCBs dechlorinate

anaerobically to lightly chlorinated congeners

● Lightly chlorinated PCB congeners biodegrade

aerobically

Figure 1. Representative PAH Ring

Structures

3-Rings (Anthracene)

4-Ring (Pyrene)

2-Ring (Naphthalene)

5-Rings (Perylene)

Figure 2. Representative PCB

Congeners

Lightly

Chlorinated

Highly

Chlorinated

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Sediment Conditions

● Contaminated sediments are anaerobic below

surface layer

● Surface layer adjacent to water is aerobic

● Slow mass transport in sediments limit

bioavailability and degradation

● Quiescent sediments favor slow accumulation of

lightly chlorinated compounds, especially mon,

di, and tri PCB homologs

● Natural turbulent mixing of sediment and water

increases aerobic degradation of PAHs and lightly

chlorinated PCBs
18-6

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Natural Attenuation

Evaluation

● Pattern of slow natural dechlorination and slow

biodegradation of aerobically degradable

contaminants

● Assessing use of natural attenuation for

managing risks

● Priority contaminants—PAHs, metals, and PCBs

● Monitoring to ensure acceptable risk management

● Monitoring methods—individual contaminant

analyses, and ecological and health effect assays

Rates of Natural Attenuation

Processes

● Anaerobic vs. aerobic

● Chemical transformation with

sediment organic matter

(humification)

● Mass transport of electron donors

and acceptors, amendments, and

contaminants

Protocol Development

● Compilation of databases on rates

of attenuation for various

contaminants and environmental

conditions

● Improvement and validation of

fate and transport models

describing integrated activity of

the attenuation processes

Research and Development

Approach

● Review and extend and summarize

current science in natural attenuation

● Review and summarize available

natural attenuation rates of sites

● Develop supplemental attenuation

rate data

● Review and improve fate and

transport models

Research and Development

Approach (continued)

● Review and summarize available

monitoring tools

● Draft protocol including

information summaries

● Validate protocol in laboratory,

pilot and field studies

● Provide technology transfer
19-1

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Source Control: Free

Product Recovery and

Hydraulic Containment

John Wilson

Office of Research and Development

National Risk Management Research Laboratory

U.S. Environmental Protection Agency

Ada, OK

Nonaqueous Phase

Liquids—NAPLS, LNAPLS,

DNAPLS

● The NAPLs define the source area

of the ground-water plume

● To the extent feasible, these

materials should be removed

before bioremediation proceeds

Site Characterization

Requirements Specific to the

Subsurface

Goals:

● Map the contaminant mass in three

dimensions

● Determine the co-distribution of

contaminant and hydraulic or

pneumatic conductivity

Problems With Monitoring

Wells

● They cannot estimate contaminant

mass in NAPLs

● They cannot estimate contaminant

mass adsorbed to solids

● They do not sample contaminant

mass above the water table

Comparison of Contaminant Mass

in Ground Water to Total

Contaminant Mass

At a pipeline spill in Kansas:

Mass in Mass in

Ground Water Subsurface

Benzene 22 kg 320 kg

BTEX 82 kg 8,800 kg

TPH 115 kg 390,000 kg

When Total Contaminant

Mass Is Unknown

● Cannot estimate requirements for

electron acceptors

● Cannot estimate requirements for

nutrients

● Cannot determine time required

for cleanup
19-2

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Relationship Between Free

Product in Monitoring Wells and

Contaminant Mass in Aquifer

● Position and quantity in wells does

not relate to position and quantity

in aquifer

● Amount of free product related to

location of water table

Relationship Between Free Product

in Monitoring Wells and Contaminant

Mass in Aquifer

● Free product is greatest when

water table is low

● Free product can disappear

when water table is high

Methods To Remove

Nonaqueous Phase Liquids

● Free product recovery

● Bioslurping

● Soil vacuum extraction

LNAPL Remediation Soil Vent System

Vent Well Vent Well

Air Flow

Contaminated Soil
19-3

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation
19-4

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Bottom Line

● 12,000 gallons of LNAPL

removed

● 122,000 gallons of LNAPL

remain
19-5

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Air Sparging/

Air Injection

John Wilson

Office of Research and Development

National Risk Management Research Laboratory

U.S. Environmental Protection Agency

Ada, OK

Need for Efficient,

Inexpensive Delivery of

Oxygen to Saturated Zone

*** Air Sparging ***

Air Sparging

● Injection of air under pressure

below the water table

● Creates transient air filled

porosity

Air Sparging System

Transient Air

Filled Porosity

Vent Radius = f(Vacuum)

Sparge Radius = f(Depth)(Pressure)

Monitoring

Probe

Vapor Extraction

Well

Air Sparger

Well

Monitoring

Probe

Vent

Radius

Sparge

Radius

Contaminated

Soil

Depth

Air

Water

Soil

Particle

Effects of Air Sparging

● Enhanced oxygenation

● Enhanced dissolution

● Volatilization

● GW stripping

● Physical displacement of GW

Enhanced Oxygenation

● Replenishes oxygen depleted by

chemical/biological processes

● Normal replenishment relies on

diffusion from water table surface

● Sparged air, distributed throughout

aquifer, has short diffusion path

● Enhanced oxygenation stimulates

biodegradation
19-6

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Air Flow Paths

● Injected air travels horizontally,

vertically

● Flow impedance by lithological barriers

blocking vertical air flow

● Channelization—horizontal air flow

captured by high permeability channels

● Small permeability differences can

change flow paths

Inhibited Vertical Air Flow Due to

Impervious Barrier

Impervious Barrier

Contaminated Soil Air Contaminant Migration

Dissolved Particles

Channeled Air Flow Through Highly

Permeable Zone

High Permeability Zone Air/Contaminant

Migration

Case Studies on Air

Sparging or Air Injection

● Worked well: Traverse City,

Michigan

● Worked well enough: Elizabeth

City, North Carolina

● Didn’t work: Plattsburgh, New

York
19-7

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Ground Water Quality after

Biosparging

Well Benzene Toluene Ethylbenzene m+p Xylene o-Xylene

 ---------------------ug/liter------------------------

3 feet <1 <1 <1 <1 <1

6 feet <1 <1 <1 <1 <1
19-8

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation
19-9

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Conditions of Sparge

Efficiency Test

● Injected air at 3 cubic feet per minute at

18 psi

● Injected air for four days over a six day

interval

● Total air injected: 17,300 cubic feet

● Total porosity to 3 feet from sparge well:

250 cubic feet

● Total porosity to 10 feet from sparge

well: 2,800 cubic feet
19-10

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Why didn’t air sparging strip

Vinyl Chloride and increase

the concentration of Oxygen?

The air moved in Ribbons,

fixed channels of preferential

flow.

● Air sparging worked well when

the contaminant was near the

water table and the sand grains

were all the same size

● Air sparging did not work well

when the contaminants were

deep, and there were a mixture

of particle sizes
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

20-1

State Review: Natural Attenuation of Ground Water and Soils

Daniel Pope

Dynamac Corporation, Ada, OK

The U.S. Environmental Protection Agency (EPA) recently conducted a survey to determine how

different states are proceeding with natural attenuation efforts. States were asked whether they

# Encourage or discourage the use of natural attenuation (NA)

# Have any formal or informal policies or guidelines that address NA

# Use any particular model when deciding on NA

# Consider any compounds other than petroleum hydrocarbons for NA

The table below summarizes the information obtained from this survey.

 State Petroleum Guidelines or Rules NA Nonpetroleum

 Encourage/ Specific 

 Discourage Models to Encourage/

 NA of Determine Discourage NA of

 Alabama Encourage No guidelines. Considers NA for Discourage

 case-by-case petroleum on a case-by-case case-by-case

 basis.

 Alaska Encourage Developing RBCA/ASTM (draft). AT123D, Discourage

 case-by-case Working with Wisconsin to SESOIL case-by-case

 develop soil guidance using NA.

 Arizona Neither Drafting interagency policy for Developing Discourage

 ground-water contaminated sites. BAN Model case-by-case

 Developing RBCA and SSL.

 Considers NA mostly at UST

 sites.

 Arkansas Neither Informal guidelines. Looks at Neither

 case-by-case property boundaries. Determines case-by-case

 NA on a case-by-case basis. 

 California Discourage Revising Resolution 92-49 to Discourage

 include "containment zones."

 Colorado Neither Meets water-quality standards at Half-lives of Discourage

 case-by-case "point of compliance" (property contaminants case-by-case

 boundary). However, (non-UST)

 water-quality standards may

 be used as "guidelines" by oil

 inspectors based on technical

 and economic feasibility.

 Connecticut Neither Remedial standards allow NA. Neither

 Uses a ground-water case-by-case

 classification system for remedial

 decision-making.
 State Petroleum Guidelines or Rules NA Nonpetroleum

 Encourage/ Specific 

 Discourage Models to Encourage/

 NA of Determine Discourage NA of

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

20-2

 Delaware Encourage Informal guidelines for petroleum Neither

 does not use RBCA. Guidance case-by-case

 uses "passive action;" after 2 

 years need permission to

 continue. Looks at property

 boundaries. Non-UST use

 ground-water management

 zones. Assesses for "no further

 action" and deed restriction.

 Have voluntary action program

 and Brownfields.

 Florida Encourage Incorporates RBCA in statutes; Discourage

 is developing NA guidelines. NA case-by-case

 now allowed if low

 concentrations. Expanding to

 allow higher concentrations, and

 more widespread contamination

 and to broaden types of sites.

 Hazardous waste section

 considers NA for soils only. 

 Georgia Neither No formal policy. Remediation Discourage

 case-by-case site specific. Threshold case-by-case

 representative standards. Looks

 at media and risk.

 Hawaii Encourage Guidance no policy. Revising SESOIL Neither

 manual on risk-based guidance. case-by-case

 Source and free product

 removal.

 Idaho Encourage Developing new ground-water Neither

 case-by-case rule. Brownfields beginning. case-by-case

 Use beneficial-use criteria.

 Illinois Neither Informal guidelines. Drafting RBCA & SSL Neither

 case-by-case RBCA and SSL approach in case-by-case

 developing guidance. RBCA for

 UST and non-UST. Looks at

 property boundaries. Brownfields

 in development.

 Indiana Neither No formal protocol. Developing Neither

 case-by-case RBCA. case-by-case

 Iowa Encourage Uses RBCA. Plans policy Neither

 case-by-case changes. Hazardous waste case-by-case

 section considers "passive

 remediation" if exposure risk is 

 low along with source removal

 and monitoring.
 State Petroleum Guidelines or Rules NA Nonpetroleum

 Encourage/ Specific 

 Discourage Models to Encourage/

 NA of Determine Discourage NA of

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

20-3

 Kansas Neither Considers NA for petroleum. AT123D, Discourage

 case-by-case Evaluates aquifer beneficial uses, SESOIL, case-by-case

 property boundaries, and VLEACH

 receptors. Has dry cleaning state

 trust fund for solvent waste.

 Kentucky Encourage Informal guidance for UST. Discourage

 Generally only considers NA for case-by-case

 UST. Monitors until plume

 dissipates. Non-UST use deed

 restrictions to risk factor of 10 . 

-6

 Louisiana Neither No guidance or protocol. Performance Discourage

case-by-case Requires site characterization, model case-by-case

 source removal, and monitoring

 before using NA.

 Maine Encourage Developing in-house guidance May use Neither

 on NA of petroleum (end of Bioplume III in case-by-case

 May). Considers NA when future

 exposure is low. Gathering

 information on non-UST for

 consideration.

 Maryland Encourage No official documents on NA. Discourage

 Uses RBCA approach. NA case-by-case

 allowed in areas not

 environmentally sensitive. Risk

 is primary factor. CERCLA does

 not promote NA. 

 Massachusetts Encourage No NA guidelines. State statutes Discourage

 use RBCA with NA implied in less case-by-case

 stringent cleanup standards

 versus water-quality standards. 

 Michigan Encourage Drafting bioremediation Bioplume II, Encourage

 guidance document with NA Modflow case-by-case

 (within year). Considers other

 wastes (e.g., solvents). Requires 

 monitoring and proof that NA

 occurs before reaching receptors.

 RBCA uses "Guidance Document

 for RBCA at LUSTs."
 State Petroleum Guidelines or Rules NA Nonpetroleum

 Encourage/ Specific 

 Discourage Models to Encourage/

 NA of Determine Discourage NA of

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

20-4

 Minnesota Encourage UST has own RBCA rules Neither

 addressing NA. Draft policy case-by-case

 statement for non-UST in early 1 site allows NA of

 development: "Site Response chlorinated solvents &

 Risk Based Guidance for metals

 Cleanup of Site Other Than

 Petroleum Waste in Ground

 Water." Uses risk and cost.

 Remedial action levels in

 drinking water aquifers, remedial

 goals for potential drinking water

 aquifers, and multiple levels for

 other aquifers. 

 Mississippi Encourage Encourages use of NA for Discourage

 petroleum only. UST section case-by-case

 adopted RBCA 6 months ago 

 and uses that to address NA.

 Hazardous waste section

 beginning to look at NA.

 Missouri Neither No policy. Expanding state RBCA Discourage

 system on NA. Source removal case-by-case

 not required if economically

 unfeasible or near cleanup

 levels. Uses property boundaries.

 Superfund uses deed restrictions.

 Montana Encourage Have informal policy in UST Discourage

 section. No degradation policy in case-by-case

 ground-water section. Superfund

 considers deed restrictions. Will

 consider NA if best or only

 technology.

 Nebraska Encourage Risk-based guidance Risk-based Discourage

 incorporates NA. Combining model being case-by-case

 EPA, ASTM, and state guidance. developed to

 Regulations based on cleanup assess NA

 levels. Superfund allows NA if

 concentrations low and no

 receptors. Determines beneficial

 uses; if drinking water aquifer no

 NA, if no potential for drinking

 water consider NA.

 Nevada Neither No formal NA policy. Adhere to Discourage

case-by-case federal UST program. Soil case-by-case

 contamination level 100 ppm.

 Cleanup required if over level.
 State Petroleum Guidelines or Rules NA Nonpetroleum

 Encourage/ Specific 

 Discourage Models to Encourage/

 NA of Determine Discourage NA of

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

20-5

 New Encourage Guidance but no policy on NA. Neither

 Hampshire Developing ground-water case-by-case

 management zones. Other

 sections are looking at NA.

 About to pass the Brownfields

 and have a voluntary action

 program.

 New Jersey Encourage Written policy on NA; involves Neither

 characterization, source removal, case-by-case

 and monitoring. Must identify

 ground water uses based on 25-

 year plan. Requires at least eight

 quarters of monitoring. Sentinel

 well 3 years time of travel

 upgradient of receptor.

 New Mexico Encourage No formal guidance. RBCA Neither

 Incorporating NA into regulations case-by-case

 as part of RBCA. Looks at 2 cases

 property boundaries,

 cost/benefit, and risk.

 Source removal and low

 concentrations use NA. Loosely

 subscribes to Chevron indices to

 determine extent of

 bioremediation. Not as many

 non-UST sites but has two using

 NA. Contaminants include

 carbon tetrachloride and

 perchloroethylene.

 North Encourage Developed NA Rules in 1993. Accepted Neither

 Carolina Over 150 sites approved. Must USGS models case-by-case

 monitor until reaching cleanup

 levels. Expanding rules to allow

 some sources to remain if no

 further leaching occurs and to

 consider more compounds for

 NA.

 North Dakota Encourage No state policy. Believes NA Encourage

 case-by-case works in significant number of case-by-case

 cases. NA approved at over

 200 petroleum and 20 solvent

 sites. Monitoring minimum of 2

 years to verify that

 concentrations are decreasing.
 State Petroleum Guidelines or Rules NA Nonpetroleum

 Encourage/ Specific 

 Discourage Models to Encourage/

 NA of Determine Discourage NA of

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

20-6

 Ohio Encourage LUST follows RBCA guidelines. Include Neither

 New Voluntary Action Program, SESOIL, case-by-case

 Brownfields. Working on draft VLEACH

 rule for hazardous waste and

 petroleum. Various models used.

 One PRP used POLLUT to

 demonstrate NA.

 Oklahoma Encourage No formal policy on NA. Neither

 case-by-case Evaluates on a case-by-case case-by-case

 basis. Property boundaries used

 as point of compliance.

 Oregon Encourage No state guidance. Revising the SESOIL, Case-by-case

 ASTM, and NA issue may arise AT123D

 when adopting rules on USTs.

 Pennsylvania Neither No NA policy. Not using RBCA. Neither

 case-by-case Developed "Act 2," which drives case-by-case

 state programs. Site-specific

 standards based on risk

 assessment. "No action" may

 be designated to sites.

 Rhode Island Neither No guidelines. NA reviewed on a Discourage

 case-by-case case-by-case basis. case-by-case

 South Encourage Intrinsic remediation written into Discourage

 Carolina RBCA in evaluating LUST sites. case-by-case

 Working with USGS on field

 studies addressing NA. Flexibility

 in modeling for NA.

 South Dakota Neither Uses ASTM RBCA system. No RBCA Neither

 case-by-case formal NA procedures. NA case-by-case

 factors include contaminant

 type/extent and beneficial uses of

 aquifer. Looks at property

 boundaries. Soil cleanup

 required. Consult handbook,

 soil cleanup regulations, and

 ground-water quality standards

 used. Must meet water-quality

 standards for 1 year before

 closure.

 Tennessee Discourage NA not encouraged, but Discourage

 case-by-case considers on a case-by-case case-by-case

 basis. Encourages an accelerated

 bioremediation approach.
 State Petroleum Guidelines or Rules NA Nonpetroleum

 Encourage/ Specific 

 Discourage Models to Encourage/

 NA of Determine Discourage NA of

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

20-7

 Texas Encourage Developing risk-based rules Neither

 addressing NA for UST; ready by case-by-case

 end of year. Volunteer cleanup

 program started. Has guidance

 on NA of soils and is developing

 guidance for ground water.

 Utah Encourage Risk-based approach. Approves Discourage

 NA for petroleum but not for case-by-case

 other compounds. Non-UST has

 two levels of industrial risk, 10-4

 and 10 . Uses deed restrictions. -6

 Virginia Neither No guidance. Recognizes NA REAMS Discourage

 case-by-case occurs with petroleum. Non-UST (SESOIL, case-by-case

 uses risk-based standards. NA AT123D) 

 depends on aquifer beneficial

 use. Have voluntary action

 program.

 Washington Encourage Actively looking at NA, Neither

 particularly soil to ground water. case-by-case

 Using SSL after EPA.

 West Virginia Encourage No definitive rule. Developing Neither

 state policy for NA incorporating case-by-case

 soil cleanup levels. Plans

 interagency risk-based approach.

 Brownfields just passed. 

 Wisconsin Neither Developing preliminary guidance Neither

 for a range of contaminants to case-by-case

 be ready by end of year for

 ground water. Aquifer

 characteristics, risk, beneficial

 uses, and aquifer type will be

 considerations. Has guidance on

 NA of soils.

 Wyoming Neither NA considers risk, beneficial Neither

 uses, aquifer characteristics. case-by-case

 Considers NA in industrial areas

 and no potential receptors.

 Developing guidance (end of

 year) looking at a range of

 contaminants. 

North Carolina and New Jersey are the only states with formal guidance or rules addressing NA as

a remediation option in both ground water and soils. Texas and Wisconsin have written formal

guidance with regard to NA in soils and are currently working on ground-water guidance. States

with informal policies or guidelines include Arkansas, Delaware, Illinois, Kentucky, Montana, North
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

20-8

Dakota, South Dakota, and Vermont. In the North Carolina Implementation Guidance, "the

Corrective Action Plan (CAP) must document that conditions at the subject site are conducive to

natural remediation processes and should present any evidence that natural attenuation is occurring

at the site." NA is generally used as part of a treatment which may include source removal or other

types of active remediation. Monitoring data are generally used to demonstrate decreases in volume

and concentration over time. For sites where the plume is still expanding, NA could also be

demonstrated if it can be shown that the rate of contaminant transport is significantly less than the

estimated rate of linear ground-water velocity. Degradation products must also be evaluated since

they can sometimes be more toxic the original contaminant of concern.

State agencies widely accept that NA does occur in petroleum-contaminated sites. EPA's Office of

Underground Storage Tanks (OUST) found that remediation at leaking underground storage tanks

has shifted to using NA across the United States. In 1993, landfilling was the predominant

remediation for soils and pump-and-treat the most common in ground-water treatment. As of 1995,

NA of soils (28 percent) was a close second to landfilling (34 percent), while NA (47 percent) is the

most common form of remediation at ground-water sites. The policy is, however, that NA is not to

be regarded as a "default" remediation technology, and free product removal is a prerequisite.

Leaks from underground storage tanks (USTs) are one of the most common causes of ground-water

contamination. Many states are using or developing a risk-based corrective action approach when

addressing these sites. The Emergency Standard Guide for Risk-Based Corrective Action applied at

petroleum release sites, issued by the American Society for Testing and Materials (ASTM), looks at

"demonstrated and predicted attenuation of hydrocarbon compounds with distance." Corrective

action goals are determined based on a tiered approach, the most conservative being at Tier 1,

where risk to human health or the environment is high. The other two tiers may allow for site-specific

goals to be developed where risk is not imminent. Revisions to RBCA are under way to incorporate

the premise that the further a receptor is from a contaminated area, the less likely it is to be affected,

consequently allowing for greater amounts of contaminants to be left in place the farther they are

from a receptor. Natural attenuation is "assumed" to occur between the source and the receptor.

In risk-based decision-making, proof of NA may not always be as important as the potential impact

on a given receptor, the classification or use of the ground-water aquifer, or simply the approaches

that are technologically feasible or cost-effective. Some states are assigning different levels of

cleanup based on these other factors. Alternate protection levels may be assigned based on the

beneficial-use designation of the aquifer. Even in highly populated areas, if the ground water is

already contaminated and is not being used as a water supply, then cleanup may not be required.

These decisions, although they may be in part based on assumed NA, may not be the main

consideration. Many states view remediation with regard to property boundaries. As long as the

contamination remains within the property boundaries, then no action may be taken. If a plume

migrates off the property, however, NA may be used to address contamination at that point. Some

states using "monitoring only" may not necessarily be basing these decisions on the basis of sitespecific NA, but on risk. Other states are claiming NA by default, simply due to the length of time

required for active cleanup. Also, not all states are requiring source removal before using NA.
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

20-9

Summary 

New Jersey and North Carolina have developed policies addressing NA as a stand-alone option for

both ground water and soils, primarily for petroleum compounds. North Carolina developed its rule

on NA in 1993 and has approved approximately 150 sites for the process. NA is only appropriate

after site conditions have been fully evaluated and it has been concluded that natural remediation

is a viable option for ground water. This involves an evaluation of all potential impacts in the vicinity

of the site, including impacts on ground water used for potable purposes, surface water bodies, and

wetlands, to ensure that receptors will not be affected as the contaminant concentrations degrade.

Source removal is generally required. Most of these are petroleum sites, but a couple of sites in

North Carolina have also included solvents and even lead. Although some of these compounds are

not readily biodegradable, North Carolina also looks at sorption and removal of the source. Source

removal may not even always be required if it can be proven that no further leaching will occur. 

Texas and Wisconsin have written formal guidance regarding NA of soils. They are in the process

of developing guidance pertaiing to NA of ground water as well. Wisconsin is currently working with

Alaska in developing guidance for soils in that state.

Other states have developed informal guidance for ground-water and soil contamiantion focusing

on petroleum waste. Delaware has informal guidelines concerning petroleum waste that allows for

a "passive corrective action" plan. Passive action is remediation through natural degradation.

Assurance that contaminants will not pose a threat to human health or the environment is required.

One year of monitoring must show that the remediation is sufficient for site closure. After 2 years,

written permission is required to continue using passive action. Florida recognizes NA and expects

this to be a big part of remediation in the future. The state intends to expand NA activities during the

next year and broaden the types of sites that will be considered. Monitoring for NA will be allowed

at sites with higher contaminant concentrations and more widespread contamination. Michigan is

developing a draft bioremediation guidance document to determine criteria considered for

bioremediation, including NA. A final version, expected within the year, will not only consider

petroleum waste but other wastes, including solvents. Texas is beginning to look at chemicals other

than petroleum to be considered for NA as well. A document was recently prepared entitled Present

Remedies Guidance Document for Soils at Texas Superfund Sites. A similar document on ground

water will soon be written and will address NA. Nebraska's Superfund section may also look at NA

by allowing it at sites with low levels and simply monitoring. New Mexico has allowed a few sites to

use NA of more refractory chlorinated compounds. For example, at one site it was found that

carbon tetrachloride was degrading fairly well to methylene chloride, another with NA of PCE

contamination. Wisconsin and Wyoming are developing some very preliminary guidance or

protocols looking at a range of contaminants; these should be ready by the end of the year.

Considerations for use of NA will be based not only on the risk and beneficial uses but other

characteristics of the aquifer as well.

Most of the states are either using RBCA or are incorporating it into state guidelines regarding NA

of petroleum hydrocarbons at UST sites. California, Iowa, Mississippi, Montana, North Carolina,

Washington, and West Virginia are the only states that were not using and did not plan to use the

RBCA at petroleum sites. Interested parties in West Virginia, however, recently met to develop a state

policy for NA incorporating soil cleanup levels. The state is in the process of accumulating

information from other states. A risk-based approach is in review for eventual incorporation into the
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

20-10

overall statewide policy. The state plans to have an interagency approach including UST, RCRA and

CERCLA. Idaho is developing a new ground-water rule. Maine is developing a guidance document

(draft by the end of May) for in-house staff to determine when intrinsic remediation of petroleum

hydrocarbons is appropriate. States have also indicated that NA may be incorporated in other

programs as well. In the survey, Illinois, Idaho, West Virginia, Texas, and Ohio are only a few of the

states that indicated they have a voluntary action program and have passed state legislation

concerning the "Brownfields" Act. 

Natural attenuation can play a role in the cleanup of Brownfields sites. Brownfields are abandoned,

idled, or underused industrial and commercial sites where expansion or redevelopment is

complicated by real or perceived environmental contamination that can add cost, time, or

uncertainty to a redevelopment project. In recent years, states have developed voluntary cleanup

programs designed to provide liability protection to private parties that clean up Brownfields sites.

EPA supports these state cleanup programs and pledges that the successful cleanup of a site under

a state program will also satisfy EPA regulations. Eighteen Brownfields National Pilots are currently

under way in Alabama, California, Connecticut, Indiana, Kentucky, Louisiana, Maryland,

Massachusetts, Michigan, New Jersey New York, Ohio, Oregon, Pennsylvania, Rhode Island, Texas,

Virginia, and Washington.

Bibliography

1. Barkan, C. 1996. State-by-state summary on RBCA approaches. Soil & Groundwater

Cleanup. April: 41.

2. Bryant, C. 1995. Recent developments in laws and regulations. Remediation. Winter:

111.

3. Copeland, T.L., R. Pesin, et al. 1995. Using risk assessment to achieve cost-effective

property transfers and site closures for former UST sites. Remediation. Winter: 1.

4. EERP. 1993. ERRP issues guidance on natural biodegradation. Wisconsin Department of

Natural Resources Emergency and Remedial Response Section.

5. NJDEP. 1996. Site remediation program, technical requirements for site remediation,

proposed readoption with amendments. New Jersey Administrative Code (NJAC) 7:26E.

New Jersey Department of Environmental Protection.

6. NCDEQ. 1995. 15A North Carolina Action Code (NCAC) 2L Implementation Guidance.

North Carolina Department of Environmental Quality.

7. NCDEQ. 1993. 15A North Carolina Action Code (NCAC) Title 15A, Subchapter 2L,

Sections .0100, .0200, .0300. Classifications and water quality standards applicable to

the ground waters of North Carolina. North Carolina Department of Environment, Health,

and Natural Resources Division of Environmental Management.
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

20-11

8. Penelope, P.A., K.D. Reece, et al. 1995. Sensitivity analysis for setting soil cleanup

standards. Remediation. Winter: 19.

9. Ritz, S.M. 1996. States speak out on natural attenuation. Soil & Groundwater Cleanup.

January-February: 18.

10. Tulis, D. 1996. The growth of remediation by natural attenuation at LUST sites in the U.S.

Presented at UST/LUST National Conference (March 11). U.S. EPA Office of Underground

Storage Tanks. 
20-12

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

State Review

Natural Attenuation of

Ground Water and Soils

Daniel Pope

Dynamac Corporation

Ada, OK

Natural Attenuation of

Petroleum Hydrocarbons

● Leaks from USTs are the most

common cause of ground-water

contamination

● As of June 1995, there have

been over 295,000 confirmed

releases

Remediation at LUST Has Shifted

to Using Natural Attenuation

● In 1993, landfilling was the

predominant remediation for soils, and

pump-and-treat the most common in

ground-water treatment.

● As of 1995, NA of soils (28%) only

second to landfilling (34%), while NA of

ground water (47%)

(information obtained from EPA’s Office of

Underground Storage Tanks [OUST])

Use of Soil Cleanup Technologies

at UST Sites

Landfilling

Natural Attenuation

Biopiles

34 Soil Vapor Extraction

20

16

9

7

Landfarming

Thermal Desorption Incineration

2 3

%

Adapted from Dana Tulis, EPA

UST/LUST National Conference

Talk, March 11, 1996

Use of Groundwater Cleanup

Technologies at UST Sites

Natural Attenuation

29

48

13

2 4

4

Pump-and-Treat

Air Sparging

In Situ Bioremediation

Dual-Phase Extraction Biosparging

%

Adapted from Dana Tulis, EPA

UST/LUST National Conference

Talk, March 11, 1996

Programs That May Look at Natural

Attenuation in Cleanup

● UST

● CERCLA

● RCRA

● State Voluntary Cleanup Program

● Brownfields Sites
20-13

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Risk-Based Corrective Action

(RBCA) and NA at UST Sites

● Emergency Standard Guide for RBCA by ASTM

● Most states using/incorporating RBCA into

guidelines

● Demonstrated and predicted attenuation of

hydrocarbons with distance

● Corrective action goals based on a tiered

approach

■ Tier 1 most conservative; high risk

■ Two lower tiers allow site-specific goals; risk not

imminent

ASTM Revisions

Currently assembling NA document

● Limited petroleum compounds

● May consider other compounds (e.g.,

solvents) in future

Document purpose

● Remove stigma that NA is equivalent to

“no further action”

● Serve as a conceptual framework in NA

decision-making and information needs

EPA’s Policy on Natural

Attenuation

Office of Underground Storage Tanks

(OUST)

● NA is not a “default” remediation technology for

LUST sites

● Supports use of the most appropriate technology

● Technology selection should be risk-based on a siteby-site basis

● NA is an active choice, includes site

characterization, risk assessment, and monitoring

● Free product removal is a prerequisite to using NA

● Cleanup not complete until reaching state or local

cleanup levels

Brownfields

● Abandoned industrial/commercial

sites

● Redevelopment complicated by real or

perceived contamination

● Successful cleanups under State

programs would satisfy EPA

regulations

● 18 States currently with Brownfields

National Pilot Studies

U.S. EPA Survey Asked

States:

(1) Whether they encourage or discourage

the use of natural attenuation (NA)

(2) If there are any formal or informal

policies or guidelines for NA

(3) If they use any particular model when

deciding on NA

(4) If compounds other than petroleum

hydrocarbons would be considered for

NA

States With Formal Guidance

on Soils Using NA

● Texas ● Wisconsin
20-14

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

States Developing Soils

Guidance With NA

● Alaska

● Arizona

● Florida

● Michigan

● South Dakota

● West Virginia

● Vermont

State Guidelines on Natural

Attenuation in Soils

HA

10

8

9

7

6

4

5 3

2

1

10

9

AK

WA

OR

CA

NV

ID

UT

AZ NM

CO

WY

MT MN

ND

SD

NE

KS

OK

TX

LA

AR

MO

IO

WI

IL IN

KY

TN

MS AL

GA

FL

SC

NC

OH

WV VA

PA

NY

DE

NH

ME

VT

MA

CT

RI

No Guidelines

Written Guidelines

Developing Guidelines

MI

NJ

States With Natural Attenuation

Policy on Ground Water

● North Carolina

● New Jersey

Each State Requires:

● Full plume definition and receptor analyses

● Appropriate modeling to predict plume

degradation

● Source removal or control

● Monitoring program to demonstrate NA

North Carolina

● Developed rule on natural attenuation in 1993

● Approved approximately 150 sites for NA

● Most are petroleum sites, but some included

solvents and even lead

● Looks at sorption and source removal as part of

NA, hence NA for Pb possible

● Assesses potential for toxic byproducts

● Source removal may not be required if no further

leaching to ground water is proven

● Future land use in the vicinity of the site

required

New Jersey Natural

Attenuation Rules

● Assess potential impacts, ensure no impact to

receptors, and remove/remediate sources

● NA may be used at sites deemed technically

impractical for active remediation

● Identify current and potential ground-water

uses based on a 25-year plan

● Costs of remedy includes long-term

monitoring

● Historical data determine the duration and

frequency of sampling

Monitoring Requirements

● New Jersey—at least eight quarters of

monitoring

● North Carolina—monitor until appropriate

ground-water quality standards achieved

● Both require sentinel wells downgradient of

plume if receptor involved

Minimum time of travel upgradient of receptor:

■ 3 years - New Jersey

■ 1 year - North Carolina

● Monitoring assesses past predictions, plume

behavior, and modification needs
20-15

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Other States Addressing

Natural Attenuation

Delaware UST Section’s

Technical Guidance Manual

● “Passive corrective action” allows NA if

no threat to receptors

● Source and free product removal a goal

● Monitor 1 year to demonstrate

sufficient remediation for site closure

● Passive action not allowed beyond 2

years without written approval

States Developing Natural Attenuation

Guidance on Ground Water

TEXAS Ground-water guidance similar to

“Present Remedies Guidance Document

for Soils at Texas Superfund Sites”

MICHIGAN Draft bioremediation guidance to

determine criteria considered for

bioremediation including NA. Not only

petroleum waste will be considered.

MAINE In-house guidance document to

determine intrinsic remediation of

petroleum

States Developing Natural Attenuation

Guidance on Ground Water (continued)

WISCONSIN Preliminary guidance based on

risk, beneficial uses, and aquifer

characteristics

FLORIDA Petroleum cleanup rules/

mandating RBCA in State

Legislative statutes

SOUTH Performing field studies with

CAROLINA USGS that address intrinsic 

remediation

WYOMING Preliminary guidance considering a

range of contaminants

Other States Approaches

CALIFORNIA Does not use NA. Revisions to Resolution 92-49 refer to “containment

zones” out of which the contaminant

is not allowed to migrate.

TENNESSEE Does not encourage use of NA. Does

encourage more accelerated forms of

bioremediation.

CONNECTICUT Use ground-water classification to

establish cleanup standards. Aquifers

with lower designation more likely to

be considered for NA as a remedial

option.

Natural Attenuation

Models

● Most states allow PRP to use any

peer reviewed model

● Some states have indicated they

use mostly SESOIL, VLEACH, and

AT123D

● One State indicated interest in

Bioplume III when available
20-16

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Survey Summary

● There are 2 states that have

developed official policy

● There are about 7 states

developing guidance

● There are about 13 states with

unofficial guidance

State Policies Regarding Natural

Attenuation in Ground Water

10

8

9

7

6

4

5 3

2

1

10

9

AK

WA

OR

CA

NV

ID

UT

AZ NM

CO

WY

MT MN

ND

SD

NE

KS

OK

TX

LA

AR

MO

IO

WI

IL IN

KY

TN

MS AL

GA

FL

SC

NC

OH

WV VA

PA

NY

DE

NH

ME

VT

MA

CT

RI

HA

Do not have any formal policy

Informal policies and guidelines

Written policies and guidelines

Informal guidelines/Do not

consider non-UST waste

Developing policies and guidelines

MI

NJ

Conclusion

● Interest in NA is increasing and being

incorporated into more state environmental

regulations and programs.

● Although NA is gaining acceptance, it should

be remembered that complete site

characterization is an essential part in

deciding if this remediation option is

appropriate.

● NA is a remedial approach that should be

based on the likelihood of success and is not

a “no action” alternative.
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

21-1

Monitoring

Daniel Pope

Dynamac Corporation, Ada, OK

Monitoring of bioremediation and natural attenuation can be considered from several viewpoints.

First are the contaminant-oriented questions: Are the contaminants disappearing, and, if so, how?

The mechanism of disappearance is of interest: Are contaminants being biodegraded, or to what

degree are volatilization, leaching, adsorption, or other mechanisms involved? 

Next, if the contaminants are being biodegraded, are the contaminants being broken down to

intermediate products (which may be innocuous or toxic), mineralized to carbon dioxide and water,

or polymerized/humified? Toxicity changes may be monitored to determine whether toxicity is

decreasing or whether degradation products may be of higher toxicity than the original

contaminants. Finally, the rate of contaminant loss helps to estimate remediation times and to

assess degradation relative to contaminant mobility to sensitive receptors. 

Geochemical factors associated with contaminant degradation may be monitored. Degradation may

cause changes in pH, redox potential, electron acceptors, and alkalinity; these changes may be

monitored to help prove remediation is taking place, to establish areas on the site where different

kinds of remediation are taking place, and to estimate remediation rates. In addition to the

geochemical factors already mentioned, temperature and salinity may affect microbial processes

and therefore degradation rates. Operational parameters require monitoring to determine whether

appropriate levels of nutrients, electron acceptors, and water necessary for bioremediation are

present. 

Monitoring of microbial parameters may be required. The various estimates of contaminant

degradation, electron acceptor change, and other geochemical changes indirectly measure

microbial activity, but there may be a need to measure certain aspects of the microbial population

directly. Microbial populations may be estimated by plate counts, most probable number techniques

(MPN), or direct microscopic examination. In addition to respiration measurements, ATP activity

measurements can estimate microbial metabolic activity. FAME profiles and sole carbon source

profiles measurements may provide information about microbial community structure. Several types

of culture tests can indicate the ability of the microbial population to degrade contaminants of

interest. Generally, microcosm tests using soil or water samples from the site under conditions as

similar as possible to site conditions are most likely to yield information about microbial activity and

contaminant degradation that can be readily used for making decisions about site activities. 

Monitoring may be required to establish the success (or failure) of bioremediation/natural

attenuation, give timely warning of the impending impact on sensitive receptors, and determine the

potential for site closure. Generally, monitoring is required for a number of years to develop

sufficient data to establish that risk to sensitive receptors is not significant, and that the site is ready

for closure.
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

21-2

Bibliography

1. Blackwood, L.G. 1991. Assurance levels of standard sample size formulas: Implications for

data quality planning. Environ. Sci. Technol. 25:8. 

2. Dragun, J. 1988. The soil chemistry of hazardous materials. Hazardous Materials Control

Research Institute, Silver Spring, MD.

3. Eklund, B. 1992. Practical guidance for flux chamber measurements of fugitive volatile

organic emission rates. J. Air Waste Mgmt. Assoc. 42:1,583-1,591. December.

4. Gilbert, R.O. 1987. Statistical methods for environmental pollution monitoring. Van

Nostrand Reinhold. 

5. Gilbert, R.O., and J.C. Simpson. 1990. An approach for testing attainment of soil

background standards at Superfund sites. In: American Statistical Association 1990, Joint

Statistical Meetings, Anaheim, CA. August 6-9, 1990. Pacific Northwest Laboratory,

Richland, WA.

6. Hawley-Fedder, R., and B.D. Andresen. 1991. Sampling and extraction techniques for

organic analysis of soil samples. UCRL-ID-106599. Lawrence Livermore National

Laboratory, Berkeley, CA. February.

7. Keith, L.H., ed. 1988. Principles of environmental sampling. American Chemical Society.

8. Lewis, T.E., A.B. Crockett, R.L. Siegrist, and K. Zarrabi. 1991. Soil sampling and analysis

for volatile organic compounds. EPA/540/4-91/001. Superfund Technology Support

Center for Monitoring and Site Characterization, Environmental Monitoring Systems

Laboratory, Las Vegas, NV. February.

9. Norris, et al. 1994. Handbook of bioremediation. Lewis Publishers, CRC Press.

10. Soil Science Society of America 1987. Glossary of soil science terms. Soil Science Society

of America, 677 South Segoe Road, Madison, WI.

11. U.S. EPA. 1985. Practical guide for ground-water sampling. EPA/600/2-85/104.

September.

12. U.S. EPA. 1986. Permit guidance manual on unsaturated zone monitoring for hazardous

waste land treatment units. EPA/530/SW-86/040. Environmental Monitoring Systems

Laboratory, Las Vegas, NV. October.

13. U.S. EPA. 1990. A New approach and methodologies for characterizing the hydrogeologic

properties of aquifers. EPA/600/2-90/002. January.

14. U.S. EPA. 1990. Handbook: Ground water—Vol. I: Ground water and contamination. Vol.

II: Methodology. EPA/625/6-90/016a,b. 
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

21-3

15. U.S. EPA. 1990. Basic concepts of contaminant sorption at hazardous waste sites.

EPA/540/4-90/053. October.

16. U.S. EPA. 1991. A guide: Methods for evaluating the attainment of cleanup standards for

soils and solid media. Quick reference fact sheet. 9355.4-04FS. Office of Emergency and

Remedial Response, Hazardous Site Control Division. July. 

17. U.S. EPA. 1991. Dense nonaqueous phase liquids. EPA/540/4-91-002. March.

18. U.S. EPA. 1991. Description and sampling of contaminated soils: A field pocket guide.

EPA/625/12-91/002. November.

19. U.S. EPA. 1991. Handbook of suggested practices for the design and installation of

ground-water monitoring wells. EPA/600/4-89/034. Environmental Monitoring Systems

Laboratory, Las Vegas, NV. March.

20. U.S. EPA. 1992. General methods for remedial operations performance evaluations.

EPA/600/R-92/002. January.

21. U.S. EPA. 1993. Subsurface characterization and monitoring techniques: A desk reference

guide—Vol. 1: Solids and ground water, Appendices A and B. Vol. II: The vadose zone,

field screening and analytical methods, Appendices C and D. EPA/625/R-93/003a,b.

May.

22. U.S. EPA. 1993. Use of airborne, surface and borehole geophysical techniques at

contaminated sites: A reference guide. EPA/625/R-92/007. Center for Environmental

Research Information, Cincinnati, OH. September.

23. U.S. EPA. 1994. Methods for monitoring pump-and-treat performance. EPA/600/R94/123. June.

24. Wiedemeier, T.H., et al. Technical protocol for implementing intrinsic remediation with longterm monitoring for natural attenuation of fuel contamination dissolved in groundwater,

Vols. I and II. Air Force Center For Environmental Excellence, Technology Transfer Division,

Brooks Air Force Base, San Antonio, TX. 
21-4

Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Monitoring

Daniel Pope

Dynamac Corporation

Ada, OK

Monitoring

Bioremediation/Natural

Attenuation

● Much information available on

monitoring technologies

● This presentation mainly a

checklist: what should be

monitored, and why?

● References for specific techniques

in handout

Monitoring To Determine

Remediation Rates (contaminant

disappearance)

● Are contaminants

disappearing?

● Rate of disappearance

Monitoring To Determine

Daughter Products

● Estimate remediation rates

● Determine toxic products (e.g.,

vinyl chloride from TCE)

Monitoring for

Operational Purposes

● Addition of electron acceptors

● Nutrients

● Water

Monitoring To Warn of Potential

Impact on Sensitive Receptors

● At or before point of compliance

● Must allow time for remedial

measures
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Monitoring Mass Balance

Approach

● Contaminants “disappear” from analytical

view without actually being remediated

● Monitor each phase (soil solids, gas, water,

and nonaqueous phase liquid) to

determine how much of each waste

component is in each phase

● Determine whether remediation is

actually taking place or whether

contaminants are merely being moved to

different phases

Monitoring Breakdown

Products

● Many breakdown products known

● Monitoring is not common, except

for breakdown products of known

high toxicity, such as vinyl

chloride, or those that are easy to

measure, such as carbon dioxide

Monitoring Toxicity –

Microtox Microbial Bioassay

● Cultures of phosphorescent (lightemitting) marine bacteria are

exposed to contaminated media or

extracts, and decline in light

output over time is measured

● Microtox assay measures general

metabolic inhibition

Monitoring Toxicity –

Microtox Microbial Bioassay

● Major advantages: quick, easy,

repeatable, inexpensive, and has a large

amount of published literature about

its uses and results

● Major disadvantage (as for most acute

bioassays): results of the assay have no

direct relationship to toxicity of the

contaminants to humans or ecology

Monitoring Toxicity –

Ames Assay

● A measure of mutagenic potential

of a sample

● High correlation between

mutagenicity (as measured in the

Ames test) and carcinogenicity

● Several days to complete, more

expensive than Microtox

Monitoring Toxicity –

Other Assays

● Many other species have been

used for assessing toxicity of

environmental samples

● EPA conducting R&D on ecological

and health assays to develop

alternative endpoints
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Monitoring Microbial

Activity

● Plate counts

● Most Probable Number (MPN)

counts

● Direct microscopic counts

● Respiration measurements

● ATP activity measurements

Monitoring Microbial

Activity

● Oxygen, carbon dioxide levels—general index

of microbial activity

● Monitoring oxygen or carbon dioxide alone

can be deceiving since abiotic processes can

affect oxygen or especially carbon dioxide

● Because the respiration estimated may not

result only from transformation of the

compounds of interest, respiration cannot be

used as a direct measure of transformation of

these compounds

Monitoring Microbial

Activity

● Soil gas concentrations of CO2, O2

fluctuate daily due to microbial

activity

● Measure CO2 and O2 at the same

time of day for each sampling

event

Monitoring Microbial

Activity

● Soil microorganisms can be cultured on

specific media to determine counts of

“specific degraders”

● If PAHs are added to a media with no

other carbon sources present, any

microorganisms that grow in the media

can be assumed to have the capability

of using PAHs as a sole source of

carbon

Monitoring Soil Moisture

● “Visual” methods—require

experience

● Gravimetric methods—accurate, but

time consuming

● Neutron probes—accurate,

expensive, use radioactive material

● Porous cup tensiometers

● Capacitance—not very accurate

Nutrients

● Several standard tests

● Carbon to nitrogen to phosphorus

(C:N:P) ratios of 100–300:10:1
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Volatilization

● Usually volatiles released from the

soil surface

● Canopy placed over defined area

of contaminated soil

● Vapors collected under canopy

swept into adsorbent for later

extraction and analysis

● Sampling pump at site perimeters

Leaching

● Porous cup and pan lysimeters

● Porous cup lysimeters work even

when soil is relatively dry

● Pan lysimeters collect only water

that is actively moving down

through soil

● Most LTUs, soil piles, compost

units are lined to collect leachate

Sampling Program Goals

● Average contaminant

concentration to +/- x ppm

● Highest contaminant concentration

< x

● Desired confidence limits

Sample Location

● Random

● Stratified random

● Grid, with random start

What Should Monitoring

Show?

● Plume type (stable, shrinking,

expanding)

● Remediation rates

● Warning of potential impact on

sensitive receptors

What is Required To Show That

Bioremediation/Natural

Attenuation Is “Working?”

● Documented loss of contaminants from site

● Daughter product appearance

● Appropriate geochemistry

● Electron acceptor disappearance/product

appearance

● Laboratory assays showing microorganisms

from site samples have potential to transform

contaminants under expected site conditions
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Monitoring – Primary

Evidence

Plume behavior (stable,

shrinking, expanding)

Monitoring – Primary

Evidence

● If the plume is stable or shrinking,

this is primary evidence that

natural attenuation is occurring

● If the plume is expanding more

slowly than GW movement

adjusted for retardation, this is

evidence that natural attenuation

is occurring

Monitoring – Secondary

Evidence

● Historical data may not be

available to indicate the plume

state

● Then, secondary evidence can be

used while information on plume

state is being accumulated

Monitoring – Secondary

Evidence

Electron acceptor/reduction

product concentrations

Monitoring – Secondary

Evidence

Alkalinity

Monitoring – Secondary

Evidence

● Inverse correlation between

electron acceptors and

contaminant concentrations

● Daughter products
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Determining Natural

Attenuation Rates

● Mass balance (for any plume type)

● Concentration versus time

(for shrinking plumes)

● Concentration versus distance

(for stable plumes)

Mass Balance Approach

Requirements

● Estimate of source area

perpendicular to GW flow

● Estimate of hydraulic conductivity

and gradient

Concentration versus Time

Approach Requirements

Wells with measurable

contaminant outside free product

zone

Concentration versus Distance

Approach Requirements

Two or three downgradient wells,

along direction of GW flow, with at

least two wells with measurable

contaminant concentrations,

differing by several fold

Warning of Impact on

Sensitive Receptors

● Sentinel wells located at

compliance point between

contaminated GW and sensitive

receptor

● Location must allow time for

remedial measures to be taken

before contamination moves past

sentinel well to sensitive receptor

Monitoring Frequency –

Factors

● Plume status

● Water table fluctuations

● Seasonal variability

● GW velocity

● Distance from plume to sensitive

receptor
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Monitoring Frequency

One year of quarterly monitoring

often sufficient to establish

relationship between readily

degraded contaminants and electron

acceptor/reduction products

concentrations

Monitoring Frequency

● More than one year may be

necessary to establish whether a

plume is stable, shrinking, or

expanding

● Previous monitoring efforts may

reduce need for more wells,

monitoring data

Laboratory Assays for

Biodegradation

● Determine biodegradation rates, but

may not reliably indicate field rates

● Establish potential for

bioremediation, but may not be

necessary for simple petroleum

contaminants

● Determining need for nutrient,

electron acceptor addition
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

22-1

Modeling

Daniel Pope

Dynamac Corporation, Ada, OK

Introduction

A mathematical description of bioremediation establishes a framework for evaluating laboratory

treatability data and field data that are useful for determining treatment potential under site and

environmental constraints. Mathematical models provide an approach for integrating simultaneous

processes of degradation, mass transport, and partitioning within subsurface and surface systems

so that an assessment can be made of the presence of target chemicals in leachate, soil, and air.

Models provide an estimate of the potential for ground-water and air contamination through a

determination of the rate and extent of contaminant transport and biodegradation as related to

specific subsurface or surface characteristics. Models also allow identification of those chemicals

requiring management to reduce or eliminate risk to human health and the environment. Thus,

mathematical models represent tools for ranking design, operation, and management alternatives

as well as for the design of monitoring programs for engineered (active) and nonengineered

(passive) biological treatment systems.

Model Types

To address the complex processes occurring at a site with regard to bioremediation, four types of

models are described: 1) saturated flow, 2) multiphase flow, 3) geochemical, and 4) reaction rate

models (1). Saturated flow models are derived from basic principles of conservation of fluid mass

and describe the flow path and rate of transport of water and dissolved contaminants (using

principles of conservation of chemical mass) through the saturated zone. In special cases,

biodegradation reactions, based on simple first-order kinetics, can be incorporated into the model.

Often, however, biodegradation processes are too complex to be simply incorporated; therefore,

special modeling tools are needed.

Multiphase flow models describe systems where two or more fluids exist together in a porous

medium. With regard to unsaturated flow, water and air are two fluids that exist together. Addition

of gasoline represent a third fluid within the unsaturated zone. Dense nonaqueous phase liquids

(DNAPLs) often occur within the saturated zone and are immiscible (nonmixing) with water.

Complex interactions among water, air, NAPLs, and solids renders multiphase flow models that are

more complex and less accurate due to the relatively large number of transport parameters required.

Geochemical models identify how thermodynamics of chemical reactions in the subsurface control

the speciation of target chemicals. Geochemical models are primarily concerned with inorganic

contaminants, for example, metal mobility. The lack of application to bioremediation of such

models is due to 1) lack of incorporation of organic chemicals, 2) equilibrium orientation (rather
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

22-2

than kinetic orientation of biodegradation models), and 3) high complexity and cost without the

incorporation of biological components.

Reaction rate models, including biological models, describe the rate of microbial transformation of

target organic chemicals. Biodegradation rate expressions can be incorporated into a model that

takes into account the rate of reaction as a function of active biomass present, contaminant

concentration, and electron acceptors present. Determination of appropriate rate expressions,

especially for the description of co-oxidation or co-metabolism, is an area of current development.

Biodegradation models are most easily combined with flow models when one rate-limiting material

can be identified. The rate-limiting material often is the primary electron donor or electron acceptor.

The biodegradation of petroleum hydrocarbons can often be modeled with oxygen as the

rate-limiting parameter.

Modeling Biodegradation

Main approaches used for modeling biodegradation include 1) first-order degradation models, 2)

biofilm models, 3) instantaneous reaction models, and 4) dual-substrate Monod models. Additional

information regarding these modeling efforts is given in Bedient and Rifai (2). Where a biofilm

approach is used, as often occurs in the subsurface, three processes are described: 1) mass

transport from the bulk liquid, 2) biodecomposition within the biofilm, and 3) biofilm growth and

decay.

Borden and Bedient (3) developed the first version of the BIOPLUME model. They developed a

system of equations to simulate the simultaneous growth, decay, and transport of microorganisms

combined with the transport and removal of hydrocarbons and oxygen. Simulation indicated that

any available oxygen in the region near the hydrocarbon source will be rapidly consumed. In the

body of the plume, oxygen transport will be rate limiting, and the consumption of oxygen and

hydrocarbon can be approximated as an instantaneous reaction.

Rifai and others (4, 5) expanded the original BIOPLUME and developed a numerical version

(BIOPLUME II) by modifying the U.S. Geological Survey (USGS) two-dimensional method of

characteristics (6). Transport of oxygen and contaminants in the subsurface is simulated, and

biodegradation is approximated by the instantaneous reaction model. The only input parameters to

BIOPLUME II that are required to simulate biodegradation are the amount of dissolved oxygen in

the aquifer prior to contamination and the oxygen demand of the contaminant determined from a

stoichiometric relationship. Other parameters are the same as required for the USGS model (6).

BIOPLUME II was used to model biodegradation of aviation fuel at the U.S. Coast Guard Station

in Traverse City, Michigan. 

Unsaturated zone modeling has been presented in Stevens et al. (7), where the model developed

by the U.S. Environmental Protection Agency, Regulatory and Investigative Treatment Zone (RITZ),

was expanded. The Vadose Zone Interactive Processes (VIP) model allows for the prediction of the

dynamic behavior of chemicals in the unsaturated zone under variation of temperature,

precipitation, and waste spill frequency (7). The VIP model accounts for biodegradation, effect of
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

22-3

oxygen concentration on biodegradation rate, volatilization, sorption/desorption, advection, and

dispersion of target chemicals within a vadose zone system. 

The BIOSCREEN model is an easy-to-use screening tool for simulating natural attenuation of

dissolved hydrocarbons at petroleum release sites (8). The software uses a Microsoft Excel

spreadsheet environment and is based on the Domenico analytical solute transport model.

BIOSCREEN has the ability to simulate advection, dispersion, adsorption, and aerobic decay as well

as anaerobic reactions, which have been shown to be the dominant biodegradation processes.

BIOSCREEN included three types of models: 1) solute transport without decay, 2) solute transport

with first order decay, and 3) solute transport with biodegradation assuming an "instantaneous"

biodegradation reaction. It is possible to modify BIOSCREEN to simulate intrinsic remediation of

chlorinated hydrocarbons.

With regard to the application of all models, the limitations must be identified and constraints

addressed. For all models, validity must be established on a site-by-site basis. No "off-the-shelf"

models are available for use on a routine basis regarding biodegradation. In addition,

measurement of input parameters often are extensive and sometimes are expensive (1). While

modeling has several limitations, the approach is a useful tool for understanding the dynamic

changes that occur in field sites during bioremediation.

References

1. National Research Council. 1993. Evaluating in situ bioremediation. In: In situ

bioremediation: When does it work? Washington, DC: National Academy Press. pp.

63-90. 

2. Bedient, P.B., and H.S. Rifai. 1993. Modeling in situ bioremediation. In: In situ

bioremediation: When does it work? National Research Council. Washington, DC:

National Academy Press. pp. 153-159. 

3. Borden, R.C., and P.B. Bedient. No date. Transport of dissolved hydrocarbons influenced

by reaeration and oxygen limited biodegradation. I. Theoretical development. Water

Resour. Res. 22:1,973-1,982. 

4. Rifai, H.S., P.B. Bedient, R.C. Borden, and J.F. Haasbeek. 1987. BIOPLUME II computer

model of two-dimensional contaminant transport under the influence of oxygen limited

biodegradation in ground-water, user's manual version 1.0. Rice University, National

Center for Ground Water Research, Houston, TX. 

5. Rifai, J.S., P.B. Bedient, J.R. Wilson, K.M. Miller, and J.M. Armstrong. 1988.

Biodegradation modeling at a jet fuel spill site. American Society of Civil Engineers. J.

Environ. Eng. Div. 114:1,007-1,019. 

6. Konikow, L.F., and J.D. Brederheoft. 1978. Computer model of two-dimensional solute

transport and dispersion in ground water. Techniques of water resources: Investigations

of the U.S. Geological Survey. Washington, DC. 
Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

22-4

7. Stevens, D.K., W.J. Grenney, Z. Yan, and R.C. Sims. 1989. Sensitive parameter evaluation

for a vadose zone fate and transport model. EPA/600/2-89/039. Ada, OK.

8. Newell, C.J., and J. Gonzales. 1996. BIOSCREEN intrinsic remediation decision support

system. In: Proceedings of the Conference on Intrinsic Remediation of Chlorinated

Solvents, Salt Lake City, UT (April 2). Sponsored by Hill Air Force Base, UT, in cooperation

with Battelle Laboratories, Columbus, OH.
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Modeling

Quantifying Biodegradation of

Subsurface Pollutants

Daniel Pope

Dynamac Corporation

Ada, OK

Modeling

● Provides framework for organizing

information about a site

● Provides an approach for

integration of degradation,

transport, and partitioning

processes

● Useful tools for managing field sites

and evaluating bioremediation

Modeling

● Contaminant loss explained by

abiotic reactions?

● Contaminant loss explained by

biological reactions using

reasonable processes

Evaluation of In Situ

Bioremediation

Model Types

● Saturated flow Water

● Multiphase flow Two or more

fluids together

● Geochemical Speciation/

thermodynamics

● Reaction rate Biological, 

chemical

Challenges

● Physical, chemical, and biological

processes must be incorporated

● Lack of field data on

biodegradation

● Lack of numerical schemes that

accurately simulate relevant

processes

Biodegradation Kinetics

Main Approaches for Modeling

● First-order degradation models

● Biofilm models (including

kinetic expressions)

● Instantaneous reaction models

● Dual-substrate monod models
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Biofilm Model Processes

● Mass transport from the bulk

liquid

● Biodecomposition within the

biofilm

● Biofilm growth and decay

Bioplume Model

● Borden and Bedient (1986)

● Microorganism growth, decay,

and transport

● Hydrocarbon transport and

removal

● Oxygen transport and removal

Bioplume Model

● Oxygen near hydrocarbon

source rapidly depleted

● Oxygen transport limiting in

the body of the plume

● Consumption of oxygen and

hydrocarbon considered

instantaneous

Bioplume Model

Major Sources of Oxygen

● Transverse mixing

● Advective fluxes

● Vertical exchange with unsaturated

zone

Bioplume II

● Rifai et al. (1987, 1988)

● Improvement

● Simulate transport of

oxygen and contaminants

Bioplume Applications

● Conroe, Texas site—PAH

contamination

● Traverse City,

Michigan—aviation fuel
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Unsaturated Zone Modeling

Vadose Zone Interactive Processes (VIP)

● EPA model

● Grenney and Stevens (1988–1989)

● Enhancement of Ritz model (EPA)

● Regulatory and Investigative Treatment

Zone

Vadose Zone Interactive Processes (VIP)

● Biodegradation

● Effect of O2 concentration on

biodegradation

● Volatilization

● Sorption/desorption

● Advection

● Dispersion

Unsaturated Zone Modeling

Vadose Zone Interactive Processes (VIP)

● Dynamic behavior under variable

conditions of:

■ Precipitation

■ Temperature

■ Spill frequency

Unsaturated Zone Modeling Model Applications

● Mass of parent compound

remaining with time and

distance

● Apparent mass of parent

compound remaining with time

and distance

● Predict effects of source

removal on lifetime of plume

Bioscreen Model

● U.S. Air Force

● Microsoft Excel spreadsheet

environment

● Based on Domenico analytical

solute transport model

Bioscreen Model

● Simulate natural attenuation of

dissolved hydrocarbons at

petroleum release sites

● Can be modified to simulate

natural attenuation of

chlorinated hydrocarbons
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Seminar Series on Bioremediation of Hazardous Waste Sites: Practical Approaches to Implementation

Bioscreen Model

Processes Simulated

■ Advection

■ Adsorption

■ Dispersion

■ Aerobic decay

■ Dominant anaerobic reactions

Bioscreen Model

Includes 3 Model Types

1. Solute transport without decay

2. Solute transport with first-order 

decay

3. Solute transport with

biodegradation assuming as

“instantaneous” biodegradation

reaction

Limitations of Models

● Validity must be established on

“site-by-site” basis

● No “off-the-shelf” models are

available for evaluating

bioremediation on a routine basis

● Measurement of input parameters

often extensive and/or expensive