Monitored Natural Attenuationand

Risk-Based Corrective Actionat

Underground Storage Tanks Sites

Mike Trombetta

Department of Environmental Quality

Environmental Quality CounselPetroleum Tank Release Fund Subcommittee

MeetingJune 4, 2008

Agenda

• Science of a Petroleum Cleanup

• Monitored Natural Attenuation

– Science Behind MNA

– Implementation Issues

• Risk - Based Corrective Action

• Long-Term Monitoring

Science Behind

Petroleum

Release Cleanup

GROUNDWATER

PRODUCT SMEAR ZONEDISSOLVED

CONTAMINATION

EXPANDING PLUME

FLOATING FREE PRODUCT

GROUNDWATER

CONCENTRATIONSBELOW DEQ-7STANDARDS

CONCENTRATIONSEXCEED DEQ-7

STANDARDS

0

505

500200

1,000

mg/kg(parts per million)

µg/liter(parts per billion)

GROUNDWATER

PRODUCT SMEAR ZONEDISSOLVED

CONTAMINATION

CLEAN FILL

TANK SYSTEM

REMOVED

FLOATING FREE PRODUCT

GROUNDWATER

PRODUCT SMEAR ZONEDISSOLVED

CONTAMINATION

CLEAN FILL

STEADY STATE

STABLE OR SHRINKING

TANK SYSTEM

REMOVED

FLOATING FREE PRODUCT

CONTAMINATED SOIL CONTINUES TO LEACH INTO GROUNDWATER

GROUNDWATER

PRODUCT SMEAR ZONEDISSOLVED

CONTAMINATION

CLEAN FILL

STEADY STATE

STABLE OR SHRINKING

TANK SYSTEM

REMOVED

• Sorption• Diffusion• Dispersion• Volatilization• Bio-degradation

Natural Attenuation Mechanisms

GROUNDWATER

PRODUCT SMEAR ZONEDISSOLVED

CONTAMINATION

CLEAN FILL

STEADY STATE

STABLE OR SHRINKING

TANK SYSTEM

REMOVED

FLOATING FREE PRODUCT

GROUNDWATER

PRODUCT SMEAR ZONEDISSOLVED

CONTAMINATION

CLEAN FILL

SOURCEREMOVAL

(EXCAVATION)

PLUME

SHRINKS

BACK

FLOATING FREE PRODUCT

GROUNDWATER

PRODUCT SMEAR ZONEDISSOLVED

CONTAMINATION

CLEAN FILL

SOURCEREMOVAL

(EXCAVATION)

NEW “STEADY STATE”

(SLOW SHRINK BACK)

FLOATING FREE PRODUCT

GROUNDWATER

PRODUCT SMEAR ZONE

DISSOLVED CONTAMINATION

BELOW DEQ-7STANDARDS

CLEAN FILL

SOURCEREMOVAL

(EXCAVATION)

GROUNDWATER

PRODUCT SMEAR ZONE

DISSOLVED CONTAMINATION

BELOW DEQ-7STANDARDS

CLEAN FILL

SOURCEREMOVAL

(EXCAVATION)

SITE

CLOSED

Natural AttenuationNatural Attenuation

A reduction in the

concentration and mass of a

substance in groundwater, due

to naturally occurring physical,

chemical, and biological

processes without human

intervention or enhancement.

Monitored Natural Monitored Natural Attenuation Attenuation (MNA)

The reliance on natural

attenuation processes (within

the context of a carefully

controlled and monitored site

cleanup approach) to achieve

site-specific remediation

objectives within a time frame

that is reasonable compared to

other more active methods.

GROUNDWATER

CLEAN FILL

MONITORING MONITORING WELLSWELLS

MNA

Implementation

Issues

U.S. Environmental Projection AgencyU.S. Environmental Projection Agency

Office of Solid Waste and Office of Solid Waste and Emergency ResponseEmergency Response

Guidance 9200.4-17P, April 21, 1999

Use of Monitored Natural Attenuation Use of Monitored Natural Attenuation at Superfund, RCRA Corrective at Superfund, RCRA Corrective

Action, and Action, and Underground Storage Underground Storage Tank SitesTank Sites

• Not a Presumptive Remedy

• Thorough Site Characterization

• Source Control

• Remove Free Product

• Performance Monitoring

• Contingency Remedies

• Reasonable Timeframe Compared to other methods

• Institutional Controls

Implementation Issues

Other Issues

• Vapor Intrusion

• Utility Impacts

• Risks to Construction Workers

VAPORS VAPORS EMINATING FROM EMINATING FROM DISSOLVED AND DISSOLVED AND FREE PRODUCTFREE PRODUCT

BURIED WATER LINEBURIED WATER LINE

• PERMEATION OF PIPEPERMEATION OF PIPE

• MIGRATION ALONG TRENCHMIGRATION ALONG TRENCH

CONSTRUCTION CONSTRUCTION WORKER WORKER

VAPOR VAPOR

INTRUSIONINTRUSION

MNA Wrap-Up• MNA is not applicable where

human receptors are at risk.

• Source removal is critical.

• Long-term monitoring will likely be required after source removal.

• Source area soils cannot always be fully removed.

• Closure requires DEQ-7 Standards.

Minimal Source Removal

Very long cleanup time

up to 100 years or more…

in tight clay-type soils.

Significant Source Removal

Reduced cleanup time

down to 10 years …

in tight clay-type soils.

Low – PermeabilityClay Soil

Low – PermeabilityClay Soil

NATURAL ATTENUATION PROCESSES EXIST AT ALL SITES

MNA can be used as a stand-alone cleanup strategy, but is typically combined with other technologies

(source removal).

MNAQuestions?

(RISK - BASED CORRECTIVE ACTION DISCUSSION -- NEXT)

Risk-Based

Corrective

Action (RBCA)

GROUNDWATER

BURRIED WATER LINEBURRIED WATER LINE

DRINKING WATER

POTENTIAL HUMAN POTENTIAL HUMAN RECEPTORSRECEPTORS

VAPORS

GROUNDWATER IS AN GROUNDWATER IS AN ENVIRONMENTAL RECEPTORENVIRONMENTAL RECEPTOR

GROUNDWATER

DISSOLVED CONTAMINATION

BELOW DEQ-7STANDARDS

MONTANA RBCA ADDRESSES

GROUNDWATER AS A RECEPTOR

Long-Term Monitoring

A method of managing release sites remaining above cleanup standards for a long period of time.

LTM generally follows MNA protocols

Long-Term Monitoring

A site-specific monitoring schedule selected by DEQ where groundwater is sampled on a frequency of 1 to 3 years, that ensures the plume is stable and shrinking and there is minimal risk to potential receptors.

Long-Term Monitoring

• Typically implemented following active cleanup

• Dissolved groundwater plume fully defined

• Low risk to all Current and Potential Receptors

• Establish Attenuation Curve (shrinking plume)

• Reduce Monitoring (least expensive option)

GROUNDWATER

CLEAN FILL

MONITORING MONITORING WELLSWELLS

Long-Term Monitoring 4414 Total Release Sites 2708 Release Sites Closed1706 Releases Sites Open

134 Release Sites Still in LTM

89 Annual Schedule24 Every 2 years3 Every 3 years18 Other Schedule

14 Monitoring 1 well14 Monitoring 2 wells

CONCLUSIONS• Source removal is critical.

• Source area soils cannot always be removed.

• Closure requires DEQ-7 Standards.

• Long-term monitoring will likely be required at sites with tight soil, even after source removal.

• Reduced monitoring protects human health and the public while limiting ongoing costs.

Questions?