Monitored Natural Attenuation and Risk-Based Corrective Action at Underground Storage Tanks Sites Mike Trombetta Department of Environmental Quality Environmental Quality Counsel Petroleum Tank Release Fund Subcommittee Meeting June 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 ZONE DISSOLVED CONTAMINATION EXPANDING PLUME FLOATING FREE PRODUCT

GROUNDWATER CONCENTRATIONS BELOW DEQ-7 STANDARDS CONCENTRATIONS EXCEED DEQ-7 STANDARDS ,000 mg/kg (parts per million) µg/liter (parts per billion)

GROUNDWATER PRODUCT SMEAR ZONE DISSOLVED CONTAMINATION CLEAN FILL TANK SYSTEM REMOVED FLOATING FREE PRODUCT

GROUNDWATER PRODUCT SMEAR ZONE DISSOLVED CONTAMINATION CLEAN FILL STEADY STATE STABLE OR SHRINKING TANK SYSTEM REMOVED FLOATING FREE PRODUCT CONTAMINATED SOIL CONTINUES TO LEACH INTO GROUNDWATER

GROUNDWATER PRODUCT SMEAR ZONE DISSOLVED CONTAMINATION CLEAN FILL STEADY STATE STABLE OR SHRINKING TANK SYSTEM REMOVED Sorption Diffusion Dispersion Volatilization Bio-degradation Natural Attenuation Mechanisms

GROUNDWATER PRODUCT SMEAR ZONE DISSOLVED CONTAMINATION CLEAN FILL STEADY STATE STABLE OR SHRINKING TANK SYSTEM REMOVED FLOATING FREE PRODUCT

GROUNDWATER PRODUCT SMEAR ZONE DISSOLVED CONTAMINATION CLEAN FILL SOURCE REMOVAL (EXCAVATION) PLUME SHRINKS BACK FLOATING FREE PRODUCT

GROUNDWATER PRODUCT SMEAR ZONE DISSOLVED CONTAMINATION CLEAN FILL SOURCE REMOVAL (EXCAVATION) NEW “STEADY STATE” (SLOW SHRINK BACK) FLOATING FREE PRODUCT

GROUNDWATER PRODUCT SMEAR ZONE DISSOLVED CONTAMINATION BELOW DEQ-7 STANDARDS CLEAN FILL SOURCE REMOVAL (EXCAVATION)

GROUNDWATER PRODUCT SMEAR ZONE DISSOLVED CONTAMINATION BELOW DEQ-7 STANDARDS CLEAN FILL SOURCE REMOVAL (EXCAVATION) SITE CLOSED

Natural 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 Attenuation Monitored Natural 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 MONITORINGWELLS

MNA Implementation Issues

U.S. Environmental Projection Agency Office of Solid Waste and Emergency Response Guidance P, April 21, 1999 Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank 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 EMINATING FROM DISSOLVED AND FREE PRODUCT BURIED WATER LINE PERMEATION OF PIPE PERMEATION OF PIPE MIGRATION ALONG TRENCH MIGRATION ALONG TRENCH CONSTRUCTION WORKER VAPORINTRUSION

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 – Permeability Clay Soil Low – Permeability Clay 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).

MNA Questions? (RISK - BASED CORRECTIVE ACTION DISCUSSION -- NEXT)

Risk-Based Corrective Action (RBCA)

GROUNDWATER BURRIED WATER LINE DRINKING WATER POTENTIAL HUMAN RECEPTORS VAPORS GROUNDWATER IS AN ENVIRONMENTAL RECEPTOR

GROUNDWATER DISSOLVED CONTAMINATION BELOW DEQ-7 STANDARDS 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 MONITORINGWELLS

Long-Term Monitoring 4414 Total Release Sites 2708 Release Sites Closed 1706 Releases Sites Open 134 Release Sites Still in LTM 89Annual Schedule 24 Every 2 years 3Every 3 years 18Other Schedule 14Monitoring 1 well 14Monitoring 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?