Monitored Natural Attenuation and Risk-Based Corrective Action at Underground Storage Tanks Sites

1. Monitored Natural AttenuationandRisk-Based Corrective Actionat Underground Storage Tanks Sites Mike Trombetta Department of Environmental Quality Environmental Quality Counsel Petroleum Tank Release Fund Subcommittee Meeting June 4, 2008

2. Agenda • Science of a Petroleum Cleanup • Monitored Natural Attenuation • Science Behind MNA • Implementation Issues • Risk - Based Corrective Action • Long-Term Monitoring

3. Science Behind Petroleum Release Cleanup

4. FLOATING FREE PRODUCT DISSOLVED CONTAMINATION PRODUCT SMEAR ZONE EXPANDING PLUME GROUNDWATER

5. CONCENTRATIONS EXCEED DEQ-7 STANDARDS mg/kg (parts per million) CONCENTRATIONS BELOW DEQ-7 STANDARDS 1,000 500 200 50 5 µg/liter (parts per billion) 0 GROUNDWATER

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

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

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

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

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

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

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

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

14. 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. Natural Attenuation

15. 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.

16. CLEAN FILL MONITORING WELLS GROUNDWATER

17. MNA Implementation Issues

18. U.S. Environmental Projection Agency Office of Solid Waste and Emergency Response Guidance 9200.4-17P, April 21, 1999 Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites

19. Implementation Issues • Not a Presumptive Remedy • Thorough Site Characterization • Source Control • Remove Free Product • Performance Monitoring • Contingency Remedies • Reasonable Timeframe Compared to other methods • Institutional Controls

20. Other Issues • Vapor Intrusion • Utility Impacts • Risks to Construction Workers

21. CONSTRUCTION WORKER VAPOR INTRUSION VAPORS EMINATING FROM DISSOLVED AND FREE PRODUCT • BURIED WATER LINE • PERMEATION OF PIPE • MIGRATION ALONG TRENCH

22. 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.

23. NATURAL ATTENUATION PROCESSES EXIST AT ALL SITES Low – Permeability Clay Soil Low – Permeability Clay Soil Significant Source Removal Reduced cleanup time down to 10 years … in tight clay-type soils. Minimal Source Removal Very long cleanup time up to 100 years or more… in tight clay-type soils. MNA can be used as a stand-alone cleanup strategy, but is typically combined with other technologies (source removal).

24. MNAQuestions? (RISK - BASED CORRECTIVE ACTION DISCUSSION -- NEXT)

25. Risk-Based Corrective Action (RBCA)

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

27. DISSOLVED CONTAMINATION BELOW DEQ-7 STANDARDS MONTANA RBCA ADDRESSES GROUNDWATER AS A RECEPTOR GROUNDWATER

28. Long-Term Monitoring A method of managing release sites remaining above cleanup standards for a long period of time. LTM generally follows MNA protocols

29. 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.

30. 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)

31. CLEAN FILL MONITORING WELLS GROUNDWATER

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

33. 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.

34. Questions?