Remaining gasoline from a tanker rollover is transferred
to another tanker. Wallagrass, Maine. May 2011
Wallagrass Gasoline Spill
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On May 31, 2011 a tanker truck hauling 8,200 gallons of gasoline rolled over in the northbound lane of Route 11 in Wallagrass, Maine. In total, an estimated 5,000 gallons of gasoline was discharged into the soil and fractured bedrock below.
DEP is regularly monitoring possible contamination of nearby wells and as a precaution, has proactively placed temporary filters on several area homes near the spill site. The Department is also overseeing work of contractors performing cleanup and remediation activities.
December 4, 2012
Ground water was sampled from monitoring wells across the site in October, and the data support the conclusion that the trend of gasoline contaminant concentrations continues to be strongly downward:
- During the most recent sampling, only five (5) wells in the immediate spill zone had concentrations of contaminants that exceeded Maine’s stringent drinking water standards.
- Three monitoring wells that were contaminated above drinking water standards in July or August 2012 were below the drinking water standards as of October 2012.
- Since the spill, contamination has been found in 13 of the monitoring wells that were drilled at the site. Since 2011, the contamination levels in these 13 monitoring wells has decreased, on average, 97%.
Since early August 2012, the north vapor recovery system has been active and the south system has been active since mid-September. During this time over 75 gallons of volatile compounds were removed from the site by these systems. The operation of the vapor extraction systems also enhances the subsurface conditions needed for natural breakdown of contaminants by microorganisms.
As the amount of gasoline in the subsurface has decreased, there is less vapor for the systems to extract. Given the decrease in contamination at the site over time, it follows that the volume of gasoline vapor that can be extracted will also decline. After reviewing the positive ground water trends across the site, the decreased performance of the vapor extraction systems and the cost of operating the systems relative to their performance, the DEP has authorized the removal of the systems.
Be assured that site conditions will continue to be monitored closely. We can and will install back-up treatment systems in the event that site conditions change, and the piping and electrical hook-ups will remain at the site in the event that active treatment is needed in the future.
Gasoline in the ground exists in several phases and can leave the site by many pathways:
There is liquid gasoline, the traditional oily liquid that you put in your car. Early on, Stantec recovered over 800 gallons of liquid gasoline. We do not anticipate further recovery of liquid gasoline from the site.
There are components of gasoline that dissolve in water. For example, approximately 10% of the gasoline that spilled (i.e., 500 gallons) was composed of ethanol. Ethanol is regular drinking alcohol. Ethanol dissolves in water and also evaporates very quickly. The ethanol component of gasoline in the soil rapidly evaporates into the air and is consumed by soil bacteria. If any ethanol remains, the new soil vapor extraction system should remove it rapidly. Other components of gasoline are poorly-soluble in water, but will dissolve into water to some degree. These compounds include many of the more hazardous components of the gasoline, such as benzene. The new upgraded water treatment systems treat a very large quantity of ground water from the spill site in order to remove these dissolved-phase contaminants. Treating ground water to remove dissolved gasoline is not an efficient method of removing gasoline from the site but it protects the neighborhood from ground water contamination that might otherwise migrate from the spill zone into residential wells.
There is vapor-phase gasoline in the air-filled pores and fractures in soil and rock at the spill site. This is a gasoline as a vapor or gas.
The current remediation systems vacuum gasoline vapors out of the soil (“soil vapor extraction”) and effectively remove gasoline from the ground. As the existing vapor-phase gasoline is vacuumed from the pores and fractures in the soil and rock, the liquid-phase gasoline that is trapped and immobile in the soil (and thus not pump-able as a liquid) and dissolved-phase gasoline will evaporate and will provide even more gasoline vapor for the new soil vapor extraction system to recover.
There is also evidence that the soil vapor extraction system is providing microorganisms in the soil with conditions that allow these microbes to digest gasoline.
- Is my water safe to drink?
- Private drinking water supply wells have been regularly sampled for contamination since the spill. As the Department gains more understanding of the concentration and distribution of contamination over time, it may change the sampling frequency and/or make decisions regarding which drinking water supplies are provided temporary treatment filters.
If water sampling results indicate that contaminant concentrations remain below health and safety guidelines and applicable drinking water standards for a sufficient amount of time to make a determination (typically a year), and evidence supports the conclusion that contamination is not expected to migrate to the well, the temporary treatment filters will be removed. Water quality sampling of untreated wells may continue as part of the overall site monitoring plan. The Department will not make the decision to remove temporary treatment filters lightly and will not put your health at risk. If the Department concludes that temporary treatment filters are no longer needed at your home, you should be confident that your drinking water supply is safe for you and your family. Even after the Department decides that filters are no longer needed, homeowners have the option of taking over the ownership, operation, and maintenance of the filters if they wish to continue using them.
If concentrations of contaminants remain above drinking water standards, the Department will seek to either replace your water supply with a new water supply that is not affected by the spill or install a long-term treatment system.
- How long will the cleanup take?
- The Department is unable to say with certainty how long it will take to completely clean up the site. Residents should understand that there will not be a recovery of all gasoline spilled as a substantial portion will volatilize into the air, dissolve into water, stick to rock and soil and/or be digested by microorganisms. The Department wants Wallagrass residents to be confident and comfortable in the fact that we will remain focused on ensuring the protection of their health and collaboratively with Stantec, other contractors and the responsible party, bringing this remedial action to a close in a way that is safest and most effective for the community and for the environment. We want the community to know we are working to ensure their continued safety and are grateful for the care and concern of the responsible party and related contractors.
Andrew Flint, Wallagrass Site Project Manager
Paul Higgins, Geologist
Nick Archer, Northern Maine Regional Director