Cleaning up a Spill from an Underground Storage Tank
Remediation Solutions for a Major Diesel Fuel Spill Below Ground
A CASE STUDY
During a project to replace two generators at a community hospital in the Appalachian Mountains, a fuel line to one of those generators suffered damage.
This damage caused a diesel fuel leak from the underground storage tank, six feet below the surface. The leak released about 3,000 gallons of diesel fuel and, instead of spreading in a wide plume, it migrated vertically through sand and even bedrock to a depth of over 50 feet.
Remediation is challenging and requires time and state approval to complete. The project is ongoing. It will take roughly three to five years to entirely remove the diesel fuel from the bedrock and groundwater, high in the Appalachians.
Project Highlights
Timeline:
Ongoing - estimated time for completion is 3-5 years
Project Scope:
Report the fuel spill to North Carolina Department of Environmental Quality (NCDEQ) because of the risk of the fuel contaminating the environment and nearby properties
Determine the extent and location of the spill
Confirm the spill didn’t further damage the environment in the Appalachian Mountains
Project Outcomes:
Located the diesel fuel spill reaching over 50 feet down into fragmented bedrock and groundwater
Gathered information to eventually clean up the spill and replace the damaged and outdated diesel fuel storage tank
Project Stats:
A 20,000-gallon fiberglass diesel tank installed 30+ years ago
A fuel spill of 3,000 gallons
Client
The client is a community hospital in the Appalachian Mountains that also has rehabilitation and long-term care facilities on-site.
Challenges and Objectives
The community hospital experienced a major diesel fuel loss during a planned construction project. The primary underground diesel tank indicated a steady fuel loss, but the hospital didn’t know where the leak started until approximately one day after the damage occurred when GHP had representation onsite.
The community hospital’s corporate entity called GHP to figure out the details of the problem and how to solve it. The immediate issues were:
Where within the subsurface did the spill originate?
How far did the spill migrate?
What are the next steps to ultimately clean this up?
The spill stopped because the fuel level in the tank became low enough that the tank’s pumps could not push the fuel any further. The damage happened at ground level, but the actual breakage of the diesel line was six feet deep and the fuel was migrating vertically into the subsurface of the Earth.
This led to an unusual challenge: a spill that went deeper than most. Many petroleum spills spread out along different pathways, but not always to great depths. For this hospital, about 3,000 gallons of fuel migrated over 50 feet below the surface.
This site is 4,100 feet above sea level, which presented another challenge. After six feet, the ground contained sandy soil – which does not hold liquids well – and then fragmented bedrock. Due to the large granular size of sand particles in predominantly sandy soils, a high degree of porosity exists that allows for the rapid vertical migration of liquids, including released fuels. This allowed the released diesel fuel to migrate to fragmented bedrock, which included the measured groundwater table.
Solution
To begin the process of investigating the migration pathways for the fuel release, GHP initially installed two monitoring wells for sampling.
Well #1 was slightly downhill from the tank.
Well #2 was near the fuel release point.
The depth made digging the wells more complicated. Installation took an entire day. After 40 feet of digging, the crew encountered fractured bedrock and changed to larger and more durable drilling equipment. The crew eventually found water and evidence of diesel fuel in the bedrock.
When lab results came back, well #2 showed significant contamination at about 50 feet. Well #1 showed lower levels of contamination that were barely over the regulatory limit, indicating the diesel fuel was migrating.
Since the initial well installation event, GHP has expanded the testing footprint with four additional monitoring wells, which so far have not indicated significant levels of contaminants. The ongoing plan is continuing to monitor these wells, in addition to private water wells within a 500-foot radius, while working towards a final remedial solution and plan of action in conjunction with NCDEQ.
When a spill like this is too deep to dig out, there are a few options for treating contamination:
Injection of a biodegradable product: This breaks down the fuel into constituents that don’t pose environmental concerns. The process involves hydrocarbon chains breaking down into carbon and hydrogen and attenuating into the environment. This method is often used depending on the timeline needed for achieving levels below regulatory limits. It is a more rapid process but also more expensive.
Soil vapor extraction: This method forces high-pressure air flow into the contaminated groundwater to speed up the process of remediation by allowing the fuel to become volatilized and passed through a carbon filtration system that exits into the atmosphere. This method often requires a longer period to achieve final remediation.
Results
To remediate the 3,000 gallons of diesel fuel 50 feet underground, GHP is proposing to the state to:
Close the existing damaged UST in-place. The mechanical contractor for the facility completed installation of two new backup power generators to replace the previous two onsite. Due to concerns with excavation near the new operating generators and based on proximity to the main existing oxygen supply line for the hospital, GHP secured approval from NCDEQ to close the UST in-place versus physical removal. This is currently approved and projected to occur in fall 2023.
Complete additional soil borings and a vertical extent well, which is a monitoring well that may extend to 100 feet below ground surface or beyond to measure vertical plume thickness. GHP secured permission from NCDEQ to complete this additional investigational work as soon as the existing UST is closed-in-place to avoid damage to underground features.
Maintain a groundwater sampling schedule for the in-place wells and private wells. GHP completed a quarterly sampling event in April and July of 2023. GHP oversaw the sampling of a supply well onsite at the facility and another off-site in a private community.
Complete the final design and successful installation of a new above-ground storage tank (AST). Construction is projected to begin July 2023 and continue into September 2023. The UST closure-in-place work is projected to follow the successful installation and change over to the new planned AST.
Use the site of the old tank to potentially provide important pathways for remediation. This will be further investigated as a possibility once the old tank is empty and no longer in use or viable.
“GHP is uniquely positioned to solve issues like these because we're so accustomed to dealing with large projects from start to finish. Beginning with planning, to surveying, to overseeing contractors, and sampling and analysis of soil and groundwater. I can look at the problem from the outside and say, ‘This is what they're facing, this is what they need to fix it, this is what I need to bring in to help me fix it.’ And then we carry out and execute a plan all the way through completion to the client’s satisfaction.”
— Bobby Kenner, CHMM, Senior Project Manager
Conclusion
Accidents like the one at the community hospital could have been avoided. Whenever you plan for construction near fuel systems or underground utilities, review plans multiple times to ensure equipment and machinery are in locations that don’t interfere with existing infrastructure. A spill like this one can be expensive and lead to an environmental emergency.