Risk-Based remediation addresses the highest level of contamination from a leaking underground storage tank, with levels reported above 10,000 PPM. According to the DEQ, a risk-based clean-up offers the greatest amount of flexibility and adaptability to site-specific conditions. However, it is also the option that might require collecting additional site data and evaluation of more alternatives than a soil matrix or generic remedy clean-up. Risk-based clean-ups are the most rigorous option, with the collection of several soil samples to determine the magnitude and depth of contaminated soil, as well as how far the contamination has spread. The most important aspect of a risk-based clean-up is the evaluation of current and reasonably likely future risks to human health and the environment.
A risk-based remediation involves a soil investigation, which examines the pathways the contamination has likely spread. This pathway evaluation determines where the contaminants were released, how the contaminants can be transported to new locations and the reasonably likely ways that people may come into contact with them. Tests need to be conducted on enough samples to determine the lateral extent, as well as the depth of the petroleum contamination.
There are three investigative phases of concern while conducting a risk-based clean-up, phase two and three may not be necessary if the area of contaminated soil is limited enough in extent. Below are the steps, in order, that are required by the DEQ in order to close a risk-based clean-up:
Risk-Based Clean-Up and Free Product, Groundwater and Soil Vapor Gas
Petroleum Contaminated Soil (PCS) and/ or Free Product:
Up to seven (7) soil samples are taken to establish a representative sample and delineate the vertical and lateral extent of the pocket of petroleum-impacted soil, and potential or actual groundwater impacts. If it is determined that there is free product or large volumes of heavily contaminated soil, from which product may continue to migrate, mitigating the risk by excavation and off-site removal would be required, to the maximum extent practicable.
Has groundwater at the site at risk or has it been impacted by the diesel contamination?
In order to determine if groundwater has been or could be potentially encountered, a 10’ separation is required by the DEQ between the vertical extent of the contaminated soil and any potential shallow groundwater.
If there is no impact or potential impact to groundwater?
The groundwater investigation is now completed, no further action would be required.
If groundwater has been or has the potential to be impacted, what are the next steps?
A representative well will be installed in the source area to determine if groundwater has been impacted by the release. The installation of the representative well is designed to obtain a groundwater sample to measure, if any, the water level elevation in the contaminated water or soil.
If the samples in the representative well have detections that exceed the DEQs Ingestion and Inhalation from Tap Water Risk-Based Clean-Up levels, a groundwater investigation is necessary. The groundwater investigation determines the extent of the contaminated groundwater, and whether it is limited only to the property of the leaking UST or if extends to neighboring properties.
If the samples in the representative well meet the DEQ Ingestion and Inhalation from Tap Water Risk Based Clean-Up levels, no further action would be required.
The 10’ separation boring cannot be obtained due to site conditions, what happens to the investigation?
If a 10’ separation boring cannot be completed due to examples like refusal (rocky soil, etc.) or if the tank is too deep, per the DEQ a Beneficial Well Survey would need to be conducted.
A Beneficial Well Survey determines if a release of hazardous substances has impacted or has the potential to impact groundwater or surface water, through contaminant migration. These determinations will be used for evaluating exposure pathways in human health and ecological risk assessment; for identifying hot spots of contamination; and for selection or approval of remedial actions at hazardous substance clean-up sites.
The DEQ requires the survey to identify all properties that are in proximity to the site with the leaking underground storage tank, which may be utilizing a water supply well. The well may be used for drinking water, irrigation, etc. DEQ regulations considers that any well on a property that is for domestic use, be accounted for. These surveys can and are conducted within and outside city limits.
Vapor Intrusion from soil or groundwater:
Vapor intrusion is the migration of Volatile Organic Compounds (VOCs) from the subsurface into buildings. Per the DEQ, vapors released from volatile substances can be slowly released from underground spills. These volatile constituents move upward through the pores in the soil, when the soil is exposed during excavation, as well as, from contaminated groundwater. The DEQ requires Soil Vapor Gas sampling (SVGs) to assess the potential of carcinogenic vapors entering residential buildings. When certain constituents of interest, such as benzene, ethylbenzene and naphthalene concentrations exceed the DEQ Vapor Intrusion into Residential Buildings Standards, soil vapor gas sampling would be required.
To assess the potential cancer risks from vapor intrusion into homes, soil vapor gas sampling is triggered if any of the following variables apply:
- The plume of contamination exceeds 65 cubic yards
- There are, or likely to be, buildings within 30 feet of Volatile Organic Compounds (VOCs)
- A 10’ separation cannot be established between the vertical extent and any potential shallow groundwater
Due to site specific factors, the number and sample locations may vary. Sampling may be conducted within the home or on the exterior of the building, either through the sub-slab and/or the soil.
For a more in-depth overview of the DEQs requirements and standards regarding Risk-Based Clean-Up and leaking tanks, please visit their Leaking Underground Storage Tank Program (LUST site) DEQ LUST Site.