Jump to main content.

Characterization Of The Source And Site




The main objectives of a site assessment are to determine the extent of the contamination, predict where it might spread, identify potential receptors, and develop a conceptual site model (CSM) Exit EPA disclaimer – that is, create a picture of what is happening underground and in the surrounding area. Understanding the source of the contamination and its characteristics is a part of this process. Site assessment involves determining where underground storage tanks (USTs) are (or were) located, where the transmission piping is, what contaminants are present, and where the contaminant may have moved (for example, into groundwater or indoor air spaces). It is also important to locate subsurface structures and utilities during the site assessment as these can act as "preferential pathways" that rapidly change the extent of a release.

Although the specific steps of a site assessment are based on fact-finding and science, each state may require unique procedures for the site assessment process.  For example, some states may require that a proposed scope of work be approved prior to beginning the assessment work or that risk-based decision-making is used to establish the scope of the assessment. The site professional will need to refer to state guidance for appropriate procedures.

A site assessment typically begins by determining the types of chemicals present and their concentrations, the lateral area and vertical depth of contamination, nearby receptors identified in the CSM, and site conditions such as groundwater flow and geology. The site professional creates the CSM and is likely to modify it during and after the site assessment as new information is gathered.

Top of page

Characterizing The Source Contaminants

The initial identification of the contaminants that are released from a leaking underground storage tank (LUST) requires an understanding of the specific properties of the fuel involved. Different types of fuels and additives present different problems at a site. For example, older gasoline releases contained lead, while newer releases contain oxygenates that promote clean air (such as methyl tertiary butyl ether, or MTBE). Recent federal mandates to add ethanol and other biofuels to gasoline and diesel fuel may require modified or additional investigation.

Different contaminants of concern have different chemical and physical properties and toxicological characteristics, causing them to behave differently underground and to present a variety of risks to human health and the environment. It is important to identify the applicable contaminants present to develop an accurate idea of how to remediate the site effectively.

More On Characteristics Of Contaminants [Show/Hide]

Contaminants Of Concern And Cleanup Levels
EPA summary page on containments of concern, including fuel oxygenates and methyl tertiary butyl ether (MTBE), and cleanup levels and regulatory standards.

Light Non-Aqueous Phase Liquid (LNAPL) Resource Center Exit EPA disclaimer
Information portal compiled by the American Petroleum Institute (API) on LNAPLs, including answers to frequently asked questions, manuals, software, and technical reports.

Light Non-Aqueous Phase Liquids (LNAPLs) Exit EPA disclaimer
U.S. Geological Survey definition of LNAPLs, including a cross-sectional diagram of a LNAPL spill.

MTBE And Underground Storage Tanks
EPA summary page about MTBE (methyl tertiary butyl ether), including its chemical properties and characteristics, use in gasoline, presence in USTs and the environment, and EPA efforts underway to reduce the health and environmental impacts of MBTE.

Fuel Oxygenates: Characteristics And Chemical Properties
Links compiled by EPA about the properties and characteristics of oxygenates.

Oxygenates Exit EPA disclaimer
Information portal compiled by the American Petroleum Institute (API) on fuel oxygenates in soil and water, including technical protocols, conference reports, and publications of ongoing research in the field.

Biofuels Compendium
EPA Web-based compendium that provides UST stakeholders with information regarding storing ethanol and biodiesel fuels.

Biofuels Compendium – Ethanol – Fate And Transport
Links compiled by EPA on the fate and transport of ethanol-blended fuels.

Biofuels Compendium – Biodiesel – Fate And Transport
Links compiled by EPA on the fate and transport of biodiesel-blended fuels.

Investigation Requirements for Ethanol-Blended Fuel Releases (9 pp, 138K, About PDF) Exit EPA disclaimer
Minnesota Pollution Control Agency guidance document which describes site investigation requirements for ethanol-blended fuel releases.

Lead Scavengers Compendium: Overview Of Properties, Occurrence, And Remedial Technologies
Compendium prepared by EPA on the state of the knowledge on additives to lead gasoline (known as lead scavengers) at LUST sites, including their occurrence in drinking water, environmental fate and transport, and case studies.

Leaking Underground Storage Tank Research
Research information from EPA related to LUST contaminants of concern and models for use at LUST sites.

Top of page


Characterizing The Site

Site conditions play a critical role in the fate of a release and how contamination can best be mitigated. The known characteristics of a site, such as geology and hydrology, are used to create a conceptual site model (CSM), Exit EPA disclaimer which is utilized to guide the collection of data and determine the type and amount of required cleanup. The location of the leak source and its extent, both horizontally and vertically, must be understood. Site conditions could affect response actions, so information is needed on the site’s proximity to receptors, such as drinking water supplies, sensitive wetlands or surface waters, schools, day-care facilities, hospitals, and residences that may complete exposure pathways.

Potential concerns may include, but are not limited to, threatened water supply sources, impaired indoor air, also known as vapor intrusion, that can be an elevated threat to children and pregnant women, and exposure for construction workers and other potential sources of public exposures. In addition, storm drains and underground utilities can create “preferential pathways” that can alter and exacerbate the migration of pollutants.  Site assessment activities may include removing USTs and piping to collect soil or groundwater samples. As specific site information is gathered, the data is used to fine-tune the CSM.

More On Characterizing The Site [Show/Hide]

Conceptual Site Model Checklist (2 pp, 62K, About PDF) Exit EPA disclaimer
Checklist provided by Triad of requirements for completing a conceptual site model.

EPA Region 8 Site Conceptual Model
Examples from EPA of conceptual site models with a focus on human health risk assessment.

ASTM E2531-06e1 Standard Guide For Development Of Conceptual Site Models And Remediation Strategies For Light Nonaqueous-Phase Liquids Released To The Subsurface Exit EPA disclaimer
Industry standard on LNAPL occurrence and behavior in the subsurface, identifying risk-based drivers and non-risk factors for action at a site and prioritizing resources based on those action drivers.

Site Characterization For Subsurface Remediation (268 pp, 4.5M, About PDF)
Seminar publication from EPA on site characterization for subsurface remediation, focusing on concepts, processes, characterization, and data analyses.

Expedited Site Assessment Tools For Underground Storage Tank Sites: A Guide For Regulators
EPA guidance document on the expedited site assessment process, a framework for rapidly characterizing UST site conditions for corrective action decisions (also known as accelerated site characterization, rapid site characterization, and expedited site investigation.)

API – Vapor Intrusion Exit EPA disclaimer
Information portal from the American Petroleum Institute (API) on vapor intrusion, the migration of vapors from subsurface contaminant sources to buildings, including training, modeling, publications, and presentations.

Public Health Assessment Guidance Manual – Chapter 6: Exposure Evaluation: Evaluating Exposure Pathways Exit EPA disclaimer
Guidance manual from the Agency for Toxic Substances and Disease Registry (ATSDR) describing how to clearly define and explain exposure pathways, including the five elements of an exposure pathway that determine the extent to which an exposure may have occurred.

State Programs
Links to State and Territorial UST/LUST program contacts.

Risk-Based Decision-Making And Underground Storage Tanks
A summary of the risk-based decision-making (RBDM) process developed by EPA that includes links to national RBDM implementation strategies and implementation support.

Results-Based Approaches To Corrective Action: September 28, 2000
EPA page linking to the draft Results-Based Approaches to Corrective Action Guidance, which is intended to help state regulators and UST owner/operators to understand and incorporate results-based approaches in cleanups.

Introduction To Risk-Based Corrective Action (RBCA) Exit EPA disclaimer
Michigan Department of Natural Resources and Environment definition of RBCA and an explanation of the tiered approach to RBCA as described under ASTM E1739.

ASTM E1739-95(2010)e1 Standard Guide For Risk-Based Corrective Action Applied At Petroleum Release Sites Exit EPA disclaimer
Industry standard on risk-based corrective action (RBCA), a consistent decision-making process for the assessment and response to a petroleum release based on the protection of human health and the environment.

Top of page


Local Navigation

Jump to main content.