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Introduction To The Physical Site Assessment Process




When an underground storage tank (UST) system fails and causes contamination, it must be removed from service and a comprehensive assessment of the system components must be completed. Typically, certification by a third-party inspector is required to bring the repaired component back into service. The assessment is a fact-finding mission focused on answering the following questions:

What was the exact source of the release?
When did the release occur, and how much was released?
Where did the contamination migrate?
What is the horizontal and vertical extent of that migration?
Will the fuel leak potentially harm public health, safety, or the environment?

This section of the compendium provides an introductory overview to the site assessment process as well as potential technical and reporting requirements.

Specific steps undertaken during the site assessment and characterization process are not rigidly defined. As discussed in earlier sections of this compendium, requirements will vary by implementing agency.

One of the first steps in the process is reviewing existing records and historical site information to create a baseline of information on the problem. Typically the owner would have this documentation on-site or be able to access it from his monitoring system. A conceptual site model (CSM) (2 pp, 62K, About PDF) Exit EPA disclaimer is also developed at this stage. The CSM describes the geological and physical setting of the release, possible migration pathways, and the potential threat to public health and the environment. Once the historic and current UST system information is understood and the CSM is established, the process of identifying the exact source, quantity, and timing of the release can be more productive. Existing inventory records or data from electronic inventory systems can be used to estimate the source of the piping, tank, or system failure and the quantity of fuel that may have been released. This information can then be used to productively guide the remedial design and help evaluate the effectiveness of the cleanup.

Under both the traditional and expedited assessment approaches, a sampling and analysis plan must be developed and samples collected as described below. Site assessment is an iterative process: as information is gathered about the site history and conditions, the CSM is modified and, in turn, the remedial action approach revised. Sample collection is also an ongoing process, and the analytical results from sampling help to inform the assessment and cleanup.

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Sampling And Data Collection

During site sampling, care must be taken to ensure that samples are representative of site contamination conditions and are handled properly so that cross-contamination does not occur or integrity of the samples is not compromised. EPA employs a Data Quality Objectives (121 pp, 1.6M, About PDF) process to identify specific requirements for each sampling event and to guide project managers in designing a sampling program. A good sampling program will record detailed site information at the time of collection, such as soil types at various depths, groundwater observations, weather conditions, equipment used, photographs, and field personnel qualifications. These notes can be important in making key decisions related to cleanup of the site.

Samples are typically sent off-site to laboratories for analysis. To more quickly identify conditions in the field, portable analytical equipment may be brought on-site during field sampling. This equipment can be used to achieve a more real-time understanding and reduce the need for iterative laboratory sampling. A specified percentage of the collected samples are then submitted to an analytical laboratory to confirm and correlate the results of the field instrument screening. Complete on-site labs can be expensive, and often site sampling and assessment becomes an iterative process whereby samples are collected and sent to an off-site lab to be analyzed with results fed into an interim assessment report. This process must be repeated until the extent of the contamination is fully characterized. Fully assessing a site may take months to complete.

Prior to collecting samples or undertaking any subsurface work, contact utility companies to identify the location of any underground utilities. Likewise, field personnel must be careful not to operate large excavation equipment where there might be interference with overhead utility lines. Furthermore, if contamination extends beyond the source property boundaries, site access agreements need to be executed to make provisions for proper access onto adjacent properties.

New sampling technologies are available to help collect samples cost effectively and to provide better protection of samples prior to analysis. For example, some new technologies allow for soil sampling in a way that minimizes the loss of any potentially volatile chemicals prior to lab analysis. Other methods or preservation techniques maintain the integrity of the sample during transit between the site and the lab.

Finally, it is important that all sampling equipment be properly decontaminated between individual sampling points. Equipment decontamination is a critical practice to ensure the integrity of each sample by preventing cross-contamination.


Determining the nature and extent of a petroleum release generally begins with characterizing soil and rock permeability and conducting soil sampling, and this is typically an iterative process. Soil samples are collected to establish the full horizontal and vertical extent of the release in the soil. Samples are often screened for petroleum hydrocarbons in the field using a portable photo-ionization detector (PID), flame-ionization detector (FID), or an ultraviolet fluorescence (UVF) instrument to quickly establish where contamination is present. Continuous sampling of soil cores allows rapid visual observations of soil staining from releases, and technologies such as UVF screening can quickly identify the exact vertical extent of a release in the soil column. Using continuous screening of soils in this way, from the ground surface to the bottom of the borehole, allows a precise understanding of the vertical extent of contamination at each boring. Identifying soil strata is critical in development of the CSM and in design of the remediation strategy. Each implementing agency will have its own acceptable field screening methods and requirements for analytical testing.


Commonly a fuel release will be present as dissolved contaminants in the groundwater. Monitoring wells are usually constructed to establish the horizontal and vertical impact to the groundwater resource. Groundwater nearly always flows in a specific direction, although the direction can change during various times of the year or be artificially impacted by underground utilities or man-made changes. Monitoring wells are typically established around a release to understand the extent of contamination, together with background up-gradient and down-gradient conditions that existed before the release. The number and location of groundwater monitoring wells are used to characterize the nature and extent of contamination. A minimum of three monitoring wells is necessary to establish the direction of groundwater flow. For sites with complex geologic conditions, man-made disturbances, or underground utilities within the groundwater, more wells will be necessary to fully understand groundwater flows.

Gauging groundwater depths periodically is important in understanding how a fuel release behaves underground throughout the year. Seasonal fluctuations in groundwater levels may impact dissolved contaminants in the groundwater. To accurately characterize groundwater contamination at a site, groundwater sampling should be conducted during different seasons to account for potential variations in dissolved contaminant levels.

When a large quantity of product has been released, pure gasoline or fuel is often found floating on the groundwater surface. This free-floating petroleum is referred to as Light Non-Aqueous Phase Liquid (LNAPL) Exit EPA disclaimer. The thickness of the LNAPL layer must be accurately gauged with an oil/water interface probe. Thickness of the LNAPL will vary considerably as the groundwater table rises and falls and the product either saturates or evacuates from the surrounding soil particles. To fully characterize LNAPL thicknesses, it is best to collect measurements at various times of the year to account for seasonal groundwater fluctuations in LNAPL depths. Before any remedial action is conducted to remove the LNAPL Exit EPA disclaimer, it is important to quantify the mass or volume of the product, evaluate the LNAPL mobility, consider any vapors that may partition from the product, and evaluate the exposure pathways of the LNAPL. These are unique considerations regarding cleanup of sites impacted with LNAPL. Federal regulations require that any free product identified during the LNAPL assessment be removed from the ground and properly managed.

Vapor Intrusion And Indoor Air

Petroleum contamination can partition from soil or groundwater and migrate into indoor air spaces where it can cause a health hazard to humans. The impact to indoor air may be affected by site conditions that increase the potential for vapor intrusion into buildings, such as direct contact between a contaminant source (groundwater or LNAPL) and a building foundation. Soil vapor can be sampled outside or underneath building slabs before it reaches indoor pathways. Alternatively, indoor air can be sampled directly for volatile organic compounds (VOCs). Care must be exercised in sampling indoor air spaces because many household and industrial products, such as paints, new carpet, or household cleaners, can lead to false positive readings.

More On Sampling And Data Collection [Show/Hide]

Monitoring, Sampling, and Analysis
EPA Web page that provides resources on groundwater monitoring and sampling, soil sampling, analytical methods, general sampling strategies, and air emissions.

Non-Aqueous Phase Liquid (NAPL) Cleanup Alliance Exit EPA disclaimer
Remediation Technologies Development Forum's Decision-Making Framework for Cleanup of Sites Impacted with Light Non-Aqueous Phase Liquids (LNAPL).

LNAPL Documents Exit EPA disclaimer
Interstate Technology & Regulatory Council (ITRC) guidance documents addressing natural source zone depletion (NSZD) for LNAPLs, the processes involved, their rates, and long-term progress.

Vapor Intrusion
EPA Office of Solid Waste and Emergency Response (OSWER) Web site that provides topical information on vapor intrusion.

EPA CLU-IN resources regarding field investigation of vapor intrusion, mitigation approaches, and current relevant initiatives. Exit EPA disclaimer

ITCR Vapor Intrusion Documents Exit EPA disclaimer
Interstate Technology & Regulatory Council (ITRC) guidance documents addressing investigating and evaluating the vapor intrusion pathway, data evaluation, and mitigation.

EnviroGroup Limited, Vapor Intrusion Solutions Exit EPA disclaimer
EnviroGroup Web site that provides information on vapor intrusion in North America.

Innovative Technologies - Characterization and Monitoring
EPA Web site on characterization and monitoring that contains links to information on technology tools, statistics and sampling design, sample collection and handling, environmental data quality, and other topics.

Using Field Analytical Methods
EPA site providing links to resources, articles, and other documents regarding site analysis.

Triad Implementation Guide (63 pp, 418K, About PDF) Exit EPA disclaimer
Interstate Technology & Regulatory Council (ITRC) guide on implementing the EPA Triad process and the challenges of and solutions to anticipated issues.

Triad: Overview And Key Concepts Exit EPA disclaimer
An overview from the Triad Resource Center that explains the Triad's three key components: systematic planning, dynamic work strategies, and real-time measurement technologies.

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

Fact Sheet: Resources for Strategic Site Investigation and Monitoring (4 pp, 309K, About PDF) Exit EPA disclaimer
EPA fact sheet that lists education, training, and guidance resources about strategic investigation and monitoring activities at hazardous waste sites.

Guidance on Systematic Planning Using the Data Quality Objectives Process - EPA QA/G-4 (121 pp, 6.1M, About PDF)
EPA guidance on the Data Quality Objectives (DQO) Process, the Agency's recommended planning process when environmental data are used to select between two alternatives or derive an estimate of contamination.

Field Sampling and Analysis Technologies Matrix Version 1.0 Exit EPA disclaimer
A Naval Facilities Engineering Command (NAVFAC) and EPA resource to assist remedial project managers in the process of evaluating alternative technologies and to enhance technology transfer among Federal agencies.

Expedited Site Assessment Tools For Underground Storage Tank Sites: A Guide For Regulators
EPA guidance on the evaluation of conventional and new site assessment technologies and the use of expedited site assessment.

ASTM Guide E1912 - 98(2004): Standard Guide for Accelerated Site Characterization for Confirmed or Suspected Petroleum ReleasesExit EPA disclaimer
ASTM guidance on how to quickly and accurately characterize a suspected or confirmed petroleum release site.

ASTM D5730 - 04 Standard Guide for Site Characterization for Environmental Purposes With Emphasis on Soil, Rock, the Vadose Zone and Ground Water Exit EPA disclaimer
ASTM guide covering a general approach to planning field investigations with a primary focus on the subsurface and on field methods for determining site characteristics and collection of samples for further physical and chemical characterization.

ASTM D6311 - 98(2009) Standard Guide for Generation of Environmental Data Related to Waste Management Activities: Selection and Optimization of Sampling Design Exit EPA disclaimer
ASTM guide to provide practical assistance in the development of an optimized sampling design.

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Quality Assurance And Quality Control

Quality assurance (QA) addresses process management and refers to the establishment of a system to plan, carry out, and review data collection activities. Quality control (QC) focuses on operational aspects and refers to the process of taking appropriate scientific precautions, such as proper calibrations and duplications, to ensure data quality of the collected samples. During site assessments, QC samples are collected in the field to check whether the site characterization samples are representative of site conditions. The samples are used to evaluate the quality and usability of data collected from soil, groundwater, and air for accuracy, representativeness, comparability, and completeness. Government agencies may require that a QA project plan be approved before sampling or that a data usability assessment be conducted after samples are analyzed at a laboratory.

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EPA's Quality System for Environmental Data and Technology
EPA page describing its system to manage the quality of its environmental data collection, generation, and use.

Guidance for Quality Assurance Project Plans - EPA QA/G-5 (111 pp, 401K,About PDF)
EPA guidance for those involved in developing Quality Assurance (QA) Project Plans that address the specifications listed in EPA Requirements for QA Project Plans (QA/R-5).

Quality Assurance Plan Exit EPA disclaimer
Oregon Department of Environmental Quality quality assurance plan for preliminary site assessment.

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Personal Protection And Safety

Fuel releases from LUSTs can present dangerous conditions to site workers and occupants of nearby buildings. Examples of these conditions include elevated concentrations of VOCs creating low levels of oxygen in confined spaces (i.e. suffocating conditions) or potentially explosive concentrations of vapors. Workers and other individuals who may be exposed to petroleum-contaminated soil or groundwater need to avoid direct contact with the material. Ingestion and inhalation exposures can be dangerous and hazardous to health.

In addition, excavation or trenching is sometimes conducted in association with a remedial action, and may pose a risk of collapse. Personnel involved in underground storage tank removals, site assessments, and remedial actions must complete Hazardous Waste Operations and Emergency Response Standard (HAZWOPER) training certified by the Occupational Safety and Health Administration (OSHA) of the U.S. Department of Labor. Local implementing agencies may require additional certifications for these individuals. OSHA has recently adopted stricter guidelines for trenching and excavation in response to a number of accidental deaths caused by excavation collapses during UST removal and installation projects. Worker health and safety should remain a continuing priority during all assessment and cleanup actions.

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Hazardous Waste Operations and Emergency Response Exit EPA disclaimer
OSHA regulations pertaining to personal protective equipment related to hazardous materials at 29 CFR Part 1910.120 et seq.

HAZWOPER Frequently Asked Questions Exit EPA disclaimer
OSHA site that lists frequently asked questions and answers about HAZWOPER training.

OSHA HAZWOPER Safety Training Online Exit EPA disclaimer
Online training provider that offers OSHA HAZWOPER safety training.

Excavations: Hazard Recognition in Trenching and Shoring Exit EPA disclaimer
OSHA Technical Manual on excavations to assist in the recognition of trenching and shoring hazards and their prevention.

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Documentation And Reporting

Field observations and results from all site assessment work should be documented in a written site assessment report. Report requirements can vary among implementing agencies; the relevant authority should be contacted for specific report requirements and guidance. In general, a site assessment report will document the environmental investigation work performed at the site and an evaluation of the findings. It may include maps and cross-sections that show the geologic and hydrogeologic conditions and the distribution of contaminants as well as recommendations for future assessment and/or mitigation of the contamination. Assessment actions generally need to be documented and filed with the appropriate government agency. Some agencies require that the scope of work of a proposed assessment action be reviewed and approved prior to implementation.

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Minimum Data Documentation Requirements for LUST Sites in Indiana Exit EPA disclaimer
Indiana Department of Environmental Management site listing the minimum data documentation requirements for LUST sites in Indiana.

Documentation Of Field Activities (8 pp, 29K, About PDF) Exit EPA disclaimer
Maine Department of Environmental Protection guidance for documenting the investigation and remediation of hazardous substance, petroleum, and landfill sites throughout Maine.

Leaking Underground Storage Tank (LUST) Subsurface Investigation Report Guide (14 pp, 216K, About PDF) Exit EPA disclaimer
Utah Department of Environmental Quality guide to conducting a subsurface investigation that defines the extent and degree of contamination and is required in order to establish site-specific cleanup levels.

EPA site on the Field Operations and Records Management System (FORMS II Lite), a resource to assist sampling personnel with generating their sample documentation and tracking samples.

ASTM E1527 - 05 Standard Practice for Environmental Site Assessments: Phase I Environmental Site Assessment Process Exit EPA disclaimer
ASTM guide that defines good commercial and customary practice for conducting an environmental site assessment (ESA) of a parcel of commercial real estate with respect to the range of contaminants within the scope of CERCLA and petroleum products.

ASTM E1903 - 97(2002) - Standard Guide for Environmental Site Assessments: Phase II Environmental Site Assessment Process Exit EPA disclaimer
ASTM guide to employing good commercial and customary practices in conducting a Phase II environmental site assessment (ESA) of a parcel of commercial property with respect to the potential presence of a range of contaminants that are within the scope of CERCLA as well as petroleum products.

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