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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), which is utilized to guide the collection of data and determine the type and amount of required cleanup.
Introduction
Site characterization is the process by which site-specific information and data are gathered from a variety of sources to characterize the physical, biological, and chemical systems at a contaminated site. A conceptual site model (CSM) integrates all lines of evidence into a three- dimensional picture of site conditions that illustrates contaminant distributions, release mechanisms, migration routes, exposure pathways, and potential receptors. The CSM uses a combination of text and graphics to portray both known and hypothetical information. The CSM documents current site conditions and is supported by maps, cross-sections, and site diagrams that illustrate human and environmental exposure through contaminant release and migration to potential receptors. Frequently, a CSM may be presented as a site map and/or developed as a flow diagram which describes potential migration of contaminants to sit. The CSM synthesizes data acquired from historical research, site characterization, and remediation system operation.
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.
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), 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 associated with petroleum contamination may include, but are not limited to, threatened water supply sources, and impaired indoor air, also known as petroleum 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 are used to refine the CSM.
One of the first steps in the site characterization 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 CSM 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.
Standard Guide for Development of Conceptual Site Models and Remediation Strategies for Light Nonaqueous-Phase Liquids Released to the Subsurface
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 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, ground water 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.
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.
Soil
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.
Groundwater
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 ground water 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 ground water levels may impact dissolved contaminants in the groundwater. To accurately characterize ground water 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). The thickness of the LNAPL layer must be accurately gauged with an oil or 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, 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.
Petroleum 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.
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Coordinator: EnvGuide Team