Index > 2 Development of a strategy, implementation and execution program to remediate radioactively contaminated sites >

2.4.8 Evaluation of historical site assessment data

Contents
2.4.8.1 Identify potential contaminants
2.4.8.2 Identify potentially contaminated areas
2.4.8.3 Identify potentially contaminated media
2.4.8.4 Surface soil
2.4.8.5 Subsurface soil and media
2.4.8.6 Surface water
2.4.8.7 Groundwater
2.4.8.8 Air
2.4.8.9 Structures

The main purpose of the historical site assessment is to determine the current status of the site or facility, but the data collected may also be used to differentiate sites or parts of a site that need further action from those that pose little or no threat to human health and the environment. This screening process can serve to provide a site disposition recommendation or to recommend additional surveys. Because much of the data collected during historical site assessment activities is qualitative or is analytical data of unknown quality, many decisions regarding a site are the result of professional judgment.

There are three possible recommendations that follow the historical site assessment:

  • An emergency action to reduce the risk to human health and the environment.
  • The site or area is impacted and further investigation is needed before a decision regarding final disposition can be made. The site may be classified as class 1, class 2, or class 3, and a scoping survey or a characterization survey should be performed. Information collected during the historical site assessment can be very useful in planning these subsequent survey activities.
  • The site or area is non-impacted. There is no possibility or an extremely low probability of residual radioactive materials being present at the site. The site can be released.

Historical analytical data indicating the presence of contamination in environmental media (surface soil, sub-surface soil, surface water, groundwater, air, or buildings) should be used to support the hypothesis that radioactive material was released at the facility or site. A decision that the site is contaminated can be made regardless of the quality of the data, its attribution to site operations, or its relationship to background levels. In such cases, analytical indications are sufficient to support the hypothesis – it is not necessary to definitively demonstrate that a problem exists. Conversely, historical analytical data can also be used to support the hypothesis that no release has occurred. However, these data should not be the sole basis for this hypothesis. Using historical analytical data as the principal reason for ruling out the occurrence of contamination forces the data to demonstrate that a problem does not exist.

In most cases it is assumed there will be some level of process knowledge available in addition to historical analytical data. If process knowledge suggests that no residual contamination should be present and the historical analytical data also suggests that no residual contamination is present, the process knowledge provides an additional level of confidence and supports classifying the area as non-impacted. However, if process knowledge suggests no residual contamination should be present but the historical analytical data indicate the presence of residual contamination, the area will probably be considered impacted.

The following sections describe the information recommended for assessing the status of a site. This information is needed to accurately and completely support a site disposition recommendation. If some of the information is not available, it should be identified as a data need for future surveys.

2.4.8.1 Identify potential contaminants

An efficient historical site assessment gathers information sufficient to identify the radio-nuclides used at the site – including their chemical and physical form. The first step in evaluating historical site assessment data is to estimate the potential for residual contamination by these radio-nuclides.

Site operations greatly influence the potential for residual contamination. An operation that only handled encapsulated sources is expected to have a low potential for contamination – assuming that the integrity of the sources was not compromised. A review of leak-test records for such sources may be adequate to demonstrate the low probability of residual contamination. A chemical manufacturing process facility would likely have contaminated piping, ductwork, and process areas, with a potential for soil contamination where spills, discharges, or leaks occurred. Sites using large quantities of radioactive ores – especially those with outside waste collection and treatment systems – are likely to have contaminated grounds. If loose dispersible materials were stored outside or process ventilation systems were poorly controlled, then windblown surface contamination may be possible.

Consider how long the site was operational. If enough time elapsed since the site discontinued operations, radio-nuclides with short half-lives may no longer be present in significant quantities. In this case, calculations demonstrating that residual activity could not exceed the derived concentration guideline level (DCGL) may be sufficient to evaluate the potential residual contaminants at the site. A similar consideration can be made based on knowledge of a contaminant’s chemical and physical form. Such a determination relies on records of radio-nuclide inventories, chemical and physical forms, total amounts of activity in waste shipments, and purchasing records to document and support this decision. However, a number of radio-nuclides experience significant decay product in-growth, which should be included when evaluating existing site information.

2.4.8.2 Identify potentially contaminated areas

Information gathered during the historical site assessment should be used to provide an initial classification of the site areas as impacted or non-impacted.
Impacted areas have a reasonable potential for radioactive contamination (based on historical data) or contain known radioactive contamination (based on past or preliminary radiological surveillance). This includes areas where:

  • Radioactive materials were used and stored;
  • Records indicate spills, discharges, or other unusual occurrences that could result in the spread of contamination;
  • Radioactive materials were buried or disposed. Areas immediately surrounding or adjacent to these locations are included in this classification because of the potential for inadvertent spread of contamination.
    Non-impacted areas – identified through knowledge of site history or previous survey information – are those areas where there is no reasonable possibility for residual radioactive contamination. The criteria used for this segregation need not be as strict as those used to demonstrate final compliance with the regulations. However, the reasoning for classifying an area as non-impacted should be maintained as a written record. Note that – based on accumulated survey data – an impacted area’s classification may change as the radiation site survey investigation process progresses.

All potential sources of radioactivity in impacted areas should be identified and their dimensions recorded (in 2 or 3 dimensions – to the extent they can be measured or estimated). Sources can be delineated and characterized through visual inspection during the site reconnaissance, interviews with knowledgeable personnel, and historical information concerning disposal records, waste manifests, and waste sampling data. The historical site assessment should address potential contamination from the site whether it is physically within or outside of site boundaries.

2.4.8.3 Identify potentially contaminated media

The next step in evaluating the data gathered during the historical site assessment is to identify potentially contaminated media at the site. To identify media that may and media that do not contain residual contamination supports both preliminary area classification (Section 2.4.9 and Section 3.3.2.1) and planning subsequent survey activities.

The following sections provide guidance on evaluating the likelihood for release of radioactivity into the following environmental media: surface soil, subsurface soil, sediment, surface water, ground water, air, and buildings. The evaluation will result in either a finding of “Suspected contamination” or “No suspected contamination,” which may be based on analytical data, professional judgment, or a combination of the two.

Subsequent sections describe the environmental media and pose questions pertinent to each type. Each question is accompanied by a commentary. Carefully consider the questions within the context of the site and the available data. Avoid spending excessive amounts of time answering each question because answers to every question are unlikely to be available at each site. Questions that cannot be answered based on existing data can be used to direct future surveys of the site. Also, keep in mind the numerous differences in site-specific circumstances and that the questions do not identify every characteristic that might apply to a specific site. Additional questions or characteristics identified during a specific site assessment should be included in the historical site assessment report.

2.4.8.4 Surface soil

Surface soil is the top layer of soil on a site that is available for direct exposure, growing plants, re-suspension of particles for inhalation, and mixing from human disturbances. Surface soil may also be defined as the thickness of soil that can be measured using direct measurement or scanning techniques. Typically, this layer is represented as the top 15 cm (6 in.) of soil. Surface sources may include gravel fill, waste piles, concrete, or asphalt paving. For many sites where radioactive materials were used, one first assumes that surface contamination exists and the evaluation is used to identify areas of high and low probability of contamination (e.g., Class 1, Class 2 or Class 3 areas).

  • Were all radiation sources used at the site encapsulated sources?
    A site where only (proven) encapsulated sources were used would be expected to have a low potential for contamination. A review of the leak-test records and documentation of encapsulated source location may be adequate for a finding of “No suspected contamination.”
  • Were radiation sources used only in specific areas of the site?
    Evidence that radioactive materials were confined to certain areas of the site may be helpful in determining which areas are impacted and which are non-impacted. This should be supported by other gathered information, e.g., interviews, documents dealing with the transport of radioactive materials and storage at the site.
  • Was surface soil re-graded or moved elsewhere for fill or construction purposes?
    This helps to identify additional potential radiation sites.

2.4.8.5 Subsurface soil and media

Subsurface soil and media are defined as any solid materials not considered to be surface soil. The purpose of these investigations is to locate and define the vertical extent of the potential contamination. Subsurface measurements can be expensive, especially for beta- or alpha-emitting radionuclides. Removing areas from consideration for subsurface measurements or defining areas as non-impacted for subsurface sampling conserves limited resources and focuses the site assessment on areas of concern.

  • Are there areas of known or suspected surface soil contamination?
    Surface soil contamination can migrate deeper into the soil. Surface soil sources should be evaluated based on radionuclide mobility, soil permeability, and infiltration rate to determine the potential for subsurface contamination. Computer modelling may be helpful for evaluating these types of situations. See also Sections 3.3.3 and 3.3.4).
  • Is there a groundwater plume without an identifiable source?
    Contaminated groundwater indicates that a source of contamination is present. If no source is identified during the historical site assessment, subsurface contamination is a probable source.
  • Is there potential for enhanced mobility of radionuclides in soils?
    Radionuclide mobility can be enhanced by the presence of solvents or other volatile chemicals that affect the ion-exchange capacity of soil (see Section 3.3.4).
  • Is there evidence that the surface has been disturbed?
    Recent or previous excavation activities are obvious sources of surface disturbance. Areas with developed plant life (forested or old growth areas) may indicate that the area remained undisturbed during the operating life of the facility. Areas where vegetation is removed during previous excavation activity may be distinct from mature plant growth in adjacent areas. If a site is not purposely replanted, vegetation may appear in a sequence starting with grasses that are later replaced by shrubs and trees. Typically, grasslands recover within a few years, sagebrush or low ground cover appears over decades, while mature forests may take centuries to develop.
  • Is there evidence of subsurface disturbance?
    Non-intrusive, non-radiological measurement techniques may provide evidence of subsurface disturbance. Magnetometer surveys can identify buried metallic objects, and ground-penetrating radar can identify subsurface anomalies such as trenches or dump sites. Techniques involving special equipment are discussed in Section 3.3.8 and Section 3.6.6.
  • Are surface structures present?
    Structures constructed at a site – during the operational history of that site – may cover below-ground contamination. Some consideration for contaminants that may exist beneath parking lots, buildings, or other onsite structures may be warranted as part of the investigation. There may be underground piping, drains, sewers, or tanks that caused contamination (see Section 3.6.3).

2.4.8.6 Surface water

Surface waters include streams and rivers, lakes, coastal tidal waters, and oceans. Note that certain ditches and intermittently flowing streams qualify as surface water. The evaluation determines whether radio-nuclides are likely to migrate to surface waters or their sediments. Where a previous release is not suspected, the potential for future release depends on the distance to surface water and the flood potential at the site. With regard to the two preceding sections, one can also consider an interaction between soil and water in relation to seasonal factors including soil cracking due to freezing, thawing, and dessication that influence the dispersal or infiltration of radio-nuclides.

  • Is surface water nearby?
    The proximity of a contaminant to local surface water is essentially determined by run-off and radionuclide migration through the soil. The definition for nearby depends on site-specific conditions. If the terrain is flat, precipitation is low, and soils are sandy, nearby may be within several meters. If annual precipitation is high or occasional rainfall events are high, within 1,200 meters (3/4 mile) might be considered nearby. In general, sites need not include the surface water pathway where the overland flow distance to the nearest surface water is more than 3,200 meters (2 miles).
  • Is the waste quantity particularly large?
    Depending on the physical and chemical form of the waste and its location, large is a relative term. A small quantity of liquid waste may be of more importance – i.e., a greater risk or hazard – than a large quantity of solid waste stored in water tight containers.
  • Is the drainage area large?
    The drainage area includes the area of the site itself plus the up-gradient area that produces run-off flowing over the site. Larger drainage areas generally produce more run-off and increase the potential for surface water contamination.
  • Is rainfall heavy?
    If the site and surrounding area are flat, a combination of heavy precipitation and low infiltration rate may cause rainwater to pool on the site. Otherwise, these characteristics may contribute to high run-off rates that carry radio-nuclides overland to surface water. Total annual rainfall exceeding one meter (40 inches), or a once in two-year-24-hour precipitation exceeding five cm (two inches) might be considered “heavy”.
    Rainfall varies for locations across Europe as also the precipitation rates during the year at each location due to seasonal and geographic factors. These value rates should be known for making a correct judgement about the migration of radio-nuclides.
  • Is the infiltration rate low?
    Infiltration rates range from very high in gravelly and sandy soils to very low in fine silt and clay soils. Paved sites prevent infiltration and generate run-off.
  • Are sources of contamination poorly contained or prone to run-off?
    Proper containment which prevents radioactive material from migrating to surface water generally uses engineered structures such as dikes, berms, run-on and run-off control systems, and spill collection and removal systems. Sources prone to releases via run-off include leaks, spills, exposed storage piles, or intentional disposal on the ground surface. Sources not prone to run-off include underground tanks, above-ground tanks, and containers stored in a building.
  • Is a run-off route well defined?
    A well defined run-off route – along a gully, trench, berm, wall, etc. – will more likely contribute to migration to surface water than a poorly defined route. However, a poorly defined route may contribute to dispersion of contamination to a larger area of surface soil.
  • Has deposition of waste into surface water been observed?
    Indications of this type of activity will appear in records from past practice at a site or from information gathered during personal interviews.
  • Is ground water discharge to surface water probable?
    The hydrogeology and geographical information of the area around and inside the site may be sufficiently documented to indicate discharge locations.
  • Does analytical or circumstantial evidence suggest surface water contamination?
    Any condition considered suspicious – and that indicates a potential contamination problem – can be considered circumstantial evidence.
  • Is the site prone to flooding?
    In national or local archives information may be available about the flood rate and occurred floods in the past. Generally, a site on a 500-year floodplain is not considered prone to flooding.

2.4.8.7 Groundwater

Proper evaluation of groundwater includes a general understanding of the local geology and subsurface conditions. Of particular interest is descriptive information relating to subsurface stratigraphy, aquifers, and groundwater use.

  • Are sources poorly contained?
    Proper containment which prevents radioactive material from migrating to groundwater generally uses engineered structures such as liners, layers of low permeability soil (e.g., clay), and leachate collection systems.
  • Is the source likely to contaminate groundwater?
    Underground tanks, landfills , surface impoundments and lagoons are examples of sources that are likely to release contaminants that migrate to groundwater. Above ground tanks, drummed solid wastes, or sources inside buildings are less likely to contribute to groundwater contamination.
  • Is waste quantity particularly large?
    Depending on the physical and chemical form of the waste and its location, large is a relative term. A small quantity of liquid waste may be of more importance – i.e., greater risk or hazard – than a large quantity of solid waste stored in water tight containers.
  • Is precipitation heavy?
    If the site and surrounding area are flat, a combination of heavy precipitation and low infiltration rate may cause rainwater to pool on the site. Otherwise, these characteristics may contribute to high run-off rates that carry radio-nuclides overland to surface water. Total annual rainfall exceeding one meter (40 in.), or a once in two-year-24-hour precipitation exceeding five cm (two in.) might be considered “heavy”.
  • Is the infiltration rate high?
    Infiltration rates range from very high in gravelly and sandy soils to very low in fine silt and clay soils. Unobstructed surface areas are potential candidates for further examination to determine infiltration rates.
  • Is the site located in an area of karst terrain?
    In karst terrain, groundwater moves rapidly through channels caused by dissolution of the rock material (usually limestone) that facilitates migration of contaminants.
  • Is the subsurface highly permeable?
    Highly permeable soils favour downward movement of water that may transport radioactive materials. Well logs, local geologic literature, or interviews with knowledgeable individuals may help answer this question.
  • What is the distance from the surface to an aquifer?
    The shallower the source of groundwater, the higher the threat of contamination. It is difficult to determine whether an aquifer may be a potential source of drinking water in the future (e.g., next 1,000 years). This generally applies to the shallowest aquifer below the site.
  • Are suspected contaminants highly mobile in ground water?
    Mobility in ground water can be estimated based on the distribution coefficient (Kd) of the radionuclide. Elements with a high Kd, like thorium (e.g., Kd = 3,200 cm2/g), are not mobile while elements with a low Kd, like hydrogen (e.g., Kd = 0 cm2/g), are very mobile. The Unted States Nuclear regulatory Commission (NRC) [USNRC-1992] and the Department of Energy (DOE)[YU] provide a compilation of Kd values. These values can be influenced by site-specific considerations such that site-specific Kd values need to be evaluated or determined. Also, the mobility of a radionuclide can be enhanced by the presence of a solvent or volatile chemical.
  • Does analytical or circumstantial evidence suggest groundwater contamination?
    Evidence for contamination may appear in current site data; historical, hydro-geological, and geographical information systems records; or as a result of personal interviews.

2.4.8.8 Air

Evaluation of air is different than evaluation of other potentially contaminated media. Air is rarely the source of contamination. Air is evaluated as a pathway for re-suspending and dispersing radioactive contamination as well as a contaminated media.

  • Were there any observations of contaminant releases into the air caused by any activity performed on the site?
    Direct observation of a release to the air might occur where radioactive materials are suspected to be present in particulate form (e.g., mine tailings, waste pile) or adsorbed to particulates (e.g., contaminated soil) or released by a chimney, and where site conditions favour air transport (e.g., dry, dusty, windy).
  • Does analytical or circumstantial evidence suggest a release to the air?
    Other evidence for releases to the air might include areas of surface soil contamination that do not appear to be caused by direct deposition or overland migration of radioactive material.
  • For radon exposure only, are there elevated amounts of radium (226Ra or one of its daughters, e.g., 210Pb) in the soil or water that could act as a source of radon in the air?
    The source, 226Ra, decays to 222Rn, which is radon gas. Once radon is produced, the gas needs a pathway to escape from its point of origin into the air. Radon is not particularly soluble in water, so this gas is readily released from water sources which are open to air. Soil, however, can retain radon gas until it has decayed. The rate that radon is emitted by a solid, i.e. radon flux, can be measured directly to evaluate potential sources of radon.
  • Is there a prevailing wind and a propensity for windblown transport of contamination?
    Information pertaining to geography, ground cover (e.g., amount and types of local vegetation), meteorology (e.g., wind speed at 7 meters above ground level) for and around the site, plus site-specific parameters related to surface soil characteristics enter into calculations used to describe particulate transport. Mean annual wind speed can be obtained from the national weather service surface station nearest to the site.

2.4.8.9 Structures

Structures used for storage, maintenance, or processing of radioactive materials are potentially contaminated by these materials. The questions presented in Table 2.3 help to determine if a building might be potentially contaminated. The questions listed in this section are for identifying potentially contaminated structures, or portions of structures, that might not be identified using Table 2.3.

  • Were adjacent structures used for storage, maintenance, or processing of radioactive materials?
    Adjacent is a relative term for this question. A processing facility with a potential for venting radioactive material to the air could contaminate buildings downwind. A facility with little potential for release outside of the structures handling the material would be less likely to contaminate nearby structures.
  • Is a building or its addition or a new structure located on a former radioactive waste burial site or contaminated land?
    Comparing past and present photographs or site maps and retrieving building permits or other structural drawings and records in relation to historical operations information will reveal site locations where structures may have been built over buried waste or contaminated land.
  • Was the building constructed using contaminated material?
    Building materials such as concrete, brick, or cinder block may have been formed using contaminated material.
  • Does the potentially non-impacted portion of the building share a drainage system or ventilation system with a potentially contaminated area?
    Technical and architectural drawings for site structures along with visual inspections are required to determine if this is a concern in terms of current or past operations.
  • Is there evidence that previously identified areas of contamination were re-mediated by painting or similar methods of immobilizing contaminants?
    Removable sources of contamination immobilized by painting may be more difficult to locate, and may need special consideration when planning subsequent surveys.