U.S. EPA Contaminated Site Cleanup Information (CLU-IN)


U.S. Environmental Protection Agency
U.S. EPA Technology Innovation and Field Services Division

Sediments

Conceptual Site Models

Conceptual site models (CSM) are part of the data quality objectives process, which is required for all site activities that involve gathering environmental data (EPA 2006). Since the fate and transport of contaminants in sediments is often complex and depends on the physical/chemical properties of the contaminant and sediments, the type of water body in which the sediments are deposited, and whether the water body is gaining or losing, it is important that the CSM be both detailed and complete.

Generally two types of CSMs are developed during the course of a site investigation and remediation. The first characterizes the hydrogeologic construct of the area of concern and depicts contaminant fate and transport (the source location of the contaminants, where are they currently found, any biological or geochemical transforms, and where will they eventually end up). This information is used to produce a CSM that addresses the potential risks that the contaminants have to human health and ecosystems.

It is important that the CSM account for the biological and geochemical activities occurring in the sediment's transition zone. Transition zones are usually found in gaining water bodies and contain areas dominated by just groundwater, a mixture of groundwater and surface water, and just surface water (EPA 2008). The transition zone is an ecologically active area beneath the sediment/water interface where a variety of important ecological and physicochemical conditions and processes may occur (EPA 2008). These processes must be taken in to account when determining risk and cleanup levels.

The CSM must also account for the complexity and uncertainty of data in the transition zone. For example, Conant et al. (2004) found that a tetrachloroethene (PCE) ground water plume changed its size, shape, and composition as it passed through the transition zone. Biodegradation in the top 2.5 meters of the transition zone reduced the PCE concentrations but created high concentrations of seven different transformation products, thereby changing the toxicity of the plume. The biodegradation was spatially variable and concentrations in the streambed varied by a factor of 1 to 5000 over distances of less than 4 meters horizontally and 2 meters vertically (EPA 2008).

The figures below illustrate the importance of considering whether the contamination is the result of current activities, historic activities or both; how groundwater flow into a seemingly uniform sediment bed is complicated by underlying aquifer geology; and finally the usefulness of a remedial design CSM for identifying potential flaws.

Figure 1 shows a variety of potential sources for contaminated sediment. Sediments can become contaminated by processes such as:

  • overland flow from an upland source,
  • discharge of contaminated groundwater
  • direct discharge of non-aqueous phase liquids
  • air deposition of contaminants (e.g., mercury from fossil fuel power plants),
  • transport of contaminated sediments from an upstream source.

A general rule of thumb is that sediments should not be remediated until all sources have been identified and eliminated so that they can no longer contaminate the sediments. As shown in the figure, sources can be current (e.g., active advection of contaminated groundwater into sediment) or historic (e.g.,1970s release of PCBs from manufacturing and storage facilities with no new upland or upstream releases occurring). Identification of active sources can also be complicated, as is illustrated by the Portland HarborAdobe PDF Logo shoreline.

Figure 1. Generalized sediments CSM showing complexity of potential contaminant sources.
Figure 1. Generalized sediments CSM showing complexity of potential contaminant sources.

In developing the CSM, assumptions should be verified. For example, in the cleanup of PCB contaminated sediment in the Ashtabula River, one of the underlying initial assumptions was that the release of PCBs was historic; however, when baseline sampling was performed prior to starting the dredging operation, it was discovered that there was an active source on one of the river's tributaries (Cieniawski 2008). Had this source not been discovered and eliminated, the expensive remedy would have eventually failed.

Figure 2 demonstrates how understanding groundwater to surface water interaction is key to the success of a remedy. This is a stylized depiction of groundwater flow into a river system with preferential pathways for groundwater to surface water discharge. If an assumption is made that the discharge of contaminated water is diffuse and uniform across the sediment bed, it may lead to an incorrect estimate of risk or potentially improper design of a cap. To ensure an accurate CSM, preferential flow paths should be identified. (See EPA 2008 for a more detailed discussion of groundwater/surface water interactions and their relationship to contaminant transport and the Fate and Transport section for a more detailed description of contaminant movement in sediment systems. View thermal images of preferential flows into surface water bodiesAdobe PDF Logo. Also note that the area of concern may have losing as well as gaining sections. Even without an upland source of contaminated groundwater, capping the contaminated sediment could still allow surface water to flow through it, thus contaminating the underlying groundwater.

Figure 2. Groundwater discharge to a river through a heterogeneous subsurface.
Figure 2. Groundwater discharge to a river through a heterogeneous subsurface.

Figure 3 is a stylized CSM of a proposed site remedy. Note that movement of the sediment contaminant sources through the subsurface (i.e., NAPL and the contaminant groundwater plume) has been stopped by a shoreline barrier wall. The cap uses organoclay to adsorb the NAPL and contaminants in the groundwater that have already entered the aquifer under the river and will discharge through the sediment bed into the cap. The barrier wall must be sufficiently deep to prevent contaminants from flowing under it or they will eventually break through the cap material. When using a barrier wall for upland containment, its weaknesses should be carefully considered. For example, sheet pile walls are prone to leakage at the seams; and slurry walls are subject to non-uniform placement and contaminant diffusion through the slurry.

Figure 3. Stylized CSM of a contaminated sediment site remedy with an active source.
Figure 3. Stylized CSM of a contaminated sediment site remedy with an active source.



Adobe PDF LogoECO Update / Groundwater Forum Issue Paper: Evaluating Ground-Water/Surface-Water Transition Zones in Ecological Risk Assessments
EPA, EPA-540-R-06-072, 30 pp 2008

Adobe PDF LogoGround-Water Plume Behavior Near the Ground-Water/Surface Water Interface of a River
Conant, Jr., Brewster
EPA, Proceedings of the Ground-Water/Surface-Water Interactions Workshop, EPA/542/R-00/007, 2000

Adobe PDF LogoGuidance on Systematic Planning Using the Data Quality Objectives Process
EPA, Office of Environmental Information, EPA QA/G-4, EPA/240/B-06/001, 121 pp, February 2006

Proceedings of the Ground-Water/Surface-Water Interactions Workshop
EPA, EPA 542-R-00-007, 204 pp, 2000

Adobe PDF LogoThe U.S. EPA's Great Lakes Legacy Act Ashtabula River Clean-Up
Cieniawski, Scott
USEPA, Great Lakes Program Office, 46 pp (PPT), 2008

Environmental Research Brief— The Impact of Ground-Water/Surface-Water Interactions on Contaminant Transport with Application to an Arsenic Contaminated Site
Ford, Robert
EPA, Office of Research and Development, 22 pp, 2005

Adobe PDF LogoGuidelines for Ecological Risk Assessment
USEPA, Office of Research and Development, EPA 630/R-95/002F, 1998



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