Because of the inherent complexity of sediment remediation projects, site characteristics (such as source areas, transport mechanisms, background and upstream areas, and key site features) should be clearly identified in a conceptual site model (CSM) before evaluating and selecting remedial alternatives.
The remedy evaluation framework presented here assists in selecting remedial technologies and evaluating remedial alternatives that are applicable to contaminated sediment sites based on site-specific conditions.
Background Condition
The term “background” typically refers to substances, conditions, or locations that are not influenced by the chemical releases from the site being evaluated. Background is usually described as either naturally occurring (consistently present in the environment and not influenced by human activity) or anthropogenic (influenced by human activity but not related to specific activities at the site).
Background conditions and concentrations of chemicals for sediment sites are typically determined from reference samples (obtained from upstream or areas unaffected by site-related sources) and may include the following:
- Sediment samples are typically surface grab samples but could also be selected from deeper sediment core intervals that represent pre-industrial horizons.
- Surface water samples are collected from lateral or upstream stations entering the site. The samples can be discrete samples (grab) or composite samples (collected over time or integrated over the height of the water column).
- Total suspended solids (particulates) samples are typically collected from upstream locations, stormwater or combined sewer overflow (CSO) outfalls, sediment traps, catch basins, or atmospheric collection traps at locations where water is entering the site or entering the watershed. These samples are used to indicate the ongoing background contributions to the sediment.
- Residue samples are typically collected from biota (fish, invertebrates).
A background data set or threshold value, once calculated, can be used in many stages of a site cleanup including:
- Determining if a release has occurred
- Determining site boundaries and evaluating site conditions (nature and extent of contamination)
- Distinguishing chemicals of potential concern from background chemicals to help refine the list of chemicals of concern
- Establishing a cleanup standard from background data
- Using reference areas that are physically, geochemically, and ecologically similar to the site to help evaluate the significance of observed effects and risks from chemical exposure
- Establishing remedial action objectives (RAOs)
- Establishing performance criteria to evaluate compliance monitoring data
- Evaluating recontamination potential after remedy implementation (applicable to all remedial technologies)
- Assisting with risk communication to the public and stakeholders
Source Control
Identifying and controlling the sources of contaminants to an aquatic system is an integral component to remediating contaminated sediments and effective source control is a prerequisite for applying any of the remedial technologies.
Multiple source is possible in large complex site
Sources that should be controlled can include the following:
- In-water sources. These sources are characterized by elevated sediment contaminant concentrations associated with current or historical releases to the water body that represent an ongoing source of contamination to downstream or adjacent areas of the water body.
- Land-based sources. Land based sources of contamination include contaminated soil that may migrate to water bodies by erosion and overland sheet flow, stormwater discharge, terrestrial activity (for example, wind-blown materials, soil or sediment creep, or improper use of engineering controls), erosion of contaminated bank soils, or episodic erosion of floodplain soils during high flow rates. In some situations, contaminated groundwater discharges may also transport contaminants to sediment and surface water.
- Watershed sources. Sediment contamination may result from regional watershed activities. Nonpoint sources resulting from atmospheric deposition, urban and agricultural activities may contribute to ambient sediment contamination at a regional or watershed level.
When multiple sources exist, they must be prioritized according to risk in order to determine where to best focus resources. Generally, any significant continuing site-related upland sources (including contaminated groundwater, stormwater, NAPL migration, or other releases) should be controlled in a manner and time frame compatible with the sediment remedy.
Site Characterization Data
Collecting site data for remedial evaluation
Evaluating remedial technologies requires site-specific data that may affect a technology’s performance. These data needs go beyond the data necessary to delineate the nature and extent of contamination and include information necessary to evaluate sediment stability and transport, contaminant mobility, waterway characteristics, hydrology and adjacent land and waterway use.
The table 1 in the presents a summary of the types of data that may be required at contaminated sediment sites, potential approaches to obtain the data, and the implications of the data types for remedy selection.
Remedial Zone Identification and Mapping
Defining remedial zones delineates the overall area and volume of contaminated sediments into workable units that are subsequently considered for remediation. Identifying these units based on site-specific conditions simplifies the evaluation of remedial technologies. Zone identification may not be applicable at every site, but the concept should at least be examined at each site.
Zones should first be identified based on the distribution of contamination and preliminary remedial goals (PRGs). These zones should be further refined based on site-specific information relevant to the evaluation of remedial technologies. At large complicated sediment sites, however, dividing the site into specific remedial zones will facilitate the focused evaluation of remedial technologies and the development, screening, and evaluation of remedial action alternatives.
Screening of Remedial Technologies
To simplify this screening step, questions are included as part of the remedy selection framework to help conduct an initial screening assessment of MNR, EMNR, in situ treatment, conventional capping, amended capping, and excavation and dredging. The screening questions may be used to evaluate and screen remedial technologies from further consideration on a zone by zone basis. Further information about screening remedial technologies can be found in other sediment remedy articles.
Evaluation of remedial technologies
Detailed evaluations of remedial technologies retained after the initial screening step are conducted using site-specific information to identify the most favorable technologies. Based on these evaluations, additional remedial technologies may be eliminated.
Table 2 lists the physical, sediment, contaminant, and land and waterway use characteristics used to establish the applicability of each of the technologies (MNR, EMNR, in situ treatment, conventional capping, amended capping, dredging and excavation). Each cell corresponds to a characteristic and technology. Each cell also contains a ranking of importance of each characteristic for specific technologies.
Evaluation of Remedial Action Alternatives
The evaluation of the remedial alternatives developed should consider a range of criteria consistent with the regulatory framework that the site is being remediated under.
Evaluation criteria for remedial action alternatives are typically organized into the following major categories:
- Ability to meet project objectives (such as RAOs)
- Effectiveness (such as long-term reliability and short-term impacts)
- Technical feasibility (which addresses the question: Can this be done?)
- Administrative feasibility (which addresses the question: Can required approvals be obtained?)
- Cost and schedule
- Ability to meet stakeholder objectives
Source: ITRC
Coordinator: EnvGuide Team