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The initial risk assessment and emission control from contaminated sediments
Vol. 6 No 1-4, 213-222 2006
The initial risk assessment and emission control from contaminated sediments
Ecohydrology for Implementation of the European Water Framework Directive
Grzegorz Malina1, Martijn P.J. Smit2, Tim J.C. Grotenhuis2 1
Institute of Environmental Engineering, Czêstochowa University of Technology, BrzeŸnicka 60A, 42-200 Czêstochowa, Poland e-mail: [email protected] 2 Section Environmental Technology, Wageningen University, Wageningen, The Netherlands, e-mail: [email protected]
Abstract Discharged persistent organic pollutants and heavy metals into the rivers are often assumed to adsorb on sediments without re-suspension, thus sediments may function as a sink for contaminants. Sediments, however, may also function as a (secondary) source of contaminants, depending on the hydrodynamic characteristics of a river. To assess whether the sediments function as a sink or a source of contaminants, a preliminary risk assessment using the so-called SEDINA tool (SEDiment INitial Assessment) is recommended. By comparison of the total concentrations of pollutants in sediments upstream, at, and downstream of the site area, the role of sediments can be determined. When the sediments functions as a sink, no specific sediments-related measures are required, although monitoring of the surface water and sediments quality is recommended. If the output of the SEDINA tool reflects that the sediments may function or function as a source of contaminants, the risk-reduction measures are necessary that may include emission control at the source(s), pathway(s) and/or receptor(s). Key words: the SEDINA tool, bioavailability, remediation, persistent organic pollutants, heavy metals.
1. Introduction Long-term diverse activities at industrial areas resulted in direct emissions of waste products to the environmental compartments (atmosphere, surface water and soil/groundwater), which in consequence led to major environmental problems. Since the 1970's, direct emissions to the environmental compartments have been continuously reduced due to a growing awareness of environmental problems. Application of off-gas and wastewater treatment plants, spill control, and
restrictions in the use of some chemicals led to a rapid improvement of both air and surface water quality. Soil and sediment systems respond much slower to these measures and need more time to improve their quality. Moreover, soils and sediments were polluted by numerous spills and landfills resulting in a diffuse pollution on a mega-scale (e.g. brownfields, post-industrial areas, military bases, etc.), which makes the active conventional remediation more difficult. Complete cleanup within an intermediate timeframe (25 years) is usually not feasible, or may even be
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impossible for technical and economical reasons. Therefore, such sites are continuous and long-term potential and actual sources of regional contamination of groundwater, surface water and sediments. Developments in environmental policy are observed since the mid nineties towards changing the approach from multi-functionality into the risk reduction for humans and ecosystems to acceptable levels (Malina 2005). Consequently, remediation goals changed from target oriented into risk oriented. The currently applied risk evaluation procedures are commonly based on the source -pathway-receptor sequence. Large amounts of sediments are dredged worldwide for both maintenance and environmental reasons. For example, 400 106 m3 of sediments are dredged annually by the US Army Corps of Engineers or their permit recipients (Linkov et al. 2002), and about 40 106 m3 are dredged annually in the Netherlands (Vermeulen et al. 2003). On the basis of the SedNet estimation, around 100-200 106 m3 of contaminated sediments might be produced yearly in Europe (Bortone et al. 2004). A substantial proportion of these dredged sediments is held temporarily at above ground (upland) disposal facilities. An upland disposal is a relatively easy and cost-effective alternative for dredged materials and is, therefore, a widely adopted sediment management practice (Vermeulen et al. 2005). As a consequence of anthropopression, water quality has become, to a great extent, dependent on human population density and its whole range of activities that lead to: (i) chemical alteration of water quality through emission of pollutants and (ii) physical degradation of environments. While the emission of pollutants can be controlled by technologies, restoration of the physical degradation of the environment related to modification of hydrological and biogeochemical cycles due to e.g., deforestation, urbanisation, canalisation, requires a new, systemic approach. For that reason the concept of Ecohydrology may be applicable, which suggests that the sustainable development of water resources has been dependent on the ability to maintain the ecosystem processes that have been established by evolution (Zalewski 2002). It concerns how to regulate the biological processes of freshwater ecosystems using hydrology; and, vice versa, how to use biotic ecosystem properties as a tool in water management. At most industrial and urbanised areas, discharges of contaminants into the surface water (rivers, watercourses) may lead to their increased contents in the water phase or unconsolidated sediments. It is often assumed that contaminants strongly adsorb to the sediments and do not resuspend. If so, sediments function as a sink for contaminants. However, sediments may function as a (secondary) source of contaminants, depending on the hydrodynamic characteristics of the water flow
and sediments transport. The goal of this work was to develop a simple tool for preliminary assessment whether the sediments in watercourses and riverbeds at a certain location are functioning as a sink or a source of contamination. In the paper the concept of initial risk assessment to define the role of sediments as a sink or a source of pollution at the site of concern is discussed. Engineering options to reduce/control emissions from sediments to comply with defined threshold values at the planes of compliance are looked at in more details. Possible engineering options can reduce the emission from sediments to receptors in three ways: at a source, during migration (path), and at a receptor. The engineering options take into account the bioavailability concept and the contaminated sediments transport as such.
2. Initial risk assessment Contaminants fluxes from sediments One of the major risks for the aquatic ecosystems and human health is the possible recontamination of the adjacent surface waters, groundwater and atmosphere due to the polluted soil or sediments. This concept for contaminated sediments is shown in Fig. 1 (Welcome deliverable 9.4, 2004). Both, risk and cost efficiency assessments are positioned centrally to manage contaminated sites in a cost effective way. Current and predicted changes of risks at a site are assessed for defined, the so called planes of compliances, i.e. a horizontal or vertical line, at which certain concentrations or mass fluxes have to be achieved, to comply with the site-specific threshold values (Malina et al. 2006). They can be defined at the boundaries of the sites, between different compartments (e.g. groundwater/surface water), and/or between different receptors. At a mega-scale, the interface between sediments and groundwater (flux 4), the interface between sediments and surface water (flux 5), and the boundary of the site (flux 7) can be distinguished as planes of compliance. To facilitate the technical management, the mega-scale can be structured into manageable units, the so-called risk clusters, which comprise areas of similar features with respect to contaminants (sources), hydrogeological conditions (pathways) and affected receptors. Contaminant fluxes into surface water The presence of heavy metals in surface water is mostly dependent upon the pH. At low pH the speciation of metals will usually lead to increased solubility. As the pH of surface water is normally near neutral, only low concentrations of free dissolved metals are expected. Therefore, metals originating from fluxes 6 and 3 will be merely
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7 1 Unsaturated Zone
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3 Saturated Zone
Surface water Sediments
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Geo-hydrological base Fig. 1. Concept of contaminants fluxes between different compartments. 1 - gas/vapour transport between the atmosphere and unsaturated zone, 2 - contaminant transport between unsaturated and saturated zones, 3 - contaminant transport between a saturated zone (groundwater) and surface water, 4 - contaminant transport between a saturated zone (groundwater) and sediments, 5 - contaminant transport between sediments and surface water, 6 - gas/vapour transport between surface water and atmosphere, 7 - contaminant transport within surface water outside - inside the site.
precipitated and present in the sediments, which function as a sink for heavy metals. When environmental conditions change, a remobilization of heavy metals may occur, and the sediments may act as a source of contaminants (flux 5). The remobilization of heavy metals from sediments (flux 5) reported by Kania et al. (2005) occurred when pH was decreased, and the liquid/solid (L/S) ratio, i.e. ratio between water and sediments, was increased. These last findings indicate that during flooding conditions, when the flow of water is increased, remobilization and, therefore, an increase of risk for heavy metal transport to the surface water as a receptor is likely to occur. For hydrophobic organic contaminants, the concentration in surface water will be low as their water solubility is low. Therefore, considering the contaminant fluxes to the surface water, the interactions with direct discharge, deposition (flux 6), the flux within groundwater (flux 3) and the interaction with sediments (flux 5), are of major importance. Mass transfer of contaminants from contaminated soil and groundwater (flux 3) is more related to the interfacial processes (redox) than mass transfer as such. Mass transfer of pollutants from contaminated sediment (flux 5) is the result of three processes: (1) transport of contaminants bound to sediment particles, (2) transport of contaminants bound to dissolved organic matter (DOM), including colloidal materials, (3) transport of dissolved contaminants. The major transport route of pollutants at a site or a cluster depends on site-specific parameters (e.g. hydraulic conditions, type of contaminants,
sediments characteristics, etc). A more thorough assessment of fate and transport of pollutants are required from risk-based viewpoint (ecological and human), as well as for emission control (remediation and engineering options). From a perspective of risk, especially the freely dissolved contaminants are of interest, as they are readily available for uptake and introduction into the food chain. The second, more thorough step in risk assessment requires information about the source and receptor(s), as well as the pathway(s) linking the source and receptor(s). This assessment intends to provide information on the mass fluxes of contaminants at the plane of compliance. Source assessment Assessment of the source involves sediments characterization, the particle size and particle size distribution, organic matter (OM) and DOM contents, quantification of contaminants including their natural attenuation (NA) potentials. Information about bioavailability and eco-toxicological properties of contaminants are particularly useful to make prioritization of the contaminants. Receptor assessment Assessment of the receptors involves a clear definition and must include their properties. In this concept, the surface water plays a crucial role as a pathway for contaminants (e.g. in the case of mobile compounds), and as an intermediate to receptors (e.g. ecosystem, humans). However, in both cases international and national legislations provide appropriate threshold values. Besides of
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being a source of contaminants, the sediments may also be a receptor. In that case target species in the sediment ecosystem must be identified and related risks must be assessed. Pathway assessment The assessment of pathways between sources and receptors involves analyzing the hydraulic conditions of the surface water, water transport through the sediments (e.g. infiltration or welling), and the NA potential in the sediments, at the interface between sediments and the adjacent water, and in the aqueous phase. As a result of the pathway assessment, the fate and transport models can be developed, including desorption of potentially available contaminants or a re-suspension of contaminated sediments, transport of contaminants in the aqueous phase, and (bio)degradation of contaminants during transport. The source, receptor, and pathway mass fluxes from contaminated sediments to the surface water give the total mass fluxes of contaminants at the plane of compliance. When they lead to concentrations above the standards, the emission control measures are needed.
3. The SEDINA tool Discharges of persistent organic pollutants and heavy metals into the rivers may lead to increased contamination of the surface water and/or sediments. Contaminants are often assumed to adsorb on sediments and not to re-suspend. Thus, sediments may function as a sink for contaminants. Observations in number of sites show, however that sediments may also function as a (secondary) source of contaminants, depending on the hydrodynamic characteristics of a river. For an initial indication whether the sediments at a site of concern may function as a sink or a source of contaminants, a preliminary risk assessment using the so-called SEDINA tool (SEDiment INitial Assessment) was developed (Welcome deliverable 9.3, 2004; Grotenhuis et al. 2005). The SEDINA tool is aimed at obtaining an initial assessment of the environmental quality and probable risks related to the sediments at a mega-scale. When further actions are needed, more detailed tools are required for modelling transport of contaminated particles and contaminant transport in water systems. Transport of contaminated particles can be described by the HEC-RAS model (Welcome deliverables 9.3, 2003; Welcome deliverable 9.4, 2004), and the effect of water erosion on the contamination of sediments can be estimated by the European Soil Erosion Model (ESEM) (Welcome deliverable 9.4, 2004). Furthermore, the contaminant transport in water systems can be determined using the bioavailability concept for heavy metals and hydrophobic organic
contaminants. These tools provide both general and site-specific information on contaminants fate and transport to, from and along with the sediments. They can be based on actual and potential risks rather than the presence of contaminants quantified by total concentrations. In the case of the SEDINA tool, three situations can be distinguished for sediments in a (surface) water system at a mega - or cluster - scale. 1. Sediments act as a sink of contaminants coming from the groundwater (flux 4), surface water (flux 5), or sediments transported from upstream (flux 7). In this case the fluxes of contaminants are above zero, and the concentrations in sediments downstream of the site are lower than upstream. 2. Sediments are in equilibrium with contaminants in groundwater (flux 4), surface water (flux 5), and/or sediments upstream and downstream of the site (flux 7). In this case the contaminant fluxes equal zero, and the concentrations in sediments are equal downstream and upstream of the site. 3. Sediments act as a source of contaminants going to groundwater (flux 4), surface water (flux 5), or together with sediments are transported downstream (flux 7). In this case the contaminant fluxes are below zero and, thus contaminants from the site may affect the receptors within and outside the site. The situation can change in time due to altering environmental conditions (e.g. flooding/dry periods, fluctuations of groundwater/surface water tables, and human activities, like dredging). Although risks can be present in all three situations, only emissions in situation 3, when sediments act as a source, must be controlled from a cost-effective point of view. For situation 1 and 2, it is likely to be more cost-effective to control emissions towards the sediments before controlling emissions from the sediments. The input of the SEDINA tool is the total concentration of dissolved and bound contaminants, both upstream and downstream of the site (plane of compliance, flux 7 in Fig. 1). The output is a proposed action, indicated by a "traffic light" approach (Table I), where the green colour represents no action and orange/red colours comprise further action. When the output of the SEDINA tool is green, no further actions are required. However, monitoring the surface water and sediments quality is recommended with respect to future spills or discharges that may affect the aquatic environment. If the output is orange or red, identification of possible sources is recommended. These sources can be the result of current discharges of anthropogenic origin (e.g. wastewater discharge, run-off, spills, etc. (flux 6)), or seepage from historically contaminated soils (flux 3), groundwater (flux 3), or sediments (flux 5).
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Table I. An overview of the SEDINA-tool for the initial risk assessment from contaminated sediments (Grotenhuis et al. 2005). Concentration upstream versus downstream Cup-stream > Cdown-stream Cup-stream = Cdown-stream Cup-stream < Cdown-stream
Concentration levels below target/background values green green orange2
Concentration above intervention levels orange1 red red
green: sediment does function as a sink; no specific actions necessary on the short term; orange: 1incoming concentrations are high, action recommended to find the source of these priority pollutants; 2sediments may function as source of contaminants; action recommended to define the sources; red: most probably the sediments function as a source of contaminants; action recommended to find the probable sources at the site; as total concentrations are above intervention levels at least samples downstream of the site should be taken to obtain insight in bioavailability of the respective contaminants.
4. Emission control concepts Emission control can generally be achieved at the source, within the pathway, or at the receptor. During the risk assessment, information is obtained regarding the characteristics of contaminants, their concentrations, properties of the sediments, and hydraulic conditions of the surface water. This information can be easily used to indicate the best engineering option for emission control of contaminated sediments. When concentrations of pollutants in surface water are (very) high, emission control at the source is preferred. Depending on the mobility and availability of the contaminants, ex-situ or in-situ remediation options can be applied. When concentrations of pollutants are relatively low, but mobility and availability are high, emission control along the migration pathway is preferred. Emission control at the receptor is not feasible when the surface water is considered as the receptor. However, when surface water is used for other purposes (e.g. drinking water) additional treatment can be required to protect other receptors (e.g. humans).
Emission control at the source The concepts of emission control at the source focus on engineering options, however management options like reconstruction of sedimentation basins, construction of wetlands, erosion prevention (e.g. by planting), local extraction and treatment of groundwater, infiltration of clean water, water table management, may also be involved (Renhnolds 1998). Highly mobile and available contaminants High concentrations of mobile and available contaminants at the source pose an increased or even acute risk for the receptors. When contaminants are present in sediments at (very) high concentrations and are strongly mobile (available), dredging is an attractive approach, followed by treatment or controlled disposal similar to the
excavated soil treatment. Ex-situ emission control of polluted sediments at the source can be especially favourable when sediments have to be dredged for nautical or other functional reasons. Currently, there are "environmentally friendly" dredging methods available causing a limited turbidity and, thus a limited re-suspension of contaminated sediments (US EPA 1999a, 2002). Moderately mobile and available contaminants High concentrations of moderately mobile and available contaminants at the source pose high but less acute risk for receptors and, therefore, allow for more extensive emission reduction options. In-situ options for emission control of high concentrations of moderately mobile and available contaminants are different for organic and inorganic contaminants because of their different physical and chemical properties. Basically, organic contaminants can be converted to harmless products (biodegradation, biotransformation), whereas inorganic contaminants cannot. Organic contaminants. Different biological and physical/chemical in-situ engineered options exist to reduce emissions from the contaminated sediments. The choice between biological, physical-chemical, or a combination of biological and physical-chemical methods depends on the biodegradability of the pollutants, and to a lesser extent on the environmental conditions. Examples of in-situ emission control at the source are: NA or enhanced NA (ENA), and (advanced chemical) oxidation. The NA generally refers to physical, chemical or biological processes, which under favourable conditions lead to the reduction of toxicity, mobility, volume or concentration of organic contaminants in soil and/or groundwater, which are all related to mass reduction (US EPA 1999b; NRC 2000). The reduction takes place as a result of processes such as biological or chemical degradation. When conditions of NA are not favourable at the site, the ENA can be an option. ENA is the
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speeding-up of natural physical-chemical and biological processes of self-purification taking place in the subsoil and aqueous environment by: enhancing the sorption capacity (immobilization, (bio)precipitation), providing electron acceptors/donors, nutrients for biodegradation (biostimulation), and/or microorganisms (bioaugmentation). Under anaerobic conditions, organic contaminants can ultimately be metabolized to methane, limited amounts of carbon dioxide, and traces of hydrogen. Advanced chemical oxidation can be applied with different oxidants. As OM content in sediments is often higher than in soil, advanced chemical oxidation may be less successful than in the case of soil treatment. The chemical oxidants most commonly employed to-date include peroxide, ozone, and permanganate. They may cause the rapid and complete chemical destruction of many toxic organic chemicals; other organics are amenable to partial degradation as an aid to subsequent bioremediation. In general, the oxidants have been capable of achieving high treatment efficiencies (e.g. >90%) for unsaturated aliphatic (e.g. trichloroethylene (TCE) and aromatic compounds (e.g., benzene), with very fast reaction rates (90% destruction in minutes) (Welcome deliverable 9.4, 2004). Field applications for soil confirmed that matching the oxidant and the in situ delivery system to the contaminants of concern and the site conditions, are the keys to successful implementation and achieving performance goals (http://w ww.frtr.gov). Heavy metals. Examples of in situ emission control from heavy metals contaminated sediments at the source are: electro-reclamation, physical immobilization, and biological immobilization (bio-sorption) (Welcome deliverable 9.5, 2004). The principle of electro-kinetic remediation relies on application of a low-intensity direct electric current through the sediments between ceramic electrodes that are divided into a cathode and an anode arrays. This mobilizes charged species, causing ions and water to move toward the electrodes. Metal ions, ammonium ions, and positively charged organic compounds move toward the cathode. Anions, such as: chloride, cyanide, fluoride, nitrate, and negatively charged organic compounds move toward the anode. The electric current creates an acid front at the anode and a base front at the cathode. This generation of acidic conditions in situ may increase mobility of adsorbed heavy metals and enhance their transport to the collection system at the cathode (http: //www.frtr.gov). Two primary mechanisms that are responsible for contaminants transport through the sediments towards one or the other electrodes are: electro-migration and electro-osmosis. In electro-migration, charged particles are trans-
ported through the substrate. In contrast, electroosmosis is the movement of liquid containing ions relative to a stationary charged surface. Of the two, electro-migration is the main mechanism of the process. The directions and rates of movement of ionic species depend on their charge and the magnitude of electro-osmosis - induced flow velocities. Non-ionic species, both inorganic and organic, will also be transported along with the electro-osmosis - induced water flow. Immobilization techniques like solidification/stabilization reduce the mobility of contaminants through both physical and chemical means. Unlike other remedial technologies, they seek to trap or immobilize contaminants within their "host" medium (i.e. soil, sediments, and/or building materials that contain them), instead of removing them through chemical or physical treatment. These techniques can be used alone, or combined with other treatment and disposal methods, to yield a product or material suitable for land disposal or, in other cases, that can be applied for beneficial use. They have been used as both final and interim remedial measures (http://www. frtr.gov). Under sulphate-reduction conditions, sulphate is converted to sulphide or elemental sulphur, which can lead to precipitation of heavy metals. As a result of successful applications for treating metal-laden industrial wastewaters, precipitation is being considered and selected for remediation of groundwater containing heavy metals, including their radioactive isotopes. Metals precipitation from contaminated water involves the conversion of soluble heavy metal salts to insoluble salts that may precipitate, which can be realized by addition of an electron donor (Malina, Kwiatkowska 2003).
Emission control within the pathway Emission control within the pathway is preferred when concentrations of contaminants are relatively low and their availability and mobility are relatively high. Emission reduction measures mainly focus on retardation of contaminants to comply with the standards set for the planes of compliance, and are applicable for organic and inorganic contaminants. Retardation measures can possibly be combined with mass reduction of contaminants. Organic and inorganic contaminants can be transported in three ways to and through the surface water: bound to sediment inorganic particles, bound to OM and dissolved in water (Welcome deliverable 9.4, 2004). Depending on site-specific parameters (e.g. hydraulic conditions, type of pollutant, sediment characteristics, etc) the major transport route can be assessed. These three ways of transport require different approaches of emis-
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sion control. Under the conditions of net natural sedimentation the mass flux of contaminants through the plane of compliance is probably negligible and no engineering options are required. Engineering options to reduce contaminant emission by blocking their transport to the surface water can be realized by physical and physicalchemical methods. Physical methods can be used to reduce emission of pollutants bound to sediment particles and OM, whereas physical-chemical options reduce emissions of dissolved pollutants and pollutants bound to DOM. Physical options Physical engineering options can be used to reduce transport of contaminated sediments as such in a river-channel, and to block the contaminants mass transfer from sediments to the surface water. Natural capping. Natural capping only occurs in net sedimentation water systems (e.g. delta areas). However, by widening a river-channel, water flow velocity decreases and sedimentation will increase. The sediment movement is reduced, which may lead to anoxic conditions, stimulating precipitation of contaminants. Some efforts on this concept were made for acid mine drainage by Evangelou and Zhang (Welcome deliverable 9.5, 2004). To prevent acidification of a river system they designed a wetland treatment system to increase the average residence time of heavy metals under sulphate reducing conditions, and thus stimulate precipitation of heavy metal complexes. Artificial capping. This approach should be used where the long-term physical integrity of the cap can be maintained, and environments with low turbulence are generally desired for in-situ capping projects. The potential severity of the environmental impacts associated with cap erosion and potential dispersion of the sediment contaminants in an extreme event should determine the level of protection against erosion (http://www .epa.gov/glnpo/sediment/iscmain). During capping the contaminated sediments are covered by non-contaminated materials, and this method can be applied individually or in combinations, both in situ and ex-situ (e.g. heavily contaminated sediments to be dredged and slightly contaminated - capped) (Lane 2002). The most common modifications of in situ capping (ISC) include: (i) capping with partial dredging, (ii) residual capping (RC) (Malina, Szwedowski 2005). During ISC with partial dredging, contaminated sediments are removed down to a certain depth, and the remaining layers are covered with non-contaminated materials. This method is used
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when a certain depth of the watercourse has to be assured (shipping), the deeper sediment layers are required to keep the stability of riverbanks, and/or the deeper layers are heavily contaminated and their dredging may pose a serious risk to the environment. In the case of RC a thin layer of clean materials is deposited on sediments remained after dredging. Due to re-suspension a part of sediments is suspended in water, creating after some time a thin (of few centimetres) residual layer. The RC speeds up natural recuperation processes in the aquatic ecosystems. The covering layers are in that case not designed as the isolating layers. Another ISC approach is to place sediments dredged from other locations with subsequent covering with non-contaminated materials, similarly as during conventional capping (US EPA 1994). This is the so-called Dredged Material Capping (DMC), and it can be realized by distribution of dredged sediments in oceans and coastal waters. The covered sediments can be both nontreated or treated. The DMC is usually used in the case of renovation of waterways, as: (i) Level Bottom Capping (LBC), (ii) Contained Aquatic Disposal (CAD). The covering layer can fully or partially reduce the effect of contaminated sediments on aquatic ecosystems due to (Malina, Szwedowski, 2005): - physical isolation of contaminated sediments from benthos (decreased hazards to organisms and bioaccumulation in the food chain), - stabilisation of contaminated sediments (protection against erosion, reduction of re-suspension and transport), - chemical isolation and limited transfer of soluble and colloidal contaminants to the water phase. The most common capping materials are: sand, clay and silt, gravel, break stone, non-contaminated sediments, as well as geotexitles (Palermo et al. 2002; Palermo et al. 1998a, b; http://www.hsrc.org/capping). Applying geotextiles requires special equipments and, in practice was done only in shallow watercourses. Currently, in the USA studies are carried out on the novel group of active materials for capping to be applied for, the so-called Active Capping (AC) technology. These materials are: (i) AquablokTM (gravel and break stone covered by clay layers), (ii) zerovalent iron - Fe0, (iii) apatite - Ca5(PO4)3OH, (iv) BionSoilTM (materials from compost), (v) Organo-Clay Sorbents, (vi) coke, (vii) activated carbon, (viii) laminated mats (Reible 2003; US EPA 2004). Physical-chemical options Physical-chemical engineered options can be used to reduce transport (i.e. mass transfer) of dissolved contaminants, including contaminants adsorbed to DOM. With the selected diffusion
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model it was shown that transport of contaminants depends strongly on the effective diffusion rate and, thus on their sorption affinity to the sediment particles (Welcome deliverable 9.4, 2004). Physical-chemical emission control is based on the reduction of the effective diffusion rate by increasing the affinity of the contaminants to the sediment particles. Chemical isolation. Even if contaminant concentrations are high in the pore water, a granular cap component may act as a filter and/or a buffer during advective and diffusive transport. These uncontaminated granular cap materials can be expected to remove contaminants or at least reduce their concentrations in pore waters through sorption, ion exchange, surface complexation, and redox mediated flocculation. The removal degree is very much dependent on the cap materials. A cap composed of quarry-run sand would not be as effective as natural sand with fine fractions and the OM content (http://www.epa.gov/glnpo/sediment/iscmain). Reduction of the effective diffusion rate can be achieved by adding an extra sorbent to the sediments. For organic contaminants such sorbent should be hydrophobic, whereas for inorganics it should have a high specific surface area. Examples of additives are: activated carbon and metal binding additives. The group of Luthy (Welcome deliverable 9.5, 2004) explored the applicability of mixing activated carbon with sediments to reduce the aqueous concentration of PCBs and PAHs.
Emission control at the receptor Emission control at the receptor, such as i.e. surface water, is not feasible. Depending on the function of surface water, an additional water treatment can be required but these aspects are beyond the scope of this study.
5. Concluding remarks As many sites are located in a delta area or along the river course, they all have to deal with the contaminated sediments regarding the quality of surface water that is considered as one of the major receptors for contaminants in the environment. To perform an initial risk assessment caused by contaminated sediments at a site the SEDINA tool is postulated. By comparison of the total concentrations of contaminants in bed and suspended sediments upstream, at the site and downstream of the site area, the role of the site as a sink or a source of contaminants can be defined. When the site functions as the sink no specific measures related to the sediments have to be undertaken, although
monitoring of the surface water and sediments quality is recommended. If the output of the SEDINA tool reflects that the site may function or functions as a source of contaminants, further investigations are necessary prior to the risk-reduction measures that may include emission control at the source(s), pathways and/or receptor(s). Several details can be found based on fate and transport modelling. The amounts of contaminated sediments that can be transported to or from the site can be calculated using properly selected sediment transport models. Also the erosion by water may affect the risk management as such. Besides the flux of contaminated particles into and from the site, information is necessary about transport of contaminants (heavy metals, hydrophobic organic contaminants) within the sediments and from the sediments to the water system. Depending on the mobility and availability of the contaminants, ex-situ or in situ remediation options can be applied. If the availability and mobility are high, dredging with subsequent exsitu treatment or proper disposal may be favourable. When availability and mobility are moderately low, in situ options can be applied. Emission reduction measures mainly focus on reduction of contaminants to comply with the standards set for the plane of compliance. Still most often risk assessment is based upon total concentrations of contaminants, which may lead to overestimation of risks. In several scientific disciplines like ecotoxicology, biology, as well as in biodegradation studies in microbiology and environmental technology, it was shown in the last decade that only the bioavailable fractions of organic contaminants are causing risks, and consequently they should be taken into account in risk assessment. Therefore, it is expected that legislation should be based on 'real' risks of contaminants in the field. For risk determination the sound-based scientific methods have to be developed. All bioavailability methods based on physical/chemical techniques show that much less than the total concentrations of the pollutants can be desorbed in limited timeframe, leading to less risk compared to the use of total concentrations. Therefore, the application of bioavailability methods may lead to an improvement of the risk assessment and, subsequently, to more cost effective remediation.
Acknowledgements This study was done within the frame of the 5 EU Project Water, Environmental, Landscape Management at Contaminated Megasite WELCOME (EVK1-CT-2001-00103). It was also supported by the Polish Ministry of Sciences and Informatization - Committee for Scientific Research (KBN) (154/E-358/SPB/5.PR UE/DZ 165/2002-2004).
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6. References Bortone, G., Arevalo, E. Deibel, I., Detzner, H.-D., de Propris, L., Elskens, F., Giordano, A., Haksteges, P., Hamers, K., Harmsen, J., Hauge, A., Palumbo, L., Veen, J. 2004. Synthesis of the SedNet Work Package 4 Outcomes. J. Soils & Sediments 4(4), 225-232. Grotenhuis, T., Smit, M., Malina, G., Kasperek, R., Szdzuj, J., Satijn, B., Joziasse, J. 2005. Management scenarios for contaminated sediments at meagsites. In: Procc. 9-th Int. FZK/TNO Conference on SoilWater Systems - ConSoil2005, Bordeaux, France, pp. 2513-2522. http://www.epa.gov/glnpo/sediment/iscmain/ http://www.frtr.gov http://www.hsrc.org/capping Kania, K., Malina, G., Grotenhuis, J.T.C. 2005. Osady denne rzeki Sto³y: wtórne ognisko zanieczyszczeñ czy bariera ochronna dla wód powierzchniowych? [Contaminated sediments in the Stola River at the Tarnowskie Gory site: the source or the sink of contamination?] In: II Kongres In¿ynierii Œrodowiska, K I Œ PAN, Monografie 33, t.2. pp. 1001-1009 [Engl. summ.]. Lane, J. 2002. EPA Issues Report on Pilot Capping Project. U.S. Environmental Protection Agency, Region IX. Linkov, I., Burmistrov, D., Cura, J., Bridges, T.S., 2002. Risk-based management of contaminated sediments: consideration of spatial and temporal patterns in exposure modeling. Environmental Science & Technology 36, 238-246. Malina, G., Kwiatkowska, J. 2003. Immobilizacja metali ciê¿kich w œrodowisku gruntowo-wodnym z wykorzystaniem dodatków organicznych. [Immobilisation of heavy metals in soil and groundwater by organic additives] In¿. i Ochr. Srod. 6 (3-4), 371-390 [Engl. summ.]. Malina, G., Szwedowski, D. 2005. Immobilizacja metali ciê¿kich w osadach dennych [Immobilisation of heavy metals in sediments]. Inz. i Ochr. Srod. 8(2), 135-158 [Engl. summ.]. Malina G. 2005. Risk evaluation and reduction of groundwater contamination from petrol stations. In: Lens, P., Grotenhuis, T., Malina, G., Tabak, H. [Eds] Soil and sediment remediation: mechanisms, technologies, applications. Integrated Environmental Series, IWA Publishing, London, UK Part IV, pp. 437-457. Malina, G., Krupanek, J., Sievers, J., Grossman, J., ter Meer, J., Rijnaarts, H.H.M. 2006. Integrated management strategy for complex groundwater contamination at a megasite scale. In: Twardowska, I., Allen, H.E., Haggblom, M.H. [Eds] Viable Methods of Soil and Water Pollution Monitoring, Prevention and Remediation. NATO Science Series. Springer, pp. 567-578. NRC. 2000. Natural Attenuation for Groundwater Remediation. National Academy Press, Washington DC.
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Palermo, M.R., Thompson, T.A., Swed, F. 2002. In-Situ Capping as a Remedy Component for the Lower Fox River. Ecosystem-Based Rehabilitation Plan - an Integrated Plan for Habitat Enhancement and Expedited Exposure Reduction in the Lower Fox River and Green Bay. White Paper No. 6B. The Johnson Company. Palermo, M.R., Clausner, J.E., Rollings, M.P., Williams, G.L., Myers, T.E., Fredette, T.J., Randall, R.E. 1998a. Guidance for subaqueous dredged material capping. Technical Report DOER-1, U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS. Palermo, M., Maynord, S., Miller, J., Reible, D.D. 1998b. Guidance for In-Situ Subaqueous Capping of Contaminated Sediments. EPA 905-B96-004, Great Lakes National Program Office, Chicago, IL. Reible, D. 2003. Comparative Validation of Innovative Capping Technologies Anacostia River. Washington DC. Renholds, J. 1998. In Situ Treatment of Contaminated Sediments. National Network of Environmental Management Studies. US EPA 1994. ARCS Remediation Guidance Document. EPA 905-B94-003. Great Lakes National Program Office. Chicago, Il. US EPA 1999a. Great Lakes Commission. Dredging and the Great Lakes. EPA Great Lakes National Program Office, Chicago, Il. US EPA 1999b. Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites. OSWER Directive 9200.4-17P, -32. 4-21-1999. Washington DC. US EPA 2002. Contaminated Sediment Remediation Guidance for Hazardous Waste Sites. Inter-Agency workgroup led by the U.S. Environmental Protection Agency Office of Emergency and Remedial Response. US EPA 2004. Active capping demonstrated on Anacostia River. USEPA Technology News and Trends, issue 12. Vermeulen, J., Grotenhuis, T., Joziasse, J., Rulkens, W. 2003. Ripening of dredged sediments during temporary upland disposal. A bioremediation technique. Journal of Soils and Sediments 3, 49- 59. Vermeulen, J., van Dijk, S.G., Grotenhuis, J.T.C., Rulkens, W.H. 2005. Quantification of physical properties of dredged sediments during physical ripening. Geoderma 129(3-4), 147-166. Welcome deliverable 9.3. 2003. Contaminated sediments transport tool. Water, Environmental, Landscape Management at Contaminated Megasite WELCOME (EVK1-CT-2001-00103). http://www .euwelcome.nl. Welcome deliverable 9.4. 2004. Modelling sediment transport and contaminant transport in water sysetms. Water, Environmental, Landscape Management at Contaminated Megasite - WELCOME (EVK1-CT-2001-00103). http://www.euwelcome.nl.
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Welcome deliverable 9.5. 2004. Concepts for engineered emission control. Water, Environmental, Landscape Management at Contaminated Megasite WELCOME (EVK1-CT-2001-00103). http://www .euwelcome.nl.
Zalewski, M. 2002. Ecohydrology - the use of ecological and hydrological processes for sustainable management of water resources. Hydrological Sciences Journal. 47, 825-834.
The initial risk assessment and emission control from contaminated sediments
Ecohydrology for Implementation of the European Water Framework Directive
Grzegorz Malina1, Martijn P.J. Smit2, Tim J.C. Grotenhuis2 1
Institute of Environmental Engineering, Czêstochowa University of Technology, BrzeŸnicka 60A, 42-200 Czêstochowa, Poland e-mail: [email protected] 2 Section Environmental Technology, Wageningen University, Wageningen, The Netherlands, e-mail: [email protected]
Abstract Discharged persistent organic pollutants and heavy metals into the rivers are often assumed to adsorb on sediments without re-suspension, thus sediments may function as a sink for contaminants. Sediments, however, may also function as a (secondary) source of contaminants, depending on the hydrodynamic characteristics of a river. To assess whether the sediments function as a sink or a source of contaminants, a preliminary risk assessment using the so-called SEDINA tool (SEDiment INitial Assessment) is recommended. By comparison of the total concentrations of pollutants in sediments upstream, at, and downstream of the site area, the role of sediments can be determined. When the sediments functions as a sink, no specific sediments-related measures are required, although monitoring of the surface water and sediments quality is recommended. If the output of the SEDINA tool reflects that the sediments may function or function as a source of contaminants, the risk-reduction measures are necessary that may include emission control at the source(s), pathway(s) and/or receptor(s). Key words: the SEDINA tool, bioavailability, remediation, persistent organic pollutants, heavy metals.
1. Introduction Long-term diverse activities at industrial areas resulted in direct emissions of waste products to the environmental compartments (atmosphere, surface water and soil/groundwater), which in consequence led to major environmental problems. Since the 1970's, direct emissions to the environmental compartments have been continuously reduced due to a growing awareness of environmental problems. Application of off-gas and wastewater treatment plants, spill control, and
restrictions in the use of some chemicals led to a rapid improvement of both air and surface water quality. Soil and sediment systems respond much slower to these measures and need more time to improve their quality. Moreover, soils and sediments were polluted by numerous spills and landfills resulting in a diffuse pollution on a mega-scale (e.g. brownfields, post-industrial areas, military bases, etc.), which makes the active conventional remediation more difficult. Complete cleanup within an intermediate timeframe (25 years) is usually not feasible, or may even be
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impossible for technical and economical reasons. Therefore, such sites are continuous and long-term potential and actual sources of regional contamination of groundwater, surface water and sediments. Developments in environmental policy are observed since the mid nineties towards changing the approach from multi-functionality into the risk reduction for humans and ecosystems to acceptable levels (Malina 2005). Consequently, remediation goals changed from target oriented into risk oriented. The currently applied risk evaluation procedures are commonly based on the source -pathway-receptor sequence. Large amounts of sediments are dredged worldwide for both maintenance and environmental reasons. For example, 400 106 m3 of sediments are dredged annually by the US Army Corps of Engineers or their permit recipients (Linkov et al. 2002), and about 40 106 m3 are dredged annually in the Netherlands (Vermeulen et al. 2003). On the basis of the SedNet estimation, around 100-200 106 m3 of contaminated sediments might be produced yearly in Europe (Bortone et al. 2004). A substantial proportion of these dredged sediments is held temporarily at above ground (upland) disposal facilities. An upland disposal is a relatively easy and cost-effective alternative for dredged materials and is, therefore, a widely adopted sediment management practice (Vermeulen et al. 2005). As a consequence of anthropopression, water quality has become, to a great extent, dependent on human population density and its whole range of activities that lead to: (i) chemical alteration of water quality through emission of pollutants and (ii) physical degradation of environments. While the emission of pollutants can be controlled by technologies, restoration of the physical degradation of the environment related to modification of hydrological and biogeochemical cycles due to e.g., deforestation, urbanisation, canalisation, requires a new, systemic approach. For that reason the concept of Ecohydrology may be applicable, which suggests that the sustainable development of water resources has been dependent on the ability to maintain the ecosystem processes that have been established by evolution (Zalewski 2002). It concerns how to regulate the biological processes of freshwater ecosystems using hydrology; and, vice versa, how to use biotic ecosystem properties as a tool in water management. At most industrial and urbanised areas, discharges of contaminants into the surface water (rivers, watercourses) may lead to their increased contents in the water phase or unconsolidated sediments. It is often assumed that contaminants strongly adsorb to the sediments and do not resuspend. If so, sediments function as a sink for contaminants. However, sediments may function as a (secondary) source of contaminants, depending on the hydrodynamic characteristics of the water flow
and sediments transport. The goal of this work was to develop a simple tool for preliminary assessment whether the sediments in watercourses and riverbeds at a certain location are functioning as a sink or a source of contamination. In the paper the concept of initial risk assessment to define the role of sediments as a sink or a source of pollution at the site of concern is discussed. Engineering options to reduce/control emissions from sediments to comply with defined threshold values at the planes of compliance are looked at in more details. Possible engineering options can reduce the emission from sediments to receptors in three ways: at a source, during migration (path), and at a receptor. The engineering options take into account the bioavailability concept and the contaminated sediments transport as such.
2. Initial risk assessment Contaminants fluxes from sediments One of the major risks for the aquatic ecosystems and human health is the possible recontamination of the adjacent surface waters, groundwater and atmosphere due to the polluted soil or sediments. This concept for contaminated sediments is shown in Fig. 1 (Welcome deliverable 9.4, 2004). Both, risk and cost efficiency assessments are positioned centrally to manage contaminated sites in a cost effective way. Current and predicted changes of risks at a site are assessed for defined, the so called planes of compliances, i.e. a horizontal or vertical line, at which certain concentrations or mass fluxes have to be achieved, to comply with the site-specific threshold values (Malina et al. 2006). They can be defined at the boundaries of the sites, between different compartments (e.g. groundwater/surface water), and/or between different receptors. At a mega-scale, the interface between sediments and groundwater (flux 4), the interface between sediments and surface water (flux 5), and the boundary of the site (flux 7) can be distinguished as planes of compliance. To facilitate the technical management, the mega-scale can be structured into manageable units, the so-called risk clusters, which comprise areas of similar features with respect to contaminants (sources), hydrogeological conditions (pathways) and affected receptors. Contaminant fluxes into surface water The presence of heavy metals in surface water is mostly dependent upon the pH. At low pH the speciation of metals will usually lead to increased solubility. As the pH of surface water is normally near neutral, only low concentrations of free dissolved metals are expected. Therefore, metals originating from fluxes 6 and 3 will be merely
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7 1 Unsaturated Zone
6
2
5
3 Saturated Zone
Surface water Sediments
4
Geo-hydrological base Fig. 1. Concept of contaminants fluxes between different compartments. 1 - gas/vapour transport between the atmosphere and unsaturated zone, 2 - contaminant transport between unsaturated and saturated zones, 3 - contaminant transport between a saturated zone (groundwater) and surface water, 4 - contaminant transport between a saturated zone (groundwater) and sediments, 5 - contaminant transport between sediments and surface water, 6 - gas/vapour transport between surface water and atmosphere, 7 - contaminant transport within surface water outside - inside the site.
precipitated and present in the sediments, which function as a sink for heavy metals. When environmental conditions change, a remobilization of heavy metals may occur, and the sediments may act as a source of contaminants (flux 5). The remobilization of heavy metals from sediments (flux 5) reported by Kania et al. (2005) occurred when pH was decreased, and the liquid/solid (L/S) ratio, i.e. ratio between water and sediments, was increased. These last findings indicate that during flooding conditions, when the flow of water is increased, remobilization and, therefore, an increase of risk for heavy metal transport to the surface water as a receptor is likely to occur. For hydrophobic organic contaminants, the concentration in surface water will be low as their water solubility is low. Therefore, considering the contaminant fluxes to the surface water, the interactions with direct discharge, deposition (flux 6), the flux within groundwater (flux 3) and the interaction with sediments (flux 5), are of major importance. Mass transfer of contaminants from contaminated soil and groundwater (flux 3) is more related to the interfacial processes (redox) than mass transfer as such. Mass transfer of pollutants from contaminated sediment (flux 5) is the result of three processes: (1) transport of contaminants bound to sediment particles, (2) transport of contaminants bound to dissolved organic matter (DOM), including colloidal materials, (3) transport of dissolved contaminants. The major transport route of pollutants at a site or a cluster depends on site-specific parameters (e.g. hydraulic conditions, type of contaminants,
sediments characteristics, etc). A more thorough assessment of fate and transport of pollutants are required from risk-based viewpoint (ecological and human), as well as for emission control (remediation and engineering options). From a perspective of risk, especially the freely dissolved contaminants are of interest, as they are readily available for uptake and introduction into the food chain. The second, more thorough step in risk assessment requires information about the source and receptor(s), as well as the pathway(s) linking the source and receptor(s). This assessment intends to provide information on the mass fluxes of contaminants at the plane of compliance. Source assessment Assessment of the source involves sediments characterization, the particle size and particle size distribution, organic matter (OM) and DOM contents, quantification of contaminants including their natural attenuation (NA) potentials. Information about bioavailability and eco-toxicological properties of contaminants are particularly useful to make prioritization of the contaminants. Receptor assessment Assessment of the receptors involves a clear definition and must include their properties. In this concept, the surface water plays a crucial role as a pathway for contaminants (e.g. in the case of mobile compounds), and as an intermediate to receptors (e.g. ecosystem, humans). However, in both cases international and national legislations provide appropriate threshold values. Besides of
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being a source of contaminants, the sediments may also be a receptor. In that case target species in the sediment ecosystem must be identified and related risks must be assessed. Pathway assessment The assessment of pathways between sources and receptors involves analyzing the hydraulic conditions of the surface water, water transport through the sediments (e.g. infiltration or welling), and the NA potential in the sediments, at the interface between sediments and the adjacent water, and in the aqueous phase. As a result of the pathway assessment, the fate and transport models can be developed, including desorption of potentially available contaminants or a re-suspension of contaminated sediments, transport of contaminants in the aqueous phase, and (bio)degradation of contaminants during transport. The source, receptor, and pathway mass fluxes from contaminated sediments to the surface water give the total mass fluxes of contaminants at the plane of compliance. When they lead to concentrations above the standards, the emission control measures are needed.
3. The SEDINA tool Discharges of persistent organic pollutants and heavy metals into the rivers may lead to increased contamination of the surface water and/or sediments. Contaminants are often assumed to adsorb on sediments and not to re-suspend. Thus, sediments may function as a sink for contaminants. Observations in number of sites show, however that sediments may also function as a (secondary) source of contaminants, depending on the hydrodynamic characteristics of a river. For an initial indication whether the sediments at a site of concern may function as a sink or a source of contaminants, a preliminary risk assessment using the so-called SEDINA tool (SEDiment INitial Assessment) was developed (Welcome deliverable 9.3, 2004; Grotenhuis et al. 2005). The SEDINA tool is aimed at obtaining an initial assessment of the environmental quality and probable risks related to the sediments at a mega-scale. When further actions are needed, more detailed tools are required for modelling transport of contaminated particles and contaminant transport in water systems. Transport of contaminated particles can be described by the HEC-RAS model (Welcome deliverables 9.3, 2003; Welcome deliverable 9.4, 2004), and the effect of water erosion on the contamination of sediments can be estimated by the European Soil Erosion Model (ESEM) (Welcome deliverable 9.4, 2004). Furthermore, the contaminant transport in water systems can be determined using the bioavailability concept for heavy metals and hydrophobic organic
contaminants. These tools provide both general and site-specific information on contaminants fate and transport to, from and along with the sediments. They can be based on actual and potential risks rather than the presence of contaminants quantified by total concentrations. In the case of the SEDINA tool, three situations can be distinguished for sediments in a (surface) water system at a mega - or cluster - scale. 1. Sediments act as a sink of contaminants coming from the groundwater (flux 4), surface water (flux 5), or sediments transported from upstream (flux 7). In this case the fluxes of contaminants are above zero, and the concentrations in sediments downstream of the site are lower than upstream. 2. Sediments are in equilibrium with contaminants in groundwater (flux 4), surface water (flux 5), and/or sediments upstream and downstream of the site (flux 7). In this case the contaminant fluxes equal zero, and the concentrations in sediments are equal downstream and upstream of the site. 3. Sediments act as a source of contaminants going to groundwater (flux 4), surface water (flux 5), or together with sediments are transported downstream (flux 7). In this case the contaminant fluxes are below zero and, thus contaminants from the site may affect the receptors within and outside the site. The situation can change in time due to altering environmental conditions (e.g. flooding/dry periods, fluctuations of groundwater/surface water tables, and human activities, like dredging). Although risks can be present in all three situations, only emissions in situation 3, when sediments act as a source, must be controlled from a cost-effective point of view. For situation 1 and 2, it is likely to be more cost-effective to control emissions towards the sediments before controlling emissions from the sediments. The input of the SEDINA tool is the total concentration of dissolved and bound contaminants, both upstream and downstream of the site (plane of compliance, flux 7 in Fig. 1). The output is a proposed action, indicated by a "traffic light" approach (Table I), where the green colour represents no action and orange/red colours comprise further action. When the output of the SEDINA tool is green, no further actions are required. However, monitoring the surface water and sediments quality is recommended with respect to future spills or discharges that may affect the aquatic environment. If the output is orange or red, identification of possible sources is recommended. These sources can be the result of current discharges of anthropogenic origin (e.g. wastewater discharge, run-off, spills, etc. (flux 6)), or seepage from historically contaminated soils (flux 3), groundwater (flux 3), or sediments (flux 5).
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Table I. An overview of the SEDINA-tool for the initial risk assessment from contaminated sediments (Grotenhuis et al. 2005). Concentration upstream versus downstream Cup-stream > Cdown-stream Cup-stream = Cdown-stream Cup-stream < Cdown-stream
Concentration levels below target/background values green green orange2
Concentration above intervention levels orange1 red red
green: sediment does function as a sink; no specific actions necessary on the short term; orange: 1incoming concentrations are high, action recommended to find the source of these priority pollutants; 2sediments may function as source of contaminants; action recommended to define the sources; red: most probably the sediments function as a source of contaminants; action recommended to find the probable sources at the site; as total concentrations are above intervention levels at least samples downstream of the site should be taken to obtain insight in bioavailability of the respective contaminants.
4. Emission control concepts Emission control can generally be achieved at the source, within the pathway, or at the receptor. During the risk assessment, information is obtained regarding the characteristics of contaminants, their concentrations, properties of the sediments, and hydraulic conditions of the surface water. This information can be easily used to indicate the best engineering option for emission control of contaminated sediments. When concentrations of pollutants in surface water are (very) high, emission control at the source is preferred. Depending on the mobility and availability of the contaminants, ex-situ or in-situ remediation options can be applied. When concentrations of pollutants are relatively low, but mobility and availability are high, emission control along the migration pathway is preferred. Emission control at the receptor is not feasible when the surface water is considered as the receptor. However, when surface water is used for other purposes (e.g. drinking water) additional treatment can be required to protect other receptors (e.g. humans).
Emission control at the source The concepts of emission control at the source focus on engineering options, however management options like reconstruction of sedimentation basins, construction of wetlands, erosion prevention (e.g. by planting), local extraction and treatment of groundwater, infiltration of clean water, water table management, may also be involved (Renhnolds 1998). Highly mobile and available contaminants High concentrations of mobile and available contaminants at the source pose an increased or even acute risk for the receptors. When contaminants are present in sediments at (very) high concentrations and are strongly mobile (available), dredging is an attractive approach, followed by treatment or controlled disposal similar to the
excavated soil treatment. Ex-situ emission control of polluted sediments at the source can be especially favourable when sediments have to be dredged for nautical or other functional reasons. Currently, there are "environmentally friendly" dredging methods available causing a limited turbidity and, thus a limited re-suspension of contaminated sediments (US EPA 1999a, 2002). Moderately mobile and available contaminants High concentrations of moderately mobile and available contaminants at the source pose high but less acute risk for receptors and, therefore, allow for more extensive emission reduction options. In-situ options for emission control of high concentrations of moderately mobile and available contaminants are different for organic and inorganic contaminants because of their different physical and chemical properties. Basically, organic contaminants can be converted to harmless products (biodegradation, biotransformation), whereas inorganic contaminants cannot. Organic contaminants. Different biological and physical/chemical in-situ engineered options exist to reduce emissions from the contaminated sediments. The choice between biological, physical-chemical, or a combination of biological and physical-chemical methods depends on the biodegradability of the pollutants, and to a lesser extent on the environmental conditions. Examples of in-situ emission control at the source are: NA or enhanced NA (ENA), and (advanced chemical) oxidation. The NA generally refers to physical, chemical or biological processes, which under favourable conditions lead to the reduction of toxicity, mobility, volume or concentration of organic contaminants in soil and/or groundwater, which are all related to mass reduction (US EPA 1999b; NRC 2000). The reduction takes place as a result of processes such as biological or chemical degradation. When conditions of NA are not favourable at the site, the ENA can be an option. ENA is the
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speeding-up of natural physical-chemical and biological processes of self-purification taking place in the subsoil and aqueous environment by: enhancing the sorption capacity (immobilization, (bio)precipitation), providing electron acceptors/donors, nutrients for biodegradation (biostimulation), and/or microorganisms (bioaugmentation). Under anaerobic conditions, organic contaminants can ultimately be metabolized to methane, limited amounts of carbon dioxide, and traces of hydrogen. Advanced chemical oxidation can be applied with different oxidants. As OM content in sediments is often higher than in soil, advanced chemical oxidation may be less successful than in the case of soil treatment. The chemical oxidants most commonly employed to-date include peroxide, ozone, and permanganate. They may cause the rapid and complete chemical destruction of many toxic organic chemicals; other organics are amenable to partial degradation as an aid to subsequent bioremediation. In general, the oxidants have been capable of achieving high treatment efficiencies (e.g. >90%) for unsaturated aliphatic (e.g. trichloroethylene (TCE) and aromatic compounds (e.g., benzene), with very fast reaction rates (90% destruction in minutes) (Welcome deliverable 9.4, 2004). Field applications for soil confirmed that matching the oxidant and the in situ delivery system to the contaminants of concern and the site conditions, are the keys to successful implementation and achieving performance goals (http://w ww.frtr.gov). Heavy metals. Examples of in situ emission control from heavy metals contaminated sediments at the source are: electro-reclamation, physical immobilization, and biological immobilization (bio-sorption) (Welcome deliverable 9.5, 2004). The principle of electro-kinetic remediation relies on application of a low-intensity direct electric current through the sediments between ceramic electrodes that are divided into a cathode and an anode arrays. This mobilizes charged species, causing ions and water to move toward the electrodes. Metal ions, ammonium ions, and positively charged organic compounds move toward the cathode. Anions, such as: chloride, cyanide, fluoride, nitrate, and negatively charged organic compounds move toward the anode. The electric current creates an acid front at the anode and a base front at the cathode. This generation of acidic conditions in situ may increase mobility of adsorbed heavy metals and enhance their transport to the collection system at the cathode (http: //www.frtr.gov). Two primary mechanisms that are responsible for contaminants transport through the sediments towards one or the other electrodes are: electro-migration and electro-osmosis. In electro-migration, charged particles are trans-
ported through the substrate. In contrast, electroosmosis is the movement of liquid containing ions relative to a stationary charged surface. Of the two, electro-migration is the main mechanism of the process. The directions and rates of movement of ionic species depend on their charge and the magnitude of electro-osmosis - induced flow velocities. Non-ionic species, both inorganic and organic, will also be transported along with the electro-osmosis - induced water flow. Immobilization techniques like solidification/stabilization reduce the mobility of contaminants through both physical and chemical means. Unlike other remedial technologies, they seek to trap or immobilize contaminants within their "host" medium (i.e. soil, sediments, and/or building materials that contain them), instead of removing them through chemical or physical treatment. These techniques can be used alone, or combined with other treatment and disposal methods, to yield a product or material suitable for land disposal or, in other cases, that can be applied for beneficial use. They have been used as both final and interim remedial measures (http://www. frtr.gov). Under sulphate-reduction conditions, sulphate is converted to sulphide or elemental sulphur, which can lead to precipitation of heavy metals. As a result of successful applications for treating metal-laden industrial wastewaters, precipitation is being considered and selected for remediation of groundwater containing heavy metals, including their radioactive isotopes. Metals precipitation from contaminated water involves the conversion of soluble heavy metal salts to insoluble salts that may precipitate, which can be realized by addition of an electron donor (Malina, Kwiatkowska 2003).
Emission control within the pathway Emission control within the pathway is preferred when concentrations of contaminants are relatively low and their availability and mobility are relatively high. Emission reduction measures mainly focus on retardation of contaminants to comply with the standards set for the planes of compliance, and are applicable for organic and inorganic contaminants. Retardation measures can possibly be combined with mass reduction of contaminants. Organic and inorganic contaminants can be transported in three ways to and through the surface water: bound to sediment inorganic particles, bound to OM and dissolved in water (Welcome deliverable 9.4, 2004). Depending on site-specific parameters (e.g. hydraulic conditions, type of pollutant, sediment characteristics, etc) the major transport route can be assessed. These three ways of transport require different approaches of emis-
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sion control. Under the conditions of net natural sedimentation the mass flux of contaminants through the plane of compliance is probably negligible and no engineering options are required. Engineering options to reduce contaminant emission by blocking their transport to the surface water can be realized by physical and physicalchemical methods. Physical methods can be used to reduce emission of pollutants bound to sediment particles and OM, whereas physical-chemical options reduce emissions of dissolved pollutants and pollutants bound to DOM. Physical options Physical engineering options can be used to reduce transport of contaminated sediments as such in a river-channel, and to block the contaminants mass transfer from sediments to the surface water. Natural capping. Natural capping only occurs in net sedimentation water systems (e.g. delta areas). However, by widening a river-channel, water flow velocity decreases and sedimentation will increase. The sediment movement is reduced, which may lead to anoxic conditions, stimulating precipitation of contaminants. Some efforts on this concept were made for acid mine drainage by Evangelou and Zhang (Welcome deliverable 9.5, 2004). To prevent acidification of a river system they designed a wetland treatment system to increase the average residence time of heavy metals under sulphate reducing conditions, and thus stimulate precipitation of heavy metal complexes. Artificial capping. This approach should be used where the long-term physical integrity of the cap can be maintained, and environments with low turbulence are generally desired for in-situ capping projects. The potential severity of the environmental impacts associated with cap erosion and potential dispersion of the sediment contaminants in an extreme event should determine the level of protection against erosion (http://www .epa.gov/glnpo/sediment/iscmain). During capping the contaminated sediments are covered by non-contaminated materials, and this method can be applied individually or in combinations, both in situ and ex-situ (e.g. heavily contaminated sediments to be dredged and slightly contaminated - capped) (Lane 2002). The most common modifications of in situ capping (ISC) include: (i) capping with partial dredging, (ii) residual capping (RC) (Malina, Szwedowski 2005). During ISC with partial dredging, contaminated sediments are removed down to a certain depth, and the remaining layers are covered with non-contaminated materials. This method is used
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when a certain depth of the watercourse has to be assured (shipping), the deeper sediment layers are required to keep the stability of riverbanks, and/or the deeper layers are heavily contaminated and their dredging may pose a serious risk to the environment. In the case of RC a thin layer of clean materials is deposited on sediments remained after dredging. Due to re-suspension a part of sediments is suspended in water, creating after some time a thin (of few centimetres) residual layer. The RC speeds up natural recuperation processes in the aquatic ecosystems. The covering layers are in that case not designed as the isolating layers. Another ISC approach is to place sediments dredged from other locations with subsequent covering with non-contaminated materials, similarly as during conventional capping (US EPA 1994). This is the so-called Dredged Material Capping (DMC), and it can be realized by distribution of dredged sediments in oceans and coastal waters. The covered sediments can be both nontreated or treated. The DMC is usually used in the case of renovation of waterways, as: (i) Level Bottom Capping (LBC), (ii) Contained Aquatic Disposal (CAD). The covering layer can fully or partially reduce the effect of contaminated sediments on aquatic ecosystems due to (Malina, Szwedowski, 2005): - physical isolation of contaminated sediments from benthos (decreased hazards to organisms and bioaccumulation in the food chain), - stabilisation of contaminated sediments (protection against erosion, reduction of re-suspension and transport), - chemical isolation and limited transfer of soluble and colloidal contaminants to the water phase. The most common capping materials are: sand, clay and silt, gravel, break stone, non-contaminated sediments, as well as geotexitles (Palermo et al. 2002; Palermo et al. 1998a, b; http://www.hsrc.org/capping). Applying geotextiles requires special equipments and, in practice was done only in shallow watercourses. Currently, in the USA studies are carried out on the novel group of active materials for capping to be applied for, the so-called Active Capping (AC) technology. These materials are: (i) AquablokTM (gravel and break stone covered by clay layers), (ii) zerovalent iron - Fe0, (iii) apatite - Ca5(PO4)3OH, (iv) BionSoilTM (materials from compost), (v) Organo-Clay Sorbents, (vi) coke, (vii) activated carbon, (viii) laminated mats (Reible 2003; US EPA 2004). Physical-chemical options Physical-chemical engineered options can be used to reduce transport (i.e. mass transfer) of dissolved contaminants, including contaminants adsorbed to DOM. With the selected diffusion
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model it was shown that transport of contaminants depends strongly on the effective diffusion rate and, thus on their sorption affinity to the sediment particles (Welcome deliverable 9.4, 2004). Physical-chemical emission control is based on the reduction of the effective diffusion rate by increasing the affinity of the contaminants to the sediment particles. Chemical isolation. Even if contaminant concentrations are high in the pore water, a granular cap component may act as a filter and/or a buffer during advective and diffusive transport. These uncontaminated granular cap materials can be expected to remove contaminants or at least reduce their concentrations in pore waters through sorption, ion exchange, surface complexation, and redox mediated flocculation. The removal degree is very much dependent on the cap materials. A cap composed of quarry-run sand would not be as effective as natural sand with fine fractions and the OM content (http://www.epa.gov/glnpo/sediment/iscmain). Reduction of the effective diffusion rate can be achieved by adding an extra sorbent to the sediments. For organic contaminants such sorbent should be hydrophobic, whereas for inorganics it should have a high specific surface area. Examples of additives are: activated carbon and metal binding additives. The group of Luthy (Welcome deliverable 9.5, 2004) explored the applicability of mixing activated carbon with sediments to reduce the aqueous concentration of PCBs and PAHs.
Emission control at the receptor Emission control at the receptor, such as i.e. surface water, is not feasible. Depending on the function of surface water, an additional water treatment can be required but these aspects are beyond the scope of this study.
5. Concluding remarks As many sites are located in a delta area or along the river course, they all have to deal with the contaminated sediments regarding the quality of surface water that is considered as one of the major receptors for contaminants in the environment. To perform an initial risk assessment caused by contaminated sediments at a site the SEDINA tool is postulated. By comparison of the total concentrations of contaminants in bed and suspended sediments upstream, at the site and downstream of the site area, the role of the site as a sink or a source of contaminants can be defined. When the site functions as the sink no specific measures related to the sediments have to be undertaken, although
monitoring of the surface water and sediments quality is recommended. If the output of the SEDINA tool reflects that the site may function or functions as a source of contaminants, further investigations are necessary prior to the risk-reduction measures that may include emission control at the source(s), pathways and/or receptor(s). Several details can be found based on fate and transport modelling. The amounts of contaminated sediments that can be transported to or from the site can be calculated using properly selected sediment transport models. Also the erosion by water may affect the risk management as such. Besides the flux of contaminated particles into and from the site, information is necessary about transport of contaminants (heavy metals, hydrophobic organic contaminants) within the sediments and from the sediments to the water system. Depending on the mobility and availability of the contaminants, ex-situ or in situ remediation options can be applied. If the availability and mobility are high, dredging with subsequent exsitu treatment or proper disposal may be favourable. When availability and mobility are moderately low, in situ options can be applied. Emission reduction measures mainly focus on reduction of contaminants to comply with the standards set for the plane of compliance. Still most often risk assessment is based upon total concentrations of contaminants, which may lead to overestimation of risks. In several scientific disciplines like ecotoxicology, biology, as well as in biodegradation studies in microbiology and environmental technology, it was shown in the last decade that only the bioavailable fractions of organic contaminants are causing risks, and consequently they should be taken into account in risk assessment. Therefore, it is expected that legislation should be based on 'real' risks of contaminants in the field. For risk determination the sound-based scientific methods have to be developed. All bioavailability methods based on physical/chemical techniques show that much less than the total concentrations of the pollutants can be desorbed in limited timeframe, leading to less risk compared to the use of total concentrations. Therefore, the application of bioavailability methods may lead to an improvement of the risk assessment and, subsequently, to more cost effective remediation.
Acknowledgements This study was done within the frame of the 5 EU Project Water, Environmental, Landscape Management at Contaminated Megasite WELCOME (EVK1-CT-2001-00103). It was also supported by the Polish Ministry of Sciences and Informatization - Committee for Scientific Research (KBN) (154/E-358/SPB/5.PR UE/DZ 165/2002-2004).
Sediment initial risk assessment and emission control
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