2 The Role of Modeling in Managing Contaminated Sediments prepared by Danny D. Reible with contributions by Sam Bentley, Mimi B. Dannel, Joseph V. DePinto, James A. Dyer, Kevin J. Farley, Marcelo H. Garcia, David Glaser, John M. Hamrick, Richard H. Jensen, Wilbert J. Lick, Robert A. Pastorok, Richard F. Schwer, C. Kirk Ziegler CONTENTS 2.1Introduction 2.1.1SigniÞcance and Objectives 2.1.2Status of Contaminated Sediment Management 2.1.3Contaminated Sediment Modeling Applications 2.1.3.1Conceptual Site Model Development and Testing 2.1.3.2Baseline Risk Assessment 2.1.3.3Evaluation of Total Maximum Daily Loads 2.1.3.4Comparative Evaluation of Remedial Management Plans 2.2State of Knowledge and Practice 2.2.1Relation between Sediment and Common Contaminants 2.2.2Sediment Transport Model Components 2.2.2.1Sediment Erosion and Deposition Processes 2.2.2.2Sediment Transport Model Minimum Requirements 2.2.3Contaminant Fate and Transport Model Components 2.2.3.1Contaminant Fate and Transport Processes in Unstable Sediments 2.2.3.2Contaminant Fate and Transport Processes in Stable Sediments 2.2.3.3Contaminant Transference via Food Webs 2.2.3.4Human and Ecological Risk Evaluation 2.2.4Model Calibration and Uncertainty 2.2.4.1Sources of Model Uncertainty L1667_book.fm Page 61 Tuesday, October 21, 2003 8:33 AM ©2004 CRC Press LLC 2.2.4.2Techniques for Calibrating a Model and Evaluating Uncertainty 2.2.4.3Measures of Model Acceptability 2.3Challenges and Emerging Issues 2.3.1Cohesive Sediment Erosion and Transport 2.3.2Contaminant Release and Availability 2.3.3Advective Processes in the Hyperheic Zone 2.3.4Bioturbation as a Sediment and Contaminant Transport Mechanism 2.3.5Contaminant Bioaccumulation and Effects in Benthic and Higher Organisms 2.4Summary of Research and Data Needs 2.4.1Sediment Transport Process Modeling 2.4.2Contaminant Process Modeling 2.4.3Biological Process Modeling 2.4.4Metals Release and Availability 2.4.5Hydrophobic Organic Contaminant Release and Availability Acknowledgments References 2.1 INTRODUCTION 2.1.1 S IGNIFICANCE AND O BJECTIVES Contaminated sediment management poses some of the most difÞcult site remediation issues today. Contaminated sediments typically reside in spatially variable and dynamic systems subject to seasonal ßow variations and episodic storm events. The volume of sediments that must be managed often exceeds 1 million yd 3 , dwarÞng many contam- inated soil sites. These sediments are also associated with equally daunting volumes of water, and efforts to remove the contamination typically entrains even more water. The National Sediment Quality Survey (United States Environmental Protection Agency [USEPA], 1998) classiÞed 26% of 21,000 freshwater and estuarine sediment sampling stations in the U.S. as Tier 1 (i.e., adverse effects on aquatic life or human health are probable) and 49% as Tier 2 (i.e., adverse effects on aquatic life or human health are possible but expected infrequently). The realization of these potential risks depends in part on the degree of conservatism built into the toxicological assumptions and in part on the processes controlling both contaminant release from the sediments and the transfer to benthic, aquatic, and land-based organisms. Observations of impairments in ecological or human health can indicate potential pollution problems; however, linking these adverse effects to contaminated sediments requires an under- standing of the processes leading to exposure and uptake. In addition, the selection of cost-effective and environmentally protective remedial alternatives is dependent upon the ability to predict the risks during implementation and into the future. Conceptual models can establish hypotheses as to the links between current or potential exposure to contaminated sediments and the risk to human and ecological health. Testing these hypotheses, however, generally requires translation of the L1667_book.fm Page 62 Tuesday, October 21, 2003 8:33 AM ©2004 CRC Press LLC conceptual model into quantitative form. Quantitative models can be used to answer such questions as the following: •Is the observed exposure and risk consistent with identiÞed sources of that risk? •What are the most important source areas and exposure processes and pathways? •What data can be collected to characterize these important processes and pathways most accurately? •What interventions can be most effective in responding to these processes and pathways? •What are the future exposure and risks if the sediments are managed by •Natural processes? • In situ containment or treatment technologies? •Removal and ex situ treatment or disposal? •How does uncertainty in processes and pathways and the parameters that characterize them translate into uncertainty in current and/or potential future risks? In making decisions about contaminated sites, the use of quantitative modeling to answer these questions is a critical link between observing current exposure and risk (i.e., deÞning baseline risk) and comparing and selecting management approaches that effectively minimize or control that risk. This chapter summarizes applications of quantitative prognostic models of contaminant processes in sedi- ments, assesses the state-of-the-art of these models with respect to accuracy and adequacy, and identiÞes research that can contribute to improvements in model development and their use in resolving sediment management challenges. 2.1.2 S TATUS OF C ONTAMINATED S EDIMENT M ANAGEMENT The goals for contaminated sediment management were identiÞed by the USEPA (1998) to include the following: • Prevent further contamination of sediments that may cause unacceptable human health or ecological risks. • When practical, clean up existing sediment contamination that adversely affects the nation’s waterbodies or their uses or that causes other signiÞ- cant effects on human health or the environment. • Ensure that sediment dredging and the disposal of dredged material con- tinue to be managed in an environmentally sound manner. •Develop and consistently apply methodologies for analyzing contami- nated sediments. Sediment modeling can assist in achieving these goals by helping to quantify the importance of potential sources of sediment contaminants and by predicting sediment fate and transport processes that inßuence exposure and risk. SpeciÞcally, L1667_book.fm Page 63 Tuesday, October 21, 2003 8:33 AM ©2004 CRC Press LLC models can be used to evaluate the effect of extreme events, the likelihood that existing sources can lead to sediment recontamination, and the contribution of sediments to the pollutant burden faced by the ecosystem. In addition, models can be used to compare the effectiveness of various sediment management approaches. Contaminated sediment sites are often poorly controlled, dynamic systems con- taining large volumes of moderately contaminated material. An analysis of Super- fund Records of Decisions from 1982 to 1997 (USEPA, 1999) showed that the average contaminated soil site considered for ex situ treatment contained 38,000 yd 3 of contaminated material; for in situ treatment, the total was approximately 105,000 yd 3 of contaminated material. Contaminated sediment sites, however, often contain in excess of 1,000,000 yd 3 of contaminated material and generally are not directly accessible. Soils can be removed in a relatively dry state for further processing, whereas sediments are removed as slurries with a high proportion of water that must be treated. The assessment and control of contaminant releases when removing submerged sediments is also much more difÞcult than when removing soil for ex situ treatment. This difÞculty is the result of limited control over the aquatic envi- ronment as well as the chemical and physical changes the sediment undergoes during removal (e.g., anaerobic to aerobic and wet to dry). Many of the potential technologies for contaminated sediment management were initially developed to manage contaminated soils. Unfortunately, many of these technologies are either difÞcult to apply or impose potentially unacceptable risks when applied to contaminated sediments. Identifying, comparing, and selecting remedial options for contaminated sediment is also complicated by the multiple technologies often involved. For example, ex situ treatment or sediment disposal typically introduces a complete train of technologies, including removing material by dredging, temporarily storing or pretreating to reduce water content or volume, treating or disposing of Þnal dredged material, and managing any residually con- taminated materials. Large contaminated sediment sites generally require applying different options at different areas on-site, each containing multiple technologies. Therefore, identifying sediment management and remediation options must recog- nize the entire train of technologies that constitute each option so that a fair evalu- ation and comparison of these options can be accomplished. Risk reduction has been generally accepted as the metric by which various options are judged and selected. Use of this metric, however, places a premium on the quantitative modeling required to link the sediments to exposure and risk. Evaluating management or remedial options requires deÞning remedial action goals and objectives and developing a valid conceptual model of the sediment system to be remediated. At all but the most trivial sites, a sophisticated quantitative model can be helpful or necessary to develop and test the conceptual model and evaluate the effectiveness of various management options in meeting the remedial action goals and objectives. Large, complicated sites posing substantial risks and potentially large cleanup costs generally require the development of an extensive database and sophisticated prognostic models in order to compare management options and eval- uate potential risk reduction adequately. There is no generally accepted option for managing contaminated sediments at all sites. Removal approaches typically have been focused on a portion of the L1667_book.fm Page 64 Tuesday, October 21, 2003 8:33 AM ©2004 CRC Press LLC contaminated sites where contaminant levels are the highest and the potential success in terms of reduction of risk is limited to the extent that the “hot spot” contributes to the overall risk of the site. Due to incomplete removal, resuspension during removal, and the resulting residual contamination, the effectiveness of removal options is closely linked to the natural setting and processes inßuencing the sediment contaminants. Nonremoval options are also closely connected to natural fate and transport processes. Linking these processes to exposure and risk is dependent on modeling. Most sediment contaminants are relatively refractory at least below the upper few inches of sediment and, therefore, are unlikely to exhibit substantial attenuation by fate processes such as microbial degradation except over very long time scales. Thus, quantitatively predicting exposure and risks far into the future is often required. In addition, most sediment contaminants are strongly associated with the solid phase, and sediment transport can control contaminant fate. Therefore, modeling natural exposure and fate processes entails describing river hydraulics and sediment migration as well as contaminant transport. The sections below discuss some of the speciÞc applications of contaminated sediment modeling. 2.1.3 C ONTAMINATED S EDIMENT M ODELING A PPLICATIONS 2.1.3.1 Conceptual Site Model Development and Testing A conceptual model is necessary to deÞne the fundamental relationship between contaminant levels in the sediment and levels of exposure and risk to human health and the environment. A conceptual model identiÞes any ongoing sources that can lead to sediment recontamination, mechanisms that can move contaminants from sediments to receptor organisms, and fate processes that can reduce available contaminant concentrations or their effects on receptor organisms. Some of the processes that deÞne exposure within a conceptual model are depicted in Figure 2.1. A valid conceptual model is necessary to identify which remedial options have the potential to address the most important contaminant processes effectively. The conceptual model is the foundation on which site management actions are identiÞed and implemented. Although a conceptual model need not be quantitative, comparison with a quan- titative model can help identify and test a conceptual model. For example, the question as to whether all sources have been identiÞed can be answered by the ability to quantitatively predict the extent of contamination in water and biota based on the recognized sources. An inability to reproduce the observed patterns of contamination can suggest that additional sources exist. Similarly, an inability to predict contami- nant ßux from sediment to water based on presumed mechanisms and processes can indicate that additional processes are operative. 2.1.3.2 Baseline Risk Assessment Within a risk-based, decision-making framework, the existing or baseline risks deÞne the signiÞcance of a contaminated sediment problem and the need for management or remediation. Although Þeld measurements can identify contaminant levels in the environment and body burdens in potentially affected species, establishing a cause- L1667_book.fm Page 65 Tuesday, October 21, 2003 8:33 AM ©2004 CRC Press LLC and-effect relationship between the two normally requires using a numerical model. Generally, it is not possible to relate sediment concentrations to risks without iden- tifying and quantifying fate and transport processes that lead to exposure, assimila- tion, and effect. Quantitative modeling can be used to assess important pathways and processes and compare various contaminated sediment management approaches. The processes and their relative importance vary widely in different sediment environments as illustrated in Table 2.1. Current exposure and risk and the predicted attenuation of contaminants as a result of these natural processes serves as a baseline with which to compare active management approaches. Constitutive relationships and measure- ments of the parameters within those relationships are critical to the quantitative descriptions of these processes. 2.1.3.3 Evaluation of Total Maximum Daily Loads Contaminated sediments fall under the purview of several regulatory programs, including the total maximum daily load (TMDL) program established by 303(d) of the Clean Water Act. A TMDL is a calculation of the maximum amount of a pollutant that a waterbody can receive and still meet water quality standards. This TMDL must be allocated to the various sources of the pollutant. A TMDL must FIGURE 2.1 Potential water column and sediment processes inßuencing contaminant trans- port and fate in a river segment. Deep Sediments Biologically Active Zone Overlying Water Ground Water Exchange Fate Processes Burial/Exposure Source Area Vaporization/Deposition Resuspension Erosion Particles Dissolved Colloidal Advection Dispersion Advection Dispersion Nonparticle Exchange Processes L1667_book.fm Page 66 Tuesday, October 21, 2003 8:33 AM ©2004 CRC Press LLC also include a margin of safety to account for uncertainty and must consider seasonal variation. Of the approximately 20,000 waterbodies currently slated for TMDL develop- ment, at least 300 are expected to be impaired speciÞcally by sediments. Many additional bodies of water can be more impaired as a result of the contribution of pollutants from contaminated sediments or through the effect of sediment processes (e.g., sediment oxygen demand) on overlying water quality. Predictive modeling is a key tool in establishing TMDLs because the model provides the needed link between pollutant loadings to a waterbody and the effect of these loadings on attaining water quality standards. Modeling is especially impor- tant in situations such as contaminated sediment sites where it is difÞcult to directly TABLE 2.1 Sediment Processes and Their Relationship to Various Sediment Environments Environment Environmental Characteristics Key Fate and Transport Processes Lacustrine Low energy environment Generally depositional environment Ground water interaction decreasing away from shore Organic matter decreasing with distance from shore Often Þne-grained sediment Sediment deposition Water-side mass transfer limitations Ground water advection in near-shore area Bioturbation (especially in near-shore area) Diffusion in quiescent settings Metal sequestration Aerobic and anaerobic biotransformation of contaminants of concern (COCs) Biotransformation of organic matter Riverine Low to high energy environment Depositional or erosional environment Potential for signiÞcant ground water interaction Variable sediment characteristics (Þne to coarse grained) Local and generalized ground water advection Sediment deposition and resuspension Aerobic biotransformation processes in surÞcial sediments (potentially anaerobic at depth) Bioturbation Estuarine Generally low-energy environment Generally depositional environment Generally Þne-grained sediment Bioturbation Sediment deposition Water-side mass transfer limitations Aerobic and anaerobic biotransformation of COCs Biotransformation of organic matter Uptake and biotransformation in plants Coastal marine Relatively high-energy environment, decreasing with depth and distance from shore Often coarse sediments Bioturbation Sediment erosion and deposition Localized advection processes L1667_book.fm Page 67 Tuesday, October 21, 2003 8:33 AM ©2004 CRC Press LLC measure how much of the pollutant loading can be attributed to speciÞc sources. However, most of the tools used to establish TMDLs, including the USEPA’s Better Assessment Science Integrating Point and Nonpoint Sources (BASINS) system, do not include state-of-the-art sediment modeling capabilities. TMDLs can also be designed to eliminate sediment toxicity. Although sediment toxicity can be determined based on biological assays, the allocation of point or nonpoint contaminant sources can be related to sediment concentrations through only a numerical model. Numerical models must be able to deÞne contaminant access and availability and the assimilative capacity of the speciÞc receptor at risk. In addition, numerical modeling is required to determine the ability of management approaches (including natural attenuation and recovery) to eliminate sediment tox- icity. Numerical models can also be used as a foundation for allocating allowable pollutant discharges to point and nonpoint sources. 2.1.3.4 Comparative Evaluation of Remedial Management Plans The primary goal of contaminated sediment management is to protect resources at risk such as human or ecological health, commercial or recreational Þshing stocks, or a particular endangered species. Ideally, management and remedial options that best protect affected resources or lead to resource recovery should be selected. Because measurements can only hope to indicate the current state, quan- titative models must be used to allow comparison of the future effect of various scenarios. SigniÞcant questions remain regarding how best to use the forward projections in time. In principle, models provide concentrations or rates of expo- sure as a function of time and place. This information provides a basis on which risk and effects can be estimated. However, both the assessment of future concen- trations and the rates of exposure, along with the assessment of future risks and effects, are subject to great uncertainty. An alternative to evaluating and comparing management options is to employ contaminant mass ßows as a surrogate measure of exposure and, ultimately, risk. That is, a technology can generally be assumed to pose less exposure and risk if it leaves less residual contamination and loses fewer contaminants to the air and water than does an alternative technology. The evaluation of contaminant mass ßows for each management option can be most useful in the comparative evaluation by providing a systematic screening tool. A comparative analysis of mass ßows can also help identify those components of an overall management strategy that largely control the overall exposure or risk and, therefore, should receive the most resources and effort for detailed evaluation. In this manner, screening sediment management alternatives can ensure that all needed components of any given option are included for subsequent evaluation. Even a comparative analysis of mass ßow generally requires sophisticated modeling of contaminated sediment fate and transport processes. Although contaminant mass ßow can be useful, exposure and risk to human and ecological health ultimately drives the need for remediation and the success or failure of any management or remedial option. In particular, contaminant mass L1667_book.fm Page 68 Tuesday, October 21, 2003 8:33 AM ©2004 CRC Press LLC ßow is not very helpful in balancing short-term acute risks with long-term risks. Contaminated sediment removal, for example, tends to lead to increased risks in the short term in exchange for potential reduced long-term risk. It is important to note that in situ sediment management approaches are always subject to potential failure because of future events. Modeling can provide a basis for identifying the magnitude of the potential future exposures and risk that the various management options can pose. 2.2 STATE OF KNOWLEDGE AND PRACTICE A variety of models have been used to predict sediment and contaminant behavior, fate, and effects in ecosystems. Thoms et al. (1995) summarized the capabilities of some of these models and showed the variety of transport and fate pathways that they describe. This section summarizes some of the key processes that characterize the fate and transport pathways. 2.2.1 R ELATION BETWEEN S EDIMENT AND C OMMON C ONTAMINANTS Most priority pollutants and other contaminants of concern (COCs) are strongly associated with solids and therefore tend to accumulate in sediments. Contaminants that do not strongly associate with solids (e.g., polar organic compounds, soluble metals) rarely represent sediment contaminants in that they are efÞciently released to the overlying water. Historical industrial and municipal efßuents and runoff are often responsible for sediment contamination because only those contaminants that tend to partition strongly to the solid phase remain in their historical location. Soluble and volatile contaminants tend to be transported away by water movement or are released to the air via evaporative processes. Similarly, when more soluble and volatile contaminants initially contaminate sediments, their mobility ultimately allows them to migrate into more mobile phases, effectively eliminating them from the sediments. There are exceptions to these general rules when contaminants are continuing to be introduced to the sediments from ground waters or from active sources, but the bulk of sediment contaminants are those that strongly associate with the solid phase. The extent to which sediments are associated with the solid phase is deÞned by the effective sediment–water partition coefÞcient, K sw . This coefÞcient is deÞned as the ratio of the concentration of contaminant on solids, W s (milligrams per kilogram [mg/kg]), to the water concentration, C w (milligrams per liter [mg/l]). Given a density of solids, r s (kg/l), the fraction of contaminant associated with the solid phase, f s, is given by the following equation: (2.1) For large K sw values, as would be expected for most sediment contaminants, the fraction associated with solids approaches unity unless the density of solids is small. f K K s ssw ssw = + r r1 L1667_book.fm Page 69 Tuesday, October 21, 2003 8:33 AM ©2004 CRC Press LLC For hydrophobic organic compounds, the commonly employed linear, reversible model is that the sediment–water partition coefÞcient is given by ( K oc )( f oc ) where K oc is the organic carbon–based partition coefÞcient and a measure of the compound hydrophobicity, and f oc is the fraction organic carbon that serves as the solution phase in the solid. A moderately hydrophobic sediment contaminant might be pyrene, a polycyclic aromatic hydrocarbon (PAH) with a K oc of approximately 10 5 l/kg. A typical sediment organic carbon is of the order of 1%, suggesting a K sw of about 1,000 by this model. Thus, in the sediment bed, where r s is large (of the order of 1 kg/l), a K sw of 1,000 l/kg suggests that about 99.9% of the contaminant would be associated with the solid phase. In the overlying water, however, if the suspended sediment concentration is low (perhaps 100 mg/l or less), the majority of the con- taminant can be dissolved and not associated with the solid phase. Similarly, changes in redox conditions for resuspended sediment can cause metal releases that might normally be associated with sediments in a stable bed. The strong association of contaminants with the solid phase in a sediment bed, however, suggests that contaminant fate often is deÞned largely by sediment mobility and fate. This section examines some of the most important sediment contaminants and their physical and chemical characteristics that relate to fate and mobility in the environment. These contaminants are described below and include heavy metals, oxygen-demanding contaminants in sediments, undifferentiated oil and grease, pes- ticides, polychlorinated biphenyls (PCBs), and PAHs. • Heavy Metals The toxic elements include antimony, arsenic, beryllium, cadmium, copper, lead, mercury, nickel, silver, thallium, and zinc. These pollut- ants are important in that they are nonbiodegradable, toxic in solution, and subject to biomagniÞcation. The chemistry of many of these com- pounds is complex in sediments. A portion is generally chemically Þxed and largely unavailable to Þsh and higher organisms without chemical changes in the sediment. Often a portion is ion exchangeable that can become available simply with the addition of a more strongly held con- taminant. Finally, a portion is soluble, mobile, and directly available for uptake by organisms. Myers et al. (1996) indicate that the partition co- efÞcient between the leachable fraction and the water is typically be- tween 3 and 10, resulting in a leachable fraction of metals that is typically less than 10% and sometimes much less. The equilibrium state for metals and other elemental species depends on the chemical state of the water and sediment, particularly the pH and ox- idation–reduction conditions. The ratio of sediment loading to equilibri- um water concentration is often very large for metals, but only a small fraction of the metals are typically available. As a result of this variability, a site-speciÞc measurement of the sediment–water partition coefÞcient is preferred over any predictive approach. The ultimate or potential avail- ability of many metals appears to be controlled by the presence of acid volatile sulÞdes (AVS) in the sediments in which they reside. The term “AVS” refers to the manner in which sulÞde presence is measured. If the L1667_book.fm Page 70 Tuesday, October 21, 2003 8:33 AM ©2004 CRC Press LLC [...]... overlying water and in the sediment bed with effective mass transfer coefÞcients ks and kw, with units of length and time, the ßux between the bulk sediment and the sediment water interface (at concentration Wsi) can be written as follows: Flux = ks rs (Ws - Wsi ) (2. 5) Similarly, the ßux between the sediment water interface (at concentration Cwi) and the overlying bulk water can be written as follows: 20 04... 2. 2 .2 SEDIMENT TRANSPORT MODEL COMPONENTS 2. 2 .2. 1 Sediment Erosion and Deposition Processes As outlined above, a critical component of any attempt to describe the behavior of sediment contaminants involves a need to describe the migration and fate of the sediments with which they are associated The dominant characteristics that control the direct exposure of Þsh and higher animals to contaminated sediment. .. past, oil and grease or TPH levels in sediments often represent excellent indicators of historical pollution Pesticides The pesticide priority pollutants are generally chlorinated hydrocarbons They include compounds such as aldrin; dieldrin; 1,1,1-trichloro2 , 2- bis(p-chlorophenyl)ethane (DDT); 1,1-dichloro -2 , 2-bis(p-chlorophenyl)ethane (DDD); endosulfan; endrin; heptachlor; lindane; and 20 04 CRC Press... distribution, and type of organisms are different 20 04 CRC Press LLC L1667_book.fm Page 96 Tuesday, October 21 , 20 03 8:33 AM Water Column 23 4 Bioturbation Styles, Depths, and Rates S70-S50, Eel Shelf S60 x1 Th XS Excess activity (dpm/g) 0.1 1 0 Db = 19 cm2 /year 2 0 to 5 cm: Biodiffusive, Db = 5 to 30 cm2 /year 10 5 to 15 cm: Bioadvection, 5- to 70-year turnover Depth (cm) 4 6 8 10 12 14 16 6 to 50... hydrolysis: Zn2+ + 3 OH ´ Zn(OH3)Bulk precipitation of insoluble species such as Fe(OH)3, MnOx, Cd(OH )2, and metal sulÞdes Complexation with inorganic and organic ligands such as Cl-, SO4 2- , CO3 2- , EDTA, and the humic and fulvic acids or other materials that comprise colloidal organic material Oxidation–reduction reactions including arsenic (As[V] ´ As[III]), iron (Fe[III] ´ Fe[II]), and manganese... demand, a parameter similar in signiÞcance to oxygen-demanding measures in the overlying water Sediment oxygen demand serves to reduce available oxygen in the sediment and encourage anaerobic conditions within the sediment This can inßuence the rate of fate processes (e.g., biological contaminant degradation) and the chemical state of metals, inßuencing their mobility In slow-moving water, the sediment. .. retarded by pore -water processes Worm tubes and other macroscopic animal burrows can signiÞcantly enhance contaminant transport by advection across the sediment water interface In addition, direct ingestion of sediment deposits can lead to rapid transport of sediment and associated contaminants to the surface Bioturbation is not fully incorporated in most system-wide contaminated sediment models and is often... such a hydrodynamic model include streambed geometry and 20 04 CRC Press LLC L1667_book.fm Page 76 Tuesday, October 21 , 20 03 8:33 AM bathymetry, ßow rates, downstream water surface elevation (e.g., boundary conditions), and water and atmospheric properties (e.g., salinity, temperature, wind) Water surface elevation and current velocity measurements and water property documentation can be used to calibrate... sediment oxygen demand can also impact oxygen levels in the overlying water No speciÞc levels of oxygen-demanding constituents are considered problematic Rather, the impact of these contaminants depends on the dynamics of the sediment and overlying water column Undifferentiated Oil and Grease Long-chain nonpolar organic compounds, such as oil and grease, associate strongly with solids and sediments Their... October 21 , 20 03 8:33 AM • crease pore -water concentrations It is important to note that the Peclet number typically exceeds unity (i.e., advection dominates) at low ground water seepage velocities Over a characteristic length scale of 1 cm and with an effective diffusivity of 0.1 cm2/day, advection dominates transport if the seepage velocity exceeds 0.1 cm/day Measuring ground water ßow velocities and, . between Sediment and Common Contaminants 2. 2. 2Sediment Transport Model Components 2. 2 .2. 1Sediment Erosion and Deposition Processes 2. 2 .2. 2Sediment Transport Model Minimum Requirements 2. 2.3Contaminant. aldrin; dieldrin; 1,1,1-trichloro- 2, 2-bis( p -chlorophenyl)ethane (DDT); 1,1-dichloro -2 , 2-bis( p -chloro- phenyl)ethane (DDD); endosulfan; endrin; heptachlor; lindane; and L1667_book.fm. Requirements 2. 2.3Contaminant Fate and Transport Model Components 2. 2.3.1Contaminant Fate and Transport Processes in Unstable Sediments 2. 2.3.2Contaminant Fate and Transport Processes in Stable Sediments 2. 2.3.3Contaminant