1
Introduction

For the last 30 years, the nation has been trying to assess, remediate, and otherwise manage thousand of acres of soil and sediment1 contaminated with chemicals produced during the industrial age. Of primary concern has been the risk that these contaminated media pose to humans and ecological receptors. Evaluation of exposure is a key component of chemical risk assessment, and understanding the factors that influence exposure enables decision-makers to develop solutions for addressing environmental contamination. This report of the National Research Council examines the bioavailability of contaminants in soil and sediment, focusing on those factors that influence the percentage of total contaminant levels to which humans and ecological receptors are exposed. The extent to which chemicals are bioavailable has significant implications for the cleanup of contaminated media.

National attention on bioavailability stems from a growing awareness that soils and sediments bind chemicals to varying degrees, thus altering their availability to other environmental media (surface water, groundwater, air) and to living organisms (microbes, plants, invertebrates, wildlife, and humans). It is also recognized that the physiological characteristics or “niche” of plant and animal species influence the availability of chemicals, such that exposure to the same contaminated material may be very different from one species to another. The altered availability of chemicals associated with soils or sediments has been variously described by such terms as partitioning, reduced desorption rates,

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The terms “soil” and “sediment” are defined in detail in Chapter 3.



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Bioavailability of Contaminants in Soils and Sediments: Processes, Tools, and Applications 1 Introduction For the last 30 years, the nation has been trying to assess, remediate, and otherwise manage thousand of acres of soil and sediment1 contaminated with chemicals produced during the industrial age. Of primary concern has been the risk that these contaminated media pose to humans and ecological receptors. Evaluation of exposure is a key component of chemical risk assessment, and understanding the factors that influence exposure enables decision-makers to develop solutions for addressing environmental contamination. This report of the National Research Council examines the bioavailability of contaminants in soil and sediment, focusing on those factors that influence the percentage of total contaminant levels to which humans and ecological receptors are exposed. The extent to which chemicals are bioavailable has significant implications for the cleanup of contaminated media. National attention on bioavailability stems from a growing awareness that soils and sediments bind chemicals to varying degrees, thus altering their availability to other environmental media (surface water, groundwater, air) and to living organisms (microbes, plants, invertebrates, wildlife, and humans). It is also recognized that the physiological characteristics or “niche” of plant and animal species influence the availability of chemicals, such that exposure to the same contaminated material may be very different from one species to another. The altered availability of chemicals associated with soils or sediments has been variously described by such terms as partitioning, reduced desorption rates, 1   The terms “soil” and “sediment” are defined in detail in Chapter 3.

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Bioavailability of Contaminants in Soils and Sediments: Processes, Tools, and Applications reduced biodegradation rates, geochemical binding, sequestration, and limited absorption through biological membranes—to name but a few descriptors. While these descriptors may all involve different chemical, physical, and biological processes, they all describe the phenomenon that chemicals in soils and sediments behave differently than when those chemicals are present in other media, notably water and air. “Bioavailability processes” are defined as the individual physical, chemical, and biological interactions that determine the exposure of plants and animals to chemicals associated with soils and sediments. One reason for adopting the term “bioavailability processes” in this document is the realization that “bioavailability” has been defined in different ways that are often discipline-specific. Instead of redefining the term “bioavailability,” the committee has chosen to recognize the value of various definitions and to focus instead on the interacting biological, chemical, and physical processes particular to the presence of chemicals in soils and sediments that influence exposure. The term “bioavailability processes” captures this idea. Currently, “bioavailability” is used in risk assessment most frequently as an adjustment or correction factor that accounts for the ability of a chemical to be absorbed by an organism—an approach that makes a number of assumptions regarding individual bioavailability processes. Unfortunately, contemporary risk assessment practice does a poor job of identifying and explaining these assumptions, such that it is generally not clear how bioavailability processes are incorporated into risk assessments. It can be difficult to know whether all of the relevant processes are addressed and whether assumptions are based on valid concepts and reliable data. In fact, there is ample reason to suspect that many bioavailability processes are dealt with inadequately or inaccurately. In order to improve this aspect of risk assessment, it will be necessary to identify relevant bioavailability processes in a more transparent way, to gain greater mechanistic understanding of these processes, and to evaluate the ability of various tools to offer information on bioavailability processes. Over the long term, such a process-based approach will improve exposure assessment, resulting in greater consistency, reliability, and defensibility in measurement, modeling, and prediction. BIOAVAILABILITY PROCESSES FOR CONTAMINANTS IN SOILS AND SEDIMENTS Several definitions for the term “bioavailability” are listed Table 1-1. Depending on the context, bioavailability may represent the fraction of a chemical accessible to an organism for absorption, the rate at which a substance is absorbed into a living system, or a measure of the potential to cause a toxic effect. Often, environmental scientists consider bioavailability to represent the accessibility of a solid-bound chemical for assimilation and possible toxicity (Alexander, 2000), while toxicologists consider bioavailability as the fraction of chemical

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Bioavailability of Contaminants in Soils and Sediments: Processes, Tools, and Applications TABLE 1-1 Definitions of “Bioavailability” and Related Terms Definition Source Bioavailability A chemical element is bioavailable if it is present as, or can be transformed readily to, the free-ion species, if it can move to plant roots on a time scale that is relevant to plant growth and development, and if, once absorbed by the root, it affects the life cycle of the plant. Sposito, 1989 Generally used to describe the extent and rate of absorption for a xenobiotic which enters the systemic circulation in the unaltered (parent) form from the applied (exposure) site. Hrudy et al., 1996 The availability of a chemical to an animal, plant, or microorganism. It may be assayed by measurement of uptake, toxicity or biodegradability. Linz and Nakles, 1997 A concept that describes the ability of a chemical to interact with living organisms. NEPI, 1997 The accessibility of contaminants to microbes from the standpoint of their metabolism, their ability to grow on these chemicals, to change cellular physiology, and perhaps modulation of genetic response. Sayler et al., 1998 A measure of the fraction of the chemical(s) of concern in environmental media that is accessible to an organism for absorption. ASTM, 1998 A measure of the potential for entry into ecological or human receptors. It is specific to the receptor, the route of entry, time of exposure, and the matrix containing the contaminant. Anderson et al., 1999 The extent to which a substance can be absorbed by a living organism and can cause an adverse physiological or toxicological response. Battelle and Exponent, 2000 Bioavailable: For chemicals, the state of being potentially available for biological uptake by an aquatic organism when that organism is processing or encountering a given environmental medium (e.g., the chemicals that can be extracted by the gills from water as it passes through the respiratory cavity or the chemicals that are absorbed by internal membranes as the organism moves through or ingests sediment). In water, a chemical can exist in three different forms that affect availability to organisms: (1) dissolved, (2) sorbed to biotic or abiotic components and suspended in the water column or deposited on the bottom, and (3) incorporated (accumulated) into the organisms. EPA, 2000a The fraction of an administered dose that reaches the central (blood) compartment, whether from gastrointestinal tract, skin, or lungs. Bioavailability defined in this manner is commonly referred to as “absolute bioavailability.” NEPI, 2000a

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Bioavailability of Contaminants in Soils and Sediments: Processes, Tools, and Applications Definition Source In the environment, only a portion of the total quantity of chemical present is potentially available for uptake by organisms. This concept is referred to as the biological availability (or bioavailability) of a chemical. Casarett and Doull’s, 2001 A measure of the potential of a chemical for entry into, or interaction with, ecological or human receptors. It is specific to the receptor, the route of entry, time of exposure, and the matrix containing the contaminant. Lanno, 2001 A term used to indicate the fractional extent to which a dose reaches its site of action or a biological fluid from which the drug has access to its site of action. Wilkinson, 2001 The degree to which a drug or other substance becomes available at the physiological site of activity after administration. American Heritage Dict., 3rd Ed. The degree and rate at which a substance (as a drug) is absorbed into a living system or is made available at the site of physiological activity. Webster’s Dictionary, 10th Ed. Absolute Bioavailability The fraction or percentage of an external dose which reaches the systemic circulation, that is, the ratio of an internal dose to an applied dose. This ratio is called the bioavailability factor (BF). Hrudy et al., 1996 The percentage of an external exposing mass that reaches the systemic circulation (the internal dose). et al., Paustenbach 1997 The fraction of an administered dose that reaches the central (blood) compartment from the gastrointestinal tract. Bioavailability defined in this manner is equal to the oral absorption fraction. Ruby et al., 1999 The fraction or percentage of a compound which is ingested, inhaled, or applied on the skin that actually is absorbed and reaches the systemic circulation. Battelle and Exponent, 2000 The fraction of an administered dose that reaches the central (blood) compartment, whether from gastrointestinal tract, skin, or lungs. NEPI, 2000a Relative Bioavailability The absolute bioavailability of an external exposing mass divided by the absolute bioavailability of the chemical compound under the conditions used to derive the toxicity criterion. Paustenbach et al., 1997

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Bioavailability of Contaminants in Soils and Sediments: Processes, Tools, and Applications Definition Source Refers to comparative bioavailabilities of different forms of a chemical or for different exposure media containing the chemical (e.g., bioavailability of a chemical from soil relative to its bioavailability from water) and is expressed as a fractional relative absorption factor. NEPI, 2000a; Ruby et al., 1999 A measure of the extent of absorption among two or more forms of the same chemical (e.g., lead carbonate vs. lead acetate), different vehicles (e.g., food, soil, water), or different doses. In the context of environmental risk assessment, relative bioavailability is the ratio of the absorbed fraction from the exposure medium in the risk assessment (e.g., soil) to the absorbed fraction from the dosing medium used in the critical toxicity study. Battelle and Exponent, 2000 Other Definitions Bioaccumulation is the total accumulation of contaminants in the tissue of an organism through any route, such as food items as well as from the dissolved phase in water. Bioconcentration is accumulation of a chemical directly from the dissolved phase through the gills and epithelial tissues of an aquatic organism. Biomagnification is the process by which bioaccumulation causes an increase in tissue concentrations from one trophic level to the next from food to consumer. Rand and Petrocelli, 1985; Schnoor, 1996; EPA, 2000a Bioavailable fraction is that portion of the bulk concentration that is available to be accumulated into an organism under a defined set of conditions. For instance, for a metal it could be the freely dissolved ion of the metal. Other forms of the metal bound in precipitates or covalent or hydrogen bonded to other ions would not be available. The available fraction is a proportion ranging from 0.0 to 1.0. The available fraction determines the reactive portion of the total mass of material, much like the activity coefficient relates activity to concentration. EPA, 2000a Bioaccessibility describes the fraction of the chemical that desorbs from its matrix (e.g., soil, dust, wood) in the gastrointestinal tract and is available for absorption. The bioaccessible fraction is not necessarily equal to the RAF (or RBA) but depends on the relation between results from a particular in vitro test system and an appropriate in vivo model. Paustenbach et al., 1997; Ruby et al., 1999 Relative absorption factor (RAF) describes the ratio of the absorbed fraction of a substance from a particular exposure medium relative to the fraction absorbed from the dosing vehicle used in the toxicity study for that substance (the term relative bioavailability adjustment (RBA) is also used to describe this factor.) Ruby et al., 1999

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Bioavailability of Contaminants in Soils and Sediments: Processes, Tools, and Applications Definition Source Absorption describes the transfer of a chemical across the biological membrane into the blood circulation.a Paustenbach et al., 1997 Biostabilization refers to the biodegradation of the more labile HOC (hydrophobic organic compound) fraction leaving a residual that is much less available and mobile. Luthy et al., 1997 aIn this report, “absorption” is used generically for non-mammalian organisms to be synonymous with “uptake.” absorbed and able to reach systemic circulation in an organism. Another view of bioavailability is represented by a chemical crossing a cell membrane, entering a cell, and becoming available at a site of biological activity. Others might think of bioavailability more specifically in terms of contaminant binding to or release from a solid phase. These different viewpoints of bioavailability create a semantic stumbling block that can confound use of the term across multiple disciplines—hence the reason that “bioavailability processes” is used in this report. Figure 1-1 is a depiction of bioavailability processes in soil or sediment; it incorporates exposure by release of solid-bound contaminant and subsequent transport, direct contact of a bound contaminant, uptake by passage through a membrane, and incorporation into an organism. “A”—contaminant binding and release—refers to the physical and [bio]chemical phenomena that bind/unbind, expose, or solubilize a contaminant associated with soil or sediment. This may include geological processes like weathering and scouring, chemical processes like redox reactions or complexation, and biochemical processes through the action of biosurfactants or hydrolytic enzymes. Binding may occur by adsorption on solid surfaces, by absorption within a phase like natural organic matter, or by a change in form as in covalent bonding. “B” in Figure 1-1 involves the movement of a released contaminant to the membrane of an organism. Transport may result from diffusion and advection to target receptors such as microbes, plants, and humans. Thus, bioavailability processes A and B comprise exposure via various chemical and biochemical phenomena that affect release and subsequent transport of dissolved contaminants. “C” involves the movement of contaminants still bound to the solid phase, which can play a role in dermal contact of soils, oral ingestion of soil or sediment, or exposure to burrowing organisms in soil or sediment. It should be noted that processes A, B, and C can occur internal to an organism such as in the gut lumen, although they are depicted in Figure 1-1 as occurring in the external environment. The bioavailability process depicted as D in Figure 1-1 entails movement across membranes. Here the contaminant passes from the external environment through a physiological barrier and into a living system. An example is transport

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Bioavailability of Contaminants in Soils and Sediments: Processes, Tools, and Applications FIGURE 1-1 Bioavailability processes in soil or sediment. Note that A, B, and C can occur internal to an organism such as in the lumen of the gut. through the gut membrane of an organism (e.g., the intestinal epithelium of a mammal). Exposure to both dissolved and solid-bound contaminants can lead to chemical interaction with the membrane of an organism and subsequent uptake or absorption (these terms are used synonymously). “E” in Figure 1-1 refers to paths taken by the chemical following uptake across a membrane. For example, after passage across a biological membrane the chemical can exert a toxic effect within a particular tissue (among many possibilities). It should be noted that A, B, and C in Figure 1-1 are sometimes considered to be fate and transport processes (which they are) rather than bioavailability processes. On the other hand, process D is more traditionally associated with bioavailability in contemporary risk assessment. The committee’s definition of “bioavailability processes” incorporates all the steps that take a chemical from being bound or isolated in soil or sediment to being taken up into an organism (A through D). Figure 1-1 makes it clear that soils and sediments can affect exposure in various ways, both external and internal to the organism. For example, solid phases influence the extent of contaminant transfer from one medium to another, thereby determining soluble chemical concentrations. There is also differential uptake of contaminants into animals and plants depending on whether they are solubilized or solid-bound. Although of great importance in determining the overall effect of a contaminant on an organism, E processes—the toxic action or metabolic effect of a chemical—are not defined as bioavailability processes per se because soil and sediment are no longer a factor. However, because E processes are often measured endpoints, they are described at length in Chapters 3 and 4.

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Bioavailability of Contaminants in Soils and Sediments: Processes, Tools, and Applications Bioavailability processes have definable characteristics that provide the foundation for this report. First, in the broadest sense, bioavailability processes describe a chemical’s ability to interact with the biological world. Second, bioavailability processes are quantifiable through the use of multiple tools. Third, bioavailability processes incorporate a number of steps (see Figure 1-1), not all of which would be applicable for all compounds or all settings. Indeed, it is because the term implies several individual interactions and processes that the committee prefers the term “bioavailability processes” to “bioavailability.” Fourth, there are barriers that change exposure at each step. Thus, bioavailability processes modify the amount of chemical in soil or sediment that is actually taken up and available to cause biological responses. HISTORICAL PERSPECTIVE That soils and sediments can impact chemical interactions with plants and pests has been known for some time by farmers and those involved in agricultural services (e.g., manufacturers of fertilizers, pesticides, and herbicides). However, in the past few decades the phenomenon has gained attention with respect to releases of hazardous chemicals to the environment. First, interest in bioavailability has been driven by a desire to reduce the uncertainties in estimating exposures as part of human and ecological risk assessment. That is, a better understanding of bioavailability processes could help identify sediment- or soil-specific factors that might influence exposure. A second impetus comes from the remediation of contaminated sites, including observations that the effectiveness of bioremediation and other treatment technologies can be limited by the availability of chemicals in soils or sediments. In some cases, the greatest opportunity for risk reduction may be to treat or contain the bioavailable fraction of the hazardous chemicals in soils and sediments and then to rely on natural attenuation approaches to treat the long-term, slow release of residual contaminants. Thus, there is considerable interest in setting cleanup goals based on the bioavailable amount rather than the entire contaminant mass. The brief history below acknowledges the varied use of the term and the extent to which bioavailability processes have been considered in different contexts. Toxicological, Pharmacological, and Nutritional Use of Bioavailability Although coinage of the term “bioavailability” is relatively recent, an appreciation of bioavailability concepts in the context of toxicology is ancient, particularly with regards to the treatment and prevention of poisoning. For example, pre-Columbian natives in South America were known to extract a powerful muscle-paralyzing agent—curare—from various Strychnos plants. They had no means of knowing that this alkaloid possesses a quaternary nitrogen atom, and that the charge on this nitrogen atom prevents its movement across the gas-

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Bioavailability of Contaminants in Soils and Sediments: Processes, Tools, and Applications trointestinal epithelium. They understood quite well, however, that this poison was harmless when ingested, but very effective when injected. As a result, they could immobilize prey with curare-tipped arrows, dispatch the prey, and safely eat the meat. From the fifth century BC to the fifteenth century AD, red clay from a specific hill on the Greek island of Lemnos was regarded as a sacred antidote for poisoning (Thompson, 1931). Called terra sigillata, it was considered effective against all poisons, no doubt acting as an adsorbent and preventing uptake in the gastrointestinal tract. The use of charcoal as an adsorbent to reduce the effect of poisons can be traced back to even earlier times, with its mention recorded in the Egyptian Papyrus of 1550 BC. In the nineteenth century, when toxicologists had the fortitude to serve as their own experimental subjects, P. F. Tourney demonstrated the effectiveness of charcoal before the French Academy of Medicine by ingesting ten times the lethal dose of strychnine combined with charcoal, and surviving (Holt and Holz, 1963). One of the most fundamental concepts in toxicology is that an adverse effect is dependent upon the dose of the toxic substance (or toxicant) reaching a target organ or tissue. With the exception of chemicals that react with the organism on contact, such as corrosive agents, the toxicant must be absorbed into the systemic circulation to reach its biological target. From a toxicological perspective then, bioavailability implies movement of a chemical into the systemic circulation because to a large extent this is a good indication of the biologically effective dose. This view is reflected in the definition of bioavailability given in toxicology texts; for example, Casarett and Doull (2001) define bioavailability as the “frac-

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Bioavailability of Contaminants in Soils and Sediments: Processes, Tools, and Applications tion of dose absorbed systemically.” From the toxicologist’s perspective, this definition applies to virtually all circumstances of chemical exposure, including exposure to chemicals in soils and sediments. Because the disciplines of toxicology and pharmacology share many basic principles, this is essentially the same way bioavailability has been defined in medicine, except of course that the focus is on the absorption of drugs from dosage forms instead of chemicals from environmental media. The tenth edition of the classic pharmacology text, Goodman and Gilman (2001), defines bioavailability as “the fraction of dose of a drug reaching the systemic circulation or site of action.” Both toxicologists and medical doctors are cognizant of the importance of events outside the body and that physical–chemical properties of the toxicant or drug and its interactions with its surroundings can affect the rate and extent of absorption. In fact, much of what is termed pharmaceutics involves an understanding of these phenomena as they pertain to drugs and manipulation of drugs and their microenvironment to therapeutic advantage. Also, toxicologists are well aware that a variety of events in the environment can affect the rate and form in which chemicals are delivered to the body. Nevertheless, the defining aspect of bioavailability, as the term is used in both toxicology and medicine, is the movement of chemical from outside the body into the systemic circulation. Bioavailability is also an important consideration in nutrition. Here the focus is on absorption of nutrients from the gastrointestinal tract, and the term bioavailability can have different meanings in different situations. For example, nutrients such as amino acids in proteins must be liberated through digestive enzyme activity in the gut. In this context, bioavailability may become synonymous with digestibility. Other nutrients, such as most vitamins, require metabolic activation in order to have nutritional value. For these substances, bioavailability is sometimes defined to include both absorption and the metabolic activation process. For still other nutrients that do not require digestion or metabolic activation, bioavailability is regarded simply as the process of absorption of the substance from the gut into the systemic circulation, as in toxicology and medicine. In considering the toxicological use of the term, it is important to recognize that systemic absorption is not necessarily equivalent to general uptake or absorption into the body, particularly from the gastrointestinal tract. Mammalian anatomy is responsible for this complication. Chemicals absorbed from the gastrointestinal tract enter hepatic portal circulation and must pass through the liver before reaching the general circulation. The liver (and to some extent, the gastrointestinal epithelium) may metabolize the chemical, converting it to substances with greater, lesser, or qualitatively different biological activity. This view of bioavailability, in terms of what reaches the systemic circulation (as opposed to just crossing a biological membrane), includes both absorption and metabolism components, and components both internal and external to the body. It can also lead to some ambiguity in how bioavailability is operationally defined for a

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Bioavailability of Contaminants in Soils and Sediments: Processes, Tools, and Applications particular chemical. Often bioavailability in toxicology is described in terms of the chemical itself, ignoring metabolites that are formed during the chemical’s transit from the gut to the general circulation. However, in some instances it is important to describe bioavailability in ways that include metabolites, such as when metabolites are formed that contribute significantly to the biological dose of the chemical. This is analogous to the expanded definition of bioavailability in nutrition to include metabolic activation of vitamins. Regardless of how it is defined, a clear articulation of the basis for the bioavailability determination (with or without metabolites) is required in order to interpret the results. Bioavailability in Agriculture Nutrient Phytoavailability The recognition that total soil concentration of a compound is not equivalent to bioavailable or effective concentration is well established in the agricultural sciences. This is well known not only for plant nutrients but also for water, where physical processes such as water tension or matrix potential control the fraction of total water that is plant-available. Attempts to maximize yields and optimize economic return have resulted in extensive research to describe the behavior of necessary plant nutrients in soil systems. Methods to determine total concentration as well as the plant-available (“phytoavailable”) fraction of the 18 required plant macro- and micronutrients (including water) have been developed across a range of soil types (Bartels and Sparks, 1996). These have been validated with field trials for multiple crops under varied soil, climate, and moisture regimes. The bioavailable nutrient pool varies significantly by soil type and by plant species (Chaney, 1994). This reflects the different complexing capacities of different soil orders as well as different plant mechanisms for accessing soil nutrients (Marschner, 1995). Availability can also depend on the source of the nutrient. For example, nitrogen can be added to soils as manure N, ammoniacal N, nitrate N, and N–P materials; each of these sources will have different release characteristics that vary by soil type, soil moisture, plant growth stage, and soil microbial activity (Pierzynski et al., 2000). The range of factors that affect nutrient availability and the methods that have been developed to predict effective nutrient concentrations potentially can be used as a model for the development of appropriate protocols to assess bioavailability processes for contaminants in soils and sediments. Although the majority of these protocols have been developed to predict phytoavailability of nutrients in potentially deficient conditions, there is a direct correlation to the development of an understanding of the bioavailable fraction of soil contaminants. In many cases, however, plants are aggressively attempting to alter the rhizosphere environment to facilitate nutrient uptake, during which they may inadvertently access soil-bound contaminants.

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Bioavailability of Contaminants in Soils and Sediments: Processes, Tools, and Applications BOX 1-2 Total Concentration vs. Bioavailable Concentration: Metals in Sediment It is important to understand the magnitude of error involved if bioavailability is not considered when evaluating sediment or soil contamination. Significant, even strong, correlations between bioavailability (measured by uptake into tissues or toxicity) and total metal concentration can be found among geochemically similar environments (Bryan, 1985) or within experiments using a single type of sediment (Lee et al., 2000). However, poor correspondence between total metal concentration and bioavailability is common when experiments are conducted with sediments or soils that differ widely in critical geochemical characteristics (Luoma and Jenne, 1977; DiToro et al., 1990, 1991). For example, in a large data set from English estuaries, metal concentrations in fine grained surface sediments (judged to be oxidized by appearance) were compared to concentrations in the tissues of a bivalve and a polychaete that lived within the sediments and ingested sediments with their food (Luoma and Bryan, 1981; Bryan, 1985; Bryan and Langston, 1992). The estuaries included a wide range of physical, biogeochemical, and pollution conditions, and co-variance among geochemical variables was rare. Some sources of variability, such as particle size, large redox differences, or dilution of tissue concentrations by reproductive tissue, were carefully controlled. The results displayed the typical variability of correspondence between metal concentrations in organisms (bioaccumulation) and metal concentrations in sediments. For example, no significant correlation was observed between cadmium in sediments and in the polychaete Neries diversicolor or between copper in sediment and copper in the bivalve Scrobicularia plana. Bioavailability in these cases was completely unpredictable from total metal concentrations in sediments. In contrast, copper in sediments predicted over 50 percent of the variance in copper in the polychaete, and cadmium in sediment predicted over 50 percent of the variance in bivalve cadmium (Bryan, 1985). Silver and lead concentrations in sediments explained about half the variance in bioaccumulation in three species, especially when these elements were extracted from sediments with 0.1M HCl. Clearly, factors that influence bioavailability can differ among metals, species, and environmental factors, and differ with different combinations of these three variables. In the above example, bioavailability processes add variance to the relationship between total concentration and bioaccumulated metal, so the importance of considering bioavailability depends upon how much variance is acceptable (Luoma, 1983, 1989; Landrum et al., 1992). In general, predictions of metal bioavailability from total concentration in sediment alone were outside the two-fold criteria for accuracy suggested by Landrum et al. (1992). If a higher threshold for variance is acceptable, then consideration of bioavailability is less important. Total concentration does appear to provide a first-order control on bioavailability. This control is (statistically) most evident if a large concentration gradient is considered. In the example, total concentration in sediment would be a feasible indicator of the exposure of deposit feeders to most metals if 2- to 50-fold uncertainty were acceptable (the implicit criteria employed by Long et al., 1995, for example). However, because the need to assure less than 50-fold uncertainty exists in many instances, much effort has gone into developing tools and techniques to better relate environmental concentrations and bioavailability.

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Bioavailability of Contaminants in Soils and Sediments: Processes, Tools, and Applications between total and bioavailable concentrations of contaminants in soils and sediments, and also the variability of these differences and their dependence upon such factors as the geological materials, the contaminant species present, exposure pathways, and the potential receptors. Despite the fact that bioavailability has gained popularity as a justification for leaving some contamination in place at hazardous waste sites, in fact the integration of bioavailability processes into risk-based cleanup has the potential to either increase or decrease currently accepted cleanup requirements for residual contamination. To understand this, it should be noted that the term “bioavailability” is often used to refer specifically to uptake or absorption. It is true that absorption efficiency can never be greater than 100 percent, and thus assessments that focus exclusively on absorption efficiency would seem to have the potential to measure only “reduced” bioavailability. However, when other bioavailability processes are taken into account, then it is possible for overall exposure to increase or decrease. That is, although one bioavailability process may suggest that less contaminant is available to a receptor, other bioavailability processes may act as counterbalances, such that the actual dose is not reduced. This is illustrated by the example in Box 1-3, where the overall dose received by an organism is dependent on many factors, including the presence of multiple exposure pathways, ingestion rates, total concentration, and other bioavailability processes. Thus, an examination of all relevant bioavailability processes may actually increase the cost of remediation or alter the remedial technology implemented. A few points can be made with the example presented in Box 1-3 and Table 1-3. First, many definitions of “bioavailability” are limited to the term in the last column of Table 1-3 (uptake efficiency or absorption). This is somewhat analogous to the terms “absolute bioavailability” and “relative bioavailability” commonly used in human health risk assessment. In the absence of compound-specific data, assumptions about absolute and relative bioavailability are made, with a common assumption being that relative bioavailability is 100 percent (see Chapter 2). Part of the goal of this report is to suggest that experiments be conducted to better define the numbers used in the final column of such a table, numbers that often are based on limited data and may not be applicable in all situations. For example, the default for the relative bioavailability of soil-bound lead via oral ingestion is 60 percent, which may be too low or high in certain situations and for certain soils. Indeed, for most compounds and soil- or sediment-types, absolute and relative bioavailability numbers are not available. Second, it should be clear from the above discussion that the committee’s concept of bioavailability processes encompasses not only the uptake term in Table 1-3, but also the concentration term and the term dealing with ingestion rates. Gaining a better understanding of all bioavailability processes can help manage contaminated sediments and soils in a way that not only protects the environment but also considers other issues such as costs, permanence, future

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Bioavailability of Contaminants in Soils and Sediments: Processes, Tools, and Applications land or water use, and community acceptance. As discussed in Box 1-3, management guidelines derived from the viewpoint of a single process can underestimate risk if other important processes are not considered, just as likely as they might overestimate risk. TASK STATEMENT AND REPORT ROADMAP Growing interest in bioavailability processes has generated numerous questions among scientists, engineers, risk assessors, managers, regulatory agencies, and other interested parties. It has highlighted a need for better understanding such processes in terms of specific pathways, contaminated media, biological receptors, and even routes of entry. This report seeks to address the most pressing issues and to contribute toward developing common frameworks and language to build a mechanistic-based perspective of bioavailability processes. Several key questions served to guide the work of the committee: What scientific understanding is missing that would provide confidence in the use of bioavailability factors for different contaminant classes? That is, what mechanisms and processes require better understanding? What are the highest priority research needs? For which contaminant classes, environmental settings, and organism classes are bioavailability assessments most important? What tools (biological, chemical, and physical methods) are available to characterize and measure bioavailability for different contaminant classes, and what new tools are needed? What criteria should be used to validate these tools? How do treatment processes affect bioavailability for different contaminant classes? How does bioavailability affect treatment processes that rely on microbial degradation of contaminants? How and when should bioavailability information be used? What are its implications for relevant regulations? How can information on bioavailability be reliably communicated, especially to the public? This report assesses our current understanding of processes that affect the degree to which chemical contaminants in soils and sediments are bioavailable to humans, animals, microorganisms, and plants. Chapter 2 discusses how the bioavailability concept is used today in solid and hazardous waste management. The legal and regulatory framework for considering bioavailability during soil, sediment, and biosolids management is evaluated as well as the technical methods devised for use in human health and ecological risk assessment. Case studies are presented that illustrate where bioavailability adjustment factors have been used to refine risk assessment calculations. Because the concept of bioavailability incorporates multiple physical, chemical, and biological processes that affect the concentration and transformation of chemicals in soils, sediments, and aquatic systems, Chapter 3 describes these processes in greater detail and weighs their relative importance in certain envi-

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Bioavailability of Contaminants in Soils and Sediments: Processes, Tools, and Applications BOX 1-3 Multiple Bioavailability Processes Affect Contaminant Intake Several environmental processes affect how a contaminant in soil or sediment is taken into an organism. Viewing bioavailability as a single factor, and then making implicit assumptions about the link between the single process and incorporation of the chemical into an organism, can lead to false conclusions. The example below illustrates how a mix of processes can be relevant to bioavailability of a contaminant in sediments, such as: the concentration the organism experiences (as influenced by the contaminant input, fate, and transport, and interactions between the organism and its environment); processes specific to the organism like the rate at which it feeds or the speed with which it passes water over an uptake surface; and processes (perhaps geochemical or biological) that affect the proportion of the total concentration that is incorporated into the tissues of the organism. Influx rate at the membrane is an unambiguous indicator of incorporation into an animal. Mathematically, influx into an organism (say a sediment dwelling, deposit feeding animal) from a dissolved source is defined as: Influxwater = C × R × A where C is concentration in water (μg/g water), R is the rate at which the animal passes water across the gills (gwater/ganimal/d) and A is the absorption efficiency (what proportion of the total concentration is absorbed into the organism) (Wang et al., 1996). A similar equation defines other exposure routes such as from food, where C is concentration in food (μg/g), R is ingestion rate (g/ganimal/d) and A is the absorption efficiency (what proportion of the total concentration ingested is absorbed into the organism). This equation illustrates the interplay among contaminant concentration, biology, and factors modifying absorption, whatever the exposure route. The importance of considering all three in combination is illustrated in the table below. Table 1-3 presents a hypothetical example using reasonable concentrations from a natural system. The goal is to compare intake from two sources with very different absorption efficiencies (often assumed to define bioavailability). The biological processes are typical of a sediment (deposit) feeding animal, like a bivalve. The feeding rate is 1 g sediment per g tissue per day; the filtration rate is 1000 g water per g tissue per day. The concentrations are typical of a moderate cadmium contaminated sediment: 4 μg Cd/g dry wt in sediment; 0.0002 μg Cd/gpore water in pore water (again, units are converted). Absorption efficiency from water is taken as 0.99 because it is often assumed that absorption from solution is highly efficient. Absorption efficiency from food is typical of cadmium availability for a bivalve

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Bioavailability of Contaminants in Soils and Sediments: Processes, Tools, and Applications (20 percent). An analysis of the values could lead to the statement that cadmium is more “bioavailable” from water than sediment (because efficiency of absorption is much higher). But if all bioavailability processes are considered, intake is similar between the sources because concentrations are much higher in sediment. Filtration rate and feeding rates can also make great differences in the ultimate exposure. TABLE 1-3 Hypothetical Intake Rates of Cadmium given Two Different Exposure Pathways   Bioavailability Processes Exposure Pathway Intake Rate (μg Cd/gtissue/day) Medium Concentration (μg Cd/gmedium) Medium Filtration or Ingestion Rate (gmedium/gtissue/day) Medium-specific Uptake Efficiency Pore water 0.2 0.0002 1000 0.99 Ingested sediment 0.8 4 1 0.2 The point illustrated by this example has important implications for setting cleanup standards. Determination of the environmental toxicity of chemicals for regulatory purposes is typically based upon bioassay exposures of surrogate organisms to a dissolved chemical, under circumstances that maximize the efficiency of bioavailability process D in Figure 1-1. For example, selenium toxicity was first determined using exposure of fish or invertebrates to selenite in solution, recognizing that selenite is the “most bioavailable” of the oxidation states (the standard condition is assumed to be close to 100 percent absolute bioavailability). Tests typically reported selenite toxicities at concentrations > 70 μg/L (Lemly, 1998). The first case studies of selenium toxicity in nature, however, showed that selenium was responsible for the elimination of most fish species in Belews Lake, but that selenite concentrations were less than 5mg/L (Lemly, 1985). Clearly, in this system “bioavailability” was greater than predicted from the (originally implied) maximum bioavailability, and the standard test had underestimated risk. Interpretation of the lake data and later experimental studies showed that an additional process was responsible for the enhanced risk. Selenium exposure was found to occur primarily from diet, but dietary exposure was not considered in the tests that set the standard (Lemly, 1985; Luoma et al., 1992). The most recent analyses suggest that understanding of selenium risks in nature requires consideration of multiple additional processes (Lemly, 1995; Luoma and Presser, 2000).

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Bioavailability of Contaminants in Soils and Sediments: Processes, Tools, and Applications ronmental settings. Solubility and sorption, burial and encapsulation, diffusion and advection, microbial transformation and degradation, and uptake into organisms are considered, among other processes. Chapter 4 of this report describes and evaluates the myriad of methods and techniques for measuring different bioavailability processes for both metal and organic contaminants in soils and sediments. For each method, the report considers what bioavailability process(es) it addresses, for what chemicals and contaminated media it can be used, what endpoint is considered, its cost, and the extent to which it has been validated. Suggestions are given for improving our ability to quantitatively assess bioavailability. The implications of more explicitly considering bioavailability processes in environmental cleanup constitute Chapter 5. In particular, the chapter discusses for which contaminants and environmental settings measurements of bioavailability are needed and likely to be most beneficial for the protection of human health and ecosystems. A section is devoted to exploring the complex relationship between contaminant bioavailability and success of bioremediation. Finally, it asks how more explicit consideration of bioavailability can be moved into the regulatory arena and also into practice. Because of the importance of regulatory and public buy-in prior to the refinement of risk assessment and the alteration of cleanup goals, the report discusses ways to effectively communicate bioavailability concepts. REFERENCES Alexander, M. 1995. How toxic are chemicals in soil? Environ. Sci. Technol. 29:2713-2717. Alexander, M. 1997. Sequestration and bioavailability of organic compounds in soil. Chapter 1 In: Environmentally Acceptable Endpoints in Soil. D. G. Linz and D. V. Nakles, Eds. Annapolis, MD: American Academy of Environmental Engineers. Alexander, M. 2000. Aging, bioavailability, and overestimation of risk from environmental pollutants. Environ. Sci. Technol. 34:4259-4265. Ali, M. A., D. A. Dzombak, and S. B. Roy. 1995. Assessment of in situ solvent extraction for remediation of coal tar sites: process modeling. Water Environment Research 67(1):16-24. American Society for Testing and Materials (ASTM). 1998. Standard guide for conducting laboratory soil toxicity or bioaccumulation test with the lumbricid earthworm Eisenia foetida. E 1676-97. Philadelphia, PA: ASTM. Anderson, W. C., R. C. Loehr, and B. P. Smith. 1999. Environmental availability of chlorinated organics, explosives, and metals in soils. Annapolis, MD: American Academy of Environmental Engineers. Bailey, G. W., and J. L. White. 1970. Factors influencing the adsorption, desorption, and movement of pesticides in soil. Residue Rev. 32:29-92. Bartels, J. M., and D. L. Sparks (eds.). 1996. Methods of soil analysis, part 3: chemical methods. Madison, WI: Soil Science Society of America, Inc. Battelle and Exponent. 1999. Guide for incorporating bioavailability adjustments into human health and ecological risk assessments at U.S. Navy and Marine Corps facilities (draft final). Part 1: overview of metals bioavailability. Port Hueneme, CA: Naval Facilities Engineering Command.

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Bioavailability of Contaminants in Soils and Sediments: Processes, Tools, and Applications Latterell J. J., R. H. Dowdy, and W. E. Larson. 1978. Correlation of extractable metals and metal uptake of snap beans grown on soil amended with sewage sludge. J. Environ. Qual. 7:435-440. Lee, B.-G., S. B. Griscom, J.-S. Lee, H. J. Choi, C.-H. Koh, S. N. Luoma, and N. S. Fisher. 2000. Influence of dietary uptake and reactive sulfides on metal bioavailability from aquatic sediments. Science 287:282-284. Lemly, A. D. 1985. Toxicology of selenium in a freshwater reservoir: Implications for environmental hazard evaluations and safety. Ecotoxicol. Environ. Safety 10:314-338. Lemly, A. D. 1995. A protocol for aquatic hazard assessment of selenium. Ecotoxicol. Environ. Safety 34: 223-227. Lemly, A. D. 1998. Pathology of selenium poisoning. Pp. 281-296 In: Environmental Chemistry of Selenium. W. Frankenberger and R. A. Engberg (eds.). New York: Marcel Dekker Inc. Linz, D. G., and D. V. Nakles. 1997. Environmentally acceptable endpoints in soil. Annapolis, MD: American Academy of Environmental Engineers. Long, E. R., D. D. Macdonald, S. L. Smith, and F. D. Calder. 1995. Incidence of adverse biological effects within ranges of chemical concentrations in marine and estuarine sediments. Environ. Mgt. 19:81-97. Lucier, G. W., R. C. Rumbaugh, Z. McCoy, R. Hass, D. Harvan, and P. Albro. 1986. Ingestion of soil contaminated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) alters hepatic enzyme activities in rats. Fund. Appl. Toxicol. 6:364–371. Luoma, S. N., and E. A. Jenne. 1977. The availability of sediment-bound cobalt, silver, and zinc to a deposit-feeding clam. Pp. 213-230 In: Biological Implications of Metals in the Environment. R. W. Wildung and H. Drucker (eds.). Luoma, S. N., and G. W. Bryan. 1981. Statistical assessment of the form of trace metals in oxidized estuarine sediments employing chemical extractants. Sci. Total Environ. 17:165-196. Luoma, S. N. 1983. Bioavailability of trace metals to aquatic organisms—a review. Sci. Total Environ. 28:1-22. Luoma, S. N. 1989. Can we determine the biological availability of sediment-bound trace elements? Hydrobiologia 176/177:379-396. Luoma, S. N., and T. S. Presser. 2000. Forecasting selenium discharges to the San Francisco Bay-Delta estuary: ecological effects of a proposed San Luis drain extension. USGS Professional Paper 00-416-RTS. 210 pp. Luoma, S. N., C. Johns, N. Fisher, N. A. Steinberg, R. S. Oremland, and J. R. Reinfelder. 1992. Determination of selenium bioavailability to a benthic bivalve from particulate and solute pathways. Environ. Sci. Technol. 26:485-491. Luthy, R. G., D. A. Dzombak, C. A. Peters, S. B. Roy, A. Ramaswami, D. V. Nakles, and B. R. Nott. 1994. Remediating tar-contaminated soils at manufactured gas plant sites. Environ. Sci. Technol. 28(6):266A-276A. Luthy, R. G., G. R. Aiken, M. L. Brusseau, S. D. Cunningham, P. M. Gschwend, J. J. Pignatello, M. Reinhard, S. Traina, W. J. Weber, Jr., and J. C. Westall. 1997. Sequestration of hydrophobic organic contaminants by geosorbents. Environ. Sci. Technol. 31(12):3341-3347. Marks, M. J., J. H. Williams, and C. G. Chumbley. 1980. Field experiments testing the effects of metal contaminated sewage sludges on some vegetable crops. Pp. 235-251 In: Inorganic pollution and agriculture : proceedings of a conference organised by the Agricultural Development and Advisory Service April 1977. London: H.M.S.O. Marschner, H. 1995. Mineral nutrition of higher plants. London: Academic Press. Massachusetts DEP. 2001. Quality assurance and quality control requirements and performance standards for SW-856 Method 9014 (modified), total and physiologically available cyanide (PAC), when utilized in support of assessment and evaluation decisions at disposal sites regulated under M.G.L. c. 21E and 310 CMR 40.0000, the Massachusetts Contingency Plan (MCP). Boston, MA: MA Department of Environmental Protection.

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Bioavailability of Contaminants in Soils and Sediments: Processes, Tools, and Applications McConnell, E. E., G. W. Lucier, R. C. Rumbaugh, P. W. Albro, D. J. Harvan, J. R. Hass, and M. W. Harris. 1984. Dioxin in soil: bioavailability after ingestion by rats and guinea pigs. Science 223:1077–1079. Mihelcic, J. R., and R. G. Luthy. 1991. Sorption and microbial degradation of naphthalene in soil-water suspensions under denitrification conditions. Environ. Sci. Technol. 25(1):169-177. Moore, D. W., T. S. Bridges, B. R. Gray, and B. M. Duke. 1997. Risk of ammonia toxicity during sediment bioassays with the estuarine amphipod Leptocheirus plumulosus. Environ. Toxicol. Chem. 16:1020-1027. Morrison, D. E., B. K. Robertson, and M. Alexander. 2000. Bioavailability to earthworms of aged DDT, DDE, DDD, and dieldrin in soil. Environ. Sci. Technol. 34: 709-713. Mueller-Warrant, G. W. 1999. Duration of control from preemergence herbicides for use in non-burned grass seed crops. Weed Tech. 13:439-449. Nash, R. G., and E. A. Woolson. 1967. Persistence of chlorinated hydrocarbon insecticides in soils. Science 157:924-927. National Environmental Policy Institute (NEPI). 1997. Bioavailability: the policy impact of emerging science. Washington, DC: NEPI. NEPI. 2000a. Assessing the bioavailability of metals in soil for use in human health risk assessments. Washington, DC: NEPI. NEPI. 2000b. Assessing the bioavailability of organic chemicals in soil for use in human health risk assessments. Washington, DC: NEPI. Page, A. L., T. J. Logan, and J. A. Ryan (eds.). 1987. Land application of sludge: food chain implications. Chelsea, MI: Lewis Publishers. Paustenbach, D. J., G. M. Bruce, and P. Chrostowski. 1997. Current views on the oral bioavailability of inorganic mercury in soil: implications for health risk assessments. Risk Analysis 17(5):533-544. Pierzynski, G., J. T. Sims, and G. F. Vance. 2000. Soils and environmental quality. Boca Raton, FL: CRC Press. Ramaswami, A., and R. G. Luthy. 1997. Measuring and modeling physicochemical limitations to bioavailability and biodegradation. Chapter 78 In: Manual of Environmental Microbiology. C. J. Hurst (ed.). Washington, DC: ASM Press. Rand, G. M., and S. R. Petrocelli, Eds. 1985. Fundamentals of aquatic toxicology: effects, environmental fate, and risk assessment, 2nd Ed. New York: Hemisphere Publishing Co. Robertson, B. K., and M. Alexander. 1998. Sequestration of DDT and dieldrin in soil: disappearance of acute toxicity but not the compounds. Environ. Toxicol. Chem. 17:1034-1038. Ruby, M., R. Schoof, W. Brattin, M. Goldade, G. Post, M. Harnois, D. E. Mosby, S. W. Casteel, W. Berti, M. Carpenter, D. Edwards, D. Cragin, and W. Chappell. 1999. Advances in evaluating the oral bioavailability of inorganics in soil for use in human health risk assessments. Environ. Sci. Technol. 33(21):3697-3705. Sayler, G. S., U. Matrubutham, F.-M. Menn, W. H. Johnson, and R. D. Stapleton, Jr. 1998. Molecular probes and biosensors in bioremediation and site assessment. Pp. 385-434 In: Bioremediation: Principles and Practice. Volume 1 Fundamentals and Applications. Subhas K. Sikdar and Robert L. Irvine (eds.). Lancaster: Technomic Pub. Co. Schnoor, J. L. 1996. Environmental modeling: fate and transport of pollutants in water, air, and soil. New York: J. Wiley and Sons. Schoof, R. A., and J. B. Nielsen. 1997. Evaluation of methods for assessing the oral bioavailability of inorganic mercury in soil. Risk Analysis 17(5):545-555. Scribner, S. L., T. R. Benzing, S. Sun and S. A. Boyd. 1992. Desorption and bioavailability of aged simazine residues in soil from a continuous corn field. J. Environ. Qual. 21:115-120. Shu, H., D. Paustenbach, F. J. Murray, et al. 1988. Bioavailability of soil-bound TCDD: oral bioavailability in the rat. Fund. Appl. Toxicol. 10:648-654.

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