,
and Aquatic Assessment
chronic concentrations. This he protection of philosophy is now being conour aquatic reW I L L I A M J. A D A M S sources has asABCLobomtoN‘es, Co]umbio, Mo 65205 sidered for sediments. The purposes of this article sumed national are to provide the reader with and global promiR I C H A R D A. K I M E R L E background information on the nence. Oil spills, J A M E S W. B A R N E T T , JR. status of sediment assessment medical wastes, and plastic in the United States, a review Monsonto Compony St. Louis, MO 631 67 debris presence on beaches, of the existing methods availocean incineration, ocean disable for assessing sediment posal of garbage and dredged quality, an analysis of the commaterials, pesticide and fertilplexity and uncertainty of the izer runoff, contaminated harsediment assessment methodbors, and diminishing fisherologies, and a proposed api e s have focused p u b l i c proach that utilizes the unique attention on the need to adeattributes of many of these quately protect marine and methods in a tiered sediment freshwater resources-includassessment strategy. The auing sediments. Sediments are thors hope that this sediment repositories for physical deassessment strategy will help bris and “sinks” for a wide vaprovide a mechanism for riety of chemicals. The concern achieving cleaner sediments associated with the chemicals and water i n the nation’s sorbed to sediments is that aquatic ecosystems. many commercial species and food chain organisms spend a Assessment approaches major portion of their life-cycle The need to develop sediliving in or on aquatic sediment assessment approaches ments. This provides a patharises from the recognition way for these chemicals to be that sediments at many freshconsumed by higher aquatic water and marine sites in the life and wildlife, including United States are contamiavian species as well as hunated with varying levels of mans. Direct transfer of chemimetals and organics. Chemicals from sediments to organcals most often reported inisms is now considered to be a clude polycyclic aromatic hymajor route of exposure for drocarbons, chlorinated many species. Concern has inorganics, and some pesticides creased over the number of in(2).Assessment methods are cidences of tumors being obneeded that interpret the sigserved in many species of fish, nificance of these contamiespecially those that have dinant levels and that account rect contact with sediments (1). for differences in chemical These issues are focusing attenbioavailability &om site to site tion on sediment contamina.-and physical and chemical tion and highlight the fact that site-specific sediment characsediments am an important reteristics. These methods are some. Direct transfer of needed to help make deciAquatic sediments can be sions about ecosystem and huloosely defined as a collection chemicals from sediments to man health protection, dredgof fine-, medium-, and coarseing and open water disposal of grain minerals and organic organisms is now considered a sediments, source control, and particles that are found at the major route of exposure for the need for remediation of bottom of lakes, rivers, bays, contaminated sites. estuaries, and oceans. Sedimany species. One of the first organized efments are an important comforts of the scientific commuponent of aquatic ecosystems nity to address emerging techbecause of the niche they pronical and regulatory issues on sediments was a vide for benthic aquatic oiganisms. Sediments provide “Pellston” workshop on the “Fate and Effects of Sedia substrate for a wide variety of organisms to live in or ment-Bound Chemicals in Aquatic Systems” held in on, including shrimp, crayfish, lobster, crab, mussels, 1984 (3). Since this time considerable technical clams, flounder, many other economically important progress has been made in assessment methods. Recent species as well as species important in the food chainpublished reports on sediment chemical contamination including many species of worms, amphipods, oliin freshwater and marine sites in the United States gochaetes, chironomids, bivalves, and insects. In recent present several approaches to evaluating sediment years, protecting sediment quality has been viewed as a quality. These approaches are briefly reviewed in this logical and needed extension of water quality protecpaper. Some of these approaches are now being intion. The basic premise that has been used to protect cluded or considered for inclusion in regulatory prowater quality has been to restrict chemicals from occnrring in water at concentrations above the known “safe” grams.
.
I I
I
0013-936)(192/0926-1864$03.00/0 Q 1992 American Chemical Society
Envimn. Sci. Technol.. Vol. 26, No. 10, 1592 1865
Evidence for establishing comprehensive programs for assessment and protection of sediments from chemical contamination can be found in an EPA publication on “Managing Contaminated Sediments” ( 4 ) and in the state of Washington Department of Ecology’s “Sediment Management Standards” (5),which were based on an approach entitled “Apparent Effects Threshold” (6,7).EPA’s Criteria and Standards Division, as authorized by the Clean Water Act (CWA, 1972,Sections 104 and 304), has begun a program to develop a regulatory sediment strategy which includes the use of sediment quality criteria (4, 8). Similar interests and programs in sediment quality have emerged in several other governmental agencies.
the existing quality of U.S. sediments is clearly needed.
I
l
and under what circumstances uu contaminated sites need to be remediated? Some scientists and regulators believe that the development of Key issues sediment quality criteria (SQC) is The extent of sediment chemical the best approach for assessing sedcontamination. There is a lack of iment quality and managing sedidocumentation of the extent and ments. These criteria could be used significance of sediment contami- to trigger additional biological and nation. This point was recently em- chemical testing to determine the phasized by the National Academy extent and significance of the conof Sciences’ report, “Marine Sedi- tamination. They could also be used ment Contamination Assessment similarly to the water quality criteand Remediation ” (9). No national ria (WQC) to develop sediment comprehensive programs have col- quality standards to be used in setlectively reviewed the numerous re- ting waste site cleanup standards, ports in the literature relative to lo- effluent discharge limits, standards cal or regional contamination. EPA for disposal of sediments in open has, however, issued a report sum- waters, and protection of marine marizing the sediment contaminant and freshwater sediments. EPA addata in the STORET data base (2). vocates the SQC approach (7,8 ) . Many specific local sites or “hot The scientific community is still despots” have been identified that ap- bating the methods used to develop pear to be the result of past events the criteria, the manner in which such as spills and discharges (10). the criteria would be used, the need Certain chemicals such as chlori- for criteria versus sediment guidenated pesticides are no longer pro- line or assessment values, and the duced and are clearly the result of accuracy and precision of the varipast practices. There is a need to ous methods (11-14). This diversity evaluate to what extent sediment of opinion is emphasized by the contamination is a result of histori- wide variety of methods proposed cal practices and the amount of pro- for assessing sediment quality and tection provided by existing regula- by the fact that the state of Washingtions. There is a general perception ton has implemented state sediment that contamination in many harbors quality standards based on the Apand below certain discharge points parent Effects Threshold approach, is pervasive. However, published whereas the EPA Office of Water reports indicate that many sites, es- and Regulations favors the Equilibpecially those some distance from rium Partitioning Approach. Past programs to protect surface urban areas, are not contaminated. A better assessment of the existing waters provide a basis for considerquality of US.sediments is clearly ing future actions relative to sediments. The primary focus of the needed. The best approach for assessing CWA is to keep “toxics in toxic sediment quality. The national is- amounts” out of the nation’s surface sues we are now dealing with are waters. Within the framework of the twofold How can we best protect CWA, WQC and the National Polluand manage our sediment re- tion Discharge Elimination System sources, and What is the signifi- of permits have evolved. These procance of sediment contamination grams focus on measuring the toxic1888 Environ. Sci. TKhnol., Vol. 26,No. 10, 1992
ity of chemicals to aquatic life and keeping concentrations of chemicals in surface waters below acute and chronic toxic effect levels. It is generally recognized that this approach has provided a reasonable means to regulate chemicals and has helped attain the legislative and regulatory goals of protecting the biological integrity of aquatic ecosystems (15-18). Within the WQC guidelines there is a provision to develop site-specific criteria that can be used in lieu of the generic national criteria when the local water quality characteristics influence the biological availability-and thus the toxicity-of a chemical. Recently it has been recognized that for industrial and municipal effluents, which contain a mixture of unknown chemicals, the chemicalspecific WQC approach needs to be supplemented with an effluent sitespecific toxicological approach to protect receiving waters (19).What emerged was an approach that tested “whole” effluent toxicity. Advantages of this whole effluent toxicity approach are that it assesses the toxicity of mixtures of unknown chemicals for which no WQC are available, is site specific, and compares exposure and effects concentrations for decision making purposes. The experience gained through many years of managing and regulating water quality to account for bioavailability of the chemical, site- and species-specific differences, and integration of multiple chemicals provides potential insights for managing sediment quality and argues for site-specific and effects-based approaches. Classification compendium EPA has prepared an analysis of 11 of the current methods available to assess sediment quality entitled “Sediment Classification Methods Compendium” (20).Not all of these methods provide the same level of assessment, nor are they at the same stage of development or use. A brief description and comparative summary of 10 of the compendium methods is presented in following sections. Two additional methods, one for metals and the other for ionic organics, are also reviewed and summarized. Comments of the EPA Science Advisory Board on the “Compendium” have been reviewed (21). Equilibrium partitioning (EP) a p proach. The EP approach is based on the theory that chemicals sorbed to sediments achieve thermody-
namic equilibrium over appropriate periods of time between sediment and sediment pore water concentrations (22,23). At equilibrium, the mass of the chemical present in either phase can be estimated by measuring the mass present in the other phase. The sediment pore water concentration and the bulk sediment chemical concentration are related by the carbon normalized sediment-water partition coefficient for that chemical. Karickhoff (24) demonstrated the linear dependence of sediment-water partition coefficients on organic carbon and that partition coefficients indexed to organic carbon are relatively invariant acxoss a wide range of sediment types. Normalizing bulk chemical concentrations for the carbon content of the sediment accounts for a majority, but not all, of the variability associated with partition coefficient measurements for sediments with organic contents greater than 0.2%. The hypothetical data in Table 1 demonstrate how the organic carbon and sedimentwater partition coefficient influence the dissolved and bound chemical concentrations in sediments. The application of these principles to sediment assessment provides a way to predict the bioavailability of non-ionic chemicals sorbed to sediments. Estimating bioavailability is an important first step in evaluating the significance of chemicals sorbed to sediments. A wealth of data demonstrates for both metals and organics that similar concentrations of a given chemical on two different sediment types most often do not produce the same toxicity result (25, 26). One sediment may be highly toxic and the other may show no toxicity to the same organism and both contain the same measured total chemical concentration. From the perspective of protecting aquatic life from toxicity it is generally believed that it is the “free” or bioavailable fraction of the total metal or organic chemical present that is most important. Therefore, many of the methods being used today are attempting either directly or indirectly to predict the fraction of the total sediment-bound chemical that is toxicologically available to aquatic organisms. Figure 1 is an illustration of how an organic chemical partitions when it reaches a body of water. Pavlou (27)first used the EP approach to estimate sediment pore water concentrations of PCBs. The EP approach coalesced when Ad-
7
.. . Relationship of chemical properties (Kay or and sediment organic carbon (f0J on sediment-specific partition coefficients
6.)
sediment Organic carboa(%)
Concenfralion Sediment Water ( W L )
c,
(KiZ)
K,’
Le
A
500 5000 50,000
1 1 1
2
B C
10 100 1000
A B C
10 10 10
2
5000 50,000
2
100 io00
2
10,000
Chemical
&.,or
500
50 500 5000
Gw>sthe octanai-water
partition coenicisnt: & is the organic carbon normaifred partition coefticlent that can be approximated With the &. Percent fraction organic carbon: “ K , is the sediment-specilic panition coeficient. ratio 01 C , to
ams et al. (25) demonstrated that for freshwater midges (Chironomus tentans) and kepone toxicity could be correlated not to the total sediment chemical concentration (kg keponelg dry weight sediment), but to the pore water concentration (kg keponell pore water) or the carbon normalized sediment concentration. These results suggest, but do not necessarily prove, that pore water is the primary route of exposure because the experiments were conducted so that all exposure pathways were at equal chemical potential (Figure 1). This finding has subsequently been demonstrated for several other chemicals including dieldrin, DDT, fluoranthene, and cadmium (28-32).The results of these studies support the conclusion that dose-response toxicity data should correlate to organic carbon-normalized total sediment chemical concentrations as well as to pore water exposure concentrations (22,23), but not to total bulk chemical concentrations. EPA is currently proposing the EP approach for deriving SQC for nonionic organic chemicals (33, 34). The use of pore water concentrations for assessing sediment quality is matter of choice and convenience because it allows for a calculation of a sediment quality value or criteria from the existing WQC by using the WQC as the pore water concentration that should not be exceeded to protect benthic life. This approach assumes that benthic invertebrates are equally sensitive to non-ionic organic chemicals, as are water column organisms. DiToro et al. (22) reviewed this assumption. The following equation is used to calculate a sediment quality crite-
rion from a water quality criterion: SQC,, = GC x WQC; where K cis the carbon normalized sediment-water partition coefficient (22, 23). This equation calculates an organic carbon normalized sediment quality criterion (SQC.,) that is independent of site-specific considerations relative to the organic carbon content of the sediments. The calculation requires only the WQC and the carbon normalized sedimentwater partition coefficient for a specific chemical. To assess a given contaminated sediment it is then a simple matter to measure the total sediment chemical concentration and percent organic carbon, calculate the concentration of chemical per unit of organic carbon, and compare that to the SQC,. to determine whether a given chemical concentration presents a risk to benthic aquatic life. The principal advantages of the EP method are that it allows for the derivation of chemical-specific sediment values (criteria) from WQC and accounts for differences in the bioavailability of non-ionic chemicals sorbed to sediments from different sites (Table 2). Major limitations of the method are that there are relatively few WQC available, the method is only for non-ionic chemicals, and limited field validation of the method has occurred. Apparent effects threshold (AETI. Empirical data (field and laboratory) are used to identify concentrations of chemicals above which biological effects are always expected. AET values are derived using a comparison of biological effects and chemical data in paired data sets from field-collected samples. In a given data set, the AET for
Environ. Sci. Technoi., VoI. 26, NO.10, 1992 1887
Interaction of a nonpolar organ1
-
a
-
rmlcal with a bodv of water *
0
Suspended particle,
n \I)
a particular chemical is the seuiment chemical concentration above which biologically adverse effects are always observed (based on statistical significance, P$0.05)relative to an appropriate reference sediment. A wide variety of organisms and biological tests can be used to obtain the effects data. These may include benthic infaunal field surveys and bulk sediment bioassays with various organisms and endpoints (6,7, 35). [Infaunal organisms are those that burrow into sediments.] The calculation of the AET for each chemical and biological indicator is conducted as follows: (1) Obtain “matched” chemical and biological effects data on numerous field sediment samples. This is done by analytical measurements of the sediment chemical concentrations, benthic infaunal measurements, and bulk sediment bioassays. (2) Identify “impacted” and “non-impacted” sediment sites. For each sediment sample determine if the biological effects are statistically different from the “clean” reference site. (3) Determine the AET for each chemical of interest. This is done by using the paired data sets for all sediment sites in a given area (e.g., Puget Sound). From these paired data, unimpacted sites (no observed biological effects) are selected and
I
r ,
rn
used to determine the highest chemical concentration present in the sediment samples not causing biological effects for each chemical of interest. This value is reported as the AET for a given chemical based on a specific biological response (e.& infaunal abundance). An AET based on a different endpoint (growth or survival in a laboratory bioassay with a field sample) can also be calculated. By calculating several AET values for different biological endpoints, a range of values can be determined and the data evaluated for consistency, accuracy, and sensitivity. The AET approach was based on data sets developed to assess and manage the quality of sediments in F’uget Sound, WA (6, 7,361.Following public comment, the state of Washington has adopted Sediment Management Standards, which are in part based on the AET approach. They were officially adopted March 27, 1991, and became legally effective April 27, 1991, in Washington (5). The AET method is probably most useful as a screening tool to provide a consistent basis on which to evaluate sediment contamination within a given region and to define which sediment samples or sites have the greatest potential to adversely impact aquatic life or, conversely, the lowest probability of ecological impact. A major limita-
1888 Envimn. Sci. Technol., Vol. 26. No. 10, 1992
tion of the AET method is its inability to directly assess which chemical contaminant causes which biological effect. Empirical data are used to derive the AET, and it is not possible to separate individual chemical effects when multiple chemicals are present in the same sample. Additionally, AETs are not always carbon normalized. This is necessary to extend the use of the values beyond the samples from which they were derived. Sediment quality triad approach. This is a biological effects-based approach for describing sediment quality (37,38).It typically consists of three components: bulk sediment chemistry, sediment toxicity information, and in situ measurements of benthic community health. This method provides both descriptive and numeric data to evaluate sediment quality. The objectives of the Triad approach are to independently measure contamination, sediment toxicity, and in situ biological alteration and then use weight of evidence to assess sediment quality. The first component of the Triad involves identification and quantification of inorganic and organic contaminants in the sediment with the lowest limit of detection possible. The second component of the Triad involves measurement and quantification of toxicity based on bulk sediment laboratory toxicity tests. Bioassays are selected to span the widest possible range of organism type, life-cycle, exposure route, and feeding type. The third component of the Triad approach typically involves the evaluation of in situ biological effects or alteration. Benthic infaunal community structure and function data are often used because the organisms are relatively immobile and site specific. Other endpoints such as bottom fish histopathological data have also been used. The three data components are compared on a quantitative basis and are normalized to “clean” reference site values by converting them to ratio-to-reference (RTR) values (37).To determine RTR values, the values of specific variables (chemical concentration, percent mortality, number of taxa, organism growth, etc.) are divided by the corresponding reference value. This provides an index of chemical contamination, toxicity, and biological alteration for each station. All of the RTR values for the chemical measurements, toxicity, and infaunal data at one station are each col-
lapsed into a single mean value. These values provide indices of contamination and are used to rank stations. The composite RTR values can be assessed in a number of different ways. For Puget Sound, the data have been used to develop numeric criteria for lead, PAHs, and PCBs (38). The principal advantages of the Triad approach are that it can be used for any sediment type; does not require a priori assumptions concerning the specific mechanism of interaction between organisms and toxic contaminants: and integrates laboratory, field, biological, and chemical data. The major limitations of the Triad approach are that statistical approaches have not been fully developed for assessing Triad data, the method does not provide direct cause-and-effect data when multiple chemicals are present, and it does not eliminate the possibility that the presence of unmeasured contaminants may be responsible for the observed toxic effects. Bulk sediment toxicity test. For this method, field sediments are brought into the laboratory, where water and aquatic organisms are added to determine if the sediments produce any adverse toxic effects on growth, survival, or reproduction of a variety of aquatic, benthic organisms (39). When sediments from numerous sites and depths within a geographical area are tested, including a “clean” reference site, a general conclusion can be reached on the extent of contamination and its significance to benthic aquatic life. It is assumed that the chemicals act similarly on the test sediments in the laboratory and on those in the field.
The advantages of the bulk sediment toxicity method are that it provides a direct measurement and integration of toxicity resulting from one or several chemicals present at a particular site, it is relatively simple and inexpensive to perform, and it can be conducted with a wide variety of organisms and life stages, often with the organisms of interest at a particular site. Bulk sediment bioassays have been the method of choice for assessing the safety of sediments for many years. This method is routinely used to evaluate the suitability of dredged sediments for open ocean or freshwater disposal, the safety of sediments below specific discharge points or in the vicinity of waste sites, and it comprises one portion of the Triad method of sediment assessment. A good review of bulk sediment bioassay procedures was recently published by Burton (40). The principal disadvantages of this method are that it does not indicate which chemicals are responsible for toxicity and it does not lend itself to developing chemical criteria. Interstitial water toxicity approach. The interstitial water approach is a procedure for measuring the toxicity of sediment bound chemicals to aquatic organisms by exposing the organisms to pore water extracted (removed) from sediments (41). The use of pore water to assess sediment quality is based on the assumption that the pore water is at equilibrium with the surrounding sediment and that the water phase provides a direct and important route of exposure for organisms that live in sediments. This method also assumes that the “soluble” or “free” uncomplexed fraction of any
chemical in the pore water is the fraction most responsible for observed sediment toxicity (22).Supporting evidence for these assumptions has been developed (23, 28, 29.42-45). Use of the pore water approach has been reported by several of the above authors and recently has been expanded to incorporate toxicity identification evaluation (TIE)procedures developed for effluent toxicity tests (46-49). This addition allows for the identification of the contaminant responsible for the sediment toxicity. This identification is achieved by fractionating the pore water sample into fractions containing major chemical groups followed by toxicity tests and chemical analyses. The interstitial water toxicity test involves three steps: isolation of the pore water from the sediment samples, conducting of toxicity tests with pore water, and application of TIE procedures to pore water fractions. Pore water toxicity tests can be performed with a variety of test organisms, both benthic and water column species. An advantage of this test is that it allows for acute and chronic tests to be performed, which use very sensitive species and life stages. The amount of pore water available is often the most limiting factor regarding the selection of test species and duration of the test. Noncompacted wet sediment is typically 50% water by weight. Large amounts of pore water can be obtained if sufficient sediment is collected and processed. However, it is difficult to obtain s a cient quantities of pore water from compacted sand and clay sediments. Advantages of this method are that it provides a direct approach to
TABLE 2
A comparison of various methods for assessing sediment quality Chsmlcal spcillc
W l m m t malhd
Equilibrium Partitioning Apparent Effects Threshold Sediment quality triad Bulk sediment toxicity
Interstitial water approach Spiked sediment toxicity Tissue residue approach Freshwater benthic approach Marine benthic approach
ionic organics Metals e The
degree of uncertain
yes yes yes
no yes yes yes no no
yes
sits
l%GY
Fleld
chemicals
validated
no
no
partially
yes yes yes
yes
yes
yes
yes yes partially
SpeClllC
yes yes yes yes yes no no
yes
yes no
no
yes yes
partially no yes
yes
Melhod
uncertainly‘
moderate high
low moderate
moderate high’ high high
iowlmoderate iowlmoderate low
moderate moderate unknown low low unknown
no low partially low moderateflarge far each methad is subjective and reflects the authors’ opinion and experience as well as previously rewnec evaluations.
yes
no no
bThe cost of this aoo,oacx would be hioh if both sediment$ and tissue were analvzed.
Environ. Sci. Technol., Vol. 26. No. 10, 1992 1889
sediment quality assessment by measuring organism exposure and effects via the sediment pore water route without normalizing bulk sediment chemical concentrations, cause-and-effect relationships can be established, and sensitive life stages of several organisms can be used. Disadvantages of this method are that the pore water toxicity testing concept and the sediment TIE procedures have not been extensively evaluated and insufficient data are available to determine whether pore water toxicity tests provide a means for evaluating a broad spectrum of chemicals and benthic species. For some species (direct sediment ingesters) and compounds (those with a high Kow) there is evidence that pore water is not the main route of exposure (50). The significance of this finding needs to be further evaluated. Spiked sediment toxicity test. In this test the toxicity of a specific chemical to one or more benthic organisms is measured by the addition of the chemical to test sediment(s) at different doses. This approach for assessing sediments uses laboratory data to establish cause-and-effect relationships in order to predict concentrations of specific chemicals that would be harmful to resident biota under field conditions (52). The method offers the opportunity to test numerous combinations of sediment types and chemicals either individually or in mixtures. This approach assumes that the laboratory experimental conditions realistically simulate the real world. This has not been adequately confirmed for an array of chemicals. It is expected that spiked sediment toxicity tests will prove to be a valuable tool as researchers gain more experience. They could potentially be used as part of a validation procedure for sediment assessment values or for site-specific SQC. The advantages of this method are that it relates toxicity to a specific chemical and dose and could be used to develop chemical-specific assessment values, it provides a means to test a range of organisms and sensitive life stages, and it most closely approximates the approach for generating WQC. Disadvantages of this approach are that it does not test “real-world” contaminated sediments or in-place organisms. The aging of the contaminated sediments often results in much slower sediment desorption rates than is experienced from unaged labora-
tory spiked samples. Additional research is needed to determine spiking a n d aging procedures for laboratory spiked samples. Tissue residue approach. This approach relies on an ability to establish a maximum concentration of a chemical in sediment that will result in an acceptable concentration in aquatic organisms or consumers of aquatic biota like humans, birds, or mammals (52). This process involves two steps: linking toxic effects to organism residues (doseresidue experiments) and linking chemical residues in specific organisms to sediment chemical concentrations. Linking toxic effects to organism residues can be done by direct laboratory experiments in which the exposure level and the resulting organism tissue levels are measured. Additionally, for chemicals for which WQC have been developed, it is possible to determine a maximum permissible tissue concentration (MPTC) by multiplying the WQC continuous conce,ntration by an appropriate bioconcentration factor (BCF)for the particular chemical. If the BCF is not known it can be estimated from the octanolwater partition coefficient for the chemical. In the absence of a WQC, the no-observed-effect level determined in a chronic aquatic toxicity test for a sensitive organism can be multiplied by the BCF to obtain an estimate of the MPTC. Linking organism residue concentrations to sediment chemical concentrations can be done from site-specific measurements of sediment-organism partition coefficients [53),by equilibrium partitioning (54, by use of organism preference factors (55), and by pharmacokinetic-bioenergetic model predictions of tissue levels resulting from water, food, and sediment contact (56). The advantages of this method are that it uses organisms and sediments of interest to arrive at tissue residue concentrations that should not be exceeded to protect aquatic life, it is not limited to one class of chemicals, and all interactive pathways between sediment and organism are incorporated in the site-specific sediment-organism partition coefficient. Disadvantages of this approach are that it is in a developmental state and has not been widely used. Both the field and laboratory components of this approach introduce uncertainties that make it of limited value until the relationship between chemicals in
1870 Environ. Sci. Technol., Vol. 26, No. 10, 1992
sediments and toxicological effects in organisms is better understood. Freshwater benthic macroinvertebrate community structure and function approach. This is a sitespecific method that relies upon the naturally occurring benthic macroinvertebrate fauna to determine if the sediments at a given location are contaminated beyond the point of being able to sustain a healthy ecosystem. The method uses qualitative and quantitative sampling of the sediment organisms, comparing the species composition, relative abundance, and community function at a study site relative to a “clean” reference site or to the assemblage that would be expected to be present if the sediment were not contaminated. This is an approach that has evolved for both freshwater and marine sediments (57, 58). For purposes of this review, marine and freshwater approaches are combined. Community structure assessment refers to numeric taxonomic distribution of individual species within the community. Functional assessment involves determining trophic (feeding) level and morphological characteristics. Community structure and function are used extensively to evaluate both water quality and sediment quality of flowing [lotic) and standing (lentic) ecosystems. Assessment of benthic community structure and function is an in situ method that can be used alone, but is often used in conjunction with the Triad or Apparent Effects Threshold methods or with sediment bioassays. The approach involves the following steps: organism collection, organism identification, quantification of structure and function, assessment of relationship with other environmental measurements, and a comparison with a reference site. These approaches have been used extensively to assess the impact of chemical or crude oil spills, point source discharges, organic enrichment, pesticide runoff, and highway runoff. They have also been used in several state regulatory programs as optimal measures of designated water use attainment and to develop both narrative and in-stream biological criteria. The state of Ohio has used this approach for developing numerical biological criteria for state water quality standards (59). The advantages of this approach are that benthic macroinvertebrates at the site of concern are used as a direct measure of sediment chemi-
cal contamination, and it integrates chemical interactions of multiple pollutants, is not dependent on route of exposure, and offers a method for prioritizing sites based on the extent of observed effects. Disadvantages of this approach are that it does not describe cause-andeffect relationships, does not provide chemical-specific criteria, and is site specific. Ionic organic chemicals. The assessment of sediment quality relative to ionic organic chemicals is not well developed. The principal reason is that the mechanisms controlling bioavailability are not well understood. The sorption of surfactants to clay and mineral particles has been extensively studied (6062) as have surfactant sorption relative to the onset of critical micelle concentration (CMC) formation (63),effects of chain length, position of the hydrophilic group, pH, ionic strength. and particle concentration (64-66). However, the sorption of ionic organics to sediments has not been reviewed in depth. Recently DiToro et al. (67) published an overall approach accounting for partition coefficient differences between particle types (soils, sediments, and sludges) for anionic surfactants. They demonstrated that CMC can be used to index hydrophobicity of these surfactants. These data also suggest that CMC could potentially be used for other classes of ionic chemicals. This model relates the partition coefficient that characterizes the linear, low- concentration portion of the isotherm to the surfactant CMC as a measure of hydrophobicity as well as to the cation exchange capacity (CEC) of the sediment particles and to the particle concentration itself. The use of CMC was shown to eliminate much, but not all, of the observed variability in partition coefficients for various absorbents. The equation showing normalization for CMC can be expressed in the following manner:
K~ = C,[CEC)~=ICMC where K = the classical soil partition coefficient and c1 and c, are constants. It was also shown that normalizing the data for CEC and particle interaction further reduced the observed variability in partition coefficients. Normalizing the data for the fraction of organic carbon (f0J was shown to be equally effective as CEC in reducing the variability in the observed partition coefficients for various soil-sediment
Direrent sediments can differby a factor of 10 or more in toxicity for the same
types. This would be expected because it has been shown that f. covaries with CEC (Le., CEC varies in proportion with the square root of f..). A lack of observed linearity of Kp to fOc may be partially explained by the fact that as the carbon content increases so does the CEC, which would increase the electrostatic repulsion between the two charged particles, that is, the surfactant and the soil particles. Orth et al. (68) have also shown for C,, linear alkylbenzene sulfonate (anionic surfactant] that both CEC and f. are important parameters for normalizing partition coefficient differences with a somewhat greater dependence on f. than CEC. This approach for assessing partitioning of ionic chemicals is an important step in determining the bioavailability of these chemicals when sorbed to sediments. The approach is in its infancy and will most likely undergo further revision, but it does provide a place to begin to focus research efforts to further define a method to assess sediment quality. Metals. Considerable published data indicate that total metal concentrations on sediments are not good estimators of the “free” and bioavailable fraction of the total chemical present (22, 23, 26). Different sediments can differ by a factor of 10 or more in toxicity for the same total metal concentration. To use toxicity estimates based on chemical measurements there needs to be a way to estimate the bioavailable fraction of the total present. A number of approaches to determine metal bioavailability associated with sediments have been reviewed or tried, including carbon
normalization and sorption of metals in oxic freshwater sediments to particulate carbon and the oxides of iron and manganese (69-72). Recently the dominant role of the sediment sulfides in controlling metal bioavailability has been demonstrated (72-75).Sulfides are common in many freshwater and marine sediments and are the predominant form of sulfur in anaerobic sediments. The ability of sulfides to complex with metal ions to form water insoluble precipitates is well known. This accounts for the lack of toxicity when even high metal concentrations are present on some sediments. The solubility of metal-sulfide complexes are well below the toxic threshold. It has been shown that the solid-phase sediment sulfides that are soluble in cold acid, termed acid volatile sulfide (AVS), are a key factor for controlling the toxicity of cadmium and nickel and potentially several other heavy metals (72,73).No toxicity is observed from these metals when bound to sediment and when, on a molar basis, the concentration of AVS is greater than the sum of the molar concentrations of the sulfidebinding metals. When the ratio of the AVS to metal concentration drops below 1.0, toxicity begins to appear. The important point here is that AVS can be used to normalize sediment metals concentrations in the same way that sediment organic carbon is used to normalize nonionic chemicals. The reason that both methods work is that they both properly account for the chemical activity of the chemical in the aqueous and the sediment phases. The advantages of this method are essentially the same as those of the EP approach for non-ionic organics. The normalization of metal concentrations by AVS would be invalid if the sediment AVS content is very low. This could occur in fully oxidized sediments. Most sediments have at least a small zone where the sediments are oxic near the sediment-water interface. The importance of this zone relative to AVS normalization has not been evaluated. A limited number of AVS measurements have been performed in both marine and h s h w a ter sediments and there is a need for additional data. Evaluation of this approach for other metals and metal mixtures is also needed. A tiered assessment strategy Significant experience has been obtained on environmental hazard
Envimn. Sci. Technol.. Vol. 26, No. 10, 1992 1871
assessment since a 1977 workshop at Pellston, MI, (76)focused on environmental chemical assessment. The conceptual framework supporting the current approaches for assessing chemical hazard uses of data on chemical exposure and biological effects on organisms that is collected in a stepwise (tiered)manner. This allows for periodic decisions to stop if adequate safety is demonstrated or the toxicity is well characterized, or to collect more data if significant questions remain. We have used this conceptual framework to develop an approach
Tier 2 (Investigative)
Tier 3 (Conlirmatory)
1872 Envimn. sci. Technol., Vol ES, No. 10. 1-2
for assessing the significance of chemicals sorbed to sediments (Figure 2).
We recommended that sediment assessment begin in Tier I by deriving Sediment Assessment Values (SAVs). These values could be obtained in a number of ways. For instance, the EP approach could be used to develop SAVs for non-ionic organics, AVS normalization could be used for metals, or cation exchange capacity and critical micelle concentration could be used for ionic organics. Other methods, reviewed earlier in this paper, could
be used for developing SAVs including t h e Apparent Effects Threshold or Triad approaches. We propose that the Tier I SAVs would be used as screening-level concentrations to be compared against sediment chemical concentrations. If the SAV is exceeded by the sediment chemical concentration then additional assessment of the sediment would be appropriate (Tier 11). If the value is not exceeded and the margin of safety is adequate (e.g., ratio between the sediment field concentration and the SAV; 10) no additional testing would be con-
ducted. Limited chronic aquatic toxicity testing and bioaccumulation estimation may be desired in some cases where the margin of safety is small ( 4 0 ) . If no SAV can be calculated for a particular chemical, then Tier I screening toxicity tests would be conducted. Tier I1 is called an investigative tier. In this part of the assessment, the determination is made whether the sediment contains chemicals in amounts toxic to aquatic organisms or whether chemicals with a high potential to bioaccumulate are below levels of concern. Additional testing may be required to define the zone or magnitude of the area that is impacted by the chemicals in the sediments. It is proposed that the zone-of-impact study would include both chemical and biological measurements (Figure 2). If the zone of impact is determined to be large, then additional testing should be considered and one would perform confirmatory tests (Tier III). If the zone of impact is small, a decision could be made that no further action is required or that limited remediation will be performed. Tier III is that part of the assessment approach that would provide in-depth testing of the sediments in the zone of impact to confirm the significance of the chemicals to aquatic life and their potential to move through the food web to other organisms. Multispecies chronic toxicity tests, spiked sediment bioassays, bioaccumulation measurements, and toxicity identification evaluations could be performed as well as infaunal investigation to determine impacts on the aquatic life in the zone of impact. Sufficient data might be collected to perform an Apparent Effect Threshold evaluation or a Triad analysis and calculate a site specific sediment quality criterion, if appropriate. This integrated biological and chemical sediment assessment attempts to provide a comprehensive approach using existing tools to evaluate the significance of chemicals on sediments without using inflexible criteria. The state of the art of sediment contamination assessment is still emerging and, although much progress has been made, it is not at the point where a single value can be generated by existing methods and be used to broadly regulate. This approach is recommended as a comprehensive approach for using existing methodologies to assess sediment quality and protect our nation’s sediments.
cx1sr1ng
methodologies should 6e incorporated into a tiered assessment approach that could be used in regulatory program5
Conclusions Over the past 10 years several methods have been developed for evaluating the environmental safety of aquatic sediments. The AET, EP, and Triad approaches are the most widely used for deriving chemical specific sediment quality values. Bulk sediment bioassays and freshwater and marine benthic macroinvertebrate community structure and function approaches have been used most frequently to determine the presence and significance of sediment contaminants. In the authors’ opinion, no one approach offers all the flexibility, reliability, and scientific credibility that is needed to develop national sediment criteria as have been developed for water. This WQC approach provides numbers which are generally recognized as protective of the environment without being overly conservative. The WQC methodology relies on acute and chronic bioassays with a wide variety of marine and freshwater organisms to calculate the continuous criterion concentration. The credibility of this value is supported by the total weight-ofevidence gathered. Sediment quality assessment is considerably more complex than water quality assessment due to the many site-specific parameters that need to be considered that are not a factor for water. These factors include bioavailability: sorption kinetics: sediment characteristics: sediment deposition, erosion, and compaction: bioturbation; and temporal and spatial differences, to mention a few. The methodologies developed to date do not adequately deal with the complex nature of
sediments when the methods are thought of as national in scope. In summary, we believe that: (1) existing methodologies should be incorporated into a tiered assessment approach that could be utilized in various regulatory programs to evaluate the significance of sediment contamination: (2) additional efforts should be made to validate sediment assessment methods with field tests as they develop: (3) research should be conducted to expand and improve existing methods for assessing metals and ionic organics and to expand chronic toxicity test capabilities: and (4) a more thorough review of the extent of sediment contamination in the United States should be undertaken with the view of establishing sites and chemicals of highest concern and delineating historical problems from current practices. Acknowledgments The authors wish to thank Susan Rh Allen Burton, and Dominic DiToro critical review of this manuscript.
e, a
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William I. Adorns I S vice president of ABC Laboratories' Aquofic Toxicology Program. He worked 14 years a t Mons a n t o Chemical Company in oquotic foxicology, sedimenf assessment, environmental fote, risk ossessment and biological wosfe treatment. He received his B.S. degree from Lake Superior State University and his M.S. degree and Ph.D. from Michigan State University.
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