I
I
S G T VIEWS
A Proposed Approach for Deriving National Sediment Criteria for Dioxin ederal and state environmental regulatory agencies are concerned about effects on human health and aquatic life resulting from exposure to contaminated sediments. A number of initiatives are under way that are designed to result in sediment aualitv criteria ISQCI
F
Criteria or numeric standards for dioxin and other pollutants are based on what is perceived to be the most sensitive endpoint [e.g.. cancer). For example, EPA currently uses a risk assessment formula [adopted in large part by state regulatory agencies) that relates the can-
day)-' [slope of the dose-response curve); W = weight of a human [kg); FC = fish consumption [g fishlday); and BCF = bioconcentration factor (Llg fish). Historicallv. EPA used a BCF to predict pollitant levels in fish and other aquatic life in order to ascertain risks to human health [from consuming fish) or aquatic life. Bioconcentration refers to the accumulation of a chemical from exposure via water only, and the BCF is defined as the ratio ofthe chemical concentration in the aquatic organism to that in the water. The BCF was developed to reflect the chemical activity of a contaminant i n water; therefore, it would be reasonable to assume that the BCF is based on the dissolved fraction of a chemical (termed BCFd). It should be noted, however, that there has heen considerable confusioii over tlie definition of the BCF and other faciors that d e wrilw hioacciimulntion of contaminants. 'l'hc use o f I ~ Cto P prrdict 'I'CI)I) fish tissue levels is prubleinatic Dioxin is a very hvdrop1iot)ir: (:oiiipound, as indicated bv the reported range of its octanol-wder
1
s o u n d science to d e v e l o p regulations, other conditions are required for effective risk management, including a regulatory agency with sufficient resources and a mandate to establish SQC; recognition of the significance of administrative and legal factors in the risk management process; willingness to be receptive to the concerns of the regulated and environmental communities: receptiveness to alternatives [which may require revisiting assumptions); and development of a direct approach that follows, to the extent possible, the existing regulatory framework.
BY ERIK RlFKlN E D W A R D J. B O U W E R w r rndpoint from consumption of contaminated fish and shellfisli to a water quality critwion:
c=
RLXW q* x FC x BCF
(1)
where C = ambient water quality criterion [mglL); RL = risk level; q* = cancer potency factor [mglkgl
0013-936X/9410927-441A$04.5010 0 1994 American Chemical Society
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Environ. Sci. Technol.. VoI. 28. No. 9. 1994 441 A
partition coefficient (Kow between 2.4 x 10’ and 8.5 x 10’) and its sorption partition coefficient (KO, between 1.1x lo3 and 3.8 x 10’) ( I ) . The amount of dissolved dioxin in water is extremely low relative to the amount that is sorbed (bound)to sediment, colloids, dissolved organic carbon (DOC), and particulate organic carbon (POC]. It would be scientifically sound to regulate dissolved dioxin levels using a water quality criterion that is computed from a BCFd; however, it is not practical because dissolved levels of dioxin are well below current analytical detection Limits. Furthermore, because the current dioxin water quality criterion is computed from the total recoverable dioxin concentration (e.g., dioxin dissolved i n the water and sorbed to organic materials), the use of a B C F ~is scientifically inappropriate. If the BCF for TCDD is based on the total chemical concentration in water (termed BCF‘) (dissolved and sorbed to POC and DOC), then it will be necessary to calculate BCF‘S on a site-specific basis. This is because site-specific differences in the amounts of DOC and POC would significantly change the BCF‘. The ratio between dioxin in aquatic organisms and dissolved dioxin is significantly greater (orders of magnitude) than the ratio of dioxin i n aquatic organisms to dioxin that is sorbed to sediment and water column particulates. Efforts to differentiate sorbed dioxin components [e.g., sediment, DOC, and particulates) from freely dissolved dioxin have limited value in a regulatory context. The BCF and bioaccumulation factor (BAF) require that the accumulation of a chemical by an organism be referenced to a water concentration that cannot be reliably measured. Consequently, the BCF and BAF should not be relied on to predict bioaccumulation for highly hydrophobic compounds, such as TCDD, that concentrate in sediments. This point can best be illustrated by BCF values for TCDD, which range from the currently used 5000 to 4,300,000 (2). The wide disparity in BCF values (approximately three orders of magnitude) exists because there is no clear definition of the term “BCF,” and dissolved concentrations of TCDD are too low to be reliably measured. Dioxin is a solids contaminant problem, and regulatory decisions regarding this pollutant should reflect this view.
Biota-to-sediment accumulation factor Because highly hydrophobic cbemicals distribute predominantly in association with organic carbon, concentrations can be reliably measured in sediments or sediment organic carbon. Therefore, a factor that relates dioxin sediment organic carbon to fish tissue concentrations could be used as a predictor of bioaccumulation. A biota-to-sediment accumulation factor (BSAF) [the term replaces bioavailability index (BI), accumulation factor (AF), and biota-to-sediment factor (BSF)] was developed (3). It is defined as the ratio of the concentration of chemical in the organism normalized to the lipid content (Clipid) to the chemical concentration in the sediment normalized to organic carbon (Cm). Therefore, BSAF =
‘lipid
coc
(2)
where: BSAF is unitless, Clipid= concentration of contaminant i n lipid of fish (mg of contaminantlkg lipid), and C,, = concentration of contaminant in bottom sediment organic carbon (mg of contaminadkg of sediment organic carbon). A recommended regulatory framework for using a BSAF to establish sediment-based dioxin criteria protective of human health and aquatic life will be discussed below. BSAF values were determined for fish in Lake Ontario ( 4 ) by measuring lipid-normalized TCDD concentrations in different fish species and TCDD in sediment organic carbon by collecting surface sediment samples from 60 locations throughout the
442 A Environ. Sci. Technol., Vol. 28,No. 9. 1594
lake. The lakewide average BSAFs calculated for several fish species appear in Table 1. The Lake Ontario BSAFs vary from 0.03 to 0.20, a much narrower range than corresponding BCF or BAF values (2). BSAF values computed for additional locations and fish are compared i n Table 1. BSAFs computed for TCDD in lake trout from Lake Ontario (0.03 to 0.07) correlate well with the BSAF measured in brown bullheads in the Fox River near Green Bay, WI (0.05). Based on the limited data, these BSAFs appear to consistently predict the bioaccumulation potential of TCDD at different sites despite large differences in ecosystem characteristics and fish species. Dioxin sediment quality criteria An SQC for TCDD and other hydrophobic contaminants can be calculated by replacing the BCF with a BSAF in the standard risk assessment formula.
SQC =
RLXW q* x FC x BSAF
The rationale for this measure of bioaccumulation is that for many extremely hydrophobic chemicals, such as TCDD, nearly all of the chemical is associated with sediment. The BSAF relates contaminant concentrations in sediment to aquatic organism contaminant concentrations. Because the BSAF addresses exposure via sediments, the resulting criterion establishes limits for sediment concentrations. If the BSAF can predict, with some degree of accuracy, fish tissue concentrations, then a rigorous accounting of exposure paths from all sources of di-
TABL
Steaay-stalemora-to-seaimentaccumulation factors for TCDDSediment
Blotalo-rediment nccumulat ion
factor
Wies
Location
characterlsllc
Brown trout Lake trout Small bass White perch Yellow perch Smelt
Lake Ontario
Lakewide mean 0.03
Sculpin
” ”
I
n
” ”
x
n
(3)
”
0.07 0.05 0.20
0.03 0.04 0.12 0.27
Carp
Reservoir mean
Bullhead F Large-mouth bass Brown bullhead catfish
cPgment mean 0.05 0.08
Reference
4 4
oxin (e.g., dissolved) is not necessary for the development of regulations. The development of an appropriate risk equation will enable regulators to determine human health risks associated with contaminated sediments based on the integration of equilibrium partitioning concepts and empirical measurements. Central to this approach is the use of a factor that accurately predicts fishlshellfish contaminant levels. The BSAF is a scientifically defensible means of predicting fish tissue contaminant concentrations based on levels in the sediment (8,9). Selection of an appropriate and conservative BSAF will require a critical review of the literature and sampling (fish and sediment) to determine whether a universally applicable BSAF can be developed or whether site-specific indices must be considered. Fish and sediment data have been recorded at sites throughout the country and could be used to initiate an evaluation of the consistency of the BSAF. Once a BSAF has been determined, sediment criteria could be calculated by substituting the BSAF for the BCF in EPA's standard risk equation. Site-specific parameters, if necessary to the calculation, could he further investigated to delineate the range of values that may exist. The resulting criterion established using the BSAF will be a sediment quality objective based on the protection of human health (aquatic life criteria could also be determined based on this metbodological approach). Compliance with this criterion could be determined by sampling surface sediments and determining whether dioxin samples are higher or lower than the SQC. If compliance were to be based on an effluent dioxin limit, it would be necessary to develop a pipe-to-sediment model. Conclusions and recommendations Dioxin is a solids contaminant problem, which should be reflected in the development of criteria designed to protect human health and aquatic life. In this light, the appropriateness of developing water quality criteria for a strongly hydrophobic compound such as dioxin should be reassessed. BSAFs should be considered as a factor to be used to predict dioxin levels in fish tissue. Although limited existing data indicate that BSAFs are consistent, further work is needed, first, in evaluating the BSAF and sediment as predictors of dioxin fish tissue levels and, sec-
ond, in determining whether SQC can be established by using a BSAF. The use of empirical data to establish dioxin SQC will avoid the problems encountered when improvements in analytical capabilities result in new compliance limits (e.g., dioxin water quality criteria). The application of an accepted risk equation, with slight modifications, should significantly facilitate the risk management process as it relates to dioxin SQC.
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Erik RiJkin is president of Rifkin and Associates, Inc., on environmental consulting firm in Columbia, MD. He hos a Ph.D. in zoology from the Universitv of Hawaii. He also had a National Acudcmt!. of Scimces Postdoctoral Associnfpsliip at the Naval Medical Research Institute in Bethesda, MD. He has publislird a number of articles on the environmentalfate of dioxin.
IL
Edward I. Bou wer is a professor of environmental engineering at the lohns Hopkins University. His research foport and fate of oranic contaminants t.sptwioll!. hiodr~yadationof hydrophoDir chemicals orid applications to in situ biorrnvdiation He has a Ph.D. from Stonford University. References
D.;Shiu. W. Y.;Ma, K. C. IIlustrated Handbook of PhysicalChemical Properties and Environmental Fate for Organic Chemicals; Lewis: Chelsea, MI, 1992, pp. 400-09. Cook, P. M. et al. "Interim Report on
(1) Mackay.
(2)
Data and Methods for Assessment of 2.3.7.8-Tetrachlorodibenzo-p-dioxin Risks to Aquatic Life and Associated Wildlife"; Environmental Protection Agency: Washington, DC. 1993: EPA/
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600/R-93/055. (3) Ankley, G. T. et al. Can. I. Fish. Aquat. Sci. 1992.49,2080-85. (4) Carey, A. E.; Shifrin. N. S.; Cook.
P. M. In Lake Ontario TCDD Bioaccumulation Study-Final Report US. Environmental Protection Agency: New York. 1990; Chapter 9. ( 5 ) Batterman, A. R. et al. Chemosphere 1989,19.451-58. (6)
@ ACS Software
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1989.16,667-79. (7) Schell, J.D., Jr.: Campbell. D.
M.;
Lowe. E. Environ. Toxicol. Chem. 1993.
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(8) R i k i n , E.; LaKind. J. I. Toxicol. Endran. Health 1991, 33. 103-12. (9) LaKind. 1.: Rifkin. E. Environ. Sci. Technol. 1991,24(7),963-65. Environ. Sci. Technol.. VoI. 28, No. 9. 1994 443 A
