Aquatic Toxicology 84 (2007) 236–246 Assessing metal bioaccumulation in aquatic environments: The inverse relationship between bioaccumulation factors, trophic transfer factors and exposure concentration David K. DeForest a,∗ , Kevin V. Brix b , William J. Adams c a Parametrix, Inc., 411 108th Avenue NE, Suite 1800, Bellevue, WA 98004, United States b EcoTox, 8963 Hampton Landing Dr. E. Jacksonville, FL 32256, United States c Rio Tinto, 8315 West 3595 South, Magna, UT 84044, United States Received 28 November 2006; received in revised form 5 January 2007; accepted 5 February 2007 Abstract Bioaccumulation potential in aquatic biota is typically expressed using ratios of chemical concentrations in organism tissue (typically whole body) relative to chemical exposure concentrations, such as bioconcentration factors (BCFs). Past reviews of metal BCFs for aquatic biota, which account for water-only exposures, have shown that BCFs are often highly variable between organisms and generally inversely related to exposure concentration. This paper further evaluates trends in metal bioaccumulation data by evaluating data for bioaccumulation factors (BAFs) and trophic transfer factors (TTFs). Bioaccumulation factor data were compiled from field studies that account for combined waterborne and dietary metal exposures. Trophic transfer factor data for metals were compiled from laboratory studies in which aquatic food chains were simulated. Natural aquatic food webs are rarely sufficiently understood to properly evaluate exact predator–prey relationships (i.e., TTFs). Results indicate that field BAFs, like laboratory BCFs, tend to be significantly (p ≤ 0.05) inversely related to exposure concentration. Bioaccumulation factors are frequently 100–1000 times larger than BCFs for the same metal and species. This difference is attributed to both lower exposure levels in the field and inclusion of the dietary exposure route. Trophic transfer factors for the metals reviewed, including selenium and methyl mercury were also observed to be inversely related to exposure concentration, particularly at lower exposure concentrations. These inverse relationships have important implications for environmental regulations (e.g., hazard classification and tissue residue-based water quality criteria) and for the use of metal bioaccumulation data in site-specific environmental evaluations, such as ecological and human health risk assessments. Data presented indicate that for metals and metalloids, unlike organic substances, no one BAF or TTF can be used to express bioaccumulation and/or trophic transfer without consideration of the exposure concentration. © 2007 Elsevier B.V. All rights reserved. Keywords: Metals; Bioaccumulation; Trophic transfer; Hazard assessment; Risk assessment 1. Introduction Current aquatic hazard identification procedures for chemical contaminants, such as those developed by the OECD, are based on persistence, bioaccumulation, and toxicity (PBT). The basic premise behind these procedures is that substances which are toxic present greater hazard when they are both persistent and bioaccumulative. Given the large number of chemicals currently in use worldwide, and the extremely large number of new chemi- cals developed every year, defining critical levels of persistence, ∗ Corresponding author Tel.: +1 425 458 6304; fax: +1 425 458 6363. E-mail address: deforest@parametrix.com (D.K. DeForest). bioaccumulation potential, and toxicity must be generalized for practicality. For metals, however, defining critical PBT levels is problematic because (1) metals are naturally persistent in the environment; (2) both essential and non-essential metals are nat- urally bioaccumulated and internally regulated using different strategies (e.g., active excretion, storage); and (3) the toxicity of metals is highly influenced by geochemical factors that influence metal bioavailability. This paper focuses on the uncertainties in using the bioac- cumulation component of PBT to regulate metals. Under the current PBT classification scheme, bioaccumulation potential is defined based on the ratio of a chemical’s concentration in an organism to the chemical concentration in the water to which the organism was exposed (typically termed a bioconcentration fac- 0166-445X/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.aquatox.2007.02.022