Metals Toxicity & the Biotic Ligand Model

Metals Toxicity & the Biotic Ligand Model

The bioavailability of metals to aquatic organisms is highly dependent on water chemistry conditions that vary from site to site. For example, complexation with dissolved organic carbon (DOC) and competition with cations such as calcium reduce metals bioavailability. Other site-specific factors, such as pH, also influence metal speciation, with different metal species having different degrees of bioavailability. Ultimately, this means that the concentration of a given metal, on a total recoverable or dissolved basis, that may present toxicity to aquatic life is highly variable among sites.

Regulatory criteria that do not account for the key factors affecting metal bioavailability could be greatly over- or under-protective of the aquatic life uses that they are intended to protect. Over-protectiveness can result in extensive effort and cost to manage a problem that does not exist, while under-protectiveness can result in unacceptable risk of detrimental effects on the aquatic community.

Properly accounting for metal bioavailability is critical to reduce uncertainty in any metals risk assessment or site-specific water quality evaluation, including the development of National Pollutant Discharge Elimination System (NPDES) permits and total maximum daily loads (TMDLs) in the United States. Members of Windward were early collaborators in the development of a tool called the Biotic Ligand Model (BLM), which allows for mechanistic predictions of bioavailability and toxicity for several metals. We are actively involved in the ongoing development of BLMs for additional metals, as well as for metal mixtures. We have extensive experience in the application of metal BLMs in support of proposed ambient water quality criteria (AWQC) updates for both the US Environmental Protection Agency (EPA) and individual states, site-specific assessments, and evaluations of whether criteria are protective against toxicity endpoints of concern for threatened and endangered species.

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