Biotic Ligand Modelling Service
The U.S. Environmental Protection Agency has published a draft update of the Ambient Water Quality Criteria for Copper (68 Fed. Reg. 75552, December 31, 2003). This document, which is available at http://www.epa.gov/waterscience/criteria/copper, contains updated freshwater and saltwater aquatic life criteria for copper. In addition to incorporating new toxicity data, the US EPA has adopted the Biotic Ligand Model (BLM; as developed by HydroQual Inc.) in the criteria derivation procedure. The revised criteria are more scientifically defensible as they account for site-specific bioavailability of metals compared to the previous freshwater criteria, which were based on empirical relationships of toxicity to water hardness alone.
What does the model predict? The BLM predicts metal toxicity (e.g., lethal concentrations, LC50s) to a variety of different aquatic organisms (e.g., fathead minnows, rainbow trout, Daphnia magna, Ceriodaphnia dubia) in defined water chemistry. The site water quality parameters required include temperature, pH, dissolved organic carbon (DOC), major cations (Ca, Mg, Na, K), major anions (SO4 and Cl), alkalinity, and sulfide.
How does the model predict toxicity? Ligands are chemical structures that bind with metals. In water, ligands are organic (e.g., DOC) or inorganic (e.g., carbonates, hydroxides, sulphates); in organisms, the biotic ligands are sites that bind metals, like the surface binding sites on fish gills. The BLM assumes that a fixed amount of metal bound to the physiologically active sites in an organism causes toxicity. Essentially, the BLM treats biotic ligands as chemical ligands and takes into account metal complexation (e.g., with DOC), competition (e.g., with calcium for sites at the gill), and concentration (e.g., site specific parameters) when predicting toxicity.
Why is the model a better predictor of toxicity? Most of the toxicity data available in the scientific literature was conducted in laboratory dilution water, which can differ greatly from natural waters and mining effluents. In addition, the measurement of total metal concentrations overestimates toxicity, as not all metal species are toxic. The BLM incorporates the combined influence of pH, alkalinity, dissolved organic carbon, sulphate, and water hardness on toxicity. It also calculates site-specific metal speciation and the concentration of dissolved metal, which will result in toxicity. The BLM can provide a better indication of toxicity on a site-specific basis, especially when used in combination with standard regulatory tests (e.g., the BLM can be used to determine fluctuations in toxicity between regulatory toxicity test submissions).
Can the model be used in Canada? Absolutely. The BLM can be applied to predict metal toxicity in effluent or receiving water. By predicting the influence of site-specific water chemistry parameters on toxicity, the model is a useful tool in designing site-specific water quality criteria, determining potential impacts of metals downstream of effluent discharge points in receiving waters, determining operational treatment options (e.g., metal chelation or pH adjustment), and understanding fluctuations in routine toxicity testing. The model has also made its way into a requirement under a Certificate of Approval (C of A) by the Ontario Ministry of the Environment.
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