BLM – Biotic Ligand Model

Model Elements

  • Cost effective and expedient compared to toxicity test procedures
  • May be able to utilize existing water quality data
  • Effective Management Tool
  • Represents on-going efforts to incorporate the latest science advancements in a tool for  ecological risk assessment
  • Scientifically defensible

Development and Testing of the Biotic Ligand Model for Predicting the Metal Toxicity

A number of HydroQual’s projects have focused on the development of a new approach for assessing metal toxicity to aquatic and other organisms.  Typically, metal toxicity to aquatic organisms is dependent on the local water chemistry.  Variations in pH, concentrations of cations such as calcium, magnesium and sodium, variations in alkalinity and the presence of natural organic matter can all have a significant effect on the toxicity of metals.  As a result, toxicity levels may vary widely for a given metal.  This presents a difficult problem for establishing regulatory guidelines because in the absence of a way to predict these effects, a conservative regulatory limit is typically selected.  The Biotic Ligand Model (BLM) is a predictive tool that can account for variations in metal toxicity using information on the chemistry of local water sources.  Several projects with various funding sources have contributed to parallel development and testing of the BLM for various metals. Versions of the BLM that have been developed or are currently in development by HydroQual include versions for Copper, Silver, Zinc, Lead, Aluminum, Nickel, and Cadmium appropriate for aquatic organisms in freshwater environments. In addition, recent development efforts have included development of Copper and Zinc versions appropriate for aquatic organisms in marine environments.  Versions of the BLM applicable to predicting metal effects in soils have also been underway for both Copper and Nickel.

The benefits of this work have recently led to the release of a revised water quality criteria document for Copper that is based on the BLM . Updates to the water quality criteria for other metals, including silver and zinc, are planned.

The Biotic Ligand Model and BLM-Monte

HydroQual is pleased to distribute the Biotic Ligand Model and BLM-Monte to interested users. Follow the links below to download the latest models.

The Biotic Ligand Model - Windows Interface Version

The Biotic Ligand Model can be used to calculate metal speciation and predict metal toxicity in aquatic systems. The BLM is currently being developed for a variety of metals including copper, silver, cadmium, zinc, nickel, and lead. The Windows Version of the BLM incorporates an easy to use spreadsheet type of environment to facilitate data-entry, model simulations, and the analysis of model output in a common application environment and in an efficient and user-friendly fashion. The BLM is designed for use on the PC family of microcomputers running Microsoft Windows. 

After downloading and unzipping the setup files, please refer to the README document for additional notes on installing the BLM application. The help function within the BLM Windows Interface provides information on the use of the interface application and the contact details for additional help/information. 


BLM-Monte combines metal bioavailability predicted by the BLM with a simple mixing calculation and a Monte Carlo generation of chemical inputs to perform a probabilistic waste load allocation calculation. BLM-Monte can be used to calculate site-specific toxicity information to design an acceptable effluent copper concentration that is below toxicity endpoints. BLM-Monte uses Microsoft Excel to organize model inputs and plot results. (Users must supply their own copy of Microsoft Excel to use BLM-Monte.)

With the availability of a more refined and faster version of the Biotic Ligand Model, BLM-Monte has been updated (BLM-Monte, Version 2.0) and is available for download. In addition to the faster program execution, some of the computational details have also undergone a slight modification. Previously, BLM-Monte used to compute a Site-Specific Water Quality Criterion (SSWQC) using the US Environmental Protection Agency's Water Effect Ratio (WER) methodology and then compute the compliance of the copper discharge with the SSWQC. However, keeping in mind the needs of a broader, international audience, BLM-Monte now predicts the toxicity of the effluent copper by comparing the instream copper concentration with the BLM predicted LC50 for a user-specified aquatic organism. Based on a frequency of occurrence of toxicity that is acceptable to the user, the effluent copper concentration is modified accordingly.