Biotic ligand models : availability, performance and applicability for water quality assessment

Biotic ligand models (BLMs) for heavy metals were developed in past years as tools for water quality assessment, following the Water Framework Directive (WFD). The essence of BLMs is that chemical speciation is incorporated in the assessment of ecotoxicological risks for aquatic species. For WFD purposes, BLMs are recognized as useful concepts to determine site-specific risks, and are allowed as second-tier assessment method. Concepts have been made operational, resulting in tools that make the approach available to a larger audience. As a consequence, these tools are in general simplifications of the original, validated concept. The goal of this study is to test the performance of available tools that aim at predicting risks, or related water quality assessment, based on the concept of biotic ligand modelling, in order to aid in the recommendation for second-tier assessment method for heavy metals in surface waters. The tools that were tested in this study are: 1 BLM EU-RAR Validated BLM models for Cu, Ni and Zn, used in the European (Voluntary) Risk Assessment Reports. Extended and operationalized by Deltares, NL. 2 BLM (HydroQual) A simplified tool (version 2.2.3) based on the BLM EU-RAR, using statistical functions. Developed by HydroQual, USA. 3 BLM (WCA) A simplified tool (version 8) based on the BLM EU-RAR which makes use of statistical functions to determine bioavailable fractions. Developed by WCA Environment ltd, UK. 4 Ni-BLM (ARCHE) A simplified method to calculate potential no effect concentrations for Ni. Version 10, developed by ARCHE, BE. 5 Transfer functions (STOWA) Statistical zero-order relations of HC5 values and DOC concentrations, based on local data sets. Published by STOWA, NL. For the five tools that were tested in this study, a large dataset of water composition measurements was composed that was used as input for all selected models. Data for water chemistry and metal concentrations were collected from the iBever National monitoring database. This database was supplemented with data from various monitoring programs of different local water managers. The database contained a grand total of 2575 records with geographical information and surface water compositions in the Netherlands. These data covered most of the water types that are described in the Water Framework Directive. The database was constructed to contain all parameters that are required to perform the chemical speciation calculations and the BLM modeling.