The importance of dissolved organic carbon in the assessment of environmental quality standard compliance for copper and zinc

The regulation of metals in the aquatic environment through the use of environmental quality standards (EQSs) presents a challenge to environmental regulators. Bioavailability corrections through the use of biotic ligand models (BLMs) allow an accurate assessment to be made of potential metal toxicity, but the implementation of such corrections requires additional data, which may not be available. This report sets out a methodology for the estimation and validation of waterbody and hydrometric area specific dissolved organic carbon (DOC) concentrations, one of the most-important parameters required to run the BLMs, for use in assessing compliance against EQSs within a tiered assessment approach.

The BLMs for both copper (Cu) and zinc (Zn) indicate that the most-sensitive conditions for exposure to these metals occur when DOC concentrations are low, and particularly if this occurs in combination with relatively extreme pH conditions (i.e. pH <6 for Cu). These extreme pH conditions are also close to the validation boundaries of the models.
The generic predicted no-effect concentration (PNEC) values provided in the Cu Existing Substances Regulations (ESR) voluntary risk assessment report are calculated for conditions that may be considered representative of several different areas or regions within Europe. Considering the range of bioavailability conditions across Europe, a generic reasonable worst case PNEC, protective to 90 per cent of the EU surface waters, was determined. Because the UK surface waters include water chemistry conditions that cover relatively wide ranges for all of the important BLM input parameters, it is necessary to consider the suitability of such a generic PNEC for a local compliance assessment. 
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