The assessment of compliance of surface waters with environmental quality standards (EQSs) for metals has to deal with a number of issues that do not apply to other substances. For example, it is known that a proportion of a given metal dissolved in water is of limited bioavailability. This non-available fraction does not exert any adverse environmental effects. In order to account for this, bioavailability models known as biotic ligand models (BLM) have been developed. Whilst these can be applied to the majority of water types that might be encountered, there are some water types in the UK for which they have not been calibrated. To this end, the Environment Agency has commissioned a report to address BLM boundary condition issues that affect compliance assessment for copper and zinc in waters of low pH and low calcium concentration.
Desk top study to evaluate options outside BLM thresholds (low pH and low calcium issues)
The Environment Agency has created a simplified version of the BLM to allow reporting of bioavailable metal as part of its routine monitoring programme within a tiered assessment approach to metal compliance. The simplified versions are known as bioavailability assessment tools. The input parameters required to determine bioavailable copper and zinc levels using these tools are dissolved organic carbon (DOC), pH and calcium. The BLM allow for unbounded inputs for DOC but only provide acceptable outputs in the range pH 6 to 9 for zinc and pH 5.5 to 8.5 for copper. Below the lower pH range, the default output is 100 per cent bioavailability for the metal. In terms of compliance reporting, this raises the possibility of step changes in water body compliance in response to a small change in pH from inside to outside the pH boundary conditions. In the case of zinc, outputs relating to calcium (Ca) concentrations less than 7 mg/l also default to a bioavailability of 100 per cent and to the lower predicted no effect concentration (PNEC) applicable to soft waters. This situation raises unacceptable difficulties for compliance reporting without necessarily reflecting the true nature of the bioavailability of the metal. This project seeks to provide the Environment Agency with a method to assess metal bioavailability in low pH waters.