Tiered approach to the assessment of metal compliance in surface waters

The aquatic environment is sensitive to damage from a wide range of chemicals.  Environmental quality standards (EQSs) are one of the instruments  to protect and improve water quality. Derived from toxicological data, the EQS values set limits for chemicals and elements in water bodies. This project has found that a tiered assessment system offers a viable option for considering metal compliance. 

Zinc and copper have been used to investigate the approach, using predicted no-effect concentrations (PNEC) from EU risk assessment reports (RAR) as the potential basis for EQS values. The tiered approach has three levels; site-specific assessments of water quality only move up to the next level when observed metal concentrations exceed the value of the preceding tier. The tiers progressively take account of the background concentrations of metals and their bioavailability: 

Tier 1: compares observed concentrations with the PNEC, akin to a provisional EQS 
for copper, but which represent a very conservative value for zinc. The copper PNEC 
includes a nominal background concentration for test conditions, but not ambient 
environment conditions, and a minimal correction for bioavailability. The zinc PNEC 
does not factor in either background concentrations or bioavailability. 
Tier 2: compares observed concentrations with the PNEC plus an accepted aquatic 
background concentration of the metal to reflect regional or local situations. 
Tier 3: compares observed concentrations with a predicted ‘bioavailable’ PNEC 
derived from biotic ligand models (BLMs)
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