Water Quality Guidelines Online - Sediment Contaminant Guideline exceeded or likely to be exceeded?

The general approach to use of the decision tree shown below is outlined in WQG - Section 3.1.5.
If the lower sediment quality guideline, the trigger value, for a particular contaminant is not exceeded, it is unlikely that it will result in any biological disturbance for organisms inhabiting that sediment. If the trigger value is exceeded, either management (including remedial) action is taken, or additional site-specific studies are conducted to determine whether this exceedance poses a risk to the ecosystem.
Should a �low risk� outcome result after continuous monitoring, there is scope to refine the guideline trigger value. It is important to compatre test sites with suitable references sites that provide background concentrations. Exceedance of a trigger value is acceptable if it is at or below the normal background concentration for a site.
Also any factors which might lower the potential bioavailability of contaminants should be examined. These inlcude sediment speciation (using dilute acid extracts), acid volatile sulphides and pore water concentrations.

Temporal variability is less relevant for sediments which are generally deposited very slowly. When comparing test results with trigger values, and with reference site values, the median may be universally suitable as an estimate of relative concentrations. What are particularly important are the spatial viability and particle size distribution at the sampling site, and the reference sites. Where sediment samples within a test site clearly exceed trigger values, and background levels for the same particle size, or are judged to be hazardous, extra sampling should be undertaken to delineate contaminated zones within a site.