Methodology for the Implementation of Bioavailability

The derivation of a single generic EQSbioavailable for nickel fulfills the need of European Member States to have a single EQS. However, the robust implementation of the EQS relies upon the use of a bioavailability factor, or BioF, to deliver bioavailability normalization in later tiers of assessment. The BioF is calculated as the generic EQSbioavailable divided by the local EQS.

Tiered Approach to the Implementation of Bioavailability

A tiered approach for implementing the nickel EQSbioavailable has been proposed by the Danish EPA (Figure 1). The first tier of the approach has been described in section Deriving a Nickel EQS Under the WFD and simply considers a direct comparison between the annual average monitored dissolved nickel concentration for a site and the generic nickel EQSbioavailable.

At Tier 2 bioavailability normalization is performed using the bio-met bioavailability tool or a similar user-friendly NiBLM tool (see section The User-Friendly NiBLM—BIO-MET as well as section What Next?).5 The bio-met bioavailability tool calculates the concentration of “bioavailable nickel” at a site and compares this value against the EQSbioavailable. Samples with bioavailable nickel concentrations exceeding the EQSbioavailable progress to Tier 3.

At Tier 3 local factors that may influence nickel risks are considered. These factors could include the use of nickel ambient background concentrations (ABCs). ABCs should be considered only after normalizing for bioavailability. Other local refinements that may be considered could include:

  • the collection of improved site-specific water chemistry data,
  • the use of the full NiBLM, and 
  • an assessment of local ecological data to determine the magnitude of any biological effects.

Tier 4 is reached when it is clear that the EQS has not been achieved and the site (or waterbody) will not attain Good Chemical Status. A program of measures to mitigate the situation may be required. 

The User-Friendly NiBLM—bio-met

The chronic Nickel Biotic Ligand Models (NiBLM) are sophisticated and represent the most advanced understanding about chronic nickel ecotoxicity in freshwaters (see Fact Sheet 4, Section BLM Software). However the data input requirements and expertise required to interpret the outputs mean that the NiBLM is too complex and resource intensive for routine regulatory use.

In order to facilitate regulatory application, a user-friendly version of the NiBLM has been developed. The bio-met bioavailability tool is based on the NiBLM but only requires data on three physicochemical input parameters: DOC, calcium and pH. These three parameters have been found to predominantly influence HC5 predictions by the NiBLM. The bio-met bioavailability tool has been developed from nearly 700 NiBLM HC5 predictions, where DOC, calcium, and pH are varied, but other input parameters are fixed.

The bio-met bioavailability tool is a multi-metal tool that also calculates bioavailable metal concentrations for copper and zinc.5

Other resources include:5

  • A comprehensive base of information on metal bioavailability and its use in the regulatory risk assessment of metals. This section also contains information on the development and validation of the bio-met bioavailability tool. 
  • A series of case studies that demonstrate the application of bioavailability-based approaches within the risk-management of metals in the aquatic environment.

The bio-met bioavailability tool is also available as an online application, which means that bioavailability calculations can be performed irrespective of corporate IT restrictions, software versions, and individual computer processing power.

The performance of the bio-met bioavailability tool against the NiBLM was evaluated using data from Great Britain and The Netherlands that cover a broad range of water chemistries. The bio-met bioavailability tool makes predictions that are moderately precautionary (over protective) when compared to the NiBLM (Figure 2). This is because the bio-met bioavailability tool was designed to behave conservatively relative to the NiBLM, which is appropriate for a second tier assessment.

Figure 2: The NiBLM HC5 predictions (WHAM Model) versus the HC5 predictions from the bio-met bioavailability tool. The solid line is the 1:1 relationship and the hatched lines are with a factor of 2.

Inputs, outputs, interpretation

The bio-met bioavailability tool is designed to be simple to use. Full instructions are provided on the website.5

If pH, calcium concentration, and DOC concentration data for a site are entered into the bio-met bioavailability tool, without corresponding dissolved nickel concentrations, the tool reports a local nickel EQSdissolved and corresponding BioF (Figure 3).

If dissolved nickel data are added to the bio-met bioavailability tool, the bioavailable nickel concentration and the corresponding risk characterization ratio (RCR) are calculated. This last term, also sometimes called the risk quotient, is calculated by dividing the bioavailable nickel concentration by the EQSbioavailable. Values equal to or greater than 1 represent a potential risk (resulting in progression to Tier 3).

The bio-met bioavailability tool can be used in two ways: 

  • to calculate a bioavailable metal concentration from monitoring data for direct comparison with the EQSbioavailable, or 
  • to calculate a site specific EQSdissolved (dissolved metal monitoring data are compared to the site specific EQSdissolved).

The bio-met bioavailability tool operates within the same validated boundary conditions of the NiBLM, which has been modified since the EU RAR as more ecotoxicity data have become available (i.e., pH 6.5-8.7, Ca 3.8-88 mg/L, DOC 0-20 mg/L).

Figure 3: Data entry screen of the bio-met bioavailability tool, the user-friendly NiBLM



Missing input data?

Not all European Member States measure DOC. However, many Member States have recently started monitoring programs. Consequently, there may be some situations where an assessment of bioavailability is required, but data on supporting parameters needed for the bio-met bioavailability tool is not available. This can present a considerable obstacle to the implementation of a bioavailability-based approach.

Alternatives to site-specific monitoring data are sometimes available, although the results from calculations using these data need to be interpreted with caution. For example, “default” values for DOC have been estimated from historic monitoring data (Environment Agency 2009), or predicted from dissolved and total iron concentrations (Peters et al. 2011a) or UV absorption (Tipping et al. 2009).

In addition to surrogate input data, robust data may be available from regulators. For example, Austrian,6 Swedish,7 and French8 data are available from their respective websites. It is possible to undertake targeted bioavailability assessments using these data in order to determine where exactly site-specific data are needed. If only water hardness data are available it is possible to convert these to calcium concentrations from relationships developed using European surface waters (Peters et al. 2011b).
    (last accessed January 2014)
5  The user-friendly NiBLM is incorporated into the multi-metal bioavailability tool available at (last accessed January 2014)

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