Incorporation of Bioavailability Models

The correction for bioavailability is applied to the effect concentrations [No Observed Effects Concentration (NOEC) and 10% Effects Concentration (EC10)]  in  the  nickel terrestrial toxicity database (see Fact Sheet 2). The steps below need to be accom-plished in order to incorporate bioavailability for the derivation of bioavailability-based’ PNECs of Ni for the terrestrial compartment for use in risk characterization (Figure 6).

Figure 3


Figure 4


Figure 5

Figure 6

In the first step, the added Ni toxicity values, i.e., [Ni]added4, are calculated by subtracting the Ni background concentration (Cb) in the test media from the reported total toxicity values, i.e., [Ni]total.

In the second step the [Ni]added toxicity values are corrected for the difference in Ni bioavailability between laboratory conditions (Ni freshly added as soluble salts) and field conditions, through the application of the L/A factor5. Because the background concentration is assumed to be already aged, the pH dependent L/A factor should only be applied to the [Ni]added which result in [Ni]aged, added6 values. The individual Ni background concentration (Cb) from the control soil is subsequently added to the [Ni]aged, added values resulting in an [Ni]aged, total7 (= [Ni]aged, added + Cb). The derivation and application of the L/A factor is discussed in Correction for Leaching and Ageing Effects.

The third step is the correction of the [Ni]aged, total values towards the specific soil properties of a given site, since it is demonstrated that abiotic factors (i.e., soil properties) affect Ni toxicity in soil. This normalization is based on the slopes of the organism specific regression models between toxicity thresholds and the driving abiotic factor (i.e., eCEC). This approach is discussed in Normalization for Soil Types and result in the calculation of [Ni]normalized, aged, total8.

Subsequently, these [Ni]normalized, aged, total concentrations are further used to calculate the PNECnormalized, aged, total9 values according to the approach described in Fact Sheet 2.

Incorporation of the bioavailability concept as described above will result in the derivation of different Species Sensitivity Distributions (SSDs) and PNEC values, depending on the physicochemical characteristics (eCEC) of the terrestrial environments under assessment.

An overview of the SSDs derived for the different selected soil eco-regions in Europe as defined in Fact Sheet 2 is provided in Figure 7.

The soil physico-chemistry and median HC5 and PNEC values calculated for the different selected eco-regions in EU soils are summarized in Table 3.

Table 3: Overview of the soil chemistry, L/A factors, and median HC5 and PNEC values for the different selected EU eco-regions

Eco-Region  Soil Use Soil Physico-Chemistry L/A
Median HC5 (µg/L)  PNEC (µg/L)10
Acid Sandy Soil in Sweden Arable Land pH 4.8, OM 2.8%, clay 7%, eCEC 2.4 cmol/kg 1.05 8.50 4.3 
Loamy Soil in The Netherlands Arable Land pH 7.5, OM 2.2%, clay 26%, eCEC 20 cmol/kg 3.01 99.20 49.6
Peaty Soil in The Netherlands Grassland pH 4.7, OM 40%, clay 24%, eCEC 35 cmol/kg 1.04 186.3 93.2
Acid Sandy Soil in Germany Forest Land pH 3.0, OM 9%, clay 7%, eCEC 6 cmol/kg 1.00 25.00 12.5
Clay Soil in Greece Woodland pH 7.4, OM 4.5%, clay 46%, eCEC 36 cmol/kg 2.75 192.3 96.2
Soils of Different Types in Denmark Arable & Forest Land pH 6.3, OM 0.6%, clay 8.9%, eCEC 10.4 cmol/kg 1.38 47.10 23.6

4.  [Ni]added = Ni concentrations in the soils beyond which toxicity occurs after subtraction of the Ni background in the test medium.
5.  The L/A factor could be derived from the following empirical chemical model as, i.e., L/A = 1 + exp (1.4(pH-7.0)4. This equation is calibrated on soil aged maximally 1.5 year and soil pH ranged between pH 3.6 and 7.7. This empirical model predicts almost no ageing (L/A < 1.2) up to pH 6 and L/A = 2 at pH 7.0 and L/A = 3 at pH 7.5.
6.  [Ni]aged, added = Ni concentrations in the soils beyond which toxicity occurs after subtraction of the Ni background in the test medium and correction for the L/A effect.
7.  [Ni]aged, total = Ni concentrations in the soils beyond which toxicity occurs after correction for the L/A and addition of the Ni background in the test medium.
8.  [Ni]normalized, aged, total = Ni concentrations in the soils beyond which toxicity occurs after correction for the L/A and normalization for differences in toxicity between different soil types.
9.  PNECnormalized, aged, total = integration of aged and normalized EC10/NOEC values from the terrestrial Ni toxicity database via the SSD (see
Fact Sheet 2 ).
10  PNEC is calculated using an assessment factor of 2.